CaK disc comparison between SS and DS

krakatoa1883 · Post by **krakatoa1883** » Wed Mar 10, 2021 11:24 am

Having definitely missed the CaK PST when it was available and therefore the possibility to assemble a CaK unit by myself, I have nothing left to increase image contrast but to stack two Lunt B1200 units. The difference between SS and DS is already apparent during the capture (through 90 mm refractor stopped to 50 mm f/10) without any processing applied

: comparison.jpg (63.89 KiB) Viewed 1422 times

and is more clear after (the same) processing

: comp processed.jpg (302.91 KiB) Viewed 1422 times

although is less than I expected but worth the purchase of a second module. For those that may be interested stacking is made by removing the blocking filter of one module (I suggest to check which one is better between the two as quality may vary) converting the final male T thread to a female one by means of a converting ring that can be attached to the second module after the eyepiece holder has been removed

: stack.jpg (106.63 KiB) Viewed 1422 times

a very simple and fully reversible mod. I tried to remove from one module only the filter assembly, without the full barrel, in order to shorten the system but I have been unsuccessful and eventually gave up.

EGRAY_OBSERVATORY · Post by **EGRAY_OBSERVATORY** » Wed Mar 10, 2021 11:42 am

Thank you Raf - for an interesting and informative comparison.

Cheers
Terry

Stardust5858 · Post by **Stardust5858** » Wed Mar 10, 2021 12:55 pm

Thanks Raf, that's a very interesting comparison. Thanks for sharing your findings.

Post by **Montana** » Wed Mar 10, 2021 1:11 pm

Very nice indeed

I don't think I could afford to buy two though

Alexandra

torsinadoc · Post by **torsinadoc** » Wed Mar 10, 2021 1:55 pm

Thanks for the information. I am considering double stacking my lunt 1800.

Bob Yoesle · Post by **Bob Yoesle** » Wed Mar 10, 2021 2:55 pm

Very good results with the Lunt DS! Some folks are skeptical about DS improvement in CaK, but for me it was obvious.

Triple stacking CaK will provide a more subtle but perceptible improvement, but just as with H alpha pushes exposure time up. However, for my DIY system TS'd CaK exposures were no more than double stacking H alpha:

viewtopic.php?t=24617&start=75#p225539

krakatoa1883 · Post by **krakatoa1883** » Wed Mar 10, 2021 4:45 pm

Thanks for comments

The idea of stacking two modules came in fact from Bob (thanks, Bob!) in the thread he linked and at least in another one. Stacking requires more gain but not so much to make the image noisy. Better to have a very transparent sky however as one can see from the above, imaging of the full disc with only a 50 mm aperture is already possibile and with the Sun at less than 30° above my horizon when I took those images. I have still to try in hires mode by using of my 90 mm scope which has a dedicated focal extender that corrects for SA in the near UV. I am planning to replace the original Lunt BF with the blue glass from the blocking filter of Quark Calcium + a KG3 glass, I already noted that this assembly works fine with the Lunt filter and passes more light than the series BF.

Post by **marktownley** » Wed Mar 10, 2021 8:50 pm

Excellent Raf! Very clearly demonstrates the benefits of DS CaK.

JochenM · Post by **JochenM** » Thu Mar 11, 2021 7:18 am

Excellent results, Raf.

Carbon60 · Post by **Carbon60** » Thu Mar 11, 2021 7:35 am

Hi Raf,

Just to be clear on this, you’ve double stacked the yellow filters in the Lunt modules, correct? The purple/blue filters (two per module) are in the long barrel (on the left of your image) and as you have only one barrel in your setup, the second pair of transmission filters from the second module have been set aside (not used). Is this also correct?

Thanks.

Stu.

christian viladrich · Post by **christian viladrich** » Thu Mar 11, 2021 9:33 am

Excellent. Thanks for sharing !

In addition to the image of yours and Bob's images, here is another comparison.

First one with a 0.37 A FWHM Ca K filter :

http://astrosurf.com/viladrich/astro/so ... ASI290.jpg

Then a 0.24 nm FWHM :
http://astrosurf.com/viladrich/astro/so ... ASI290.jpg
The layer of the chromosphere is a bit higher, so the small pore is less visible.The frontier of the bright plages become a bit fuzzier (because magnetic tubes are expanding with height).

