Tilting the PST etalon?

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Tilting the PST etalon?

Post by Rusted »

Hi,

After a short period with more even lighting across the image I am back to a dark ring, sweet spot on my 150/10 PST mod.
I have tried different frame sizes in SharpCap. I have tried my range of GPCs and 2x Barlow. Nothing helps.
I have wrapped and rewrapped the etalon element with plumber's tape. There are no reliable results.

My PST etalon has no outer, rubber band. I use a rod in one of the screw hole in the inner ring to tune and tilt the etalon element.
The PST etalon is tuned by physical pressure on a rubber ring. Some have them, but there are no packing pieces on my orange, rubber ring.
My ability to tilt the etalon suggests that tilting is also important to spreading the sweet spot.
I can usually rely on turning and tilting until the entire field goes as dark as possible. Not any more.

My FT focuser backplate has four [push-pull] collimation screws. So I could play with tilting of the etalon that way.
Has anyone actually improved their PST image by tilting the entire optical train from right back at the focuser base?

Here is an example of the highly asymmetric image lighting and off-band, lower left sector this morning:
The detail is fine. It just isn't consistent across the image. I keep thinking I need a Quark to make further imaging progress.
The Daystar "QC lottery" prevents me from investing. I hate returning things! :( Brexit has made dealing with the UK impossible.

Is there a European, continental dealer who will replace a poor Quark without quibbling? Any [affordable] alternatives to the Quark?
Thanks.

2.07.2021 08.13.42 2.6x gpc red rings.jpg
2.07.2021 08.13.42 2.6x gpc red rings.jpg (167.51 KiB) Viewed 4604 times


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Re: Tilting the PST etalon?

Post by MalVeauX »

Hi,

Looks like there's likely pressure gradient across your etalon. What you're seeing is uniformity issues, which is going to come from there not being the same gap across the entire surface within the jacquinot spot which changes things. Tilting will not fix uniformity issues like this, it will change the tuning a little but the uniformity issue will remain. There's no obstructing spacers in the PST etalon plates, so I would take a look at the etalon itself if you're not concerned with it at this point. You could decontact it and clean it and make sure its spacer is there and recontact it and check it again if you're feeling adventurous.

Very best,


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Re: Tilting the PST etalon?

Post by Rusted »

Thanks Marty. :bow

The etalon isn't de-contacted at present. I am getting far too much fine detail for that.

As you say, it's the uniformity I can no longer adjust for.
It has always been like that but could usually be wiggled to darken a smaller field evenly.
Larger frame sizes would never become evenly lit. No matter what I tried.

I'm now wondering whether having the etalon constantly exposed has exacerbated the problem.
I removed the rubber band and outer shell some time ago.
A bright red glow can always be seen though the tuning ring slot.

Thanks to your response I was able to search around the subject more usefully.
I may just try applying finger pressure to the etalon. While wearing disposable gloves.
Nothing ventured... Safer than lifting the bonnet. [US: hood]


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Re: Tilting the PST etalon?

Post by marktownley »

Hi Chris.

I hope you don't be cross at me for saying this, but your PST etalon is not a very good specimen...

It's natural centre wavelength is not 656.28nm, when you tune it to get it on band, you get this banding effect which your picture above shows well. You get this with front mounted etalons too. It's what etalons do... There's nothing you can do to fix it. I would suggest the changes you see is due to variation in thermal loading on it throughout the year.

My advice would be to look around on the second hand market for PSTs, only considering those where the seller shows pictures taken through it that show completely uniform and on band full disks.

Mark


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Re: Tilting the PST etalon?

Post by Rusted »

Now I really am cross! :D

Thanks Mark. ;)

I paid good money for this secondhand PST from the UK.
The ITF was a mess so a new Maier added to the cost.

I note you are not suggesting I cut my losses and buy a Quark?


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Re: Tilting the PST etalon?

Post by marktownley »

I'm not suggesting you buy a Quark ;)


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Re: Tilting the PST etalon?

Post by Rusted »

Thanks.

Are PST etalons perfectly symmetrical so that they are reversible?

I may have swapped mine around while taping it [repeatedly!]


