Developing NGC7000 using ST: can I do better?

Questions and answers about processing in StarTools and how to accomplish certain tasks.
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Re: Developing NGC7000 using ST: can I do better?

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F-Astro wrote: Wed Sep 23, 2020 3:51 pm If I wanted to create a purely mono image with my OSC, which RGB channel would you suggest processing? I assume that by processing a single channel it would eliminate the focus/wavelength issues discussed above, creating a sharper image than a full color image converted to black and white?
Your OSC's Bayer matrix allocates 2 green samples vs just 1 red and blue sample to each 2x2 patch on the sensor. The green channel is therefore the best channel to record/process if you absolutely have to and really cannot live with the other channels being slightly out of focus. There are other means to mitigate chromatic aberration however (hardware and software).

Of course, the green channel will only record one part of the spectrum and you will miss out on Ha and SII. That said, the latter statement isn;t enitrely true for many OSCs as green channel sensitivity goes up again (see here). To avoid recording detail at those wavelengths (which may be out of focus), use a UV/IR cut filter.

Really though, the loss of so much signal (e.g. the parts of the spectrum no longer recorded) makes all this not something that many find practical. Living with and mitigating the CA is usually the lesser of the two evils.
Ivo Jager
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Re: Developing NGC7000 using ST: can I do better?

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polslinux wrote: Wed Sep 23, 2020 8:01 am
admin wrote: Wed Sep 23, 2020 2:38 am
You did not do anything wrong at all! It's indeed a characteristic of your lens. It's a very common thing with lenses meant for terrestrial use. It's not the end of the world, just something to be mindful of. The blue channel suffers from precisely the same thing. It's textbook axial/longitudal chromatic aberration.
Interesting, thank you very much! I'm happy to know that I did everything right, but way less happy to know that, for astro, my lens is not as good as I thought :cry:
I guess it's time to start looking around for a lightweight APO refractor then :mrgreen:
That's fantastic - the AP community is full of passionate people! If not too much trouble, would you be able to re-post the stack by any chance with that setting disabled? I'm keen to see the difference. If it indeed fixes that issue, that would move ASTAP a lot higher up the list of preferred/recommended stacking solutions. That would allow me to update the docs accordingly.

Thank you!
Sure, no problem :thumbsup:

Stacked with DSS: https://drive.google.com/file/d/1B__N92 ... sp=sharing
Stacked with ASTAP: https://drive.google.com/file/d/1Ru3xV5 ... sp=sharing

This is the full ASTAP process:

1. Select "Stack"
1.jpg

2. Select "Images" and then "Browse" . Add all your lights.
2.jpg

4. Repeat procedure for Darks, Flats and Flat darks (I didn't have any :oops: )

5. Go to "Stack Method" and then:
5a: select "sigma clip average"
5b: disable auto levels
5c. select "bilinear interpolation"
3.jpg

6. Click on "Stack (Sigma clip average)

Thanks!
This is super helpful! Thank you also for sharing the new stack - it indeed looks perfectly suitable for StarTools.
Can I take this content and put it on the website?
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Re: Developing NGC7000 using ST: can I do better?

Post by polslinux »

Feel free to use all my images however you want and where you want! :thumbsup:
I am a big supporter of the super permissive license called WTFPL :mrgreen:
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Re: Developing NGC7000 using ST: can I do better?

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Thanks again for the ASTAP settings/instructions! I now added this to the links & tutorials section. :bow-yellow:
I've also been using your dataset as a development test case for an upcoming re-write of the Wipe module.
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Re: Developing NGC7000 using ST: can I do better?

Post by polslinux »

Thank you Ivo :thumbsup:
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