Double stack 0.37 A + 0.24 A filters :
http://astrosurf.com/viladrich/astro/so ... ASI290.jpg
Here we get higher in the chromosphere. The pore is virtually hiden by the canopy. Vertical fibrils become visible in the bright plages.

krakatoa1883 · Post by **krakatoa1883** » Thu Mar 11, 2021 9:59 am

Hi Stu

Both modules have the same long 2" barrel containing (I presume) the blocking assembly as most of the light is blocked here, I only removed one of them. The rear barrels with the Lunt logo including the "yellow" filter remain untouched.

pedro · Post by **pedro** » Thu Mar 11, 2021 10:03 am

Excellent comparison Raf, thanks for sharing

krakatoa1883 · Post by **krakatoa1883** » Thu Mar 11, 2021 10:10 am

Thanks to everyone for comments.

Interesting comparison, Christian, the difference between 0.37 and 0.24 is already striking but the DS still add something more.

Post by **marktownley** » Thu Mar 11, 2021 4:36 pm

Very good thread everyone! I'll leave a while longer here for further discussion and then move to the solar reference library so it doesn't get lost.

MAURITS · Post by **MAURITS** » Thu Mar 11, 2021 4:39 pm

Excellent comparation Raf.

Post by **marktownley** » Sat Mar 13, 2021 7:01 am

The other thing to throw in the mix you may want to consider in your trials Raf. I found when I did this with CaK PST filters that there was a preferred order which gave a more contrasty view, so, try swapping the 2 filters round and seeing if it makes any difference.

krakatoa1883 · Post by **krakatoa1883** » Sat Mar 13, 2021 3:18 pm

Thanks for your suggestion, Mark. I didn't consider swapping the filters so this will be my next try. The good thing of this mod is that Lunt modules (I mean the filter assembly) are T-threaded at both ends so it's easy to experiment with different combinations, one needs just a couple of inverting rings.

justapictureposter wrote: ↑Sat Mar 13, 2021 1:25 pmAs seen on this image, the small filter becomes the "aperture" and the image space and object space become parallel through it..

I am not sure to have understood the mod you suggest, it seems you mean the following: sun--> scope --> ERF --> barlow --> filter --> DCX --> camera, however a Barlow is a diverging lens while in the pictures I see the aperture placed between two converging lenses. Do you mean is the second filter that shall be placed between the DCX's ?

Bob Yoesle · Post by **Bob Yoesle** » Sat Mar 13, 2021 5:31 pm

The narrowband filter itself, should be placed between these 2 optics. It is a quick an easy double telecentric and This made the spicule layer and prominences explode into view... I cannot explain how it works, but it does.

Hi Apollo,

These optical systems are no different than exists with H alpha (or any other) filter or etalon.

In the above diagrams for the telecentric lens arrangements, if the first lens (positive or negative) is placed with its focus coincident with that of the objective you will have the exiting rays parallel, and placing the filter after this lens results in that you are using a collimator lens/filter system, not a telecentric system. The second convex lens merely becomes a refocusing lens - which may or may not be telecentric. Having the filter in a collimated system likely reduces the FWHM of the filter compared to the usual non-collimated objective-filter system.

: machinevisionsystems4.png (70.14 KiB) Viewed 1217 times

If you place the filter after the second convex lens, and the axial rays exiting it are also parallel, you then have a telecentric lens system.

Having the filter receive parallel axial rays in the collimator configuration will, just as with H alpha filters in a collimated system, result in field angle magnification, and therefore might produce a Jacquinot spot depending on the FWHM of the filter. In this case it might be unnoticeable due to the wide FWHM and the resulting larger Jaquinot spot size.

If the optics produce the parallel axial output from the second positive (refocusing) lens, and it is sufficient long convergence, then just as with an H alpha mica filter system, results in a reduced FWHM across the filter. This might be the preferable configuration. But just as with H alpha filters, may result in excessive EFL.

If the axial rays exiting the second positive lens are not parallel, then it is not a telecentric lens/filter system, and it is effectively a Barlow lens system, and will result in a widened FWHM and possibly a Jacquinot spot as well.

: appearance01.jpg (227.57 KiB) Viewed 1217 times

The "aperture" in these diagrams simply indicates the position where no vignetting occurs.

krakatoa1883 · Post by **krakatoa1883** » Sat Mar 13, 2021 7:51 pm

Very useful clarification, thanks. I have a Edmund -100 mm PCV that I can place inside the focal point to generate a collimated beam. If I have understood well both filters shall be placed in this bundle as the etalon in a H-alpha system. Which focal length should I use for the refocusing lens ?

Bob Yoesle · Post by **Bob Yoesle** » Sat Mar 13, 2021 9:35 pm

That depends on what the objective focal length is. Here's an example for what you are currently using:

If your objective has a 500 mm FL (i.e. 50 mm f10), and your collimator has a (+/-) 100 mm FL, your field angle magnification is 500 mm / 100 mm = 5 x.