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Re: Tilting the PST etalon?

Post by marktownley »

Rusted wrote: Fri Jul 02, 2021 8:11 pm Thanks.

Are PST etalons perfectly symmetrical so that they are reversible?

I may have swapped mine around while taping it [repeatedly!]
I've done the same...


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Re: Tilting the PST etalon?

Post by DeepSolar64 »

I have noticed my Coronado 60 is a bit like this. Nice details in some areas and not so much in others. And the disc is not evenly illuminated. It's good enough for visual, you can move the image around and pick up detail but can never see it well over the entire disc. The normal sweet spot is allowed for but I still find the 60 never approaches the 90, even allowing for the aperture difference. The 90 isn't perfect but gives a much more even view.

I wish I knew someone who had a good pressure tuned Lunt to look through.

James


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Re: Tilting the PST etalon?

Post by Rusted »

An update:

The sun came out this morning once the cloud had cleared.
I reversed the etalon in its housing and removed the plumber's tape.
The first images were detailed but odd. Off band?

4.07.2021 10.56  2x Barlow.jpg
4.07.2021 10.56 2x Barlow.jpg (170.87 KiB) Viewed 4521 times

So I retuned and then fitted the 2x Barlow as the seeing slowly improved. :mrgreen:

4.07.2021 11.13  2x Barlow.jpg
4.07.2021 11.13 2x Barlow.jpg (168.03 KiB) Viewed 4521 times
Then a good dollop of orange marmalade: :D

4.07.2021 11.31  2x Barlow col.jpg
4.07.2021 11.31 2x Barlow col.jpg (225.38 KiB) Viewed 4521 times


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Re: Tilting the PST etalon?

Post by EGRAY_OBSERVATORY »

Whether they are "off-band" or otherwise Chris, they are excellent images.

Terry


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Re: Tilting the PST etalon?

Post by Rusted »

Thanks Terry. :bow

I would not have thought that reversing the etalon element would make such a difference.
Nor the difference after re-tuning. I have marked the etalon with a pencilled arrow.
The light and dark banding, which prompted this thread, has disappeared.

Thin, high cloud has moved in now. So I'll have to get back to building the new observatory.

Chris


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Re: Tilting the PST etalon?

Post by marktownley »

Hi Chris.

First image definitely off band, number 2 most definitely not. Sounds like you had some slop in the system somewhere?

Mark


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Re: Tilting the PST etalon?

Post by Rusted »

marktownley wrote: Sun Jul 04, 2021 4:04 pm Hi Chris.

First image definitely off band, number 2 most definitely not. Sounds like you had some slop in the system somewhere?

Mark
Thanks Mark.

You are probably right. I was able to easily tilt the etalon with the rod I have projecting from the inner tuning ring.
The need for plumber's tape around the etalon element is indicative of unnecessary slop.
When I reversed the etalon I tightened the housing slightly more than usual. Snug, not tight. No rattle.
Seeing the images after retuning felt like a win after the struggle over the last couple of days. :cool:

It was interesting how the off-band images were littered with Ellerman Bombs.
No sign of them on-band.

Thanks again for your input. :bow


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Re: Tilting the PST etalon?

Post by Bob Yoesle »

The initial image looks like the "sweet ring" resulting from the field angle magnification that occurs with a short a focal length collimator lens.


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Re: Tilting the PST etalon?

Post by Rusted »

Thanks Bob.

That is precisely what I am struggling with most days.
Though I have no idea where the collimation lens comes in.
There are only the original PST lenses sandwiching the etalon.

While rotating the Lunt B1200S2 BF I thought I had found the problem. It wasn't that.
It was just the BF being tilted by my clumsy, three-handed adjustment.
I had the camera image on SharpCap to monitor any change.

Today I tried every GPC and even the 2x Barlow at lots of frame sizes. [SharpCap Resolutions]
There just wasn't any "lucky" combination which would give me smooth image brightness across the entire frame.

I dismantled the etalon again, reversed it and re-tightened the internal ring.
Today, it made no observable difference to the image. Still a diagonal, darker, on-band stripe.
With paler, less interesting patches on either side.
These are the only images I bothered to post today.
The rest were even worse!

https://fullerscopes.blogspot.com/2021/07/13072021.html


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Re: Tilting the PST etalon?

