Can I have an 2020 update on where we are with Ca-K filter controls.

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solarGain
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Can I have an 2020 update on where we are with Ca-K filter controls.

Post by solarGain »

Can I have an 2020 update on where we are with Ca-K filter controls.

Well I am trying to improve on my imaging capabilities, and we all know that Imaging is only as good as the tools at the disposal of the operator.

On two fronts I need to tighten up the definition of the final processed image that is beginning to show ‘reversed granulation’ etc. But more importantly on my images there is a band of pronounced softening from the edge of the disk and inwards. This softening blurs and spoils the definition of the details on the edge including but not limited too Wilson effect, sunspots and areas of brightening. This I am calling the outer zone. This appears to be a coma effect associated with the large f/ number.
I am working with a 102mm and a 150mm objectives, and I have had a few success with ~f/50 but these have been in the perfect sphere of the inner zone.

By double stacking my Lunt Ca-k I can tighten up the physical features of the overall disk. This would then show a narrowing of the footings of the FWHM and partial achieve an improvement in definition.


The second area of improvement comes from filters of one sort or another and with varying qualities attached to them, and at a price.
All modern filters are now with parallel sides and flat tops where in the earlier days of use of the ‘cold filters’ i.e. my Lunt they were inclined to a rolling apex, a Gaussian curve graph model.

“High aperture CaK imaging with full aperture "ER"-like filter?” And there are many more that go back as far as 2015 on solarchat!

And then there is the subject of a dedicated full aperture ERF for Ca-K. Of which only Valery shows any interest in manufacturing this glassware.
Which filters might I use for this. Does this still include the Baader K-Line filter # 2458355 as a secondary internal ERF in removing the tails of the FWHM?

Presently I am using only a Baader RG ccd-filter B item # 2458475B, to block the heating of the Ca-K filter.

Thanks in advance for your comments.
John


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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by marktownley »

Hi John,

The softening you describe sounds to me like it is the image amplifier you use. There is no magic answer to this apart from try a range and see what works best, unfortunately it is not as simple as saying 'which is the best' and buying that as any scope has a unique and inherent amount of spherical aberration, the same applies for the particular image amplifier. The trick is marrying an image amplifier with a particular scope that works constructively and synergistically, rather than having negative effects. e.g. C11 & 2.5x powermate combo. Can you post some pictures to show what you are talking about.

Using a full aperture ERF, or even a sub aperture ERF is not going to reduce continuum leakage from the wings of these (type 1 gaussian) filters - the ERFs bandpass is way too high - there role is to reduce thermal load on the filter.

With a double stack approach with these filters you need to be able to tilt them to get them to work to tune for the FWHM with the steepest sides (and least wing leakage) - you're options here are the Skybender from Apollo, or other filter tilter (like the one on the nose of the HaT telecentric) - these are hard to come by.

For me, reflecting on CaK in 2020, spherical aberration, correct sampling at the chip and seeing are the biggest hurdles to quality CaK imaging.

Mark


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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by solarGain »

Mark Hi. This is but one image from the 13th. It is very clear this outer band softening at the limb and inward by 30mm, and I cannot remember seeing anything like this on Christian’s images.

I have the magimax! magimix at 1.6x, I have 2 of these and I use them as a single or as a pair.
2.5x power mate with a ‘pothole effect’ which I described in cloudynights many years ago.
And a revelation 2x simple 2 lens item.

I have a SAFIX a brilliant piece of art, built like piece of from a tank.
Subject to Valery’s recent suggestion in it use I don’t need it with a native 100mm f/10 set up but up around f/30 + I am beginning to think that I do. But I cannot get it into the configuration previously advised by V. I run out of light and image becomes too grainy/ noisey. So have a different arrangement. Conditions have been poor since the13th so my attempts to explore have been hampered some what.
I am on my way with piece being machined for the coupling of the HaT telecentric to several telescopes so in a few weeks I can experiment on that.
I still think I have a good IMX174 and the written piece by Marty on the difference between the 174 and the 290 are enlightening.

John
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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by marktownley »

Hi John,

I think it is a stacking issue. You have the histogram set too high and so the white areas are over exposed. The stacking software will be honing in on these areas in the top left and also around the active region, so, the area to the right of the image will be 'moved around' more by the stacking software to accommodate the high contrast areas. This will make limb areas softer, similar effect can happen with Ha. Do you get the same effect mid disk?

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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by solarGain »

Well thank you for very much for dissecting this image and pointing out my histogram. It is very true that I like a wide histogram. I will take this into account for Ca-K imaging and possibily for Ha as well.
"Do you get the same effect mid disk?" not enough to distract me to become critical of the treatment of the image. I'll take note of these ideas in the future.

