Experimenting calcium imaging with small refractors

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krakatoa1883
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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.

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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 TheSkyBurner » Wed Oct 31, 2018 2:35 am

I can confidently say the material of the objective is not really the issue below 400nm... It is the coating applied to the objective that ruins it. If the coating reflects 380-397 perpendicular to the objective than there goes your below 400 image quality. That purple fringe is vital to operation below 400nm!

If i were to post my 100mm fluorite next to my 90mm flint, you would literally not be able to tell the difference. Because there is not one visible sign in sharpness. The fluorite might have 5% better transmission unfiltered, but you are not gonna see it when the first filter installed cut's that away.

I have concluded without a doubt that certain filters ruin the sharpness. My 2" beloptik ir/uv cut filter killed my k-line image quality which means the coating on olivers filter is not well suited for the 390's. When I took it out , it made an incredible difference (i will never use it again for k-line i wouldnt even use it for g-band....) I thought my scope was bad, but it wasnt the scope; it was the coating on the ir rejection. Uncoated KG3 is the way to go, or plain old BBAR .

You will notice the lunt CaK system does not use infrared absorption filters, lunt has a single full spectrum blocking bandpass filter.. Also take notice the coronado cak scopes never used infrared absorption filters. The ir cut in the coronado is a thin film 24k gold coating at the eyepiece, and pure bandpass ITF based filtration before that.

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Re: Experimenting calcium imaging with small refractors

Post by TheSkyBurner » Wed Oct 31, 2018 2:50 am

Look at all those very different objective coatings! Can you guess which ones were designed to remove color aberrations below 400nm?

Can you guess which coating is the best for 393nm transmission?
uncoated 203mm.jpg
uncoated 203mm.jpg (264.2 KiB) Viewed 323 times
red 50mm.JPG
red 50mm.JPG (459.05 KiB) Viewed 323 times
purple 100mm.JPG
purple 100mm.JPG (460.54 KiB) Viewed 323 times
magensium fluoride 60mm.JPG
magensium fluoride 60mm.JPG (435.95 KiB) Viewed 323 times
green coating 120mm.jpg
green coating 120mm.jpg (168.67 KiB) Viewed 323 times
gold 40mm.JPG
gold 40mm.JPG (556.63 KiB) Viewed 323 times
blue coating 120mm.jpg
blue coating 120mm.jpg (245.47 KiB) Viewed 323 times

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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 309 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 TheSkyBurner » Wed Oct 31, 2018 10:57 pm

Here is an interesting article about sol gel doped magnesium fluoride.

https://www.photonics.com/Articles/Seco ... gF2/a19440

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