Narrowest 393.37 ever by Apollo Lasky

[Valery]

If anybody knows a crew to manufacture some 150mm objectives to be .98 strehl for this filter as a primary dedicated scope , now is the time to step up out of the shadows and help please!

Mike Jones has designed some nice scopes @ f8 and f10 3 years ago:

https://www.cloudynights.com/topic/5141 ... ke-filter/

Of course, f10 or longer is better, especially for getting closer to the narrower bandpasses...

Hi Bob,

It is easy like 1-2-3 to design such an objective. 10 minutes maximum. Unfortunately, it is much more difficult to make it and then keep it optimized at this wave length. In addition only a very few individuals will really pay for such a telescope. Lucky if 5 amateurs will buy such a specialized telescope for CaK imaging.

[mdwmark]

Bob,
You should ask about the design. It is a single cavity filter. The space layer is important to how narrow the filter will be at different F#. If they designed it with a High index space ,that would be the best. With a low index you can get narrower easier but will broaden out faster in F# lower then F/30.

If they can turn on the coating machine and make 1 filter for under $1500, that cutting close on cost( i'm not sure how they can do it, with the machines they have) .

Also there no magic in correcting a doublet for K-line. If you know the design of the lens,or you could guest the design. All you will be doing in increasing the spacing of the two lens. But then it will not be corrected for anything else.
With a mica etalon at 1 Ang. The contrast jumps up , but that is with a camera , It is dim visually . With this filter, I would only use a camera .

Mark W.

[mdwmark]

What is all this about .1nm or .14nm at K-line. They will both give a great image. With the amount of T ,you will probably need some color glass or an ND filter to drop the transmission down or the exposure will be to fast for the camera.
I ask Bob to check if the spacers where high or low index. So I did a little design for a .1nm single cavity filter.
The design with the high index(2) came out at .09nm in the perfect world(100%). At F/30 it drop to .1nm and the T was 90%, at F/20 the Hw went to .15nm and 78%, then at F/15 it was .27nm and 52%T.
So I flip the design and made the spacer a low index(1.5). The same number of layers gave a .07nm perfect world (100%), at F/30 it drop to .1nm and 68%,at F/20 it was .24nm and 41%T,at F/15 it was .35nm and 25%T.
So either way if you keep this filter near F/30 it will be a killer filter at K-line
If your worried about IR blocking , just add some KG5. That will cut the IR after the 1200nm.
This is the same problem with fast F# you will have with any brand that is a single cavity filter.
Mark W.

[krakatoa1883]

A single energy rejection filter will also remove those wings to negligible levels. This filter can also be used with baader planetarium astrosolar film and it will still outperform anything else on the market even at .01% transmission. It will also work in a herschel wedge as is.

very interesting... does this means that it can be used with a large scope with only a front astrosolar film ?

[Valery]

Mark,

Thanks for your detailed explanations and results of your calculations. Very much appreciated.

So, as I understand (I easily may be wrong) this filter has perfect transmission, but the goal was not really only the transmission alone.
The goal was (or should be) improvements in what can be imaged with such a filter vs widely available filters like Lunt B1200/1800 CaK.

Therefore the question: if such a filter (low index spacers) can have 90% transmission and 0,8A bandwidth, then why not to make a double stack of two such filters at the same glass piece? In this DS case we can expect about 0,55A Hw and 70-80% transmission with very well supressed parasitic light from the skirt of the filter? Such a filter may start to show the details like filamets and may be even fibrils. If not, why not add one more SS filter on the separate glass. Total transmission may drop to 50% and there still be enough light to work with. But the parasitis light will be completely eliminated and spiculaes and fibrils sloud becomes visible.

Can you, please, describe some possible hidden difficulties in the realization of such a next generation imaging device for CaK?

Also, what we can expect from the same approach to a 854,21nm filter?


Thanks,


Valery.

[Valery]

does this means that it can be used with a large scope with only a front astrosolar film ?

No in a real world. I tried this with Lunt CaK and can not obtain enough light to see anything on the monitor with full gain of the camera and exposure of 100ms. The same with Ha filter.

[MalVeauX]

I for one definitely would like to see how a 0.1nm bandpass filter behaves. Having used the 1nm filter from Chroma, it's much better than just a typical near UV filter and made for very nice contrast with the PST CaK filter. I would think the 0.1nm filter would be just like a double stack. I"m super curious about two of them too. The idea of sub-angstrom CaK is very interesting, since nothing really out there allows for this on larger scopes outside of something like a research facility and/or SHG, etc.

Very best,

[Bob Yoesle]

Hi Mark,

I checked with Dick at Chroma, and he confirmed the CT393.37bp0.1 is a single cavity and high index filter design. As Mark has pointed out, a larger f ratio is better than a shorter, and Dick also mentioned use in a collimator type system.

