PST mod is it worth it?

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PST mod is it worth it?

Post by minhlead »

Hi guys,
After seeing tons of great PST mod shot, I am really considering getting 1 modded. I mean who can say no to this ? https://www.astrobin.com/full/6q51hx/0/ ... c&mod&real
The mod seems straight forward enough and most of the expense seems to be from the ERF.
Could anyone with experience with PST mods can shed some light here?
1. Cost/performance/durability of the mod (should I bother or just straightout buy a commercialized solar scope if cost is a big concern? Is the PST have some kind of "etalon lottery" that I should be aware of? Is the scope after mod safe/reliable/durable enough?
2. Double stack vs single stack: is single stack enough for imaging? (I do not do visual) and what double stack configuration is the best if double stack is a must.

3. How big is the FOV conmpared to say a Quark?
4. Stage 1 vs stage 2 mods? As I can see, stage 1 seems to be simpler. Are there advantages to stage 2 mod?
Thanks!


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Re: PST mod is it worth it?

Post by thesmiths »

Having built a PST mod recently viewtopic.php?f=9&t=32108, I am sceptical of the relative advantages. I had all the parts around so the incremental cost of building it was small. I think using a coloured glass filter inside the tube seems to work fine so the cost of the ERF can actually be fairly small.

The big issue is what you call the "etalon lottery". If the etalon is significantly off-band, a ring "sweet spot" will appear that limits the useful FOV. There is nothing you can do to fix that except try another etalon. My suspicion is that the used PSTs you can buy are not the best examples (the "golden" ones are presumably hoarded). In my (limited experience), the good FOV for a typical etalon will be quite small. In which case, it has no advantage over a Quark.

In some sense, the Quark is functionally doing the same thing as a PST mod. As I understand it, there is an "etalon lottery" with Quarks as well. The best etalons are binned to use in their more expensive instruments. But my guess is the variance is smaller than with PST etalons.

Also, the DayStar etalon binning has to do with the "finesse" of the etalon (i.e. the bandwidth). Since the tuning mechanism is totally different than the PST, there is no "sweet ring" effect. So I believe you will always find the available Quark FOV will be on-band, just some examples may not have as narrow a bandwidth as others. DayStar is very upfront about the Quark having no guaranteed bandwidth https://www.daystarfilters.com/Quark/Qu ... mity.shtml:
A Quark will never be warranted to perform precisely to the standard of a different Quark, due to the range of acceptable FWHM bandpass within the stated specification standard. More precise certification and smaller FWHM category ranges greatly increase the production and qualification costs of Quark filters due to degree of optical precision and assoicated cost required.
If you want the very long focal length, high magnification views as shown in the Astrobin link you mention, I would go down the Quark route. Also, I think you can always resell a Quark and get back most of the money you spent. But I don't think you will recover any of the money invested in a PST mod if you decide you don't like the results.


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Re: PST mod is it worth it?

Post by christian viladrich »

There is indeed some variability in the PST etalons.

I recently tested two of these etalons (spectrometer + interferometry with Ha lamp) :
- one etalon has a FWHM of 1 A, which is in line with the spec (FWHM < 1 A),
- the other was much better with a FWHM of 0.61 A.

I refer here to the average FWHM measured over the whole surface of the etalon, and for a collimated beam.

I think the second etalon was hand-picked (I am not sure, it is not mine).


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Re: PST mod is it worth it?

Post by thesmiths »

christian viladrich wrote: Tue Jul 13, 2021 8:30 pm I recently tested two of these etalons (spectrometer + interferometry with Ha lamp) :
- one etalon has a FWHM of 1 A, which is in line with the spec (FWHM < 1 A),
- the other was much better with a FWHM of 0.61 A.
Christian: do you also measure how well tuned the PST etalons are? My impression is that the big problem with the PST etalons are that if the natural tuning of the airgap is quite far off resonance for H-alpha, then the tilt must be large and therefore you get a severe on-band annulus. Or does your measurement of average FWHM take that into account in some way?

With the Quark, the heating of the mica can get you to a precise resonance; their issue with finesse is, I assume, up to the flatness and parallelism of the mica.
Last edited by thesmiths on Wed Jul 14, 2021 2:23 pm, edited 1 time in total.


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Re: PST mod is it worth it?

Post by minhlead »

christian viladrich wrote: Tue Jul 13, 2021 8:30 pm There is indeed some variability in the PST etalons.

I recently tested two of these etalons (spectrometer + interferometry with Ha lamp) :
- one etalon has a FWHM of 1 A, which is in line with the spec (FWHM < 1 A),
- the other was much better with a FWHM of 0.61 A.

I refer here to the average FWHM measured over the whole surface of the etalon, and for a collimated beam.

I think the second etalon was hand-picked (I am not sure, it is not mine).
Thanks, Christian. Look like I will have to stick to my Quark for now.


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Re: PST mod is it worth it?

Post by minhlead »

thesmiths wrote: Tue Jul 13, 2021 2:49 pm Having built a PST mod recently viewtopic.php?f=9&t=32108, I am sceptical of the relative advantages. I had all the parts around so the incremental cost of building it was small. I think using a coloured glass filter inside the tube seems to work fine so the cost of the ERF can actually be fairly small.
Shouldn't the ERF be polished to 1/4 wavelength or better? I can remember Daystar explain why their erf which is pretty much a colored glass so expensive and one of the main reason is that it have to be polished to 1/4 wavelength.
Also, I am nervous about using a absorbs filter in focused light cone. Is it okay doing that? Would it work better if I use a DERF or a halpha 7nm filter as erf instead. I use the Ha DSO narrowband fikter with my Quark and it works great.


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Re: PST mod is it worth it?

