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Use of ADC when using RGB filters and a monochrome camera

astrophotography planet accessories filters
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#1 Helvetios

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Posted 21 October 2020 - 07:54 AM

I understand that some astrophotographers use an atmospheric dispersion corrector (ADC) when shooting separate RGB videos with a monochrome camera. In principle, even though the final RGB images get aligned in processing and this automatically takes out the dispersion to a large extent, dispersion varies continuously with wavelength and there must be some dispersion and resultant bluring within each R-G-B image. Hence, there may be sharpening to be had by adjusting for dispersion in each color band. The question I have is how much difference does this make in practice and how do you do it? By trial-and-error until it looks sharper, or is there a tool or a more systematic approach?

 

Thanks,

Richard

 



#2 otocycle

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Posted 21 October 2020 - 08:16 AM

I found that it doesn't make a big enough difference to make the effort worth it because my skies are seeing limited.   I just touch up focus for each color.   If you are going to de-rotate multiple video captures and therefore have time to adjust the ADC and focus, it might be worthwhile.  

 

See the ADC dispersion graph by color on this site to get an idea of how much difference each color channel has, and this will depend on other factors as well.

 

https://skyinspector...-corrector-adc/

 

The red and green channels are more similarly impacted compared to blue.


Edited by otocycle, 21 October 2020 - 08:21 AM.

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#3 happylimpet

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Posted 21 October 2020 - 08:48 AM

Its a biggie! Always worth doing.

 

Recently when imaging in mono I tend to adjust the ADC by eye, viewing via the flip mirror. Ive used it so much though that i can estimate the positions pretty well. Remember, it doesnt have to be perfect - if its a small effect anyway you can be 20% off and still basically knock it on the head.


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#4 descott12

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Posted 21 October 2020 - 08:57 AM

Very good article. Thanks for the link


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#5 Helvetios

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Posted 21 October 2020 - 09:24 AM

Great responses, and really great web page otocycle! Extremely informative. The Prost graph tells the story. Right now, Mars peaks at 56 deg for me, so the correction is < 0.6 arcsec and I don't think correcting the individual colors will improve the images much. But based on what happylimpet writes, I may give it a try.  Always fun to experiment.


Edited by Helvetios, 21 October 2020 - 10:05 AM.


#6 AstroEthan

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Posted 21 October 2020 - 12:26 PM

I use a #47 violet filter for adjusting my ADC with a monochrome camera. The filter has a strong infrared leak, so you see two images when the ADC is not set. I can say it helps with Jupiter and Saturn at 40°.

This is the one I use. Worth the $9 in my opinion: https://optcorp.com/...filter-case-f47

Edited by AstroEthan, 21 October 2020 - 12:27 PM.

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#7 Helvetios

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Posted 21 October 2020 - 02:42 PM

I use a #47 violet filter for adjusting my ADC with a monochrome camera. The filter has a strong infrared leak, so you see two images when the ADC is not set. I can say it helps with Jupiter and Saturn at 40°.

This is the one I use. Worth the $9 in my opinion: https://optcorp.com/...filter-case-f47

Luckily for me I have one of these and it's going to be clear tonight!  Thanks very much.




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