OK, how further can I shield my module from the full blast of the sum at f/30 and beyond?
This is the killer question.
John


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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by marktownley »

Hi John,

I like a wide histogram too - anything above about 85% saturation in capture is going to result in clipping when the image is sharpened,

I use one of these to keep the sun off! https://smile.amazon.co.uk/Notebooktent ... uk-21&th=1

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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by solarGain »

Haha, i remember the notebooktent from many years back. I was looking for something similar. I am now indoors most of the time.
No what I mean is that light path thru' the system is only 'blocked' by a baader RG ccd-filter B item # 2458475B, do you have any further advice to attenuate the feed when going well over f/30 upto f/60?
All the best.
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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by marktownley »

Ahhh, I understand! I'm using the very same one. I currently have on order and await delivery of an Astronomik L1 UV/IR cut filter https://www.firstlightoptics.com/astron ... ilter.html which in theory offers more light throughput than the Baader, will let you know how I get on!

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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by solarGain »

Do let me know, hope it goes well, bad weather for several days to look forward to now.
John


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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by MalVeauX »

solarGain wrote: Tue Jun 16, 2020 7:38 am we all know that Imaging is only as good as the tools at the disposal of the operator.
Hey John,

I would say that imaging is only as good as seeing conditions allow, and its the operator's choice to image within the limits of the seeing. Seeing is everything, especially for 393nm, as these short wavelengths and their high angular resolution are heavily effected by the atmosphere, so excellent seeing is required. No magical devices, no correctors or undercorrectors, no energy/thermal management, etc, is going to do anything for seeing conditions in the atmosphere. Unless you have access to adaptive optics, you're subject to whatever your seeing conditions are. You can eliminate everything possible locally to make sure no local environmental conditions are influencing the final blur received at your sensor. But you cannot do that to your atmospheric seeing. It is geographically based so location matters.

You can eyeball it, image with various kits and make assumptions. I highly recommend you pick up a solar scintillation monitor, or build one. The information is invaluable because you can put a metric on seeing, and it correlates and the numbers are pretty reliable. You can at least start measuring your seeing conditions at different times of day over time and build a metric based data base of your location seeing conditions. This will tell you more often when to attempt short wavelength imaging in high resolution and this will tell you essentially what your "seeing ceiling" is. This will at least guide you to what image scale and aperture you're likely able to use reliably. You can gamble going bigger and hope for brief moments of better seeing to be captured via lucky imaging, and it works, but if you want to walk away with usable data nearly every time, you image at your seeing ceiling, the limit of your actual measured seeing.

So instead of getting more filters, stacking under-correctors, SA correctors, going for really long focal-ratio (producing a fine image scale that demands even more excellent seeing conditions), looking for energy management, etc, I suggest you start with measuring your seeing conditions. Put a number on what you can work within. Then start looking at what you can do to fine tune your 393nm system to work within what you measure to be your most common conditions.

One more thing to mention is that not all optics are well suited to imaging 393nm. Most are simply not well figured for this wavelength being outside of visible spectrum for most people. Faster optics in general are going to be poor at this. Long focal-ratio optics will likely have better figure, not knowing their measured figure, for this wavelength. So if using a mirror, a newtonian long focal-ratio like F6 or more is going to be superior to something like a F2-ish mirror found in SCT/Mak. That's why people need under-correctors for these fast mirrors, or SA correctors, etc. Mean while, a long focal-ratio refractor doesn't need it, nor does a long focal-ratio newtonian. Unfortunately mirrors need full aperture D-ERF usually (it's possible to do it without, but its not easier, it's harder frankly as it has to do with the secondary mirror). But refractors simply don't need big D-ERF's, you can do it internally and the lenses pass all the energy, they don't reflect, so they don't absorb or get hot, the energy is transmitted. So if you want the best chance of success in 393nm, I suggest a refractor with long focal-ratio first with internal D-ERF. It's the least expensive and simplest way.