Post by minhlead »

thesmiths wrote: Tue Jul 13, 2021 2:49 pm
Also, the DayStar etalon binning has to do with the "finesse" of the etalon (i.e. the bandwidth). Since the tuning mechanism is totally different than the PST, there is no "sweet ring" effect. So I believe you will always find the available Quark FOV will be on-band, just some examples may not have as narrow a bandwidth as others. DayStar is very upfront about the Quark having no guaranteed bandwidth.
From personal experiences, my Quarks does have sweet spot and that sweet spot does move around the FOV while you "tune" the knob. Mine has significant part of the FOV just washed out without any detail and that part moves in a "scanning" fashion from top to bottom when you tune the knob
The size, location of the sweet spots is varies from unit to unit and also the bandpass varies too which make any given Quark performance is quite up to the coin toss.


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Re: PST mod is it worth it?

Post by Rusted »

PST mods can provide endless frustration and fairly regular elation.

I am still at a loss to explain the variability of my single PST etalon.
Though Mark has mentioned an etalon's lifetime heat load.

My own 6" f/10 PST mod can go from utterly amazing to extremely irritating and is recently getting worse!
For some reason [possibly heat load] it has recently developed a severe ring.
I can no longer find a rotation and tilt setting which provides even lighting across the entire frame.
Which often means heavy cropping to be rid of brighter [off-band] corners.
Which is exactly how it started out when I bought the PST secondhand to make a 6" f/8 mod!

I have my etalon open so that I can apply gentle pressure to the inner etalon ring via metal rod.
In the past I could gently tilt the etalon and the whole field of view would magically darken on band.

Note that I am using a Baader, full aperture D-ERF. Not the earlier, internal sub-aperture.
The D-ERF does not provide a cool beam as claimed. Not with a 150mm [6"] aperture.
I have checked the pencilled arrow is pointing towards the sun more times than you would believe!

So the heat protection is further reinforced by a 2" Baader 35nM H-alpha filter and a BelOptic KG3.
These are pushed well up into the light cone on extension tubes forwards of the focuser.
The H-a filter is placed first to reflect heat away before the absorption provided by the BelOptic KG3.

I am absolutely certain that my entire filter-etalon-BF stack is far stiffer than it has ever been.
Gone are the days of a drooping focuser, drooping mishmash of different sized fittings and floppy camera support.

The PST etalon is at exactly 200mm inside focus. Not some arbitrary guessing distance as in the past.
I use a range of GPCs [individually] on the camera nose to increase scale for my close-ups.
These narrow the field of view. So should provide more even lighting from any sweet spot.
Only very rarely can I use a 2x Barlow because of the seeing conditions.

Despite all of this I have enjoyed a number of SPODs from my endless hours at the telescope.
I don't think the forum judges were being particularly charitable. :mrgreen:

Over time my expenditure on my PST mod has climbed and climbed.
Moving tirelessly towards a large fraction of the retail price of larger, commercial H-a scope.

The Lunt B1200S2 BF was another large investment to replace the original BF5 of the PST.
I still need the PST BF for proms because of the much lower transmission of the Lunt.
The Lunt demands SharpCap gain settings through the roof for proms.
This, despite constant fiddling with the etalon tuning for maximum brightness.

As a lifetime meddler and fiddler I have enjoyed the PST Mod, H-a journey immensely.
Those of a nervous character, or sporting several thumbs, should probably not follow my lead. :D

If I could trust a retailer to supply a decent quality Quark I would buy one like a shot.
I spent far too much money buying a poor, secondhand PST last time.
I'm not about to buy another £500 lottery ticket for a £25 consolation prize!
Nor pay £1500 for a ticket to a Quark no better than a poor, secondhand PST.
I have far too much invested in time and money already!


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Re: PST mod is it worth it?

Post by MalVeauX »

Hi,

PST mods are ok, if you have one already. It's easy to get it and the adapters are commercially made (AOK Swiss) and it's meant for rear-mount use and it's super easy to mount a small D-ERF in front (2" 50mm DERF works on apertures upwards of about 120mm and lower basically). But I would argue it's not that attractive after you get more into and realize the limitations. The primary problems with PST etalons are: (1) highly variable quality, most of them are bad with poor uniformity; (2) they're only 20mm clear aperture which means very small jacquinot spot (sweet spot) and it's exacerbated with larger apertures (field angle vs acceptance angle).

Most of the images you see from a PST mod are from someone who got a good PST etalon to begin with; hand picked. Either they got lucky on their first sample or they went through a few samples to get that good one. They're very variable. The uniformity is a huge gamble on them. A good measurement doesn't show the uniformity and the uniformity is argulably the most important parameter of the etalon. So while prominent imagers using PST etalons in mods are a great advertisement for this, it's simply not that simple since most PST etalons are quite bad. If you're ok with this gamble, and you're willing to go through several of them to get an average or good one, or if you're ok with whatever your first results are, then sure, it's something to do. But I would not argue that it's the ideal way to go at all, anymore.

And so I would ask.... what are your total goals, aperture goals, etc, and ultimately what's the price difference between a full PST mod and just buying a Quark or similar rear-mounted HA filter. PST etalon ($450ish USD used usually) + adapters ($120~150 after shipping usually) puts you in that $600~750 USD range after any other bits you may find you need to get it working. It's not that much cheaper than a Quark and the sweet spot is smaller on the PST mod than the Quark. Just something to think about, and it's all too often completely left out of these discussions the size of sweetspot.

I would weigh your options between Quark, PST mod, Lunt 35 Mod, Lunt 50 Mod, Lunt 60 Mod, etc. The better mod is going to come from the largest etalon (for larger sweet spot result) and the greatest uniformity of the etalon.

+++++++++++++++++++++

Here's what you don't see, how much FOV you lose to the small sweet spot of PST etalon. This is with a mere 100mm aperture scope with a large 1.1" IMX253 sensor. Within the sweet spot, I have great contrast and on-band, a very good sample of a PST etalon (pre-Meade; took 3 gambles to get one for me). But you can see the extremely limited FOV in the jacquinot spot where it quickly falls off band. I have to crop all that. It's small even with a tiny sensor too. I still lose 20~30% easily from my FOV with an IMX290 sensor on a larger scope (120mm, 150mm and 200mm apertures). I often have to do a mosaic for some large features when using my larger apertures due to the limited FOV of the jacquinot spot result of the PST etalon size.