So now a 100 mm FL refocusing lens will result in having 5 x its normal FL. Knowing this, you can get an identical original EFL by using a refocusing lens with a 100 mm FL. If you want and EFL of 750 mm, you would solve X x 5 = 750. X = 750 / 5 = 150; so a refocusing lens with a 150 mm FL would yield an EFL of 750 mm.

The issue that will now come up is the B1200 CaK filter diameter versus the placement of the collimator lens in the objective's converging beam and possible vignetting (as well as a smaller Jacquinot spot). Just a with H alpha etalons, it might be better to go with a telecentric lens system and use a focal reducer...

Bob Yoesle · Post by **Bob Yoesle** » Sun Mar 14, 2021 5:20 am

Hi Apollo,

Your lens system is neither a telecentric nor a collimator. But it has the same basic layout of of one which uses a negative lens or lens group for producing collimated rays, followed by a positive lens or lens group, which produces telecentric ray bundles.

You are using telecentric lens diagrams intended for industrial process to make incorrect extrapolations about how this applies to your Barlow lens with a following positive biconvex lens for an object lying at infinity.

All you really are doing with the 2 x Barlow is presenting you filter(s) with a longer f ratio, or closer to parallel light cones. They are most likely not resulting in collimated bundles of light, as this would again require the coincident foci of your negative Barlow lens and the objective. Nor are they telecentric. If the negative collimator lens of the typical Lunt or Coronado internal etalon telescope lies at a different distance other than its FL from the objectives focal plane, it to ceases to be a collimator lens and just becomes an negative amplification lens. Therefore this in not the same as the diagrams you posted.

Instead, you're presenting your filter(s) with a 2 x longer f ratio (FL 1200 mm > EFL 2400 mm, which likely results in a narrower FWHM aka bandpass - and of course there would be no change in wavelength) via the Barlow. Even if the positive biconvex lens had a telecentric output (highly unlikely), all you are using it for is a short focus focal reduction lens. And that is why your EFL is shorter than either the 1200 mm of the objective, the EFL or 2400 mm of the 2 x Barlow. Your positive lens is acting as a telecompressor - not a telecentric lens.

It's that simple.

For a more in-depth look at how a true collimator/refocuser is quite the same as the traditional astronomical telecentric lens system, see Peter Zetner's excellent work here:

: TelecentricLens2cd crp.jpg (280.1 KiB) Viewed 1313 times

https://pbase.com/p_zetner/opticsdesign1

MapleRidge · Post by **MapleRidge** » Sun Mar 14, 2021 10:16 pm

Hi Raf...

A very interesting post with the added CaK blocker

I need to get the club CaK module back and try to DS this way myself.

Brian

Bob Yoesle · Post by **Bob Yoesle** » Mon Mar 15, 2021 4:31 pm

... the collimated bundle, forms a parallel image at the center of the aperture. The terminology is called "tandem operation" in the optics world. and industrial vision schematics is no different from telescope vision schematics; albeit one is macro and one is telephoto. Lens operation becomes very different when multiple groups are incorporated outside of he standard pcx singlet + pcv combo. We are dealing with a focal point in this situation.

An achromat objective + the barlow creates a new "positive lens group" with a new focal point (the focal point being the image of the sun cast onto the aperture), the barlow also alters the focal point operation of the 100mm element ; from what i see with my eye it has reduced down to just 25mm (the size of my extension).

The design on the apochromatic triplet barlow brings all points of light from the objective to the same focal point at the center of the aperture in the center of the barlow, and when the DCX lens is focused on the center of this aperture point of focus it produces "tandem system operation", . Emphasis added.

Sorry Apollo, but since you have chosen to put this up on a public forum I feel obligated to let folks know your posts regarding macro telecentric lens systems applied to what you are doing with a Barlow lens (regardless of the number of elements) and a single element telecompressor lens are most surely inaccurate: While the triple element Barlow might coincidentally be reducing aberration at 394 mn, there is no way to know without a real-word optical element ray-trace analysis. Additionally, there is no such thing as a "parallel image;" "telescope vision" optics design results are different from "industrial vision schematics;" there is no way to tell that your triple element Barlow and positive singlet "produces 'tandem [telecentric] system operation';" nor would it be applicable to what you are doing for the placement of the CaK filter inside the Barlow lens housing where you appear to assume you are getting a collimated output, followed by the singlet telecompressor lens - which therefore is totally irrelevant to the performance of the filter, and is also likely not a telecentric output.

Without knowing the actual lens prescriptions and their placement in the optical system you can not make ANY statements regarding whether or not the Barlow-telecompressor system you are describing creates ANY collimated rays or telecentric rays at any point in the optical system.