I think Apollo deserves credit for passionately pursuing the "holy grail" of CaK filters. I think the 1 nm and KG may make for an ideal CaK ERF system. I will objectively evaluate to the best of my ability how well the 0.1 nm filter performs "real world" at f9 to f18 with an ED100. I hope to try single and DS'd with PST CaK filters, and even try and see what a triple stack might produce. Wish me luck for some good sunspots, weather and seeing...

[marktownley]

Totally agree Bob, credit and respect due to you here Apollo (I hope you are reading this!). People are most definitely interested.

I quite like the idea of incorporating into a collimator system, it was something myself and Walter were looking at, should be easy to do.

I'm guessing running the filter in a telecentric configuration will yield results somewhere in between a collimated and converging light cone setup?

[krakatoa1883]

I for one definitely would like to see how a 0.1nm bandpass filter behaves.

me too... I would be very interested. I understood that it should be used together with the 1nm as ERF, is it correct ? could a Baader K line filter be used instead ? I already have one...

[Bob Yoesle]

Hi Mark,

I'm assuming the same limitations r/t collimator vs. telecentrics apply to the very narrow CaK filter as they to to H alpha.

Since the filter uses a high index spacer construction, I assume this would result in better field angle performance for a collimator system compared to an air-spaced (etalon) system. Despite this, the filter is so small that any large aperture collimator system will result in high field angle magnification. This would make such a system only usable for small field hi-res use. A properly configured telecentric system might thererfore be a bit better, but here again you have the constraints of the EFL to deal with. A smaller aperture collimataor system would seem reasonable for full disc work, and per Mark W's work a telecentric system for larger aperture hi-res work with and EFR of f20-f30.

Raf, the filter is very robust according to Chroma.com. I would think the Baader CaK filter would be more than adequate. For my apples-to-apples comparison I will use a 2 inch Baader Blue CCD ERF, followed by the Badder CaK or Edmund 400 nm short pass, and possibly a KG3 for far IR. BTW, the transmission plot for the filter I just received shows a peak T of ~ 92% (!)

Filter curve SM.jpg (184.94 KiB) Viewed 1019 times

[krakatoa1883]

@Bob - Thanks for you reply, I think I will add also a KG3 to the K line for IR.

@Apollo - thanks for confirmation, I am already using a Quark CaH with my solar newtonian and works fine with my Chameleon. I also noted that it passes enough light to allow imaging of the full disc with a 80mm refractor through a front ND3.8 Astrosolar foil, so I am planning to use your filter with my Mewlon 210 and the same full aperture Astrosolar grade. I'll contact you soon, thanks.

[Bob Yoesle]

My first results with the CT393.37pb0.1 are disappointing.

However, a caveat is that I did not tilt any of the filters, and spurious reflections are apparent in my old system, and even more so with the Chroma Tech filter. I will therefore have to reserve a final judgement until I can do some additional experimentation and tinkering with tilting in order to mitigate the reflection issues. All images were identical in the use of an ED 100/900 (e.g. @ f9), Baader Blue CCD, KG3, and Baader K line filter as ERF and blocking filters. Seeing was below average, exposures and processing were identical except for the actual exposure length itself, which was varied to obtain similar histogram levels:

PST single stack:

PST.jpg (301.84 KiB) Viewed 938 times

Chroma single stack:

CT .jpg (253.97 KiB) Viewed 938 times

PST/PST double stack:

PST PST.jpg (281.87 KiB) Viewed 938 times

PST/Chroma double stack:

PST CT.jpg (260.97 KiB) Viewed 938 times

Note there obvious brightness gradient, indicating a secondary out-of-focus reflection issue:

PST CT reflection.jpg (69.66 KiB) Viewed 938 times

PST/PST/Croma triple stack:

PST PST CT.jpg (257.71 KiB) Viewed 938 times

Some additional machining work needs to be done in the next week or so before a better result might be obtained... and I may opt for the 400 nm sortpass cut-off instead of the Badder K line (gets rid of an extra element), and put the CT filter in a Skybender to see what effect it may have. Otherwise the fact that this is a single cavity filter may indicate it does indeed require an f20 or longer EFR for best results, while the PST filter does better at the lower focal ratio of f9 -- perhaps using a multiple cavity design with much steeper slopes of the transmission profile.

As I've discovered years ago, noting is ever as easy as we would like with narrow band filter systems...

[Bob Yoesle]

That is my suspicion as well. The Badder K line has one flat component and one tilted. I'm working on tilt wedges of 2 and 4 degrees, and better spacing...