100mmF10_PST_IMX253_test2.png
100mmF10_PST_IMX253_test2.png (714.79 KiB) Viewed 6063 times
Very best,
Last edited by MalVeauX on Wed Jul 14, 2021 4:53 pm, edited 2 times in total.


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Re: PST mod is it worth it?

Post by thesmiths »

minhlead wrote: Wed Jul 14, 2021 4:21 am
thesmiths wrote: Tue Jul 13, 2021 2:49 pm I think using a coloured glass filter inside the tube seems to work fine so the cost of the ERF can actually be fairly small.
Shouldn't the ERF be polished to 1/4 wavelength or better? I can remember Daystar explain why their erf which is pretty much a colored glass so expensive and one of the main reason is that it have to be polished to 1/4 wavelength.
Also, I am nervous about using a absorbs filter in focused light cone. Is it okay doing that? Would it work better if I use a DERF or a halpha 7nm filter as erf instead. I use the Ha DSO narrowband filter with my Quark and it works great.
With regards to internal ERF: the high quality photographic filters like Hoya seem to have very good (but unspecified) flatness. I have seen Hoya specify lambda/4 for some of their specialty filters. My own experience was using a telescope of 90mm diameter and the filter placed where the light cone was 70mm diameter. The increase in energy density would then be approximately (90/70)^2 = 1.65 times vs the filter just sitting in the sun. I don't think this is a very dramatic increase. I also used a reflective 2" IR filter closer to the etalon (which is much cheaper than a H-alpha filter). The bandpass between the 25A long pass filter and the IR low pass filter nicely overlaps the H-alpha line.


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Re: PST mod is it worth it?

Post by thesmiths »

MalVeauX wrote: Wed Jul 14, 2021 2:36 pm PST mods are ok, if you have one already.
But I would argue it's not that attractive after you get more into and realize the limitations.
I agree completely with this assessment on the attractiveness of the PST mod currently. Perhaps in the old days, when the PST etalons were made in the USA, the chance of getting a good one was higher. These days, I think the etalon lottery favours the Quark (still made in the USA) vs the typical PST (made in Mexico). The larger Lunt pressure tuned etalons likely have a much higher chance of mod success but the cost of those is quite high.

As I mentioned, I assume you can resell an "average" Quark, return a "poor" quality Quark to DayStar, and eventually stumble upon a "nice" Quark.


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Re: PST mod is it worth it?

Post by MalVeauX »

thesmiths wrote: Wed Jul 14, 2021 3:18 pm
MalVeauX wrote: Wed Jul 14, 2021 2:36 pm PST mods are ok, if you have one already.
But I would argue it's not that attractive after you get more into and realize the limitations.
I agree completely with this assessment on the attractiveness of the PST mod currently. Perhaps in the old days, when the PST etalons were made in the USA, the chance of getting a good one was higher. These days, I think the etalon lottery favours the Quark (still made in the USA) vs the typical PST (made in Mexico). The larger Lunt pressure tuned etalons likely have a much higher chance of mod success but the cost of those is quite high.

As I mentioned, I assume you can resell an "average" Quark, return a "poor" quality Quark to DayStar, and eventually stumble upon a "nice" Quark.
That's the issue in general with this, especially with HA. The options are limited and all of it is costly. But, the cost increase for what you get doesn't match the cost in increase. This is why more people use low tier etalons that are just good samples, rather than everyone on there saving pennies to have a top shelf etalon for $10k+ cost. There's no guarantee from any of them. And no standard. So it really doesn't make economy sense to go for a $16k Quantum PE 0.3A filter just to find out it's not perfect and has no guarantee and only achieves that at like F50 anyways, and still has a double limb, and there's no guarantee that it will be uniform; we would like to assume it, but they're all gambles even at the top level. So, many people spend more time trying to mod low tier etalons and just gamble sample quality until they get an ok one.

So really, for etalons that can be mounted with a larger aperture, only internal or rear mounted options, we have limited choice. At sub-$1k you have some PST etalons, used Quarks, used Lunt 35's and Lunt 50's. At $1k+ you have the same options, including some Lunt 60 etalons. At $2k+ you really don't have any more options that are clear aperture after the internal Lunt etalon options (like the Lunt 80 DS module). After that, it's pretty much $4k~5k options.

I don't think $2k is expensive for a good etalon modded for rear mount use with a larger aperture (35mm+ if air gap; 30mm+ mica-spaced). A Lunt 60mm or Lunt 80mm internal etalon is going to be generally a higher quality etalon in general than any gambled Quark, with much larger aperture that is clear, and better tuning (no electronics). $2k is probably the sweet spot for odds of a good etalon. Under this is big gamble. Over this is getting into higher grade etalons that still have gamble (yet cost twice as much, sadly).

Very few places will let you return a solar product that is opened. So it's always a huge gamble. And so this is also not really a viable option (to buy and return to find a good sample of a filter). Most people buy used filters, test them, then re-sell them if they're not what they want. It's not too bad, since you pay a bit and lose a bit per transaction, but it's very minor compared to the overhead of buying a commercial high quality etalon (with questionable quality and just as much gamble basically). But there are big risks with used etalons, especially depending on payment options and insurance from shipping. A decontacted etalon for example is dead and so everyone is unhappy with that and you better have insurance and better read the fine print if they will even cover it (not all will cover this). This is a hassle. In my opinion, its annoying as all get out. Anyone that is ok with it, probably hasn't had a decontacted etalon as a result of shipping (yet). I have. It's a huge hassle with insurance. I don't want to do it much anymore, so I'm done with rotating inexpensive used etalons (PST, Quark, etc) and shipping them around buying & selling. I'm at the point where I'd rather get a commercial etalon with the best odds of high quality (still a gamble) and get support from the commercial entity if its not up to spec (so, something like an internal Lunt).

