Experimenting calcium imaging with small refractors

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Re: Experimenting calcium imaging with small refractors

Post by marktownley » Tue Oct 30, 2018 9:06 am

george9 wrote:
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
Hi George,

Frederic has done optical reports on both the 130 and the 155 on his website https://airylab.com/astronomy-test-reports/ Looking at the 130 it shows pretty much perfect spherochromatism in the blue when masked down to about 50% aperture, above about 70 things start to get worse. At full 130mm aperture I think CaK would be quite hazy.
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Re: Experimenting calcium imaging with small refractors

Post by Bob Yoesle » Tue Oct 30, 2018 10:57 pm

Hi George,

Not sure my results will be that informative - although I am looking forward to getting things up and running with a new CaK module. My circa 1996 130 EDT f/8 uses an oil-spaced glass combination of ZKN-7 crown sandwiching an FPL-53 center element. The later f/8.35 EDF scopes used FPL-53 and a different crown glass(es - BK-7 & ?). Later lens designs may also have gone to air-spaced. Not sure which would be better for UV @ 393 nm. A little more information here:

https://www.cloudynights.com/topic/6014 ... try8276184

Also see:

https://www.telescope-optics.net/semiap ... amples.htm
Last edited by Bob Yoesle on Wed Oct 31, 2018 2:51 am, edited 1 time in total.
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Re: Experimenting calcium imaging with small refractors

Post by krakatoa1883 » Wed Oct 31, 2018 9:59 am

The color of the coating corresponds to the color or combination of colors that is most reflected out. The CIE chromaticity diagram may help in understanding what colors pass least through a given lens. If one knows the index of refraction of the coating the %T (or the % reflectance) can be easily calculated for normal incidence as it can be assumed for sun rays. Unfortunately only rarely the characteristics of the coatings are explicitly declared by manufacturers but some assumptions can be made by using of available data, for example for a MgF2 coating, which seems not bad in the violet (Edmund Optics data, but the same can be calculated by using of the above linked page):
mgf2.jpg
mgf2.jpg (33.2 KiB) Viewed 1162 times
Anyway based on the images one can see on the Internet it seems that actually most telescopes can be successfully used for calcium imaging so lens or plate or mirror coatings don't seem so critical to me. Spherical correction at short WL is much more important, I think, as this leads to huge differences between different refractors and the need to stop down the least corrected ones.
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Re: Experimenting calcium imaging with small refractors

Post by george9 » Wed Oct 31, 2018 4:16 pm

Thanks, all. I know that stopped down to 67% (4 inches), the AP155 looks very nice in my coarse R2 video camera. Thinking of getting a better camera in the spring.

George

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Re: Experimenting calcium imaging with small refractors

Post by Bob Yoesle » Sat Mar 30, 2019 4:20 pm

Hi everyone,

It seems in general that f-ratio for f-ratio, achromatic refractors have better longitudinal chromatic aberration control, while APO's have better lateral chromatic aberration. However, even my lowly Synta ED100/900 semi-APO doublet does well at CaK, which is one of the most difficult wavelengths for most refractors to image well. The fact that it is f9 as opposed to f6 likely helps (and the CaK filters likely perform much better).

Somewhere I recall reading that you can change the performance of refractors for specific wavelengths by simply re-spacing the elements. A couple of years ago I had planned a larger-aperture solar telescope project using my ED100/900's, and asked Mike Jones (now retired from Lockheed-Martin and proprietor of Precision Optics of Azle LLC) to work up an OSLO simulation of this objective, as it was not only well suited for 656 nm, but gave good results for 393 nm. Mike is a very experienced optical designer and what he came up with is probably the best that can be done without having the actual specs from the OEM:

ED100 MIJ w CaK image.jpg
ED100 MIJ w CaK image.jpg (301.61 KiB) Viewed 627 times

Having the OSLO file allowed me to adjust the spacing of the two elements. Increasing the lens spacing from 0.5 mm to 0.95 mm results in reducing 393 nm longitudinal chromatic aberration by half, and would seem to significantly improve CaK performance:

ED100 393 Nom v Rspcd.jpg
ED100 393 Nom v Rspcd.jpg (176.03 KiB) Viewed 627 times

Close up on at-focus spot, Modulation Transfer, and Point Spread Function plots:

ED100 393 compares.jpg
ED100 393 compares.jpg (542.99 KiB) Viewed 627 times

I have another ED100/900 to play with, so I'm hoping to have some new spacers made and give it a try. Given the already pretty good imaging in CaK, it will be a good side-by-side experiment to see if there are any real-world improvements under good seeing conditions.