Recall that the angular resolution of 393nm is much higher than long wavelength, such as red, such as 656nm. Use this to figure out how to produce similar resolution and then match an image scale that records that angular resolution at the critical sampling limit. And have that be at your seeing ceiling via seeing measurements. I say this because it's so easy to think we can image 393nm with a 150mm aperture refractor at critical sampling (or over sampling even!), but you're asking a lot more because its higher angular resolution and so to hit critical sampling or over that, you're going to need very fine image scales, and fine image scales demand excellent seeing. If you can reliably image 656nm with 150mm aperture at critical sampling, and you then switch to 393nm on the same 150mm aperture and increase image scale to critical sampling, it's a 60% increase in angular resolution and a massive demand on the system's need for excellent seeing. So your 150mm aperture is acting like you now have a 240mm aperture 656nm system if you were to increase it's resolution by 60% as a thought experiment, when you're doing 393nm on 150mm. That's likely too much demand for excellent seeing and most people's seeing simply will not support a scale like that just any day. And this is exactly why everyone with big aperture CaK systems cannot just post a perfectly sharp image any and every day they attempt to image, because the seeing conditions simply do not support it all the time likely unless you're geographically located somewhere with excellent seeing (sub-arc-second seeing). Agian, no magic bullet, and no magical time of day. If that were true it would a simple thing for everyone to simply buy a big aperture, a big D-ERF and easily image CaK at high resolution, but that's not the case in reality. So that said, if you want to match resolution of 656nm on your 150mm aperture refractor with a 393nm system, you would use a 95mm aperture (recall ~60% angular resolution difference). Using a 95mm aperture in 393nm will produce the same resolution output at critical sampling limit of the wavelength as a 150mm aperture in 656nm. So again, keep this in mind. Going to 150mm on your 393nm system is asking a lot of your seeing. Thus, seeing is everything.

By the way, Oliver at BelOptik makes excellent CaK D-ERF if you feel the need to get one in this aperture size range. He takes Paypal and his country is friendly with the world in terms of exchanges and shipping. Super guy. I would reach out to him if you're truly after a dedicated full aperture D-ERF for CaK. Good to have competition and options.

That said, you simply do not need a full aperture D-ERF for 150mm aperture refractor, and less. You do for mirror systems. But for a refractor you do not. Can you benefit from one? Everyone can. But is it needed? No. And this is not just some anecdotal experience thing. It's data driven. How is it possible that some people need full aperture D-ERF, but others do not? How can sub-aperture internal D-ERF do the same job? There's so much myth out there about dark paint job tubes being bad, internal tube turbulence being bad and all this and so you need an external full aperture D-ERF. But you don't. Several people have proven this by imaging in 393nm with nothing but an internal D-ERF. Christian included. Myself included. And we have lots of measurements and data and outcomes to share. As do several others I'm sure. So before plunking down money on some huge D-ERF for this, I highly suggest you measure seeing first, get an idea of what you can work within, and explore how your seeing conditions and imaging system match up with lucky imaging, and keep in mind you don't need any elaborate energy management system to do it. Now, if your goal is huge aperture, 8", 11", or more, high resolution imaging, a refractor becomes too big and difficult (but can still do it). Your trade the expense of the D-ERF needed for the mount and lens needed.

You're already using what I would recommend for energy management, so I think you're set. I highly recommend a Baader 2" Blue CCD-IR Block Filter as an internal sub-aperture D-ERF. In an out of focus beam, deep if your focuser, it will handle the thermal load of a 150mm aperture no problem. No heat is retained, most of the UV & shorter IR is rejected and reflected out passing only a bit of UV-A and blue wavelengths. If you wish to further limit what's coming through, you follow it up with a Baader K-line filter, with some tilt, to avoid reflections. But this is not even needed frankly, it's mostly for anyone worried about their rare filters that are irreplaceable (which I totally understand given it's so hard to find narrowband CaK these days, it's simply not popular). You can splash in a little KG3 IR absorption in the mix, if you want to handle long IR wavelength (it's not needed for thermal purposes for internal seeing conditions in the OTA, but it may be nice insurance for your mind if you have unicorn filters to project over a long period of time). Just giving alternatives to big full aperture D-ERFs. Don't let opinions guide things. Ask for data and make a choice based on data.

Something else to keep in mind is that when using barlows/extenders/amplifiers, their glass can have influence on the correction, figure, etc, in wavelengths near UV like this. The longer the natural focal-ratio, the better, versus effective focal-ratio generated from more optics added. This is why masking aperture is done by almost everyone who does high res CaK and have better results, as you're making it easier on the following optics in the pathway that are not as well corrected. Everything optically figured is better in a super narrow light cone, like F50~F100. But that's not reasonable for imaging with respect to sampling. Anyhow, get the longest focal-ratio you can first (mask if needed), then use a long barlow or similar extender to achieve final focal-ratio for critical sampling based on your pixel size at the wavelength your imaging.

