Experimenting calcium imaging with small refractors

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Experimenting calcium imaging with small refractors

Post by krakatoa1883 » Tue Sep 18, 2018 9:18 am

Last Sunday and Monday I spent some time in trying to understand which one of my small 80mm refractors performs better in calcium light. Made it just for fun :D but hope it may be useful for other imagers too.

Worst of the group was a TS 80ED, a doublet with an element in FPL-51 glass
ts80.jpg
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At full aperture it almost refused to form a sharp image on the screen while capturing a video of a small portion of chromosphere through my Quark
TS80ED full.jpg
TS80ED full.jpg (30.69 KiB) Viewed 1131 times
it actually needs to be stopped down to 50 mm in order to get a decent image as this reduces sperochromatism in the near UV and improves seeing too but at the expense of resolution
TS80ED reduced.jpg
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My Skywatcher 80 ED (FPL-53 glass) proved to be better but not dramatically, however it allows to take at least good full disc images without the need to reduce the aperture
se80.jpg
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sun20180509_FD_CaH.jpg
sun20180509_FD_CaH.jpg (358.15 KiB) Viewed 1131 times
while the best performer was my TMB 80mm f/11.2, the noted "Planet Hunter" designed by the late Thomas Back with Ohara glasses. It is classified as an "enhanced" achromat but it is actually much better corrected than most cheap ED doublets
tmb.jpg
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Capturing and processing the same region of the above screenshots gave this result which seems fine to my eyes for an 80 mm scope
2018-09-17-1023-RB-CaH-AR.jpg
2018-09-17-1023-RB-CaH-AR.jpg (97.05 KiB) Viewed 1131 times
The Planet Hunter is no longer produced but it can still be found in the used equipment market.

While experimenting with my small tubes I checked that the Quark calcium really needs a high focal ratio to perform best, f/9 or f/10 is a good starting point, the higher the better.

The Baader K line filter proved to be useless for calcium imaging, at least with my simple setup and the large BP of the Quark. I noted a slight increase in sharpness if used single stacked than double stacked as provided by the manufacturer.
Last edited by krakatoa1883 on Tue Sep 18, 2018 12:49 pm, edited 2 times in total.
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Re: Experimenting calcium imaging with small refractors

Post by Montana » Tue Sep 18, 2018 11:14 am

That is really interesting. I get exactly the same as your first scope with the TEC140, it seems pretty hopeless at 140mm, marginally better at 120mm and OKish at 100mm but nothing like what you get with your last scope.

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Re: Experimenting calcium imaging with small refractors

Post by krakatoa1883 » Tue Sep 18, 2018 1:45 pm

Calcium imaging is indeed a challenging task for every refractor, however stopping down the aperture seems a good cure for both apos and achros.

Navigating into Christian's web site I understand that he uses his TOA 150 at full aperture, if so it should be one of the best corrected instrument existing for calcium imaging....
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Re: Experimenting calcium imaging with small refractors

Post by marktownley » Tue Sep 18, 2018 2:27 pm

Interesting comparison Raf. I did something similar several years ago with CaK.

I get some reasonable images with my ED80 at higher magnifications I find, once I'm upto about f15-f25.

Frederics website has some great measurements of spherochromatism https://airylab.com/astronomy-test-reports/

Regards the TEC, when you compare it in the blue to other scopes it is indeed quite a poor performer, nearly 0.5mm difference in focal position from the centre of the fov compared to the edge. Stopping it down to about 70mm should give some stunning results, with the slower light cone also getting some more contrast from the Lunt module.

The TOA150 is an excellent performer, and while the 'curve' seems extreme the scale is smaller by a factor of 10 compared to most other reports on there. I think if I was on a cost no object mission then I would go for the TOA130 as this offers even better spherochromatism than the 150, and certainly on all of Freds graphs the best spherochromatism of any scope on there. It has pretty much zero spherochromatism at all wavelengths.
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Re: Experimenting calcium imaging with small refractors

Post by krakatoa1883 » Tue Sep 18, 2018 7:34 pm

Thanks Mark, the link is very informative. Looking at the TOA a Strehl ratio close to 0.9 in the blue means that the scope can still be diffraction limited in the violet and near UV, really a terrific performer.

