CaK imaging with different aperture masks

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arnedanielsen
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CaK imaging with different aperture masks

Post by arnedanielsen »

Dear all,

Following up on Mark’s excellent suggestion to stop-down my William Optics ZenithStar 61II for CaK imaging I got a good excuse to buy a 3D-printer for creating different aperture masks . 😊
202918523_224650786141324_6916936905496195674_n_resize.jpg
202918523_224650786141324_6916936905496195674_n_resize.jpg (382.85 KiB) Viewed 3532 times
200208242_328287718898191_5873735058406300343_n_resize.jpg
200208242_328287718898191_5873735058406300343_n_resize.jpg (417.43 KiB) Viewed 3532 times
After having designed and printed 3 different masks (40, 46 and 50mm) I did a back-to-back test earlier this week.
The seeing was quite decent in the afternoon and the conditions quite stable. The setup being used was the William Optics Zenithstar 61II equipped with a 2” Lunt CaK B1200 Straight Trough module and a ZWO ASI178MM. Everything mounted on a SkyWatcher HelioFind mount. I focused using the 40mm aperture filter and immediately noticed how much easier it was to find focus with the aperture mask compared to using full-aperture. I did not refocus between the recordings.

I did one 30s recording for each of the aperture mask plus one with no mask (full aperture) using the same camera settings of Gain=0, HighSpeed=on, Gamma=off and ROI=1800x1800. The shutter of course needed to be adjusted for the different apertures (40mm = 1.30ms, 46mm=1.00ms, 50mm=0,85ms and full aperture=0.60ms) to keep the histogram similar. This gave an FPS of 70 and 2100 frames for each recording.
I then stacked the best 20% (420 of 2100) using AutoStakkert before sharpening the images in ImPPG using the exact same parameters. No other image-processing.

Here’s a cropped section from each of the resulting image.
SunCaK60_155232_CaK_(3934nm)_20210615_ZWO ASI178MM_lapl4_ap2817_Sharpened_Cropped.jpg
SunCaK60_155232_CaK_(3934nm)_20210615_ZWO ASI178MM_lapl4_ap2817_Sharpened_Cropped.jpg (141.96 KiB) Viewed 3532 times
SunCaK60_155325_CaK_(3934nm)_20210615_ZWO ASI178MM_lapl4_ap2830_Sharpened_Cropped.jpg
SunCaK60_155325_CaK_(3934nm)_20210615_ZWO ASI178MM_lapl4_ap2830_Sharpened_Cropped.jpg (135.72 KiB) Viewed 3532 times
SunCaK60_155421_CaK_(3934nm)_20210615_ZWO ASI178MM_lapl4_ap2822_Sharpened_Cropped.jpg
SunCaK60_155421_CaK_(3934nm)_20210615_ZWO ASI178MM_lapl4_ap2822_Sharpened_Cropped.jpg (122.19 KiB) Viewed 3532 times
SunCaK60_155508_CaK_(3934nm)_20210615_ZWO ASI178MM_lapl4_ap2839_Sharpened_Cropped.jpg
SunCaK60_155508_CaK_(3934nm)_20210615_ZWO ASI178MM_lapl4_ap2839_Sharpened_Cropped.jpg (109.22 KiB) Viewed 3532 times
As well as the full size images.
SunCaK60_155232_CaK_(3934nm)_20210615_ZWO ASI178MM_lapl4_ap2817_Sharpened.jpg
SunCaK60_155232_CaK_(3934nm)_20210615_ZWO ASI178MM_lapl4_ap2817_Sharpened.jpg (778.4 KiB) Viewed 3532 times
SunCaK60_155325_CaK_(3934nm)_20210615_ZWO ASI178MM_lapl4_ap2830_Sharpened.jpg
SunCaK60_155325_CaK_(3934nm)_20210615_ZWO ASI178MM_lapl4_ap2830_Sharpened.jpg (741.07 KiB) Viewed 3532 times
SunCaK60_155421_CaK_(3934nm)_20210615_ZWO ASI178MM_lapl4_ap2822_Sharpened.jpg
SunCaK60_155421_CaK_(3934nm)_20210615_ZWO ASI178MM_lapl4_ap2822_Sharpened.jpg (674.34 KiB) Viewed 3532 times
SunCaK60_155508_CaK_(3934nm)_20210615_ZWO ASI178MM_lapl4_ap2839_Sharpened.jpg
SunCaK60_155508_CaK_(3934nm)_20210615_ZWO ASI178MM_lapl4_ap2839_Sharpened.jpg (591.57 KiB) Viewed 3532 times
Stopping down certainly helps for this scope for this type of imaging. Both the 40mm and the 46mm looks really good (not sure which one I prefer), but the sweet spot looks to be around this area.