A final note on critical sampling. It depends on what you subscribe to. Here's how I calculate it:


Here's 120mm aperture, 2.9um pixels (290MM):

CaK_Critical_Sampling_290_01.jpg
CaK_Critical_Sampling_290_01.jpg (38.58 KiB) Viewed 3411 times


And here's 120mm aperture, 5.86um pixels (IMX174):

CaK_Critical_Sampling_IMX174_01.jpg
CaK_Critical_Sampling_IMX174_01.jpg (37.74 KiB) Viewed 3411 times


Note, you can change the aperture all day, but the effective critical sampling focal-ratio associated with the wavelength and pixel pitch will remain the same (see the equation). I'm using 120mm aperture here because I use that often, it's within my "seeing ceiling" for daily use. I can image every day at this aperture in my location and my seeing conditions at critical sampling (this is equivalent to using a 192mm aperture Hydrogen Alpha system in terms of resolution and seeing needs).

So as you can see, you need ~F18 for the 290MM's 2.9um pixels at 393nm. No matter the aperture, the combo of 393nm wavelength with 2.9um pixels means you're always targeting F18 for critical sampling, the most resolution you can get from the system. So this is great on instruments around F8~F10 because a single 2x gets you there (long barlow is more ideal). Or a F6 (long mirror) with a 3x will get you there, if using mirrors. With SCT, you mask the aperture (this is why its commonly suggested to mask their aperture for this purpose, since they have poorly figured performance at 393nm with their F2+ mirrors).

For the IMX174's 5.86um pixels, you need ~F37. Same as above. Whatever system you're using, if it's 393nm and using IMX174, you need F36 to critically sample. Notice, this is twice the pixel size and resulting in twice the focal-ratio needed. It's truly simple like this. This is why I moved to the 290MM to have shorter imaging trains which are much easier to generate than messing with really long F37+ stuff on big pixels. This still works great. The exposures are virtually the same too. It just often takes more glass to achieve this. But it's often difficult to nail F37 (odd number). Most instruments will be F6, F8, F10, etc. Masking aperture and using 2x, 2.5x or 3x. Masking makes it easier to generate the right focal-ratio since its hard to find good variable barlows. Again this is why I went from IMX174 to 290MM, the smaller pixel makes it much easier to generate a good focal-ratio for sampling.

Hope that helps at least get the ball rolling.

Very best,


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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by marktownley »

Really excellent post Marty, I couldn't agree more on all points you raise.


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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by solarGain »

Marty, what a storming reply. Hats off to you.

I have a SSM and I have enjoyed the use of this monitor for several years now. And I watch and watch and by eye I trigger the capture. With my recent captures I have triggered more by eye than the ‘auto’ setting capture button. Such are the conditions in central London.

I am using an f/10 Vixen with a FL1000mm, even before I start filling the focuser with filters and imaging kit!
And I have a CR-150 HD such as yourself which I might also use. But I have not used this scope at the moment.

I agree with your comments on the Ca-K ERF its cost that I can do without.

I highly recommend a Baader 2" Blue CCD-IR Block Filter as an internal sub-aperture D-ERF.

I am using this filter!

These two tables are very interesting, - Wilmslow Astro - (maybe it isn’t) I know the site that you found these on but will have to find it again and I have. Lost in a software update/ transfer, really bad news. I run a VPN and it tells me I cannot use it so will use another computer to access this. I cannot use my own computer to access solarchat! because of the VPN and have to use another computer for access!

So your using a 1.6x factor for your critical sampling of a 120mm aperture. Good to know. I’ve seen 1.7x and possibly 1.8x.

I have a copy of ‘explaining the difference in the IMX174 versus the 290’ from cloudynights in reply to PhotoJohn that I have digested, a very useful.

This is very thoughtful writing and I appreciate your abilities.

Here is an image from May 25 taken at ~f/55.

AND Mark thank you for all you advice too.

John
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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by MalVeauX »

Hi John,
solarGain wrote: Wed Jun 17, 2020 5:16 pm And I watch and watch and by eye I trigger the capture. With my recent captures I have triggered more by eye than the ‘auto’ setting capture button.
I too do not use automated capture; I merely record seeing and watch the metrics. Seeing the trend is more important to me. Seeing values tells me when I likely should start a burst. I will often record 500 frames, 1k frames, or 2k frames depending on what I'm doing based on seeing a few spikes of good seeing with high FPS knowing I will likely get some moments of nearly sub-arc-second (in my conditions). Automation can be nice. But I find manual is "smarter."
And I have a CR-150 HD such as yourself which I might also use. But I have not used this scope at the moment.
It's surprisingly good glass for 393nm, well figured. Would never have guessed until I used it for it. This scope is 150mm F8, but its stock cap makes it a 120mm F10 which is really great for CaK.
These two tables are very interesting, - Wilmslow Astro - (maybe it isn’t) I know the site that you found these on but will have to find it again and I have. Lost in a software update/ transfer, really bad news. I run a VPN and it tells me I cannot use it so will use another computer to access this. I cannot use my own computer to access solarchat! because of the VPN and have to use another computer for access!
I used that formula to show the calculations, but I made my own calculator out of it via excel so that I can quickly calculate this stuff without needing to access a website. I attached the Excel file if you wish to use it, or anyone. No macros. Just fill in the inputs and click somewhere or press enter and it will auto-calc. Nothing fancy.