Again, if your filter is performing better, it is most likely due to operating at a twice as long focal ratio, and perhaps due to additional aberration correction at 394 nm by the multi element Barlow. The positive singlet is doing exactly what one would expect - reducing the EFL to what you are observing. Neither the front Barlow lens elements (with objective?), nor the rear positive telecompressor are acting as telecentrics, and therefore it is not a "tandem [telecentric] system operation." And it is highly improbable that the Barlow is producing a collimated output to the refocusing telecompressor lens.

All you seem to be attempting to do is create a collimator for your CaK filter by using a multi-element Barlow lens instead of a simple single concave lens, and calling the combination of the objective with this collimator lens a "telecentric" - with the Sun lying at infinity as the object. The Barlow will not act as a collimator unless it is fixed with respect to the objective focal plane at a specific distance which outputs truly collimated axial rays.

I know little about lens theory, and maybe someone with actual prescription knowledge can figure it out;

This is an accurate statement. While we all appreciate your endeavors to seek "new" solutions to CaK and H alpha filter implementations, given the self-admitted lack of knowledge about optical theory, unless one can produce a valid ray-trace that shows what is actually happening with your specific optics, one should not be making any specific optical system claims or conclusions. This is the exact type of unfounded speculation and misinformation we should be striving to discourage, not encourage.

"The wise know too well their weakness to assume infallibility; and he who knows most knows best how little he knows." Thomas Jefferson.

Post by **marktownley** » Tue Mar 16, 2021 6:38 am

Lets get back to the OP thread please.

MalVeauX · Post by **MalVeauX** » Tue Mar 16, 2021 11:53 am

On the topic of narrowband filters and focal-ratio for the purpose of transmission profile, does anyone know how to calculate or estimate the relationship simply?

Example would be, how a 2.4A Calcium K filter, like Lunt's, behaves in an F10 focal-ratio compared to F16 focal-ratio compared to F32 or longer focal-ratio?

I'm curious how a 2.4A and 1~2A second filter would behave in a F16 light cone, compared to a F8~F10 light cone; as a 3rd step in the idea of double stacking CaK but how to get the most out of it. And the idea of how it would compare to have a single very narrow filter (single stack) in a very long focal-ratio (F32+ or longer) compared.

Very best,

christian viladrich · Tue Mar 16, 2021 12:57 pm

MalVeauX wrote: ↑Tue Mar 16, 2021 11:53 am On the topic of narrowband filters and focal-ratio for the purpose of transmission profile, does anyone know how to calculate or estimate the relationship simply?

Yes Marty :-)
The basic formulae are there (near the bottom of this page):

As a matter of fact, I've done an extensive study (kind of feasibility) for a project of a 1 A Ca K filter. There are various issues at stake :
- CWL of the Ca K filter => this is in relation with the issue of sweet spot formation with tilt,
- change of CWL with temperature => this is related to change of FWHM with tilt and the need or not to have a thermoregulated filter,
- change of FWHM and CWL with f-ratio and field angle => this is related to the need (or not) for a telecentric beam.

Basically, filters with FWHM < about 1.5A are a bit tricky to use, there is no free lunch ;-)

If you think this could help, I can put some info/data in a web page.

MalVeauX · Post by **MalVeauX** » Tue Mar 16, 2021 1:55 pm

christian viladrich wrote: ↑Tue Mar 16, 2021 12:57 pm
MalVeauX wrote: ↑Tue Mar 16, 2021 11:53 am On the topic of narrowband filters and focal-ratio for the purpose of transmission profile, does anyone know how to calculate or estimate the relationship simply?
Yes Marty :-)
The basic formulae are there (near the bottom of this page):

As a matter of fact, I've done an extensive study (kind of feasibility) for a project of a 1 A Ca K filter. There are various issues at stake :
- CWL of the Ca K filter => this is in relation with the issue of sweet spot formation with tilt,
- change of CWL with temperature => this is related to change of FWHM with tilt and the need or not to have a thermoregulated filter,
- change of FWHM and CWL with f-ratio and field angle => this is related to the need (or not) for a telecentric beam.

Basically, filters with FWHM < about 1.5A are a bit tricky to use, there is no free lunch ;-)

If you think this could help, I can put some info/data in a web page.

Thanks Christian,

(I think the link or image you posted broke?)