So going back to the original point, after you fool around with a PST etalon, which can be good, they're only good for tiny sensor cameras and high res because the limited FOV secondary to the jacquinot spot is obvious. You lose a significant portion of your FOV even on a tiny sensor because the sweet spot just gets to small and continues to decrease in size as you increase the aperture of the OTA (field angle vs acceptance angle) since the PST etalon aperture is only 20mm. So even on my 8 inch solar scope with DERF, with PST etalon, the FOV is very tight and I still have to crop away a big portion of the FOV due to sweet spot limits. This huge loss in real estate of your FOV and sensor size kills any efficiency of the PST mod savings in general in my opinion. It works, but then you realize fast that it's super limited FOV in that tiny sweet spot.

I think when shopping an etalon, or thinking to mod an etalon for this, it's super important to consider what the goal is for full disc or not and what the jacquinot spot size will be in the system to know if you're spending a ton on a small sweet spot, or a larger one, and where the most efficient combination is between cost and quality and etalon size. I think the best combination of these factors lies in the $2k range, with things like the Lunt 80mm DS internal pressure tuned etalon (huge etalon size, air gap) or Lunt 60mm internal. You're going to get a bigger jacquinot spot with them than a PST etalon and bigger even than a Quark (despite mica spaced being around 60% larger jacquinot spots, the limited 20mm aperture of the Quark still is smaller than what you'd get from a 35mm+ etalon that was air gap).

And honestly, with things like Quantum or similar expensive rear mounted mica-spaced etalon, the FWHM number is just marketing in my opinion, compared to how much more important it is to simply have a good uniformity across the entire entalon and very specifically that it is is a much larger clear aperture etalon so that you get a much larger jacquinot spot to work with (which is even larger secondary to being mica material rather than air gap, so a big 30~40mm etalon that is mica spaced has a much bigger jacquinot spot). The problem is..... the gamble on uniformity even on expensive Quantums is very real, with zero guarantee, zero standard.

Very best,


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Re: PST mod is it worth it?

Post by Rusted »

Thank you, Marty, for sharing your valuable experience with our limited etalon options. :bow

It seems an internal pressure tuned, Lunt 60 etalon is the next best upgrade from the PST mod.
Thereby safely sidestepping the Daystar Quark lottery.

Thanks again,
Chris


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Re: PST mod is it worth it?

Post by thesmiths »

Rusted wrote: Wed Jul 14, 2021 5:57 pm It seems an internal pressure tuned, Lunt 60 etalon is the next best upgrade from the PST mod.
As they say, the best scope is the one you use the most. Two of my favourite pieces of astronomy equipment ever have been the Lunt 60 Pressure Tuned and the Coronado CaK PST. The reason is they give great full disk images with no fussy around at all. No stress and extra work, just fun and consistently high quality viewing/imaging. I actually have the 60mm front etalon to double stack the Lunt 60 but it requires some fiddling around to get a uniform image so I end up not using it as often as I expected.


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Re: PST mod is it worth it?

Post by Rusted »

Thanks. A search suggests that Lunt don't sell the internal etalon alone.
They bundle it with one of their various BFs. Which I already have.
Is there a specific serial number for the 60mm internal, pressure tuned etalon?


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Re: PST mod is it worth it?

Post by Rusted »

Here are a couple of images confirming the PST etalon's, ring-shaped, sweet spot.
The etalon was rotated 90° between captures. I also wrapped the etalon element for the second image.
Considering the poor seeing there is surprising detail to be seen in the darker regions.
While the paler regions are off-band and show much less detail.
I also tried using the 1.125 Baader GPC in case it helped. It didn't.

15.07.2021 10.50 1.125 gpc 800x800.jpg
15.07.2021 10.50 1.125 gpc 800x800.jpg (123.93 KiB) Viewed 6022 times
15.07.2021 11.48 2.6x gpc 800x800 etalon wrapped.jpg
15.07.2021 11.48 2.6x gpc 800x800 etalon wrapped.jpg (125.84 KiB) Viewed 6022 times
15.07.2021 10.50 1.125 gpc 800x800.jpg
15.07.2021 10.50 1.125 gpc 800x800.jpg (123.93 KiB) Viewed 6022 times


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Re: PST mod is it worth it?

Post by MalVeauX »

Rusted wrote: Thu Jul 15, 2021 10:07 am Here are a couple of images confirming the PST etalon's, ring-shaped, sweet spot.
The etalon was rotated 90° between captures. I also wrapped the etalon element for the second image.
Considering the poor seeing there is surprising detail to be seen in the darker regions.
While the paler regions are off-band and show much less detail.
I also tried using the 1.125 Baader GPC in case it helped. It didn't.
Regarding the jacquinot spot size, it's calculable based on field angle and acceptance angle relationship of the objective and the aperture of the etalon (along with other parameters such as sub-straight and FWHM, etc). Smaller etalons simply will have a tiny jacquinot spot with bigger objectives, so PST mods will always have a known smaller jacquinot spot relationship as the aperture of the scope you're using goes up. The only way to make use of it, is with a tiny sensor, so that most of the FOV on such larger instruments and long focal lengths end up being in the jacquinot spot which is tiny. If you use a large sensor, you will see right away just how limiting this is (such as example above with 1.1 inch sensor, instead of tiny 5mm sensor). Increasing the focal-ratio will not change this (such as using a GPC) unless its masks the objective aperture, but it will not improve things with respect to jacquinot spot size. This is true of all rear mounted etalons in terms of the jacquinot spot size; PST mod, Quark, Lunt mod, etc. It's different depending on the sub-straight such as air gap vs mica spacing cyrstal, but it's still a known relationship that if you increase the objective aperture and use a tiny etalon aperture, bottom line is, the jacqinot spot will just be small. And increasing etalon size is very costly, which diminishes any cost savings of using a rear-mounted etalon in this way.

It's just an important concept to be familiar with when considering a PST mod or any mod, since the jacquinot spot size dictates what the on-band FOV will be and that's far more important than just picking a larger aperture and thinking it will all work out when using these tiny 20mm air gaped PST etalons.

Very best,


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Re: PST mod is it worth it?