CR150HD at 393nm (there's no need or usefulness going beyond critical sampling level for your system):

Image

Very best,
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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by marktownley »

Is the last image taken at 150mm or stopped to 120mm Marty?


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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by solarGain »

Good question Mark!
Marty, thank you for the .xlsx very handy indeed. The CR-150 ED is 'almost brand new' (brown paper under the rings as per original shipment etc.) which is hard to believe, it is an interesting scope, and the cork? pads with the focuser are new and the rack is still holding gluey grease! however my focuser tube is like proverbial wagging dog's tail.
Not good but I need to test the glass non the less and see what we have.
John


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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by MalVeauX »

marktownley wrote: Thu Jun 18, 2020 7:43 am Is the last image taken at 150mm or stopped to 120mm Marty?
I think it was 150mm if I recall with an IMX174 (this was before I moved on to the 290MM); seeing was incredible, like 0.3~0.4 arc-seconds. I will see if I still have my capture settings file from back then when I get home. I hate to give false info, trying to run off memory, its been a hot minute on this one.

Very best,
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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by MalVeauX »

solarGain wrote: Thu Jun 18, 2020 11:01 am Good question Mark!
Marty, thank you for the .xlsx very handy indeed. The CR-150 ED is 'almost brand new' (brown paper under the rings as per original shipment etc.) which is hard to believe, it is an interesting scope, and the cork? pads with the focuser are new and the rack is still holding gluey grease! however my focuser tube is like proverbial wagging dog's tail.
Not good but I need to test the glass non the less and see what we have.
John
It's a good old lens and well figured for virtually everything. Great for solar. Handles 393nm and 430nm without any fancy stuff or additional optics.

Absolutely a must to replace its stock focuser. I'm using an inexpensive GSO linear bearing. I'd love to have something that would accept the ZWO AEF for it. For now I still manually focus with fingers!

Very best,


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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by hopskipson »

Hi Marty
As far as auto focus goes have you tried nfocus-nstep by Rigel? I have a plain dc motor for all my gso, ES and Lunt focusers. He even has one for the Coronado helical focuser at least until I upgrade to a moonlite.

I have been thinking of double stacking my Lunt Ca K filter with another Lunt. Has anyone tried this?

I tried DS’ing with an Omega filter. It did create a little more contrast but not much. After going through this thread would it help to tilt the Omega? If so, how is this accomplished? Or would I have better luck with the Lunt.


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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by marktownley »

hopskipson wrote: Sat Jun 20, 2020 4:34 am I have been thinking of double stacking my Lunt Ca K filter with another Lunt. Has anyone tried this?

I tried DS’ing with an Omega filter. It did create a little more contrast but not much. After going through this thread would it help to tilt the Omega? If so, how is this accomplished? Or would I have better luck with the Lunt.
What scope are you using?


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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by hopskipson »

Hi Mark
I’m using an ES 102 f/7 . I’ve been experimenting with some masks to get it to f/9-10.


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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by MalVeauX »

hopskipson wrote: Sat Jun 20, 2020 4:34 am Hi Marty
As far as auto focus goes have you tried nfocus-nstep by Rigel? I have a plain dc motor for all my gso, ES and Lunt focusers. He even has one for the Coronado helical focuser at least until I upgrade to a moonlite.

I have been thinking of double stacking my Lunt Ca K filter with another Lunt. Has anyone tried this?

I tried DS’ing with an Omega filter. It did create a little more contrast but not much. After going through this thread would it help to tilt the Omega? If so, how is this accomplished? Or would I have better luck with the Lunt.
I haven't had any mechanized focusers. Just waiting on ZWO EAF to have more options with different focusers (like GSO).

Yes, several people have double stacked CaK filters. It can be done. It does increase contrast.

I double stacked PST CaK filters at one time. They're unicorn rare now though.

For tilting a filter, just put a little rubber band or gasket strip on one side of the filter holding cell, it will tilt a bit. But no need to do this with an Omega filter. The bandpass is too loose, it doesn't do anything to help with a 2.4A filter.

Very best,


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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by hopskipson »

Do you know if the Lunt filter can be removed from its module and stacked with another Ca K module with blocking filter? And if one gets a 600 model will it be acceptable for DS?

If you believe the ad on EBay the Omega has a bandwidth of 2A. Since I have the filter I might just try tilting it.