I think it would be great to see your information. I am very interested in the relationship of a single stack or double stack filter set, narrowband, of various FWHM and the effect of isolated variable, like focal-ratio, in a given temperature & tilt scenario, and how it effects things (such as, does it change the FWHM?). And also the differences in a collimated vs telecentric beam. And is the sweet spot calculated the same or similar as an etalon? Any metrics and formula to help establish a metric for things would be great. I think it would be very interesting to see what the most efficient means to get a high contrast highly selective CaK system would be, given the severe lack of commercial options. A digital SHG is one thing. But there are common filters (lunt CaK) and some available commercial filters (chroma, etc) and I'm curious what could be done with these with respect to temperature, tilt and focal-ratio (collimated or telecentric beam influence, etc) and the FWHM with various means to stack the filters and adjust them. No one will ever be able to get PST CaK filters again, so there's just limited options and even the custom options don't seem to be compelling on their own.

So yes, data data data pertaining to CaK FWHM variation based on the variables on a page would be awesome!

Very best,

Bob Yoesle · Post by **Bob Yoesle** » Tue Mar 16, 2021 4:39 pm

I'll let Christian provide the quantitative analysis, but qualitatively, smaller filters generally perform better in telecentric as opposed to collimator based system due to the excessive field angles collimators produce with smaller FL collimator lenses and etalons. The downside of telecentric systems are the larger EFRs needed. But given the larger FWHM of Lunt and similar CaK filters, this is less of a constraint, and indeed "normal" f10 scopes can and do perform quite well, especially if well corrected at 393 nm. However, such data is usually not available, so the g line at 436 nm is a good indicator.

As I have demonstrated, double and triple stacking commercial CaK filters will indeed work, but the reduced FWHM may push to the need to use telecentric filter positions and better filter control via tilts and temperature for optimum FWHM performance. As Christian has discovered, getting a narrower FWHM filter is doable, but getting an accurate CWL out of production is more difficult with normally coated filters - thus the need for tilting and heating to optimize this parameter. Or you can just go to a standard etalon filter for this wavelength used in the long f ratio or telecentric position.

MalVeauX · Post by **MalVeauX** » Tue Mar 16, 2021 4:54 pm

I guess that's what I'm wondering.

In a collimator type system, with a really long focal-length (F16~F32+) and relatively small aperture (say 60mm~80mm tops), one can generate a really long focal-ratio. So I'm curious if this can be further used to benefit the FWHM of a narrowband filter, and if so, by about how much (the calcs). Then, would it be useful to then introduce telecentriscm to this train and its effects. Then, could a 2nd or 3rd filter be stacked, that are not ultra rare and hard to get, to get a very selective final transmission profile centered on CaK line.

It would be interesting to reverse calculate these, to know the approximately FWHM a filter needs to be, for stacking, to be of benefit in various configurations with long focal-ratio or telecentric systems, to get the result.

My interest stems from a thread Mark made about something similar a while back. I liked the contrast I was getting with just a single Lunt B1200 CaK filter module with a 60mm F16.7 refractor (no barlow, no extra glass, a 1 meter scope at 60mm aperture). And it wasn't even at critical sampling for the pixel size (on IMX253's 3.45um pixels). But I've been curious ever sense about the FWHM of the Lunt CaK Filter, when used in really long focal-ratio beams, like what if I took it to F25 with a 1500mm scope at 60mm aperture, etc, and how a telecentric could come into play if it were beneficial to aid in generating a longer focal-ratio into the filter (and reduce after), and of course, how stacking in a more narrow beam is works out for final transmission profile, etc.

Very best,

Bob Yoesle · Post by **Bob Yoesle** » Tue Mar 16, 2021 5:29 pm

Hi Marty,

In a collimator type system, with a really long focal-length (F16~F32+) and relatively small aperture (say 60mm~80mm tops), one can generate a really long focal-ratio. So I'm curious if this can be further used to benefit the FWHM of a narrowband filter, and if so, by about how much (the calcs). Then, would it be useful to then introduce telecentriscm to this train and its effects. Then, could a 2nd or 3rd filter be stacked, that are not ultra rare and hard to get, to get a very selective final transmission profile centered on CaK line.

I'm not sure I'm fully understanding what you mean by using a collimator system to create a long EFR and introduce a telecentric aspect after that. You would typically just employ a telecentric system to create the long EFR and and use one or more filters in the telecentric output of the system. I guess you could use a collimator for one filter, and make the refocusing lens a telecentric element for an additional filter or filters. But you could just as easily avoid the Jacquinot spot effects of the collimator filter - and just employ a long EFR telecentric system alone with multiple filters.

I believe Christian does this when he uses his DayStar and Coronado SMn quartz etalon double stacked H alpha filter system.