[marktownley]

Can you describe the order of the filters Bob please. Where was the Chroma when those secondary reflections were occurring? I think you could safely do away with the Baader K in your stack safely with the chroma replacing it, it offers greater blocking range.

[Valery]

A single energy rejection filter will also remove those wings to negligible levels. This filter can also be used with baader planetarium astrosolar film and it will still outperform anything else on the market even at .01% transmission. It will also work in a herschel wedge as is.

very interesting... does this means that it can be used with a large scope with only a front astrosolar film ?

Transmission is so high enough on this filter that YES, regardless of what Valery tried and says, it is going to work no problem with astrosolar film, solar newtonians, and also herschel wedges. You are limited by the sensitivity of your ccd camera at this point.

People use the baader planetarium kline filter all the time and it requires astro solar film. That transmission is only 75%.

This filter is a game changer for most instances.

Even an elementary school pupil can easily understand why the Baader K-line 75% transmission filter is OK with the AstroSolar and why your CHROMA with 92%(!) will fail. It is obvious now - you, unlike that pupil, do not understand why. This your inability to understand such elementary things perfectly confirms an earlier Frederic's Jaber judgement - you are not credible enough to discuss these elementary questions. You should remenber that your first apperance here with attempt to sell here a 1nm Chroma filter.


To krakatoa1883

Raf,

Do not waste your time with a free lunch from Apollo Lasky (SkyBurner). Astrosolar will not work the way he promised. With refractors up to 6-7 it will work with Herschel prism. But only at a small scale (to keep light density higher) and never will work OK with large scale where we need extremely short exposures to freeze the seeing at such a short wave length. When image in CaK at large scale exposures longer than 6nm do not allow high res. So, such light eaters like AstroSolar and Herschel prism are not suitable for high resolution solar imaging in ultranarrow band filters.

[Carbon60]

Bob,

Instead of using identical processing, if you were to work the heck out of the Chroma Single stack image data, what do you think would be the best possible image obtainable? From what you've posted it looks soft compared with the others. Adjusting Imppg and optimising contrast etc are you able to obtain anything approaching a good result?

Stu.

[Valery]

Bob,

Instead of using identical processing, if you were to work the heck out of the Chroma Single stack image data, what do you think would be the best possible image obtainable? From what you've posted it looks soft compared with the others. Adjusting Imppg and optimising contrast etc are you able to obtain anything approaching a good result?

Stu.

Hi Stu,

I believe Bob doing the comparision correctly. Harder processing can be applied to both - better and worser filters and better filter always will deliver better results.

After many years (now) experiments, I came to the obvious conclution - the better the initial filter, the better and easier achievable results will you get.

What is really important is a finesse and a transmission profile of a filter. Not only it's transmission peak % and a FWHM.

The results Bob obtained are exactly what were predicted. Single cavity filter will always loss vs similar FWHM filter with much steeper sides and heavily supressed skirt - as in a DS combination.

For CaK imaging I'd better stick with two PST or two Lunt filters as DS, as well as in a SS. There are no easy and cheap solution which will best old, well known and well developed solutions. And the best filter in performance/price ratio is two PST filters stacked as DS paired with the Baader K-line or Edmund 390nm bandpass filter. No one showed results which outperforms this combo. The only filter wich is close in performance is Barr filter owned by Christian Viladrich.

CHROMA filter needs a lot of work to improve it and to reach even the results peoples achieved with a simple Omega CaK filter, not say about what can be achieved with Coronado and Lunt filters.


Valery

[Bob Yoesle]

Hi Mark, the CT filter was just behind the Baader K line - probably not ideal ;-) But I don't think it would have performed much differently without the reflection. Outside the reflected area still suffers compared to the PST double stack.

Hi Stu, I will be attempting a lot more with the filter prior to making the decision on whether to keep it or not. I think part of the issue of softness might be the extremely short exposure due to the high transmission, and therefore might try to knock this up a bit with some ND filters.

But my first task will be to finish my revised CaK module, which will allow for both multiple filter arrangements and tilting of all filters to hopefully mitigate reflection issues. With the highly mirrored filters such as the CT393, Baader K Line, and/or Edmund 400 Short pass, this is apparently essential and it seems pointless to continue without the implementation of small amounts of tilt.

CaK mod prelim SM.jpg (349.78 KiB) Viewed 890 times

What we appear to need for CaK is a 3+ cavity filter to get the results Valery is describing:

Multi-cavity filters.jpg (88.06 KiB) Viewed 887 times

Since Chroma Tech has now gotten into Asronomy filters, I will see what they think it might cost to get there...