Post by GreatAttractor »

Good summary, Marty, but a small correction - the Jacquinot spot is not an issue for etalons in a telecentric beam (like a Quark), is it? In this configuration, all light beams from the whole telescope's FOV end up as same-width converging cones, passing through the etalon at normal incidence. So you're limited only by the etalon's (and its blocking filter's) clear aperture. With a Quark you could use a sensor up to 21 mm (...getting a fully-uniform Quark is another question).


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Re: PST mod is it worth it?

Post by MalVeauX »

GreatAttractor wrote: Thu Jul 15, 2021 8:20 pm Good summary, Marty, but a small correction - the Jacquinot spot is not an issue for etalons in a telecentric beam (like a Quark), is it? In this configuration, all light beams from the whole telescope's FOV end up as same-width converging cones, passing through the etalon at normal incidence. So you're limited only by the etalon's (and its blocking filter's) clear aperture. With a Quark you could use a sensor up to 21 mm (...getting a fully-uniform Quark is another question).
http://astrosurf.com/viladrich/astro/in ... lar/FP.htm

http://astrosurf.com/viladrich/astro/in ... -spot.html

viewtopic.php?f=9&t=30881

Yes, most of the above is with respect to an air-gap, like the PST and Lunt etalons mentioned. The mica-spaced etalons in a telecentric beam are indeed different and will allow greater angles between field and acceptance angles. But you can still get a jacquinot spot with mica-spaced etalons.

And yes, getting any of these etalons to have good uniformity is a bigger problem in general as mentioned above; having a big jacquinot spot doesn't matter much if the whole FOV uniformity in that spot is poor and all over the place. And that's common with these low tier etalons sadly since there's no standard. These topics are often completely missing from discussion about these mods most of the time as the center of the discussion usually just revolves around the scope's aperture and D-ERFs.

Very best,


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Re: PST mod is it worth it?

Post by Rusted »

Thank you very much, Marty [and others] for continuing to improve our understanding of modding with inexpensive etalons. :bow

Those of us who are presently struggling with "limiting" PST etalons now need expert advice to avoid wasting further funds on predictable failures

A "better" PST etalon will always have its limitations. Particularly if it leads to inflationary pressures on pricing for highly unpredictable quality.

Perhaps we now need a new wave of improved designs for better H-a mods based on sounder, optical principles?

No "free lunch" but at least we will better understand the true cost of building larger aperture, H-a telescopes.
Last edited by Rusted on Fri Jul 16, 2021 5:44 am, edited 1 time in total.


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Re: PST mod is it worth it?

Post by Rusted »

Afterthought: Would increasing the focal length of the following [positive] collimation lens behind the etalon help increase the sweet spot radius? It would need to have a clear aperture similar to the etalon group. A weak, 2" Barlow?

Does it have to be achromatic for H-a? What about separating a binocular achromat for its negative lens?


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Re: PST mod is it worth it?

Post by GreatAttractor »

Rusted wrote: Fri Jul 16, 2021 5:39 am Afterthought: Would increasing the focal length of the following [positive] collimation lens behind the etalon help increase the sweet spot radius?
Yes (e.g., I use a weak Barlow to reach close-to-critical sampling with my camera), but note that it only up-scales the image. The sweet spot's linear size will be larger, but the angular size of the Sun fragment in the spot remains the same. Had only 1/4 of the disk on-band? You still have 1/4, it's just larger on the focal plane.


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Re: PST mod is it worth it?

Post by Rusted »

Thanks. :bow

By coincidence I had a 1.25X Baader GPC in my collection. This has more than enough aperture for the PST etalon.

By opening up the baffle slightly [to 28mm Ø] in the AOK Swiss, rear etalon adapter, the Baader GPC just dropped into place.
It is then safely retained by screwing the PST etalon housing back into place as normal.

My first, quick capture suggests that the ring-shaped sweet spot remains a problem.
Though I shall continue to try etalon tuning adjustments to try and spread out the "goodness."
Sorry about the poor seeing conditions. It was very hard to find best focus.

My ASI174 camera is now some 12cm further away from the etalon. Easily managed with a 2" extension.
I am now wondering whether I should keep the etalon 200mm inside the native focus. [1500mm.]
Or adjust the telescope focuser to place the etalon at some new point inside the new focus.
Any thoughts on this? I'm thinking that the original PST negative collimation lens remains the same.

16.07.2021 09.47 Baader  1.25 GPC  after etalon.jpg
16.07.2021 09.47 Baader 1.25 GPC after etalon.jpg (136.02 KiB) Viewed 5958 times


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Re: PST mod is it worth it?

Post by Rusted »

The seeing is very soft but I am quite pleased with progress so far.
I am not even seeing any detail coming out of AS!314.
The etalon tuning and sideways pressure is still haphazard.
Blowing out the gain in SharpCap is a good guide to brightness distribution across the frame.

16.07.2021 10.22  Baader 1.25x GPC after etalon.jpg
16.07.2021 10.22 Baader 1.25x GPC after etalon.jpg (141.99 KiB) Viewed 5956 times
16.07.2021 10.29   1.25x PST etalon GPC plus 1.6x GPC at camera.jpg
16.07.2021 10.29 1.25x PST etalon GPC plus 1.6x GPC at camera.jpg (126.46 KiB) Viewed 5956 times
16.07.2021 10.49  1.25x PST etalon GPC plus 2x GPC at camera 2.jpg
16.07.2021 10.49 1.25x PST etalon GPC plus 2x GPC at camera 2.jpg (129.95 KiB) Viewed 5953 times


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Re: PST mod is it worth it?

Post by christian viladrich »

thesmiths wrote: Wed Jul 14, 2021 2:08 am
christian viladrich wrote: Tue Jul 13, 2021 8:30 pm I recently tested two of these etalons (spectrometer + interferometry with Ha lamp) :
- one etalon has a FWHM of 1 A, which is in line with the spec (FWHM < 1 A),
- the other was much better with a FWHM of 0.61 A.
Christian: do you also measure how well tuned the PST etalons are? My impression is that the big problem with the PST etalons are that if the natural tuning of the airgap is quite far off resonance for H-alpha, then the tilt must be large and therefore you get a severe on-band annulus. Or does your measurement of average FWHM take that into account in some way?