MalVeauX · Post by **MalVeauX** » Tue Mar 16, 2021 5:36 pm

Bob Yoesle wrote: ↑Tue Mar 16, 2021 5:29 pm Hi Marty,

In a collimator type system, with a really long focal-length (F16~F32+) and relatively small aperture (say 60mm~80mm tops), one can generate a really long focal-ratio. So I'm curious if this can be further used to benefit the FWHM of a narrowband filter, and if so, by about how much (the calcs). Then, would it be useful to then introduce telecentriscm to this train and its effects. Then, could a 2nd or 3rd filter be stacked, that are not ultra rare and hard to get, to get a very selective final transmission profile centered on CaK line.
I'm not sure I'm fully understanding what you mean by using a collimator system to create a long EFR and introduce a telecentric aspect after that. You would typically just employ a telecentric system to create the long EFR and and use one or more filters in the telecentric output of the system. I guess you could use a collimator for one filter, and make the refocusing lens a telecentric element for an additional filter or filters. But you could just as easily avoid the Jacquinot spot effects of the collimator filter - and just employ a long EFR telecentric system alone with multiple filters. I believe Christian does this when he uses his DayStar and Coronado SMn quartz etalon double stacked H alpha filter system.

Sorry, just worded poorly, I'm just curious if these can be effectively utilized or even combined with a narrow filter to effect its FWHM and how a stack could interact when both are treated this way. For example, could a single filter be utilized with combinations of the above; versus two filters stacked (but at what FWHM and in what setup would be ideal). A lot of optics just seem really poorly corrected for 393nm, so I've tried to avoid amplifiers, barlows, etc, and resorted to simply using truly long focal-lengths from long physical scopes and reducing aperture to generate a small light cone instead that way. So I'm curious how this drives the bandpass of the filter (my Lunt CaK for example) and what I could do to improve it (if I can), versus adding another filter (and if so, what FWHM would be needed, versus ideal).

Very best,

Post by **marktownley** » Tue Mar 16, 2021 8:06 pm

I really liked the results I got with my DS PST module used after the Airylab telecentric in my f10 scopes last year, i'm waiting for conditions to improve this year so I can explore this a little more...

Bob Yoesle · Post by **Bob Yoesle** » Tue Mar 16, 2021 9:29 pm

Well, I'm thinking of the KISS principle.

First you start with an objective that works well at 394 nm. I have an AP130 EDT f/8 that should fit the bill. Next - like Mark - you have a good telecentric that is well corrected for CaK, and that's the Badder TZ3. Next you double or tripple stack your CaK bandpass filter(s). I seem to recall David Lunt stating that doing a double stack reduces the bandpass by 0.7 for a Lorentzian filter profile, and this seems correct for H alpha: 0.7 x 0.7 = 0.49 Angstrom. So extrapolating, which may or may not be accurate, would yield for the CaK PST filters 2.2 x 0.7 = 1.6 A for double stacking, and 1.6 x 0.7 = 1.1+ Angstrom triple stacked. For the sake of argument we'll just call it 1.2 A. This hopefully gets you into the spectrohelioscope range of contrast performance, provided one is in a properly configured collimated or telecentric system. At f/24 with a TZ3 telecentric, 1.2 +/- A should be doable. You can also add the Baader TC 0.4 "research grade" telecompressor specifically designed for use with the TZ3 to get back to a reasonable FL at the camera (~f/9.6). My previous attempts at triple staking had exposures of about 2 ms at f/9, so I'm thinking this actually might work.

I have yet to finalize my other filter components, and I have some new filter-tilters to play with and I hope to have this together this year sometime (fingers crossed - lots of other projects around the home going on).

Failing this, and getting more serious, the other choice is a Solar Spectrum CaK mica etalon filter with a narrower bandpass which can be had at about $4-5k, utilized with the same TZ3 and TC 0.4. As Christian noted, there's no free lunch - especially in narrow band filters.

The real-deal project would be the building of a spectrohelioscope as in Ken's excellent book.

MalVeauX · Post by **MalVeauX** » Tue Mar 16, 2021 10:25 pm

Thanks Bob,

Interesting; that's the kind of thing I'm wondering is what can be done without resorting to a $6k setup that may or may not even be superb.

I bought Ken's book, been wanting to build a digital SHG for a while now. I need to get back on this one, as I think this is the only reliable way to achieve filaments in CaK affordably with a little work.

Very best,

george9 · Post by **george9** » Wed Mar 17, 2021 1:49 am

Just to point out that an f/30 refractor is better than f/7.5, but neither is as good as an f/7.5 with a TZ4 telecentric element (see Christian's web site). So it is not just getting to the specified f/ratio, but how you get there.

On Bob's comment, I do use a collimator followed by a telecentric for H-alpha just because that was easiest to cobble together, not because it is ideal. The sweet spot drops from 32mm SS to about 28mm DS, which is good enough for me.

