Replacing Lunt CaK BF with something better

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Replacing Lunt CaK BF with something better

Post by krakatoa1883 » Sun Sep 13, 2020 7:25 pm

In this discussion I described the problem I had with the blocking filter assembly of my Lunt module, mainly coma and astigmatism due to the tilted filters. Placing a focal extender before the BF helped in obtaining a better definition however this cannot be a solution if one wants to image the full disc. Moreover I never liked the CaK module design with that absurdly long and heavy 2" inch tube that easily tilts in the focuser adding further problems, and the etalon barrel difficult to precisely clamp in the eyepiece holder due to its shape.

Eventually I had the idea of using the blocking filter of my deceased Quark calcium, it is composed by a clear glass and a blue bandpass filter glued together
bf.jpg
bf.jpg (15.93 KiB) Viewed 299 times
The glue degraded months ago after a couple of years of use of the device so I separated and cleaned the glasses and put them in a Edmund Optics T-mount for 25 mm lenses, this is now my BF
bf q1.jpg
bf q1.jpg (60.43 KiB) Viewed 299 times
bf q2.jpg
bf q2.jpg (65.44 KiB) Viewed 299 times
The complete assembly looks as in the image below
bf q3.jpg
bf q3.jpg (40.2 KiB) Viewed 269 times
it can be separated for inserting a focal extender before the etalon for high resolution imaging. Today the seeing was horrible but the first light through the device is promising and definitely solved the problem of having half of the sun in focus and half out of focus due to the tilting of the original BF
2020-09-13-1330-RB-CaK-fulldisc.jpg
2020-09-13-1330-RB-CaK-fulldisc.jpg (206.02 KiB) Viewed 299 times
Replacing the BF with a Baader K-line filter gives similar results as I already showed in the above mentioned discussion but with a slightly less definition.
Last edited by krakatoa1883 on Mon Sep 14, 2020 10:27 am, edited 1 time in total.


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Re: Replacing Lunt CaK BF with something better

Post by marktownley » Mon Sep 14, 2020 5:57 am

That looks promising Raf, good solution!


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Re: Replacing Lunt CaK BF with something better

Post by Montana » Mon Sep 14, 2020 7:29 am

The image looks great but I am confused, are you using a Lunt Calcium K or are you using the Quark Calcium H filters? one is at 393nm and the other is at 396nm? The Baader K line has a bandpass of 8nm so should work with both Calcium K or H as there is only 3nm between them.

Alexandra



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Re: Replacing Lunt CaK BF with something better

Post by krakatoa1883 » Mon Sep 14, 2020 8:21 am

Hi Alexandra, I am using of the Lunt etalon and Quark BF, they match fine. My calcium Quark no longer goes in tune so I am using the pieces for assemblying something else. Next step will be to remove the Lunt etalon from the original barrel for mounting it in a shorter one.


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Re: Replacing Lunt CaK BF with something better

Post by Merlin66 » Mon Sep 14, 2020 9:01 am

RAF,
As far as I can tell, the Lunt CaK filter doesn’t involve an etalon solution, just multi-coated filter elements.
The Quark obviously (I think may have a micra solid etalon) is different.
What f ratio scope are you using??


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Re: Replacing Lunt CaK BF with something better

Post by krakatoa1883 » Mon Sep 14, 2020 10:20 am

Yes, just my habit to call "etalon" any device of that kind... :lol:

The scope I used for the full disc is f/6.9.

PS, thanks for offering advices on the SHG I would like to build, I am still in the very preliminary stage (i.e. book reading...).


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Re: Replacing Lunt CaK BF with something better

Post by Merlin66 » Mon Sep 14, 2020 10:49 am

Raf,
Like Mark and others, I find the CaK filter gives better (repeatable) results when used at f10 (or greater).

I respect anyone who considers building a SHG!
I'm finding that there's always a "twist" in the final design! Finding suitable sized gratings for instance.....


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Re: Replacing Lunt CaK BF with something better

Post by krakatoa1883 » Mon Sep 14, 2020 11:16 am

Merlin66 wrote:
Mon Sep 14, 2020 10:49 am
I find the CaK filter gives better (repeatable) results when used at f10 (or greater)
Totally agree, it's also my experience, however for imaging the full disc I would prefer to avoid making mosaics so I am using the scope at the native f/ratio to contain the disc within the sensor. I could increase the ratio with a weak barlow but then I should place a focal reducer just before the camera.


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Re: Replacing Lunt CaK BF with something better

Post by marktownley » Mon Sep 14, 2020 4:52 pm

krakatoa1883 wrote:
Mon Sep 14, 2020 11:16 am
Merlin66 wrote:
Mon Sep 14, 2020 10:49 am
I find the CaK filter gives better (repeatable) results when used at f10 (or greater)
Totally agree, it's also my experience, however for imaging the full disc I would prefer to avoid making mosaics so I am using the scope at the native f/ratio to contain the disc within the sensor. I could increase the ratio with a weak barlow but then I should place a focal reducer just before the camera.
Stop it down to 80mm to give you f10.