With the Quark, the heating of the mica can get you to a precise resonance; their issue with finesse is, I assume, up to the flatness and parallelism of the mica.
I have found that the two PST etalons I tested were not tuned by tlting but by mechanical compression (with the rotating knob), just like the SM III Rich View. So there is no tilt.
This being said, one of them is not very uniform (change of CWL accross the surface of the etalon).
More here :
viewtopic.php?f=8&t=32687


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Re: PST mod is it worth it?

Post by Rusted »

The PST etalons are indeed tuned by straight compression.
Between a Teflon O-ring and quite a soft sponge ring.

P1460766 rsz.JPG
P1460766 rsz.JPG (127.8 KiB) Viewed 4262 times
P1460764 rsz.JPG
P1460764 rsz.JPG (103.5 KiB) Viewed 4262 times
There is normally a textured rubber band over a shell type of ring.
The inner and outer ring are joined by a simple screw.
I have removed the rubber band and shell ring to allow direct access to the inner ring.
The inner ring is a protrusion on the edge of a machined, "top hat" component which houses the etalon.


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Re: PST mod is it worth it?

Post by christian viladrich »

Great ! Thanks for sharing.

I was wondering how much pressure is applied to the etalon.
Is it possible to assess how much the orange ring is compressed (in mm) when the etalon is tuned ? Rough figures would be fine.
Then, it would be interesting to see how much weight you need to put on the ring to compress it. Or put on it a given weigth and see how much compression there is (in mm).
Then we could calculate the pressure on the four triangular spacers separating the two plates of the etalon.


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Re: PST mod is it worth it?

Post by Rusted »

christian viladrich wrote: Sun Jul 18, 2021 3:00 pm Great ! Thanks for sharing.

I was wondering how much pressure is applied to the etalon.
Is it possible to assess how much the orange ring is compressed (in mm) when the etalon is tuned ? Rough figures would be fine.
Then, it would be interesting to see how much weight you need to put on the ring to compress it. Or put on it a given weigth and see how much compression there is (in mm).
Then we could calculate the pressure on the four triangular spacers separating the two plates of the etalon.
It feels as if it goes completely solid when adjusted too far. Even a coarse screw thread can put tremendous pressure on any object.
So I'd need to make an identical sized "plunger" to avoid stressing the etalon with hefty weights placed on top. :D
I'll turn up the end of a bit of brass rod in the lathe and measure the load required to flatten the sponge ring.
Or I cold use a fisherman's spring balance with a hooked plunger.
It may simply be that the the nested components are designed to contact each other internally before the etalon is crushed.
I can check for that without the etalon element in place.


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Re: PST mod is it worth it?

Post by Rusted »

Well, that was interesting.

The etalon is 30mm Ø x 16mm thick.
The sponge rubber ring is 33mm OD x 22mm ID x 4.5mm thick.

I applied loads via a 30mm brass plunger to the sponge ring.
It showed no visible deformation with up to 6kg on top.
Which was the only useful weight I had easily to hand.

How do I judge the required degree of deformation?
Do I keep adding more weight until the desk collapses? :D
I cannot noticeably deform the rubber ring with a 30mm disk on the edge of the desk using all my grip strength.

NB. There is no physical contact between the two metal components of the etalon housing.
The inner ring can be screwed right down out of sight without the etalon present.
The thread is quite coarse but I haven't tried to measure the pitch.


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Re: PST mod is it worth it?

Post by christian viladrich »

That is very interesting ! This orange sponge is indeed very rigid.

With some rough calculation, we have :
delta lambda / lambda = delta gap / gap = pressure / Young module of the spacers

We can estimate the force in Newton to be applied to the spacers for a 0.5 A shift of CWL :
- let's assume 4 spacers, each spacer is a triangle of 3 mm on a side (I am not sure of that)=> total surface = 18 10-6 m2
- fused silica young module is 70 10^9 Pascal
so the pressure to be applied to the spacers is : 70 10 ^9 * 0.5 / 6563 = 5.3 10 ^6
and the force to be applied on the spacers : 5.3 x 18 = 95 Newton (=9.5 kg).

Given you saw no deformation at 6 kg, this 9.5 kg for 0.5 A delta CWL looks quite possible.


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Re: PST mod is it worth it?

Post by christian viladrich »

I've found some formulae giving the axial force applied by a screw for a given torque. It depends on a number of assumptions (thread / diameter / friction coefficient / etc.). Still, with a very small torque, it is possible to have a 95 Newton axial force.


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Re: PST mod is it worth it?

Post by marktownley »

You could try 'packing out' the pST etalon Chirs, it is 30mm diameter, the foam ring is 33mm, so, you might be getting some 'slop' of the etalon in its sleeve housing. This can cause the etalon to tilt in it's housing which can cause the banding you see.

Wrap the etalon in PTFE tape to get an interference fit in the sleeve and see if this gets rid of the banding you are experiencing. It's completely reversible and non destructive.

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Re: PST mod is it worth it?

Post by thesmiths »

I think Mark is on to something here. If it's true the tuning is by compression rather than tilt, then the annular sweet spot is an aberration caused unintended tilt (rather than an intentional tilt). Having taken apart my PST, the rubber was in rather poor shape after a decade (most of it not being used much) so I could imagine it would not provide a very stable base which would compress uniformly.


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Re: PST mod is it worth it?

Post by christian viladrich »

My guess is that the etalon "sits" on the teflon O-ring. In other words, this is the reference surface setting the position of the etalon. This being said, depending on the amount of play in the thread, the threaded "cover" could be a little bit tilted (or not).


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Re: PST mod is it worth it?

Post by Rusted »

Thanks for the suggestions. :bow

I have been wrapping the etalon element[s] in plumbers tape in varying degrees for some time.
There is lots of play between the etalon element[s]s and their housing.
I have also been deliberately tilting the etalon.
There is quite some play between the two threaded, housing components themselves.