Best regards,
Arne


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Re: CaK imaging with different aperture masks

Post by Bob Yoesle »

Excellent comparison Arne!

The next step might include re-spacing the objective to optimize spherochromatic aberration, and see the effect of f-ratio on contrast performance.


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Re: CaK imaging with different aperture masks

Post by Stardust5858 »

That's brilliant Arne, thanks for the comparisons. I do think the 40mm mask is the better image.


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Re: CaK imaging with different aperture masks

Post by thesmiths »

Really nice comparison! Interestingly, I believe the CaK PST is 40mm f10. I think your best image is the 40mm f9. Maybe those Coronado guys knew what they were doing.


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Re: CaK imaging with different aperture masks

Post by p_zetner »

An impressive study, Arne.
Stopping down to 40mm makes a stunning difference.
Bob’s suggestion might be a good one to try and improve resolving power with the wide open objective.
Cheers.
Peter


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Re: CaK imaging with different aperture masks

Post by thesmiths »

I'm also curious how much of the effect is due to reduction of aperture vs f-ratio (angle of the light on the filter).


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Re: CaK imaging with different aperture masks

Post by eroel »

Arne:
A very clear information and comparison.
Beautiful made different aperture masks.
Best regards.
Eric.


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Re: CaK imaging with different aperture masks

Post by H-Alpha »

Excellent comparison Arne!
Very useful and informative for all of us. Mark, Marty and other experienced people here repeatedly note how important masking is.
I wanted to do the same (going from 130 down to 80 mm), but I am very happy you showed it so clearly, so that there is no need to repeat what you did. Many many thanks!
Best wishes,
Alexandros


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Re: CaK imaging with different aperture masks

Post by pedro »

I always image (Ca-K) with an F/10 focal ratio


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Re: CaK imaging with different aperture masks

Post by thesmiths »

pedro wrote: Sat Jun 19, 2021 7:28 pm I always image (Ca-K) with an F/10 focal ratio
Pedro: is it the f ratio or the aperture that most determines the quality?


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Re: CaK imaging with different aperture masks

Post by EGRAY_OBSERVATORY »

Interesting subject and thanks vm

Terry


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Re: CaK imaging with different aperture masks

Post by marktownley »

Super comparison Arne and really shows the effectiveness of masking. The 40mm aperture shows considerably finer detail than 61mm full aperture. Glad it was a good excuse for you to buy a 3D printer too ;)

I think will give this topic a few more days here to get more coverage and then will pop it in the reference library...


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Re: CaK imaging with different aperture masks

Post by arnedanielsen »

Thank you everyone for all your kind comments and suggestions!

It sounds like the consensus is that the 40mm (360mm f/9) has the edge in this comparison.

Bob: Re-spacing the objective is not something I feel competent to try doing. However, I might be tempted to start looking for a new dedicated scope for CaK imaging. A scope with a focallength of 430-450mm would still give nice FD images on the 178mm sensor, but with an increased sampling.

Best regards,
Arne


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Re: CaK imaging with different aperture masks

Post by christian viladrich »

Well done ! The 40 mm is the clear winner.


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Re: CaK imaging with different aperture masks

Post by pedro »

thesmiths wrote: Sat Jun 19, 2021 7:49 pm
pedro wrote: Sat Jun 19, 2021 7:28 pm I always image (Ca-K) with an F/10 focal ratio
Pedro: is it the f ratio or the aperture that most determines the quality?
The focal ratio. You should use F/10 every time. More aperture is always better, but usually seeing is bad in CaK


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Re: CaK imaging with different aperture masks

Post by AtiCelestron »

Bob Yoesle wrote: Sat Jun 19, 2021 4:17 pm Excellent comparison Arne!

The next step might include re-spacing the objective to optimize spherochromatic aberration, and see the effect of f-ratio on contrast performance.
Hello Bob!
I don't really know the re-spacing thing. Can IT improve the image quality , contrast if I place the lenses closer or farther apart?
I have a 120/1000 skywatcher telescope with Lunt LS50 system


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