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Re: Replacing Lunt CaK BF with something better

Post by krakatoa1883 » Mon Sep 14, 2020 5:26 pm

True, however my scope is well corrected in CaK even at full aperture so I would prefer to maintain resolution if I can. However I'll do some tests to see what gives better results, full aperture or higher f/ratio.


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Re: Replacing Lunt CaK BF with something better

Post by Merlin66 » Mon Sep 14, 2020 10:30 pm

Raf,
A compromise would be to stop the scope to at least f10 and then use a reducer just before the camera to give you full disk coverage.
(With my ASI 1600MM on the ED80 I use either a x1.6 barlow or the very good x2.25 Baader Hyperion barlow to give me a slightly larger image on the sensor)


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Re: Replacing Lunt CaK BF with something better

Post by Bob Yoesle » Tue Sep 15, 2020 4:24 pm

Like Mark and others, I find the CaK filter gives better (repeatable) results when used at f10 (or greater)... True, however my scope is well corrected in CaK even at full aperture so I would prefer to maintain resolution if I can. However I'll do some tests to see what gives better results, full aperture or higher f/ratio.
Hi Raf,

I'd be very interested in seeing your results. How do you know your scope is well corrected in CaK? At f6.9 your full disc is good, but a little "mushy." There are two issues that might come into play - contrast and resolution.

Does the increased f ratio improve the contrast performance of the filter - e.g. does the filter have a narrower FWHM and reduced continuum leakage at the longer f ratio? Perhaps. You could try a CaK double stack to see if this significantly improved the f6.9 full-disc results:

Single v Double stacked CaK.jpg
Single v Double stacked CaK.jpg (290.41 KiB) Viewed 215 times
Single versus double stacked PST CaK filters at f9.


Does the increased f ratio improve the optical performance via reduced spherochromatic aberration at 394 nm? A well corrected objective (i.e. TOA 130 or 150 see Christian Viladrich's CaK images viewtopic.php?t=25486) seems to not need a longer f ratio for good performance. However, for typical objectives reduced SCA might be why most of the improvement occurs.

I have observed the Synta ED100/900 f9 objectives also perform quite well for CaK:


CaK full disc mosaic.jpg
CaK full disc mosaic.jpg (412.61 KiB) Viewed 215 times
Six-pane DS CaK mosaic.


Continuum 540 nm, CaK 394 nm, Ha 656 nm at f 18 via 2 X Barlow:

Compare Cont CaK H alpha.jpg
Compare Cont CaK H alpha.jpg (405.48 KiB) Viewed 215 times


The hi-res with the Barlow shows a bit more detail at 394 nm than 540 nm, which one would theoretically expect. I therefore might conclude the effects of SCA at 394 nm are minimal with this particular objective. I'd have to balance reduced aperture effects on resolution and image brightness to see if it would be worth it - going from f9 to f10 might be pointless.

However, it very likely will be quite different going from f6.9 to 10...

Don't let the little extra work of mosaic's deter you if it gives you better overall full disc images. The mosaic route gives you effectively a larger sensor with more pixels-per-disc than a focal reducer will with the same sensor. Solar Cycle 25 is predicted to be almost identical to Cycle 24, so the activity shown above is what you can hope to see and image !-) https://www.nasa.gov/press-release/sola ... hat-means/


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Re: Replacing Lunt CaK BF with something better

Post by Merlin66 » Tue Sep 15, 2020 11:34 pm

Bob,
You mention resolution..
What is your typical plate scale, arcsec/pixel?
I find down here at the coast that working around 0.79 arcsec/pixel is about my limit. Using ED80 (80/600) for Ha and WL and stopped to f10 for CaK.


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Re: Replacing Lunt CaK BF with something better

Post by Bob Yoesle » Wed Sep 16, 2020 7:26 am

Hi Ken,

That's similar to what I have. Most all of my imaging to date has been done with a PGR (FLIR) Chameleon with ICX445 CCD @ 3.75 um pixels.

With the ED100/900 this gives me 0.86 arcsec/pixel. When seeing allows, the TeleVue 2x Big Barlow yields ~ 0.43 arcsec/pixel.


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Re: Replacing Lunt CaK BF with something better

Post by krakatoa1883 » Wed Sep 16, 2020 7:09 pm

Thanks Bob for your useful advice. Since I removed the original Lunt BF seeing was always poor here, this may be the reason why the disc appears soft, however I agree that further tests at various reduced apertures are needed to evaluate the effect of SCA, if relevant. Whatever the aberrations of the objective I think I need something better than my Lunt module to improve results, at least another one for stacking as you suggest.

You're totally right about mosaics, it's only my laziness that prevents me trying this technique.

Ah, I too have a Celestron 100ED, it would be interesting to compare this scope with the 115, I'll try.