There was time when I could simply tilt the inner ring and the entire field would darken.
Revealing the surface texture in fine detail: Sadly, this is no longer the case.

It is quite possible that I am responsible for this situation.
If the etalon is resting on a rigid Teflon O-ring.
Then it probably doesn't like being tilted.
It may even produce a shearing action at the etalon joint.
The original design incorporated an outer shell and textured rubber, adjustment ring.
The single screw connecting the inner and outer components would not allow tilting.

The rubber ring may be a protective compliance.
Not just for knocks and the PST being dropped.
But preventing the worst forces from desperate users being applied. :oops:


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Re: PST mod is it worth it?

Post by thesmiths »

christian viladrich wrote: Mon Jul 19, 2021 4:16 pm My guess is that the etalon "sits" on the teflon O-ring. In other words, this is the reference surface setting the position of the etalon.
Christian, you're quite right, the Teflon ring would provide a flat reference surface. Another way the incoming light could become tilted would be if the collimating lens were not aligned correctly.


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Re: PST mod is it worth it?

Post by Merlin66 »

This is the reply I got on the subject:

"""The PST uses compression tuning. Probably the worst way imaginable to tune an etalon. The Teflon ring in the chamber is the main point of pressure. The silicone ring is designed to smash the etalon until it comes into tune. Since it never compresses evenly the silicone ring is trimmed and cut so that it applies pressure as evenly as possible. This can only be done while applying pressure with the rotational compression ring and trimming parts of the silicone while looking through the chamber at a hydrogen spectrum light. It is not compressing the feet at all it is deforming the plates around the feet. That is why the silicone is trimmed to even out etalon while it is crushed. The compression itself is not capable of enough force to compress the feet. And compression the feet since they are fuse silica as well would in fact crack crush or powder the feet to a point of loss of contact. Thus destroying the etalon. """


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Re: PST mod is it worth it?

Post by Rusted »

And yet most PSTs continue to function for years or even decades. Rust permitting.
While my own, unmodified, rubber/silicone [?] ring appears absolutely pristine.
It cannot have been replaced because the whole PST was cemented with hard, thread locker.

It would be difficult to imagine how the etalon collimation lenses could tilt.
Both are screwed firmly into the same housing.
Any tilt of the inner, etalon housing would go completely unnoticed.

My use of larger and better quality fittings no longer allows the filter stack to droop.
Perhaps there lies the secret? It may perform better as a floppy assembly.
Though the etalon is very early in the stack and relies directly on the stiffness of the focuser.
I went from an indecently wobbly, vintage 2" Vixen to a 2.5" FT.

Previous sag may have provided exactly the correct degree of tilt?
Though that would surely have been rotationally critical. [?]
As the OTA progressed from east to west. With a meridian flip midway.

I should add that I have tried tuning over a wide range of [very] non-200mm inside focus, etalon positions.
Just in case there was something non-standard about the collimation lenses.
Such is my desperation to continue imaging with this PST etalon before giving up.

European prices for Lunt etalons are far higher [1.6x minimum] than USD on straight currency conversions.
Bresser has the sole LUNT monopoly. While the EU applies a minimum of 25% importation + purchase tax to all goods.


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Re: PST mod is it worth it?

Post by marktownley »

Have you still got the 'trimmings' on the silicone ring Chris?


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Re: PST mod is it worth it?

Post by Bob Yoesle »

This is the reply I got on the subject:

"""The PST uses compression tuning. Probably the worst way imaginable to tune an etalon. The Teflon ring in the chamber is the main point of pressure. The silicone ring is designed to smash the etalon until it comes into tune. Since it never compresses evenly the silicone ring is trimmed and cut so that it applies pressure as evenly as possible. This can only be done while applying pressure with the rotational compression ring and trimming parts of the silicone while looking through the chamber at a hydrogen spectrum light. It is not compressing the feet at all it is deforming the plates around the feet. That is why the silicone is trimmed to even out etalon while it is crushed. The compression itself is not capable of enough force to compress the feet. And compression the feet since they are fuse silica as well would in fact crack crush or powder the feet to a point of loss of contact. Thus destroying the etalon. """
Hi Ken,

The relevant Coronado patents, US 7,054,518 B2 and US 7,149,377 B2, state that the etalon is tuned by compressing the spacers (aka "feet") which thereby effects the change in the etalon cavity length; the foam ring makes the compression forces symmetrical. Some people find it hard to believe that the thin glass spacers can be compressed, but on the micro-scales involved it indeed apparently works as intended - and patented.


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Re: PST mod is it worth it?

Post by christian viladrich »

Merlin66 wrote: Tue Jul 20, 2021 2:45 am This is the reply I got on the subject:

"""The PST uses compression tuning. Probably the worst way imaginable to tune an etalon. The Teflon ring in the chamber is the main point of pressure. The silicone ring is designed to smash the etalon until it comes into tune. Since it never compresses evenly the silicone ring is trimmed and cut so that it applies pressure as evenly as possible. This can only be done while applying pressure with the rotational compression ring and trimming parts of the silicone while looking through the chamber at a hydrogen spectrum light. It is not compressing the feet at all it is deforming the plates around the feet. That is why the silicone is trimmed to even out etalon while it is crushed. The compression itself is not capable of enough force to compress the feet. And compression the feet since they are fuse silica as well would in fact crack crush or powder the feet to a point of loss of contact. Thus destroying the etalon. """
Hi Ken,
Very interesting. And still a bit strange.

Calculation shows we just need about 10 kg force to tune the etalon by 0.5 A. I can't imagine how 10 kg could crush the spacers. Spacers are stuck with molecular force.
If the plate was deformed between the spacers, it would be easily seen with the Ha lamp. I can't see anything like this. I will double check. For the record, the deformation of plate due to the central spacer is obvious in the SM 40 etalon.