Post by marktownley »

Try another PST etalon and see how it varies ;)


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Re: Tilting the PST etalon?

Post by Rusted »

marktownley wrote: Fri Jul 16, 2021 5:32 am Try another PST etalon and see how it varies ;)
Thanks but not as easy as you think.
Even if I could find a good one [another lottery] it would probably come from the UK.
Which [since Brexit] means 25% VAT + import, postal and customs clearance charges even for secondhand goods.
I am looking at alternatives.


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Re: Tilting the PST etalon?

Post by Rusted »

Bob Yoesle wrote: Mon Jul 12, 2021 8:58 pm The initial image looks like the "sweet ring" resulting from the field angle magnification that occurs with a short a focal length collimator lens.
Thanks Bob.

My PST will not provide a smooth rendition across the frame any more. No matter what I try.
I had some limited success with a Baader 1.25x GPC fitted right behind the etalon.
An easy mod to extend the light cone behind the etalon. Even if it just spreads out the "ring."
At least this makes it possible to use the PST etalon with image cropping but is still not ideal.
Unfortunately the seeing is too poor again to achieve much.
At least it gives me an excuse to keep playing with different ideas and settings.

16.07.2021 18.54 Baader 1.25x  GPC after etalon cropped.jpg
16.07.2021 18.54 Baader 1.25x GPC after etalon cropped.jpg (92.4 KiB) Viewed 4327 times


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Re: Tilting the PST etalon?

Post by AndiesHandyHandies »

Hi

I am using a Mak F10 with original lenses moved out for PST to reach Majik 200mm in focus and allow for other collimator lenses for my Meade triplet. Its 1800mm FL and from a simple formulae Valery posted the sweet spot is 8mm. And its the fully illuminated field anyway.

Sometimes the sweet spot has to be a ring of mottling and the area of interest in the ring.

For visual thats adequate.

Cheers. Andrew.


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Re: Tilting the PST etalon?

Post by Rusted »

AndiesHandyHandies wrote: Sat Jul 17, 2021 9:35 am Hi

I am using a Mak F10 with original lenses moved out for PST to reach Majik 200mm in focus and allow for other collimator lenses for my Meade triplet. Its 1800mm FL and from a simple formulae Valery posted the sweet spot is 8mm. And its the fully illuminated field anyway.

Sometimes the sweet spot has to be a ring of mottling and the area of interest in the ring.

For visual that's adequate.

Cheers. Andrew.
But not for imaging. I rarely look through my Solar telescopes.


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Re: Tilting the PST etalon?

Post by AndiesHandyHandies »

Hi

For Quarks buy SH asking for an image through it first. That will filter out obviously poor ones straight away. And they should have done the returns loop already.

If you want to try buying UK ones and avoid import duties on every one I could buy it on Ebay for you, I have a Concierge account and have always got money back. Me and Rod can check it out.

Cheers Andrew.


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Re: Tilting the PST etalon?

Post by AndiesHandyHandies »

Hi

And for PSTs.

Alternatively you buy and ship to me. I just need a return label if needed.

Cheers. Andrew.


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Re: Tilting the PST etalon?

Post by Rusted »

Thanks for the kind offer Andrew.

Unfortunately customs applies to all parcels. Nor are they idiots.
They will have a way to discover the true value of a parcel's content's..25% VAT on top of everything including purchase price, postage and customs charges. Plus some minor import charge + £16 equivalent, flat rate, for the Danish Post Office handling the customs clearance.

I am going right off Quarks if only for their long delay during tuning. That would drive me nuts!

So now I am leaning towards a Lunt internal, pressure-tuned etalon. Only if I can't fix the present PST etalon problems.
I have been very pleased with the results from the PST etalon when the seeing was in my favour.


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Re: Tilting the PST etalon?

Post by AndiesHandyHandies »

Hi

I meant for ordering UK ones till a good one turned up and then despatch abroad to you. Not trying to save fees.

Cheers. Andrew.


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