Greenish background when imaging the Needle Galaxy

Discussion of using SharpCap for Deep Sky Imaging
Moonstruck
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Greenish background when imaging the Needle Galaxy

#1

Post by Moonstruck »

Hello,
I was imaging the Needle Galaxy the other night, and there was a kind of dark milky green background in the captured image. I used DSS to stack approx 60 2-min subs, and also included 20 flat, 20 dark and 20 bias frames in the stack. This greenish background was very noticeable after stretching the image using startools, and the image itself was also greenish. What could be causing that?

I attached a word file with Google drive links to the raw stack, and one each of single light, bias, flat and dark frames. Hopefully they will help diagnose this. I also included the Sharpcap camera setting file for the light frames.

Note- the flat frames were sky flats, and I adjusted the exposure so that the white curve on the histogram was at approx. 50%.

Thanks!
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Needle Galaxy Image files.docx
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oopfan
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Re: Greenish background when imaging the Needle Galaxy

#2

Post by oopfan »

Hi,

You've got Raining Noise:
Moonstruck-NGC-4565.png
Moonstruck-NGC-4565.png (653.83 KiB) Viewed 1689 times
This can be corrected by:
1. Don't stretch this image so much.
2. Increase exposure (requires re-imaging).
3. Dither (requires re-imaging).

Most people jump into Dithering but I prefer to increase exposure. You've got great guiding so increasing exposure isn't a hardship. You certainly must have dark skies!

Brian
Moonstruck
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Re: Greenish background when imaging the Needle Galaxy

#3

Post by Moonstruck »

Thanks Brian. What would you recommend I increase exposure to? This image had 2 min exposures, and I think around 60 subs. Should I have 3 or 4 min subs? Or maybe just more of the 2 min subs?

Also will that green color go away with increased exposure time?

Note - I have never tried dithering before, and don’t know how to do that.

Thanks again.
John
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oopfan
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Re: Greenish background when imaging the Needle Galaxy

#4

Post by oopfan »

Hi John,

I have a general rule for CCD's but I think it can be extended to CMOS. I call it the "1000 ADU Rule". I came upon it a couple years after looking at a retailer's website whose name you would recognize. They have an exposure calculator. I was curious about the formula they used, and fortunately for me, they didn't "minify" their JavaScript code.

The idea is that you must treat the sky background with respect by exposing it sufficiently. It makes sense if you think of it this way: there are faint wisps of nebulosity mixed in with the sky background. In order to capture it you need to sufficiently expose it.

So for CCD's the general rule is that the sky background should be 1000 ADU above Bias. For CMOS you should be able to go with 200 ADU above Bias because Read Noise is so much lower.

Your average Bias level looks to be around 385 ADU.

Oops! The link you gave for your single Light frame is the same as your Bias frame. Can you reply with the link to your Light frame on your Google drive.

Thanks,
Brian
Moonstruck
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Re: Greenish background when imaging the Needle Galaxy

#5

Post by Moonstruck »

Sorry about that! Here is a link to a single light frame:
https://drive.google.com/file/d/1k5L2Xo ... sp=sharing

Also, I noticed that the single light frame, when stretched, doesn't have the same milky-green hue that the stack does... could the greenish color therefore have been caused by the flat or bias frames mixed in? If so, how can I correct that in the future?

I'm not familiar with how to determine what the ADUs are for any given image, or how to control that... any advice? I'm still relatively new to imaging in general. Can you give me a better exposure number I can try next time when imaging, like the #subs I should try taking, or a better overall length exposure, without reference to ADUs? I can easily understand that! :-) Or maybe let me know how to measure ADUs?

Thanks,
John
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oopfan
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Re: Greenish background when imaging the Needle Galaxy

#6

Post by oopfan »

John,

Regarding the green background, that is something taken out by StarTools when you press the right buttons. There is a couple reasons why it's green cast:
1. The Bayer pattern consists of two green pixels, one red and one blue.
2. Your camera's sensor is more sensitive to green (that's the QE).

Let's me take a look at your Light frame now.

Brian
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Re: Greenish background when imaging the Needle Galaxy

#7

Post by oopfan »

John,

Your average Light level looks to be around 2000 ADU of empty space, therefore 2000 ADU minus 385 ADU is 1650 ADU.

Question: I know you used SharpCap but did you capture them while Live Stacking? I'm just wondering if these Lights were pre-processed by SharpCap in any way. Do you remember if the SharpCap panel said "Darks: none" and "Flats: none"?

Depending on your answer, I'm inclined to say that your exposure isn't the problem. It looks like you should start looking into dithering like a lot of people using CMOS cameras. However, it is strange that my $400 Altair 290M CMOS mono camera doesn't suffer from Raining Noise unless the exposure is super short. It could be because the 290M is not cooled.

Look into dithering. Your mount performs very well, so my guess is that it can dither like the best of them.

Brian
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Re: Greenish background when imaging the Needle Galaxy

#8

Post by oopfan »

John,

Your Camera Settings file says:

Apply Flat=None
Subtract Dark=None

So SharpCap did not pre-process your Lights. That's good. It's looking more and more like Dithering is your answer. I'm looking at your Flats and Darks now.

EDIT: Your Flats and Darks look good. I'd run the Stack through StarTools again. I don't know what version you have but I was never happy with the control it gave me with regards to stretching. I always felt that it stretched too far which exaggerated every imperfection. Somehow you need to find a way to raise the black level to clip that Raining Noise. Also you need to find a way to neutralize the background (the green).

Brian
Moonstruck
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Re: Greenish background when imaging the Needle Galaxy

#9

Post by Moonstruck »

Thanks, I appreciate it. I have had great luck using sharpcap’s polar alignment tool, which really helps me to get a good alignment by the way.

Can you give me some quick guidance on how to dither? Is it easy to do, and is there a good basic video, or some simple instructions somewhere 🙂
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oopfan
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Re: Greenish background when imaging the Needle Galaxy

#10

Post by oopfan »

John,

I wish I could help but my 50-year old mount can't dither so I have no experience with it. There are a number of active posts on the topic. One that is active right now was started by Ginger. I think the topic's title is simply "Dithering" or something like that. Check it out and ask questions.

Another possibility is to sign-up for a 30-day trial of Astro Pixel Processor (APP). It replaces DSS and StarTools. It is super easy to use and it comes with a lot of videos. I use it and Dave (@turfpit) too.

Brian
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