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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by MalVeauX »

hopskipson wrote: Sun Jun 21, 2020 1:07 pm Do you know if the Lunt filter can be removed from its module and stacked with another Ca K module with blocking filter? And if one gets a 600 model will it be acceptable for DS?

If you believe the ad on EBay the Omega has a bandwidth of 2A. Since I have the filter I might just try tilting it.
Heya,

Sorry, I don't think the Omega filter is a true 2A. I may be wrong, and I'm not trying to discredit anything, but I'm skeptical that the Omega 2A CaK filter is really 2A, I've yet to see an image that made me think it was remotely close to 2A. I've used several 2~2.4A filters from PST CaK filters to Lunt's Cak filter and they are tremendously higher contrast and show chromosphere features. The Omega CaK filter looks more like a wide bandpass filter that is mostly showing photosphere features but does at least help with contrast on faculae. But I've not seen k-grains or filaments, spicules, etc, via the Omega filter in various combinations. It seems people use them more often as an ERF more than an actual narrowband filter.

The internal Lunt filters can be removed and situated in a double stack. There's lots of examples of this and it does increase contrast considerably. It will not be super high transmission, but it should still work well. These filters are just narrowband filters, not etalons, so you don't need a "blocking filter" in the sense of how etalon systems work. You just need ERF/D-ERF to protect the narrowband filter and block early UV and long IR really. The special thing is blocking UV but not eliminating 393nm and keeping it high transmission.

I plan to eventually pick up another Lunt CaK module; I currently am using a B1200 CaK and it's fantastic, and I can get spicules & prominences with it alone and its contrast is very good. I would certainly like to see a second Lunt filter and remove all the ERF materials and simply double-stack it on its own. I have several Skybenders that I hope to incorporate to make tilting very easy. I got my filter used, so just waiting to see another one pop up hopefully that is also 12mm in size. Don't want to go too small so I can use larger sensors.

Very best,


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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by hopskipson »

I think I’m getting it right. The Lunt number is the size of the filter. The reflective filter in front is a Derf to protect the Ca filter. In order to double stack it I would need another Lunt Ca filter only to tilt behind the one with the Derf. I searched for a skybender but only came up with a diy or 3D print sketch. Do you know where to find one?

I did try out tilting the omega filter but I don’t think it had much of an impact. I will post pictures later. Thanks.


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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by MalVeauX »

hopskipson wrote: Tue Jun 23, 2020 3:36 pm I think I’m getting it right. The Lunt number is the size of the filter. The reflective filter in front is a Derf to protect the Ca filter. In order to double stack it I would need another Lunt Ca filter only to tilt behind the one with the Derf. I searched for a skybender but only came up with a diy or 3D print sketch. Do you know where to find one?

I did try out tilting the omega filter but I don’t think it had much of an impact. I will post pictures later. Thanks.
Heya,

You can tilt it with a filter holder cell and glue it into place with 1 degree tilt or so. Skybenders are not commercially available, they were a custom thing a few years ago so only are found from anyone who happened to get one back then. But again you can tilt a filter by just gluing it tilted inside of something to act as a holder since you don't need to tilt to tune it, just tilt to avoid reflections, so no need to be able to move it freely.

Very best,


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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by solarGain »

Valery Hi. PM sent.
All the best
John


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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by marktownley »

And, as always, silence was the response... :(


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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by marktownley »

I thought I would feedback and share with the group my initial experiences of using a 'new' sub aperture D-ERF for my CaK system. Previously I had been using the Baader Blue CCD filter for this purpose, and while this is fine for shorter focal ratios, as the f ratio goes the light throughput rapidly goes down, never good!

I bought an Astronomik L1 Uv/IR filter whose transmission is high at 393nm:
ak_uv-ir_l_transmission_chart.png
ak_uv-ir_l_transmission_chart.png (1.66 MiB) Viewed 3406 times
Well the graph is right, I have oodles of light at longer focal ratios, that have allowed me to reduce both exposure time and gain which should help freeze the seeing. Yes, it naturally lets more energy through than the Baader Blue, but this will offer about a 60% reduction in thermal load compared to no sub aperture DERF, so, should be safe to use with a Lunt CaK at apertures up to about 150mm.

Just need some decent clear skies to try some imaging with it now.

Mark


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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by solarGain »

By my reckoning the comparison between the two is at most 2% is that enough "oodles of light at longer focal ratios".
Indeed I need all of this light but have nothing to compare with the Baader blue filter. More opportunities in front of the scope would be good to hear about this. Many thanks.
All the best
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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by marktownley »

solarGain wrote: Wed Jul 08, 2020 7:04 am By my reckoning the comparison between the two is at most 2% is that enough "oodles of light at longer focal ratios".
I'm confused? What is the 2% and where have you got it from?