[marktownley]

Interesting to see how your experiments go Bob. My gut feeling is the Baader K-line is not helping the synergy. The chroma on it's own is certainly (in that configuration) not working as good as a SS CaK PST filter. The CaK PST filter is not Gaussian in it's profile I don't think either so doesn't have the wide(r) skirt in the wings as the Chroma. What was the exposure time Bob?

Valery, Apollo, please stop the flaming with each other too. This is a great thread that I don't want to be spoiled through unnecessary personal banter! Thanks guys

[christian viladrich]

Interesting tests Bob !

It looks as if the filter is out of band, while it is supposed to be at 393.37 nm with no drift with temperature ?

The resolution is definitly lower compared to the Coronado CaK, but it might just be an artefact of variable seeing conditions.

Diffuse light and reflections is a big issue. On my Baader K-line, I removed the front (tilted) element since is was a major source of reflection. It is much better without it. Maybe something worth trying ?

Looking forward for your next tests with better conditions.

[christian viladrich]

Bob, you should probably have to jump to F/20 for the next tests. F/9 is definetly to short for a 1 A filter.

[krakatoa1883]

Thanks Bob for your update. Agree that Baader K-line filter can be an issue as it suffers from ghosting and this may interfere is some way

ghost.jpg (6.32 KiB) Viewed 1326 times

Although the single stack with Chroma is not comparable with SS/DS PST and also lacks in definition, in terms of contrast of the bright plages it is not so much different from what I get through my 5A Quark calcium. Considering it is a "simple" filter still under development I would say it is a promising result worth of further "tuning".

@Valery - thanks for your advice, I will consider all options before purchasing anything for converting my Mewlon to Calcium imaging.

[mdwmark]

Hi group,
The point I was trying to show ,was the Hw of a filter can change depending how it is used and how it is designed. The perfect word calculations is the light is parallel to the filter. The transmission is just for references to show how it decreases with F#.
There are at least 5 company's that I know of that can make a hard coated single cavity .1nm HW filter at K-line. But you would need to order quite a few to get the price down to $1250.
One of the main problems you will come across is that one axis may have a wavelength shift from edge to edge. This would be the side that is facing the center of rotation of the holder. It will get increasingly harder to keep uniform as the CA of the filter just goes from 25mm to 38mm.
The idea of using white light solar film will not work,been there try it. The hard coating can take the heat if you are in the F/25-30 range. Then use a ND filter or color glass to cut the transmission down. The KG3or5 in not a bad idea either.
Bob, I was surprised with your results. You think you may have been off band? This filter should be far superior then any of the one you tried. Just being close to the center, it should have good contrast.
Also do you have a way to measure the CW of the filter? To make sure that it is really at 393.4nm.
This interest in K-line surprises me. I have been offering K-line filters for 35+ years and they have never been that common . These are usually 38.1mm and 50mm etalons, so they have a higher cost. But in the 25mm range, these hard coated filters should be the way to go for imaging.
Mark W.

[Valery]

There are at least 5 company's that I know of that can make a hard coated single cavity .1nm HW filter at K-line. But you would need to order quite a few to get the price down to $1250.


This interest in K-line surprises me. I have been offering K-line filters for 35+ years and they have never been that common. These are usually 38.1mm and 50mm etalons, so they have a higher cost. But in the 25mm range, these hard coated filters should be the way to go for imaging.

I know the sixth which can make a 0,1 - 0,15nm 1" filter much cheaper if a quantity is 100 or more in order.


This interest is as high as Lunt sells his CaK filters. Due to specific details size and their structures CaK starts to be really interesting only at a very high resolution. Minimal diameter of a telescope for such impressive CaK pictures is 150mm which is equal in resolution to a 250mm telescope in Ha. The larger the aperture, the more interesting CaK images become.
However all the major details are well seen on the pictures taken with a single PST filter (paired with K-line Baader) or single Lunt CaK. Double stacking improves the contrast very little, much less than in H-a double stacking. This is because PST and Lunt filters already have very steep sides with effectively supressed skirt.

It was my wrong expectation that at subangstom FWHM we will be able to image new details - filaments and fibrills as well as bushes of spiculaes. As leading pros astronomers explained me, this is not the case with the CaK.
Such a new for Ca details can be imaged only at 8542,1A line and only if the filter will have very steep sides, well supressed parasitic skirt and FWHM will be 0,25A maxumum (much better if 0,15A). I believe these requirements are too strict and even two 0,7A etalons being DS will show almost as much as professional filters. I have a solid factual evidences what kind of filter will be enough for a new view of the sun in a Ca. And this is infrared Ca II I line.

Again, my experiments showed clearly - I can image 95% of CaK details with a cheap PST CaK filter vs DS PST or Lunt CaK filters. Only prominences becomes a bit better seen and spiculaes at the limb.


Valery D.