PST CaK's do show up occasionally. I got one on AM about two years ago. They pop up about that often. Mine was rusted in every element except the desired #1 yellow filter. I inserted it in my B1200 CaK. There was extra room on the eyepiece end, so a properly threaded ring holds it in place, with some shims for slight tilt.

I should really get a TZ3 for my AP155 (potentially stopped down). Same lens prescription as the AP130 f/8.3. I guess something like a TOA would be better, but that's a real expense. My trial-and-error attempts at Barlows have never worked in CaK.

George

MalVeauX · Post by **MalVeauX** » Wed Mar 17, 2021 12:03 pm

george9 wrote: ↑Wed Mar 17, 2021 1:49 am Just to point out that an f/30 refractor is better than f/7.5, but neither is as good as an f/7.5 with a TZ4 telecentric element (see Christian's web site). So it is not just getting to the specified f/ratio, but how you get there.

On Bob's comment, I do use a collimator followed by a telecentric for H-alpha just because that was easiest to cobble together, not because it is ideal. The sweet spot drops from 32mm SS to about 28mm DS, which is good enough for me.

PST CaK's do show up occasionally. I got one on AM about two years ago. They pop up about that often. Mine was rusted in every element except the desired #1 yellow filter. I inserted it in my B1200 CaK. There was extra room on the eyepiece end, so a properly threaded ring holds it in place, with some shims for slight tilt.

I should really get a TZ3 for my AP155 (potentially stopped down). Same lens prescription as the AP130 f/8.3. I guess something like a TOA would be better, but that's a real expense. My trial-and-error attempts at Barlows have never worked in CaK.

George

Thanks George, interesting!

So maybe a TZ3/TZ4 telecentric and then reduce after the filter (thinking full disc options).

Very best,

Valery · Post by **Valery** » Wed Mar 17, 2021 4:19 pm

christian viladrich wrote: ↑Thu Mar 11, 2021 9:33 am Excellent. Thanks for sharing !

In addition to the image of yours and Bob's images, here is another comparison.

First one with a 0.37 A FWHM Ca K filter :

http://astrosurf.com/viladrich/astro/so ... ASI290.jpg

Then a 0.24 nm FWHM :
http://astrosurf.com/viladrich/astro/so ... ASI290.jpg
The layer of the chromosphere is a bit higher, so the small pore is less visible.The frontier of the bright plages become a bit fuzzier (because magnetic tubes are expanding with height).

Double stack 0.37 A + 0.24 A filters :
http://astrosurf.com/viladrich/astro/so ... ASI290.jpg
Here we get higher in the chromosphere. The pore is virtually hiden by the canopy. Vertical fibrils become visible in the bright plages.

Hello Christian,

I believe you did mean 0.37nm and 0.24nm not Angstroms.

Excellent images.

Valery

christian viladrich · Post by **christian viladrich** » Wed Mar 17, 2021 5:46 pm

christian viladrich wrote: ↑Thu Mar 11, 2021 9:33 am Excellent. Thanks for sharing !

In addition to the image of yours and Bob's images, here is another comparison.

First one with a 0.37 nm FWHM Ca K filter :// correction of units made on March, 13

http://astrosurf.com/viladrich/astro/so ... ASI290.jpg

Then a 0.24 nm FWHM :
http://astrosurf.com/viladrich/astro/so ... ASI290.jpg
The layer of the chromosphere is a bit higher, so the small pore is less visible.The frontier of the bright plages become a bit fuzzier (because magnetic tubes are expanding with height).

Double stack 0.37 nm + 0.24 nm filters :// correction of units made on March, 13 /u]
http://astrosurf.com/viladrich/astro/so ... ASI290.jpg
Here we get higher in the chromosphere. The pore is virtually hiden by the canopy. Vertical fibrils become visible in the bright plages.

christian viladrich · Post by **christian viladrich** » Wed Mar 17, 2021 5:50 pm

Hello Valery,
Yes, indeed. The units were in nm and not A. Unfortunately, I am not able to correct my initial post.
Thanks !

krakatoa1883 · Post by **krakatoa1883** » Wed Mar 17, 2021 5:56 pm

What an interesting discussion, many thanks to all of you!

An image of today with the same setup of those in the opening message (open image in another window and enlarge to see at full size). I have still to solve the problem posed by the long BF barrel of the Cak assembly that tends to tilt in the Taka focuser, I think a Baader clicklock eyepiece holder should fix this issue.

Post by **Merlin66** » Thu Mar 18, 2021 7:09 am

I’ve measured my DIY CaK filter assembly to be close to 1A.
One of the benefits of having a high resolution spectroscope.