PM me if you'd like to have Mike's OSLO file to play around with the objective for other wavelengths. H alpha performance can be slightly improved, but doesn't appear to be worth the effort.
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Re: Experimenting calcium imaging with small refractors

Post by marktownley » Sun Mar 31, 2019 8:16 am

Great data there Bob, thanks for sharing. I look forward to your spacer experiment when conditions and time allows.
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Re: Experimenting calcium imaging with small refractors

Post by bart1805 » Sun Mar 31, 2019 9:50 am

Hi Bob, very interesting! When increasing the lens spacing is there any negative effect on other wavelengths? And I assume that is more difficult than some layers of aluminium foil to replace the original spacers. Is that correct?
Good luck experimenten! Bart.

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Re: Experimenting calcium imaging with small refractors

Post by marktownley » Sun Mar 31, 2019 2:38 pm

bart1805 wrote:
Sun Mar 31, 2019 9:50 am
And I assume that is more difficult than some layers of aluminium foil to replace the original spacers. Is that correct?
Good luck experimenten! Bart.
No, absolutely not Bart; use a micrometer. If you measure the thickness of potential new donor spacers beforehand you can get the ideal material, just something non compressive. There are a number of different options. I used foil from a 'take away' style container on my AR5 as this was thicker than the stock spacers. If you have 'thin' aluminum foil you can always fold it in half to double the thickness.
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Re: Experimenting calcium imaging with small refractors

Post by krakatoa1883 » Sun Mar 31, 2019 7:20 pm

Thanks Bob, very informative post. My 100ED - I suppose your one to be similar - has a ring spacer that can easily be replaced with three small foils 120° apart as in classical achromats, I made this change on my 120ED which has the same objective, only scaled up. It's a very good scope for calcium however I noted that there is still room for improvements (my TMB 80, for example, is better).

Increasing spacing works for achromats too but improving blue-violet correction has an adverse effect on H alpha.
marktownley wrote:
Sun Mar 31, 2019 2:38 pm
If you have 'thin' aluminum foil you can always fold it in half to double the thickness.
I make use of those aluminum discs that are made to stop dropping from wine bottles, those that are sold here have a very uniform thickness of 0.16mm or 0.20mm.
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Re: Experimenting calcium imaging with small refractors

Post by bart1805 » Mon Apr 01, 2019 5:11 pm

Thanks Mark and Raf.

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Re: Experimenting calcium imaging with small refractors

Post by Bob Yoesle » Wed Apr 03, 2019 12:21 am

When increasing the lens spacing is there any negative effect on other wavelengths?
Increasing spacing works for achromats too but improving blue-violet correction has an adverse effect on H alpha.
Raf is correct, but it might not be as bad as it sounds. I think the acid test is needed in the form of experimental real world evaluations - respacing for much improved CaK performance, while not optimal, might still prove acceptable 546 nm and 656 nm performance. While I'm certainly no expert, consider the following...

Here's the nominal spacing of 0.5 mm and the 656 spots:
MIJ ED100 656 Nominal.jpg
MIJ ED100 656 Nominal.jpg (271.93 KiB) Viewed 513 times

Here's a respace to 0.4 mm to optimize 656 nm:
MIJ ED100 656 respace.jpg
MIJ ED100 656 respace.jpg (239.15 KiB) Viewed 513 times
Not sure this would provide a perceptible improvement.


If we re-space for 393 nm to 0.95 m, here's the spot diagrams for 656 nm:
MIJ ED100 656 CaK respace.jpg
MIJ ED100 656 CaK respace.jpg (309.49 KiB) Viewed 513 times

Compare this to the respaced/optimized 393 nm spot perfromance:

MIJ ED100 393 respaced.jpg
MIJ ED100 393 respaced.jpg (290.83 KiB) Viewed 479 times

And here's 546 nm with a re-space to 0.95 mm:
MIJ ED100 546 CaK respace.jpg
MIJ ED100 546 CaK respace.jpg (298.54 KiB) Viewed 513 times

Below I've plotted 656 nm for the nominal objective, and the respaced objective optimized for 393 nm. I've also increased the aperture divisions from 33 to 50 to increase the intensity of the spot diagrams.