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Re: Replacing Lunt CaK BF with something better

Post by krakatoa1883 » Thu Sep 17, 2020 6:18 pm

Seeing improved a little today, however not enough for hires testing. Anyway I made further experiments with the 115mm apo f/6.9
at various apertures and the modified CaK module. Here below my results by using of the ASI 178MM and a slight reducer (0.7x). All images with same seeing, frames, fps, and so on. I don't like oversharpening so I applied only a moderate high frequency wavelet by using of Astrosurface and a slight UM with GIMP.
1 fullaperture.jpg
1 fullaperture.jpg (112.89 KiB) Viewed 152 times
2 100mm.jpg
2 100mm.jpg (113.85 KiB) Viewed 152 times
3 90mm.jpg
3 90mm.jpg (113.87 KiB) Viewed 152 times
4 80mm.jpg
4 80mm.jpg (104.94 KiB) Viewed 152 times
It's hard to find any difference, however looking carefully at the first and fourth images this last one (slightly enlarged) shows a modest contrast improvement. Differences start to better emerge by imaging more closely although still far from the ideal sampling, as usual due to the turbulent air (just look at the contrast, not detail):
5 fullaperture.jpg
5 fullaperture.jpg (51.97 KiB) Viewed 152 times
8 80mm.jpg
8 80mm.jpg (63.5 KiB) Viewed 152 times
the latter is more sharp and contrasty (same capture parameters, stacked frames and post processing for both) but not dramatically.

It seems to me that the improvement from 115 to 80 mm is only marginal and probably due to better SCA correction at the reduced aperture. To determine if the filter is also working better at f/10 than at f/6.9 would probably require a narrower filter than mine.


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Re: Replacing Lunt CaK BF with something better

Post by marktownley » Sun Sep 20, 2020 7:48 am

Interesting thread Raf!

If I can throw something into the mix here too, sampling. Your camera has 2.4um pixels and so at 393nm (my rule of thumb, others might go for a higher multiplier) is that the pixel size x 5 should put you in the f-ratio ball park you need to be for ideal sampling. So, for the full disks, essentially they are all heavily undersampled, the smallest detail that the aperture can theoretically resolve is never revealed due to less than ideal sampling. Add in you are using a focal reducer and in effect this enhances the undersampling effect. This is why all the full disks above show the same level of detail.

I think that if you did 70mm and a 60mm aperture stop that you would get the same level of resolution. Throw in the 0.7x reducer you're using and you might find that you get down to maybe 50mm or even as low as 40mm before the full disks start to show reduced resolution. The point where resolution decreases as the aperture gets smaller is now where you are over sampling, so, for a given system finding this point defines where ideal sampling should be. You want to be at the smallest aperture where going above this no more detail is visible at a given image scale.

What is visible very well on the full disks is how as aperture gets smaller the contrast of the disk increases. This is just a simple function that the smaller the f-ratio the wider the fwhm of the filter. If you were imaging at 50/60mm aperture, which given the size of the FD above, this would give you f15-13 at the filter, I think you would start noticing a further increase in contrast on the 80mm disk above. I do my CaK fD by stopping my 100mm f10 Tal refractor down to 60mm with 3.69um pixels.

Worth a try if you have the time / sun / inclination for a bit of experimenting!

BTW what barlow were you using for the closeups? It's quite apparent there is a shift of focus radially outward with the scope at full aperture compared to the 80mm shot.

Mark


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Re: Replacing Lunt CaK BF with something better

Post by krakatoa1883 » Sun Sep 20, 2020 12:11 pm

Hi Mark, thanks for your comments. You're right in everything you wrote, all images are undersampled and can't show differences in resolution. I wanted to see if making the scope slower makes also the filter more effective. It appears this is the case however from 115 mm to 80 mm I see only a modest increase in sharpness, this means, I think, that the gain in definition due to containing spherochromatism by stopping down the aperture is present but not fundamental. Scopes that are poorly corrected in terms of SCA can be immediately recognized even if one images only the full disc.

For imaging at the theoretical sampling I would need a better seeing compared to the one I had this summer which was anomalous from this point of view. Last year I was able to obtain much better images like this one
2019-09-21-0810-RB-CaK-gran b.jpg
2019-09-21-0810-RB-CaK-gran b.jpg (75.44 KiB) Viewed 75 times
so I am waiting for something similar to boost resolution.

Anyway images taken with fiters of different FWHM can't be directly compared in terms of sharpness or resolution. For example my images in CaK through a SS Lunt module are light years from those posted by Bob and would be so even if I had used the best corrected scope in the world.
marktownley wrote:
Sun Sep 20, 2020 7:48 am
BTW what barlow were you using for the closeups? It's quite apparent there is a shift of focus radially outward with the scope at full aperture compared to the 80mm shot.
Last two images in my previous message was not amplified, just at the prime focus (800 mm). I noted that streaks too, may be something tilted in the train, don't know.


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