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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by solarGain »

This was just a reading of the graphs for Baader Blue and your new Astronomik filter.
Baader looks like say ~95% and the Astronomik ~97%. Or have I misunderstood something.
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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by Bob Yoesle »

In my home brew CaK module the Badder B CCD filter lets way too much off-band energy trough for my tastes (which seems to be converted to lots of heat), and the Astronomick filter would be worse in that regard. I'm now considering one of these two filters as an CaK module ERF, perhaps combined with a KG for longer IR:

Edmund 390 45 v 394 10.jpg
Edmund 390 45 v 394 10.jpg (160.82 KiB) Viewed 3377 times

https://www.edmundoptics.com/p/390nm-cw ... ter/28795/

https://www.edmundoptics.com/p/394nm-cw ... ter/19839/

The differences come down to substrate - fused silica versus optical glass, OD, FWHM, and wavefront, scratch/dig, etc., and of course price - though both are on the $pendy side! The 394 10 seems to be similar to the Baader K line, and might eliminate the need for that filter in the stew.


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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by christian viladrich »

Just for info, we have measured the transmission of a Baader B CCD filter last year with a photometer. We found 18% at 393.3 nm.
The old B Astronomik filter has a transmission of 87% at 393 nm. This is the one as use as a 50 mm sub-apeture filter. Unfortunatly, the new version has a very low transmission at 393.3 nm.
I also use a 25 mm EO 394-10 nm filter. But it can't be used as a 50 mm ERF. The fluorescence filter pointed by Bob has much better optical quality (still L/4 RMS).


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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by marktownley »

solarGain wrote: Wed Jul 08, 2020 12:43 pm This was just a reading of the graphs for Baader Blue and your new Astronomik filter.
Baader looks like say ~95% and the Astronomik ~97%. Or have I misunderstood something.
John
Ahh got you. Here is the Baader trsnsmission curve for the blue CCD filter:
BAA-blau-G.gif
BAA-blau-G.gif (34.4 KiB) Viewed 3360 times
I think that graph above is very ambitious, and for me the transmission is in line with what Christian measured, certainly that is reflected in the difference in exposure times with it and the Astronomik L1.

What's the optical train in your filter Bob? I hear what you're saying about more energy coming through the L1.

I've recently reconfigured mine and my first element is the EO 387/11 filter this offers OD6 blocking and is a laser filter hence my choice of it as first element, then it goes CaK PST#1 filter, then EO 394/10 (OD4) then another CaK PST #1 in the tilter of the skybender to tune. I originally tried the setup with only a singular EO filter (for more transmission) but there is quite clearly, in both filters, an out of band leak, but combined it is gone... I replaced the K-line for the EO394/10. I've certainly got more light to play with in CaK than I have in Ha now.


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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by christian viladrich »

Here are some measurements we made :
Version of Astronomik B filter previous the current one:
Image
Image

Baader B CCD :
Image
Image
Last edited by christian viladrich on Wed Jul 08, 2020 4:06 pm, edited 1 time in total.


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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by christian viladrich »

ZWO B filter. I have no idea of the optical quality in 50 mm diameter :
Image


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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by george9 »

The other half of the equation, if you are using a Lunt CaK, is what it the transmission of its ERF? Depending on that profile, there may not be much to gain by adding another ERF in front. If the Lunt is already narrow and sharp, then adding another filter with high transmission in front of it ends up putting the same amount of heat through the Lunt ERF to the filters behind it. So if the 4" aperture limit on the Lunt is due to the heat passed through its ERF, this won't help. You will instead need to attenuate the signal (like the Herschel wedge on Lunt's 6" CaK) or put an even narrower ERF in front of it (like Bob's suggested 10nm).

On the other hand, if the problem is long IR, then yes a KG3 will help. The Lunt looks reflective, not absorbing, so that is not an issue to fix.

I need to double check my Baader blue-CCD. It is just a couple of years old but I think the gain did not change so perhaps I just made it. My Astronomik UV-IR L-2c does cut CaK quite a bit, which is what pointed me to the L1. My no-name UV-IR does pass CaK well (but perhaps it passes all UV well).

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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by george9 »

I stand corrected. I had a little time before sunset when the clouds cleared. In CaK:

Baader blue-CCD - passed 33%
Astronomik UV-IR cut L-2c - passed 50%
Baader double stack K-line 8nm - passed 66%+

I didn't measure it, but my PST #1 yellow filter passes the least light, but that boosts contrast, so it is worth it. (That is, the light it blocks is preferentially outside the CaK line.)

I forgot to try tilting the blue-CCD, which should shift it bluer and perhaps clearer.