While the spots are certainly not as good, they don't appear terrible. Indeed, the CaK respaced 656 nm plots look much better than the unoptimized nominal CaK, and quite similar to the the respaced/optimized 393 nm spots. Therefore the real world H alpha with the optimized for CaK might work pretty well under most seeing conditions:

MIJ ED100 656 Nominal v CaK respaced CMP.jpg
MIJ ED100 656 Nominal v CaK respaced CMP.jpg (548.24 KiB) Viewed 498 times

Spots with gaussian blur applied:

Spot compare gz blr.jpg
Spot compare gz blr.jpg (227.53 KiB) Viewed 486 times
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Re: Experimenting calcium imaging with small refractors

Post by krakatoa1883 » Wed Apr 03, 2019 4:43 pm

My comment was referred to achromats, based on your plots a 100ED seems a more flexible project. Optimizing the ED for CaK seems certainly worth the effort.
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Re: Experimenting calcium imaging with small refractors

Post by bart1805 » Wed Apr 03, 2019 7:37 pm

Hi Bob, is the nominal value of 0,5mm for the original spacers an estimate or did you measure them? I thought they were about half of that, so 0,025 mm. CS! Bart.

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Re: Experimenting calcium imaging with small refractors

Post by Bob Yoesle » Wed Apr 03, 2019 10:43 pm

I have not measured it or taken one of the objective out of it's cell. The value is based solely on Mike Jones penciling it out theoretically.

That said, a general rule for Achromats may apply for the curves and spacings being scalable to a percentage of the focal length, and therefore if the actual value is about half of Mike's design, the CaK re-space would be just about twice that.
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Re: Experimenting calcium imaging with small refractors

Post by krakatoa1883 » Wed Apr 10, 2019 10:42 am

Just to point out, again, that not always a low dispersion glass can be beneficial for narrowband imaging. Here are two almost-RAW images taken of a small group appeared last March , all parameters are the same (native focal = 900mm for both scopes, same camera, focus, no. of stacked images, same very slight UM, seeing, Quark calcium tuned to -5). Note that despite the lower aperture (78 mm effective) the image taken with the achromat is slighly sharper than the one provided with my 100ED (which is a very good and selected sample).
TMB.jpg
TMB.jpg (20.3 KiB) Viewed 397 times
100ED.jpg
100ED.jpg (16.93 KiB) Viewed 397 times
However the ED scope can regain contrast either by slightly reducing the aperture or by simply processing with a stronger UM.
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Re: Experimenting calcium imaging with small refractors

Post by bart1805 » Thu Apr 25, 2019 7:18 am

Opened the lens cell of the Celestron 100mm f/9 ED. There was a shim ring between the lenses. So cut 6 small pieces, glued them together to make three small shims and put everything back. Very tight lens cell so it took quite some time. Distance is now twice as big, now waiting for a moment to test it.

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Re: Experimenting calcium imaging with small refractors

Post by krakatoa1883 » Thu Apr 25, 2019 4:20 pm

Keep us informed, Bart, I am very curious to see the results.
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Re: Experimenting calcium imaging with small refractors

Post by bart1805 » Sun May 05, 2019 10:55 am

krakatoa1883 wrote:
Thu Apr 25, 2019 4:20 pm
Keep us informed, Bart, I am very curious to see the results.
First try this morning, it seems an improvement.
viewtopic.php?f=4&t=26041

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Re: Experimenting calcium imaging with small refractors

Post by Dennis » Sun May 05, 2019 2:43 pm

so a powermate works for enhancing f ratio? why then bothering with expensive refractors when you could use a big newton reflector with an aries front-d-erf and pump the f-ratio with a powermate to f/11 or so?

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Re: Experimenting calcium imaging with small refractors

Post by bart1805 » Sun May 05, 2019 3:02 pm

Dennis wrote:
Sun May 05, 2019 2:43 pm
so a powermate works for enhancing f ratio? why then bothering with expensive refractors when you could use a big newton reflector with an aries front-d-erf and pump the f-ratio with a powermate to f/11 or so?
These small refractors are not expensive, compared to the big Erf's and other filters you need.

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