I forgot to mention that in my tests I used a barlow or powermate in front of the Quark trying to maintain the working f/ratio about the same and not less than f/20.
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Re: Experimenting calcium imaging with small refractors

Post by christian viladrich » Tue Sep 18, 2018 7:49 pm

Hello Raf,
Your image with the TMB80 F/11.7 is quite good and is representative of the resolution that can actuallly be reached with a 80 mm in Ca K.
On my side, I had found that the (old) Taka FSQ 106 was good when stopped down to 80 mm.
The new Taka FSQ seems to have an improved blue correction.

The only drawback of the TOA 150 is its the price and ... its aperture. I wish I have a TOA 200 ;-)
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Re: Experimenting calcium imaging with small refractors

Post by marktownley » Wed Sep 19, 2018 6:40 am

christian viladrich wrote:
Tue Sep 18, 2018 7:49 pm
The only drawback of the TOA 150 is its the price and ... its aperture. I wish I have a TOA 200 ;-)
:D :D :D
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Re: Experimenting calcium imaging with small refractors

Post by pedro » Wed Sep 19, 2018 3:03 pm

I got interesting results with the TEC140 (stopped down to 110mm) and a LUNT152 CaK module

I found out that with my CaK modules work better with a focal ratio of F/10 (LUNT CaK modules)

Check out some images below

Image
IMAGING SETUPS (TEC140 F/7, LUNT152 CaK Module)

Image
IMAGING SETUPS

Image
SUN (20180826) CaK. TEC140 F/7, LUNT152 CaK module, X1.6 2" Antares Barlow, PGR GRASSHOPPER 3 GS3-U3-28S4M

Image
SUN (20180825) CaK. TEC140 F/7, LUNT152 CaK module, X1.6 2" Antares Barlow, PGR GRASSHOPPER 3 GS3-U3-28S4M

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Re: Experimenting calcium imaging with small refractors

Post by Bob Yoesle » Wed Sep 19, 2018 5:47 pm

Thank for posting your results Raf ;-)

Great images Pedro!
The only drawback of the TOA 150 is its the price and ... its aperture. I wish I have a TOA 200 ;-)
Hi Christian, my thought is that this seems to be pointing perhaps to using a reflector telescope such that you already have with an appropriately configured and UV corrected telecentric for CaK imaging ;-)
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Re: Experimenting calcium imaging with small refractors

Post by krakatoa1883 » Wed Sep 19, 2018 7:56 pm

Fine images, Pedro.
christian viladrich wrote:
Tue Sep 18, 2018 7:49 pm
Your image with the TMB80 F/11.7 is quite good and is representative of the resolution that can actuallly be reached with a 80 mm in Ca K.
On my side, I had found that the (old) Taka FSQ 106 was good when stopped down to 80 mm.
thanks Christian, I had an excellent sample of Sky 90-II with its Q-extender that performed very well in blue and violet, I sold it as at the time was not so much interested in solar imaging.
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Re: Experimenting calcium imaging with small refractors

Post by MalVeauX » Thu Sep 20, 2018 1:10 am

Heya,

I've been using an ancient achromatic doublet, the old Celestron CR 150 HD (150mm F8) refractor for high resolution stuff (found this thing for $250). I've used it at full aperture and also at F10 (stopped down to about 120~127mm aperture). I generally use it at F10 because it's more appropriate with the various filters in CaK that I was using. I also use a humble old Orion ST80 (400mm F5) refractor ($100 all day), stopped down to F10, for full disc CaK.

I've used my other scopes too, including an Orion ED80 (FPL53), Celestron Omni XLT 120 F8, etc. The results were all comparable I found. The defining difference I found was pretty much the seeing and being close to F10~F12.

I would expect the Celestron Omni XLT 102 F10 (natively F10) is probably an excellent inexpensive frac for everything frankly in narrowband.

++++++++++++++++++++++++++++
++++++++++++++++++++++++++++

Celestron CR150HD 150mm F8 Refractor (operating at F8 & F10) with PST CaK filter & Chroma 393.4 1nm filter.

Image

Image

Image

Image

Image

Image

Orion ST80 (stopped down to F10) with PST CaK filter & Chroma 393.4 1nm filter.

