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Sarahs Galaxy - Luminance (17.5 hrs)


Mr_42tr0nomy

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Hey ya'll!

Hope everyone's having a good week! I started working on Sarah's Galaxy (NGC 3628) last week as my next project. I think I may finish up here with luminance data and move on to RGB on the next clear night. The image below consists of 17.5 hours via 300s subs. The noise is really starting to die down after applying calibration frames, almost to the point where the extra 5 or so hours I WOULD get may not be worth it (thoughts? Still getting used to this camera). Anyways, I plan on getting about 4 hours RGB each totaling 29.5 hrs.

iOptron CEM60
Astro-Tech 8RC
ASI1600mm-Pro
Astrodon Lum (300s / Gain: 139 / Offset: 21)

Bortle 5

Sarahs galaxy lum stack.jpg

Edited by Mr_42tr0nomy
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Do RGB. I have that camera and surely a sky worst than your (also less experience and a smaller setup....) but trust me, you got stunning details that deserve the color. Not sure you can get more increasing the luminance. 

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2 minutes ago, Paul73 said:

It is looking good. I’ve never heard it called Sarah’s Galaxy before. Do you know the origins of the name?

Paul

Funny you ask, Paul. I was talking to my wife (Sarah) about this exact thing. She noticed that I stopped calling it "Sarah's" Galaxy and returned to its more popular name "Hamburger" Galaxy. I told her that I wasnt sure it was called Sarah's Galaxy because I had only seen it referred to as that name a few times. I'm not sure the origin but after some research...I did find this to be an alternate name. So now I have to continue calling it what my wife wants me to call it haha!

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Seriously good work!

Given the gear used to acquire it, I'm not surprised :D

I do have few recommendations. For your next project raise your offset. This is really low and you are running the risk of clipping low values. You might be ok for L due to 300s exposure and Bortle 5, but it might cause problems with color and it almost certainly will with NB.

If you change it (and my recommendation would be to get it to about 60, I personally use 64, no particular reason except maybe some sort of slight binary OCD :D ), make sure you take another set of darks as current one won't be usable any more with change in offset.

Second suggestion would be to bin your subs in software. This will boost your SNR by factor of x2 - to be able to do that with more exposure, you would need additional 50 or so hours. Your image scale will be less, but odds are you won't be loosing anything in detail. I just looked your image at 1:1 zoom (certified pixel peeper here) and stars look like more than 2" FWHM in your stack. You can safely bin your data to 1"/px

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26 minutes ago, GiorgioF said:

Do RGB. I have that camera and surely a sky worst than your (also less experience and a smaller setup....) but trust me, you got stunning details that deserve the color. Not sure you can get more increasing the luminance. 

I think that's the plan. This is my first photo with this camera so unless you got yours a month ago, youre sure to have more experience!

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48 minutes ago, vlaiv said:

Seriously good work!

Given the gear used to acquire it, I'm not surprised :D

I do have few recommendations. For your next project raise your offset. This is really low and you are running the risk of clipping low values. You might be ok for L due to 300s exposure and Bortle 5, but it might cause problems with color and it almost certainly will with NB.

If you change it (and my recommendation would be to get it to about 60, I personally use 64, no particular reason except maybe some sort of slight binary OCD :D ), make sure you take another set of darks as current one won't be usable any more with change in offset.

Second suggestion would be to bin your subs in software. This will boost your SNR by factor of x2 - to be able to do that with more exposure, you would need additional 50 or so hours. Your image scale will be less, but odds are you won't be loosing anything in detail. I just looked your image at 1:1 zoom (certified pixel peeper here) and stars look like more than 2" FWHM in your stack. You can safely bin your data to 1"/px

I was curious about my offset settings to begin with. I will give 64 a shot and see how it looks. As for NB, I will be using gain 200 and offset 50 or so. I have read that to be a good starting point.  Is there a method to bin in pixinsight? I dont know how i feel from going to 16mp to 4mp though. My FWHM is at least 5.0. Seeing here in the mountains is very poor.

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5 minutes ago, Mr_42tr0nomy said:

I was curious about my offset settings to begin with. I will give 64 a shot and see how it looks. As for NB, I will be using gain 200 and offset 50 or so. I have read that to be a good starting point.  Is there a method to bin in pixinsight? I dont know how i feel from going to 16mp to 4mp though. My FWHM is at least 5.0. Seeing here in the mountains is very poor.

What do you intend to do with your image? For most computer displays, 4mp is more than enough. If you intend to print it, that could be another matter, but again, if your FWHM is that high (and what units would that be in? It's certainly not 5 arc seconds FWHM - maybe you are referring to pixels, in that case, because of 0.48"/px, that would be 2.4" FWHM - and that is more in line with what I'm seeing in the image), you certainly won't loose any detail if you bin.

Btw, I'm no PI expert, don't even own a copy, but in PI, there is operation called integer resample - which is in fact binning. Just choose average and factor of x2.

You can do this on your subs prior to stacking and after calibration (if I'm not mistaken, run above command on bunch of subs and it will process them all), or you can bin your stacked image just to try it out.

Close to ideal sampling rate is FWHM / 1.6, or in this case it would be 1.5"/px if your FWHM is indeed 5 pixels or 2.4". This means that you can even bin 3x3 without loss of much detail. This will improve your SNR by x3 - image will go seriously deep, but you probably won't love resulting image size. Anyway, give it a go and see if you gain anything by deeper data in terms of image depth and lack of noise.

Remember to process it all in 32bit.

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