MalVeauX · Post by **MalVeauX** » Thu Mar 18, 2021 1:38 pm

Merlin66 wrote: ↑Thu Mar 18, 2021 7:09 am I’ve measured my DIY CaK filter assembly to be close to 1A.
One of the benefits of having a high resolution spectroscope.

Hi Ken,

Does it show filaments on the surface? Or does it need to be even more narrow?

Very best,

george9 · Post by **george9** » Thu Mar 18, 2021 7:03 pm

I notice a Coronado 70mm CaK just sold on AM. It was "missing part of the rear blocking filter." I wonder if that was that the narrowband filter, in which case it was less useful.

George

Bob Yoesle · Post by **Bob Yoesle** » Thu Mar 18, 2021 8:43 pm

Hi George - more like useless...

I'd also like to know the answer to Marty's question to Ken, and what Ken is using for his DIY system...

If you look at what Peter Zetner did with his SHS to simulate what different CaK FWHM's would look like, and concluded a FWHM of around 0.5 Å or less would be optimum to see filaments well defined.

Below I have revised Peter's diagram for the CaK FWHM milli-angstrom simulation figures :

: CaK FWHM study montage P Zetner SM.jpg (437.54 KiB) Viewed 830 times

However, were simulated single filter Gaussian transmission profile images. Multiple Lorentzian stacking images might approach a Gaussian profile. Hints of filaments start to reveal themselves at 1.1 Å FWHM - which is in triple stack range in a properly configured and corrected telecentric (Baader TZ3):

Bob 71741 on Cloudy Nights supplied me with a spreadsheet indicating the FWHM for stacked Lorentzian single cavity profiles, which if applicable derive the following values for stacking the nominal 2.2 Å Coronado CaK PST filters:

Single: 2.20
Double: 1.42
Triple: 1.10
Quadruple: 0.92
Quintuple: 0.80

Given the spreadsheet allows for stacking two filters with differing FWHM's, the actual performance of multiple filters might be better due to the previously calculated stacked FWHM's having better "wing" or "tail" suppression than a single filter would have - again assuming the filters have Lorentzian profiles.

I do have enough filters and tilters to do a quintuple stack (1 Chroma and 4 PST). The issue will be available light transmission levels.

Therefore I have procured an Edmund 390/40 OD 6 florescence filter with > 90% transmission for use as a sub-diameter ERF, and currently putting together long and short pass OD 4 filters which will have > 90% transmission @ 375 & 400 nm respectively. I'm hoping that combined with enough aperture (130 mm), there will be sufficient light at f24 to do reasonable imaging for at least quad if not quint stacking.

Time will tell.

Post by **Merlin66** » Thu Mar 18, 2021 9:17 pm

Marty,
When the seeing cooperates I can record filaments in CaK as well as the occasional prom.

MalVeauX · Post by **MalVeauX** » Thu Mar 18, 2021 10:47 pm

Merlin66 wrote: ↑Thu Mar 18, 2021 9:17 pm Marty,
When the seeing cooperates I can record filaments in CaK as well as the occasional prom.

Interesting, so your surface filaments on the disc are easy to notice and not just subtle?
Would love to see an example of that (unprocessed vs processed would be ideal to get an idea of things).
Mind sharing your imaging train & filter config to achieve 1A FWHM?

Very best,

Post by **Merlin66** » Thu Mar 18, 2021 11:01 pm

Marty,
I'll try to get some worthwhile images representative of the performance.
The set-up is:
SW ED80pro 80/600 (Stopped to 60mm) >2" Baader Blue CCD filter, Omega 390nm cut, Omega 40nm 394nm, Omega 2A 393.7nm (all 25mm) and a #1 "yellow" CaK PST filter.
There's about a 150mm spacing from the Baader blue to the other filters.
I use an ASI 1600MM for imaging/ Firecapture/AS3!/ Imppg/Irfanview
EDIT: I should add that tilting any filter elements moves the CWL towards the blue. My CWL is currently JUST below the optimum CWL so additional tilt would just move it further into the blue wing.....

george9 · Post by **george9** » Fri Mar 19, 2021 12:57 am

Merlin66, why not skip the Baader Blue CCD and gain transmission? I think mine is about 30%. If you tilt it enough, it comes on band, but it may be so much that it distorts the view.

George

Bob Yoesle · Post by **Bob Yoesle** » Fri Mar 19, 2021 1:17 am

When the seeing cooperates I can record filaments in CaK as well as the occasional prom.

Hi Ken,

According to Bob 71741 calculations for a two-filter Lorentzian, you have the following parameters for your double stacked CaK system:

: Image1.jpg (11.05 KiB) Viewed 785 times