Image

Image

++++++++++++++++++++++++++
++++++++++++++++++++++++++

Here's the two scopes mounted in a side by side setup for rapid scale changing.
fulldisc_skybender_cak.jpg
fulldisc_skybender_cak.jpg (66.16 KiB) Viewed 1001 times
highres_CaK_setup.jpg
highres_CaK_setup.jpg (128.96 KiB) Viewed 1001 times
Very best,

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Re: Experimenting calcium imaging with small refractors

Post by marktownley » Thu Sep 20, 2018 7:02 pm

I would hazard a guess that the 115mm/f7 triplet is a reasonable performer in CaK (or Ha or Na) based on this spherochromatism chart
115trip2.jpg
115trip2.jpg (24.21 KiB) Viewed 954 times
http://shop.tecnosky.it/Articolo.asp?Se ... 15&Score=1

The curve is about as close to the TOAs as i've seen in a lot of other scopes.
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Re: Experimenting calcium imaging with small refractors

Post by krakatoa1883 » Thu Sep 20, 2018 8:04 pm

Peter made some excellent images using a 115mm triplet, may be with optics similar to Tecnosky apo but I am not sure. I had this triplet in its first version with a different focuser but same objective and I remember it was really good. At the time I was not interested in calcium imaging but I am now seriously considering to purchase one again.

My last experiment in calcium H (I have finished the small refractors for now :lol: ), 100mm f/7.5 achromat brought to f/28.5 through the excellent Baader VIP Barlow, the same I am using to image Venus @350 nm with good results:
2018-09-20-0835-RB-CaH-AR.jpg
2018-09-20-0835-RB-CaH-AR.jpg (86.46 KiB) Viewed 947 times
It is an extreme setup because the starting f/ratio is really low for a 100mm achromat, however I noted that Skywatcher doublets tend to be fairly well corrected in the blue while in the red are not so. Moreover reducing the aperture cuts the top end of the color curve where it bends most.
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Re: Experimenting calcium imaging with small refractors

Post by marktownley » Fri Sep 21, 2018 6:39 am

General rule is with achromats longitudinal aberrations are worse that latitudinal aberrations, and with apochromats latitudinal aberrations are worse than longitudinal aberrations. There will be some exceptions to the rule, the 115/7 being one and the TOA ortho apochromats for example. I found this diagram on telescope optics that I though gave a good comparison of different configurations. https://www.telescope-optics.net/semiap ... amples.htm
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Re: Experimenting calcium imaging with small refractors

Post by marktownley » Fri Sep 21, 2018 6:43 am

TheSkyBurner wrote:
Fri Sep 21, 2018 2:23 am
You can pop the achromat lens out if its cell and increase the air space gap with cardboard strips to Decrease blue astigmatism , A bit of trial and error plus tedious guesswork involved but it works.

Use a cheap scope!
I did this with my Meade AR5 and improved perfomance in CaK, it's far from perfect but it offered an improvement on the original shims between the 2 objective elements.

This is why I love solar astronomy - it's all based around math(s) and physics - numbers and engineering then becomes our friend in achieving what we want,
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Re: Experimenting calcium imaging with small refractors

Post by krakatoa1883 » Fri Sep 21, 2018 10:27 am

Very informative table. Looking at the spot diagrams for the g-line (light blue) it seems that only few of the objectives are fairly corrected for this wavelength (calcium H and K lines should give even worse results) but of course some are better than others. Note that semi-apo designs are not much different from achromats in this respect.

I already changed the original cardboard spacers of my 150mm with thicker ones made from an aluminum foil but I didn't make any check of the resulting correction in calcium light compared to the original configuration. Mark, do you remember which thickness have you used ?
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Re: Experimenting calcium imaging with small refractors

Post by marktownley » Fri Sep 21, 2018 10:34 am

krakatoa1883 wrote:
Fri Sep 21, 2018 10:27 am
I already changed the original cardboard spacers of my 150mm with thicker ones made from an aluminum foil but I didn't make any check of the resulting correction in calcium light compared to the original configuration. Mark, do you remember which thickness have you used ?
I used foil that is used for catering cartons - like you get in a take away, no idea how thick they are, they were just thicker than the original ones on the object lens.
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Re: Experimenting calcium imaging with small refractors

