Double stack Ca K / October 6 / comparing telecentric mount or not

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christian viladrich
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Double stack Ca K / October 6 / comparing telecentric mount or not

Post by christian viladrich »

Hi,

Here are some old images taken in a test session comparing telecentric and not telecentric mounts for a double stack Ca K configuration.

It started with a FD image suffering from seeing effect due to the low elevation of the Sun :

http://astrosurf.com/viladrich/astro/so ... SI1600.jpg

The same with prominences (and a lot of glare ...) :

http://astrosurf.com/viladrich/astro/so ... 00-ddp.jpg

But my main objective was to compare my usual setup using Baader FFC as a Barlow lens, and the Airylab f/27 telecentric I use for Ha observation with my Celestron 8 EdgeHD. A double stack combination (Alluxa 0.37 nm + Alluxa 1 A Ca K filters) was used.

Measurements with the SHG Sol'Ex show there was indeed a small advantage to the telecentric combination :
- the bandpass of the combination at 10% peak transmission is 3.3 A in a telecentric beam,
- while it is in the range of 4.5 A with the FFC.
This is because the 0.37 nm filter needs to be tilted to be on Ca K.

In real life, the difference is very small.

With the FFC :
http://astrosurf.com/viladrich/astro/so ... AR3116.jpg

With the telecentric :
http://astrosurf.com/viladrich/astro/so ... entric.jpg

Christian


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Re: Double stack Ca K / October 6 / comparing telecentric mount or not

Post by pupak »

Great work.


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Re: Double stack Ca K / October 6 / comparing telecentric mount or not

Post by ffellah »

Hi Christian: beautiful images. Interesting comparison: hard for me to see any differences unless I see the images side by side, but good to know. The 2.7x Airylab telecentric is very good quality.

Franco


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Re: Double stack Ca K / October 6 / comparing telecentric mount or not

Post by marktownley »

Excellent comparison Christian. It is close, but like Franco says good to know the Airylab telecentric is good, which mirrors my findings with CaK.

Also interesting about the measured bandpass of the filter in the FFC / Airlylab optical trains.


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Re: Double stack Ca K / October 6 / comparing telecentric mount or not

Post by Dennis »

Hi Christian,
thx for the comparison, to me the closeup image with the FFC is clearly crisper then the other one for whatever reasons.


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Re: Double stack Ca K / October 6 / comparing telecentric mount or not

Post by nicspenceryork »

Gorgeous images and plenty for me to learn from,
Thanks
Nic


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Re: Double stack Ca K / October 6 / comparing telecentric mount or not

Post by Carbon60 »

Thank you, Christian. Great images and nice comparison. If only I could get anywhere near this quality :roll:

Stu.


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Re: Double stack Ca K / October 6 / comparing telecentric mount or not

Post by Montana »

All of these are absolutely terrific :bow :bow :bow :hamster: but I do think I prefer the FFC image, no idea why but I just do :)

Alexandra


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Re: Double stack Ca K / October 6 / comparing telecentric mount or not

Post by christian viladrich »

Thanks to all !

Franco and Mark, indeed it was following your various comments and discussions that I finally decided to test the Airlab telecentric in Ca K :-)

Denis, yes the version with the FFC is a bit crisper. This may be why you prefer it Alexandra.

At this stage, there are three possible (and not exclusive ...) explanations :
- seeing was better when the image with the FFC was done,
- in Ca K, the optical quality of the FFC is a bit better than the optical quality of the telecentric,
- this is the "normal" results of a narrower bandpass. Indeed, in CaK when we narrow the bandpass, we sample a higher layer of the chromosphere. And because the magnetic lines are "divergent", the details become fuzzier and larger when observed with a smaller bandpass.

And now the Sun is too low for any more testing....

Christian


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Re: Double stack Ca K / October 6 / comparing telecentric mount or not

Post by MAURITS »

That's a wonderful Calcium image Christian.


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Re: Double stack Ca K / October 6 / comparing telecentric mount or not

Post by marktownley »

Interesting the exposure / gain were the same for both images. Did you adjust spacing with the FFC to get the same image scale?


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Re: Double stack Ca K / October 6 / comparing telecentric mount or not

Post by christian viladrich »

Thanks Maurits :-)

Mark, yes and no. By chance, in just happens that I have the same image scale with both setup.


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Re: Double stack Ca K / October 6 / comparing telecentric mount or not

Post by marktownley »

christian viladrich wrote: Wed Nov 23, 2022 8:58 pm Measurements with the SHG Sol'Ex show there was indeed a small advantage to the telecentric combination :
- the bandpass of the combination at 10% peak transmission is 3.3 A in a telecentric beam,
- while it is in the range of 4.5 A with the FFC.
This is because the 0.37 nm filter needs to be tilted to be on Ca K.
Thanks Christian.

Any idea, even mathematically, how the filters would perform in a collimated beam?


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Re: Double stack Ca K / October 6 / comparing telecentric mount or not

Post by christian viladrich »

marktownley wrote: Sat Nov 26, 2022 6:39 am
Thanks Christian.
Any idea, even mathematically, how the filters would perform in a collimated beam?
Basically, there could be a "sweet spot", but some calculation is need to evaluate it.

I am developping a little piece of software to answer this kind of question. Still, the project is a bit on hold since I am busy on other stuff.


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Re: Double stack Ca K / October 6 / comparing telecentric mount or not

Post by marktownley »

I look forward to it Christian!


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