If the teflon ring is indeed "trimmed", it means that a difference in rotation of the ring would change the uniformity of the etalon. I am not sure this is a case.
My guess is that Teflon is quite homogenous. Non even compression would come from non even shape. I don't know what are the tolerance in shape for this kind of ring.
Last edited by christian viladrich on Tue Jul 20, 2021 7:58 am, edited 1 time in total.


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Re: PST mod is it worth it?

Post by christian viladrich »

BTW, what was the source of information Ken ?


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Re: PST mod is it worth it?

Post by christian viladrich »

Just wondering ...Rusted can you measure the thickness of the O-ring in different locations (i.e. four different azimuths) ?
Maybe with 0.01 mm accuracy ?


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Re: PST mod is it worth it?

Post by Rusted »

christian viladrich wrote: Tue Jul 20, 2021 8:13 am Just wondering ...Rusted can you measure the thickness of the O-ring in different locations (i.e. four different azimuths) ?
Maybe with 0.01 mm accuracy ?
EDIT: Apologies and a correction to my earlier statement:

I have found my mislaid, metric micrometer.
Measuring the Teflon O-ring, in situ, in its housing, I find 0.02mm variation.
The high is in one spot only. Not two, diametrically opposite.
With 0.01mm at roughly the halfway points. So slightly tilted?

There is no sign of any modification, nor addition to the orange, rubber ring.
Some users have reported added spacer tabs on opposite sides. Mine has none of these.
The orange ring seems perfectly uniform.


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Re: PST mod is it worth it?

Post by Merlin66 »

The reply and subsequent discussions (!!! - I didn't believe it...) were "off the record" with Brian Stephens.
My initial message:

Quote
30 Mar 2020, 14:27

to BRIAN
Brian,

I'm hoping you may be able to assist me. I'd like to know how the etalon in the PST is tuned....
We've struggled with this for over ten years! If you can put me out of misery I'd be grateful.

The Lunt Patent from 2005 https://patents.google.com/patent/US7149377 and the referred Figs 13 and 14 infer some type of "pressure tuning" but the mechanism is unclear.

-The O ring and foam ring don't seem strong enough to support squeezing of the etalon spacers.
-The sealing arrangement around the etalon assembly doesn't seem robust enough to support atmospheric tuning.
We therefore infer that somehow the etalon is tilt tuned....aided by the front foam ring...
The options I think are:

1. The PST etalon tuning is achieved by tilting of the etalon
2. The PST etalon is tuned by pressure applied to the spacers in the etalon
3. The PST etalon is tuned by increasing/ decreasing the atmospheric pressure around the etalon
4. Another method????

Can you confirm which one is correct??
I really appreciate your help and assistance to clarify this 15 year old mystery.

Unquote

Subsequent discussion:

The distortion of the etalon plates due to the pressure point difference between the rear ring and the front foam is used for tuning.
The spacer flakes are NOT distorted/ squeezed/ made thinner. The offset between the pressure circle of the Teflon ring and the silicon ring causes the bending of the etalon plates and hence the tuning.

Don't shoot the messenger! I was as surprised as you to hear the "correct answer".
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Re: PST mod is it worth it?

Post by AndiesHandyHandies »

Hi

If the pressure is only applied round the edge it can only logically work by compessing the feet slightly.

The orange ring will compress over time I would think to even out the force.

Frank Hourigan made me a 1" plate F4 mirror for the Birmingam AS. Designed for the scope to fit in a family hatchback. I mounted it on a thin sheet of Rover panel damping foam on birch ply. The foam just adapted to the pressure. I had a lovely view of Mars polar caps with a Pretoria with it.

Surely the modulus of compression for glass is available.

I designed a big horseshoe mount, Nautilus, using 3/4 ply and was able to calculate how much it would deform under load. Small enough.

Cheers. Andrew.


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Re: PST mod is it worth it?

Post by Merlin66 »

Andrew,
“..the pressure is only applied round the edge….”
I’m now convinced that the “feet” act as support pivots allowing the outer section of the etalons to bend and provide tuning.


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Re: PST mod is it worth it?

Post by thesmiths »

It sounds to me that even the designers may not entirely understand the detailed mechanism how it works, just that given a particular setup, it will work. Having worked in engineering at several companies, that is often the way it happens. However, it you want to debug or improve something, it is much better (even essential) to understand the actual mechanism is before you change anything that may (or may not) improve the performance.


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Re: PST mod is it worth it?

Post by AndiesHandyHandies »

Merlin66 wrote: Wed Jul 21, 2021 9:03 am Andrew,
“..the pressure is only applied round the edge….”
I’m now convinced that the “feet” act as support pivots allowing the outer section of the etalons to bend and provide tuning.
Hi Merlin,

I assume the spacers are centred under the orange foam, is that so? I have not noticed a shadow or 'dust bunny' because the feet are in the field of view.

You idea would result in the two thick etalon plates bowing so the bandwidth would vary across the etalon.

With equal sized feet being identically compressed the etalon plates stay flat but move together enough. I thought that CV above had calculated the force reported on the etalon and so the feet, forced them to compress enough to give 0.5A of tuning due to the gap change.

Also if the force is not placed orthogonally on the feet the molecular bond would fail surely.

Cheers. Andrew.


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Re: PST mod is it worth it?

Post by AndiesHandyHandies »

Hi

I spent about £500 on a SH PST, paid £250 and a dirt bike to Rodastro for it, maybe some sellers regret.
Comparing with Daystar bandwidth example images looks like 0.7A.

A couple of Swiss adapters aprox £150.

1.5A Omega Bob solid etalon on hand for a blocker about £75.

Various filters on hand and or useful additions.

Rod kindly contributed bits out of his bits box and some lathe work for a nosepiece for the moved PST Collimator to T2 adapter.

DeepSky64 rates my visual observations with a Mak180 mod as the best.

Montana says my reported details match her 100mm Solarscope.

Tuning the blocker by tilting I see surface datails similar to Stardusts DS images.

I can see more detail in prominences than posted images.

And more realistic details, like granulation in bright areas and fractal borders on them, rather than over-processed lines.

Quiet happy.

Cheers. Andrew.


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