Post by krakatoa1883 » Thu Oct 18, 2018 8:09 pm

An exceptionally detailed image posted somewhere in this forum (I have been unable to locate it again, may be from Bob Yoesle) made me curious to image the Sun in Ca-H light with a 100ED f/9 refractor. In the used market I could find one that accidentally was already mine five years ago :lol: and now has returned home. It belongs to the Omni XLT series with optics selected by Synta for the Celestron brand. I don't remember why I sold it but I am very happy to have it back because it proved to be a very good scope at these short wavelengths.
celestron-omni100ed-xlt1.jpg
celestron-omni100ed-xlt1.jpg (22.9 KiB) Viewed 643 times
Today was sunny but hazy however I tried some preliminary imaging with the above scope. It is really very sharp and don't need to be stopped down because it is evidently well corrected for spherical aberration in the near UV. With just a minimum of UM I got this image
cah2 100ed.jpg
cah2 100ed.jpg (158.84 KiB) Viewed 643 times
With a 2.5x Powermate in front of the Quark I obtained the following image after a very slight processing with IRIS, no image overstretch no any abuse of the unsharp masking
cah3 100ed.jpg
cah3 100ed.jpg (117.47 KiB) Viewed 643 times
contrast can be further enhanced of course but I preferred to leave the image quite soft.

Hope to have some day with a more transparent sky to make further tests.
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Re: Experimenting calcium imaging with small refractors

Post by marktownley » Thu Oct 18, 2018 9:21 pm

Yup. that looks to be a winner Raf.
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Re: Experimenting calcium imaging with small refractors

Post by MalVeauX » Thu Oct 18, 2018 10:27 pm

Looks like it handles it great!

Very best,

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Re: Experimenting calcium imaging with small refractors

Post by eroel » Fri Oct 19, 2018 3:18 pm

Hi:
For CaK I use at home a 102mm f/9 Meade doublet (achromat), looks like it is fairly corrected for that wavelenghth, though I have tried it with a Safix that got many years ago and it really performs better, even tried it on the Moon and the Safix works incredibly well.
So I think that for HR imaging in CaK, a Safix is the way to go with an achromat to get the best of scopes that are not up to par for that wavelength as the apochromatic objectives.
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Re: Experimenting calcium imaging with small refractors

Post by Bob Yoesle » Sat Oct 20, 2018 1:13 am

"An exceptionally detailed image posted somewhere in this forum (I have been unable to locate it again, may be from Bob Yoesle) made me curious to image the Sun in Ca-H light with a 100ED f/9 refractor. In the used market I could find one that accidentally was already mine five years ago :lol: and now has returned home. It belongs to the Omni XLT series with optics selected by Synta for the Celestron brand. I don't remember why I sold it but I am very happy to have it back because it proved to be a very good scope at these short wavelengths."

The Orion (Synta) 100ED f/9 scope is indeed what I have used. Perhaps it was one of these images(?):

https://www.cloudynights.com/gallery/im ... 1733-comp/
https://www.cloudynights.com/gallery/im ... labled-sm/
https://www.cloudynights.com/gallery/im ... 37-jpg-sm/
https://www.cloudynights.com/gallery/im ... sm-labled/

I'll also be trying my AP 130 EDT f/8 with a Baader TZ3 in the near future to see if I can obtain a bit more CaK resolution when the seeing permits...
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Re: Experimenting calcium imaging with small refractors

Post by krakatoa1883 » Sat Oct 20, 2018 4:12 pm

Bob Yoesle wrote:
Sat Oct 20, 2018 1:13 am
The Orion (Synta) 100ED f/9 scope is indeed what I have used. Perhaps it was one of these images(?):
got it, it was here, anyway thanks for the additional links. Look forward to see your images with the AP 130.
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Re: Experimenting calcium imaging with small refractors

Post by krakatoa1883 » Mon Oct 29, 2018 12:15 pm

wow... really, those images show much contrast as if they were taken through a DS filter. The scope shall be certainly good but it seems that most of the result has to be attributed to the specific K module used.
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Re: Experimenting calcium imaging with small refractors

Post by george9 » Mon Oct 29, 2018 2:06 pm

I am curious to hear how the AP130 EDT works out because it has implications for my AP155 EDF. Bob, you have the f/8 I take it and not the f/8.3. The f/8.3 is the same lens as the AP155 but stopped down to 130. But even the f/8 is somewhat similar.

George

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