Perhaps the blue-CCD could be helpful for large apertures by removing some of the energy getting into the Lunt CaK, at least on the blue side.

George


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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by MalVeauX »

george9 wrote: Thu Jul 09, 2020 12:20 am I forgot to try tilting the blue-CCD, which should shift it bluer and perhaps clearer.

Perhaps the blue-CCD could be helpful for large apertures by removing some of the energy getting into the Lunt CaK, at least on the blue side.

George
Hi George,

That's what I use the Blue CCD-IR Block Filter for, with bigger apertures, 120mm~150mm, with the Lunt CaK B1200 module. I'm not sure what exactly the Lunt's primary D-ERF is, it indeed looks reflective, but not sure how much visible spectrum is passing and how much IR is passing. Since it's only rated per Lunt up to 100mm apertures, I can only imagine what it's doing. That said, I have no issues with transmission with 150mm F16 on 2.9um pixels with my current system, more transmission would be welcome, but I'm not hurting for it at the moment with a single filter; maybe a double stack would cause me to want to increase transmission significantly.

That is an interesting point about tilting the filter. I wonder how it would change the reflection to throw the energy out of the tube and if tilting it does indeed shift it blue enough to increase transmission or not. Would be neat to test it. I have some Skybenders, so I will try this next time my weather allows and just do a few tests (but I can only measure by what exposures are needed, which may not be ideal since transparency fluctuation could throw up some inconsistent numbers).

Very best,


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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by marktownley »

Interesting, seems like our respective Baader Blue filters seem to have quite different transmissions...


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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by george9 »

I did better measurements today, and I added tilting. Titling 3 degrees did little. But at 25-30 degrees, many of the filters passed a lot more light. At 30 degrees, I assume the image is degraded.

This is all measured with a Lunt CaK B1200 and PST #1 yellow filter double stack on an ASI290, assuming pixel brightness is linear.

Untilted:

35% = 2" Baader blue-CCD
50% = 2" Beloptic UV-IR cut on KG3
55% = 1.25" Baader K-line
60% = 1.25" generic UV-IR cut filter
65% = 2" Astronomik UV-IR cut L-2c

Tilted 25-30 degrees:

80% = 2" Baader blue-CCD
80% = 2" Beloptic UV-IR cut on KG3
90% = 2" Astronomik UV-IR cut L-2c

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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by marktownley »

Interesting you got something out of the L2 George, I bought one of these initially (over the L1) but found it just wasn't getting any light through for me in CaK despite the transmission graph suggesting it should.


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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by george9 »

I wonder if my L-2c is a previous generation before the L-1, L-2, L-3 series. For example, I see no "c" mentioned anywhere any more. I got it about six year ago. It actually says:

L filter
UV IR cut
Type 2c

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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by LTHB »

One more data point for Baader blue CCD filters: about 35% transmission in CaK for my copy (measured by stacking it with a Lunt unit and comparing the exposure times).


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Re: Can I have an 2020 update on where we are with Ca-K filter controls.

Post by Bob Yoesle »

What's the optical train in your filter Bob? I hear what you're saying about more energy coming through the L1.
Hi Mark,

I don't have it all worked out as of yet. I'm working with a double-stacked pair of PST CaK filters, and going from there. My primary goals are to 1 - reduce reflections, and 2 - reduce the heat load at the PST filters:

1. PST CaK
2. PST CaK
3. Edmund 394-10 or Baader K-line or Edmund 400 lo-pass or Apollo L/Chroma 394.01
4. Andover KG3
5. Edmund 390-45 or Edmund 394-10 or Baader B-CCD.

Italicised filters are what I have on hand, bold filters are what I'm considering the best alternatives for the system. Keeping the number of filters as low as possible would seem to mitigate against the Baader K line and in favor or the other single filter alternatives at position 3, but the Edmund 400 nm short pass might not work as a blocking filter, and the Chroma results in a triple stack that has greatly reduced image brightness and necessitates using a Skybender to come on-band. In position 5 I already have experience that a significant amount of heat from the 400 to 500 nm transmission of the Baader B-CCD filter makes it less than ideal for an ERF.

CaK module 2 clr.jpg
CaK module 2 clr.jpg (260.1 KiB) Viewed 3171 times

I've collected a bunch of filter-tilters and tubing inserts to hopefully address the reflection issues for each individual filter, and a Baader TZ3 (suitable for CaK) should I need it, but have yet to assemble an integrated system... lots of projects underway ;-)

CaK components.jpg
CaK components.jpg (231.16 KiB) Viewed 3171 times

Any insights or suggestions greatly appreciated...


Diagonally parked in a parallel universe.

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Dark-Sky Defenders
Goldendale Observatory
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