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Moravian G3 16200 bright band on top part of image

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#1 Manav

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Posted 25 August 2019 - 05:25 AM

Hi guys,

 

I'm new to CCD imaging and am using the G3 16200 camera with Stellarvue SVQ100.

 

I've noticed that all of the images I'm taking have a really bright band on the top part.

 

Have attached example herein:

 

1. https://www.flickr.c...03/48616379356/

2. https://www.flickr.c...03/48616364391/

 

Stretching the images shows saturation evenly starting from the top of the image.

 

Can anyone help point me in the right direction of how I could troubleshoot this?

 

Regards

Manav



#2 BenKolt

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Posted 26 August 2019 - 02:38 PM

Hi, Manav, and congrats on the new 16200 camera!

 

Are these calibrated or raw frames?  I'd make sure to go through the full calibration process, step by step to compare images, dark frames, flat frames, etc.

 

What filters were you using?

 

Is it possible that you are simply seeing sky gradient?  What were your sky conditions like?

 

Best Regards,

Ben



#3 BKMaynard

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Posted 26 August 2019 - 03:00 PM

I have the same ccd camera and was seeing the same thing when using Maxim DL. I switched to The Sky X and that all went away. My guess is the ASCOM driver for the camera has some issues. Are you using the SIPS program that came with the camera? 



#4 buckeyestargazer

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Posted 26 August 2019 - 03:01 PM

Sure looks like sky gradient to me.  



#5 Ron in Michigan

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Posted 26 August 2019 - 03:10 PM

If I see that it was due to my dome slit being in the frame.  FLATS will eliminate that - but I prefer not to use flats.  Then astroflat pro would eliminate it or gradient removal tool.



#6 Manav

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Posted 26 August 2019 - 04:19 PM

Hi Guys - First of all thanks for your response. I'm new to CCD imaging so I apologise in advance.

 

I've linked the FITS file for Dark frame which is a 300sec exposure at -20 taken using SIPS software as well as a 0.2sec bias frame taken at -20 in the below link. (Ben - I used L filter for those raw images but the gradient is present in all LRGB)

 

https://drive.google...whJ?usp=sharing

 

Unfortunately, I do not yet have a flat field generator so I did not have any flat field shots.

 

I didnt even consider light pollution; I have been shooting all images at South - South West at 40deg above horizon from the backyard so that could be a very high possibility. I'm using a SVQ100 so the image is inverted which could explain why the light gradient is on top of the image.

 

Just to confirm if this was a CCD issue we should see the same glow in the darks and bias am I right?

 

Once again thanks for your responses.


Edited by Manav, 26 August 2019 - 04:29 PM.


#7 BenKolt

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Posted 26 August 2019 - 05:00 PM

Manav:

 

Sorry, I probably won't have time to look at your file anytime soon.

 

But to answer your own question, tell us if you see this glow in your dark frame.  If it shows up there, then you may indeed have a CCD issue.  But if it's absent, then this could be something else: sky gradient, something reflecting in your scope, horrible light leak, etc.

 

Getting back to calibration, you will want to process with darks and flats.  Don't skip this step!  You'll see why!

 

Ben



#8 Manav

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Posted 27 August 2019 - 04:42 PM

Hi Ben - I stretched it using Nebulosity "Levels/Power stretch" tool and here is what it looks like.

 

https://www.flickr.c...03/48632123968/

 

There is some brightness on both sides; but it seems common for 16200 cameras for what I've seen on the net.

 

I tested for light leaks i.e. took some darks with room light on and the dark frame was consistent so I'm now narrowing it down to LP gradient.

 

I'm sure you will agree.

 

On a side note how do I get rid or these LP gradients? Should i shoot in Ha or are there ways to do that?


Edited by Manav, 28 August 2019 - 01:07 AM.


#9 BenKolt

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Posted 27 August 2019 - 05:56 PM

Hi Ben - I stretched it using Nebulosity "Levels/Power stretch" tool and here is what it looks like.

 

https://www.flickr.c...03/48632123968/

 

There is some brightness on both sides; but it seems common for 16200 cameras for what I've seen on the net.

 

I tested for light leaks i.e. took some darks with room light on and the dark frame was consistent so I'm not narrowing it down to LP gradient.

 

I'm sure you will agree.

 

On a side note how do I get rid or these LP gradients? Should i shoot in Ha or are there ways to do that?

 

Manav:

 

Your dark frame looks normal to me, very similar to my KAF-16200's darks.  The edge glow along the two horizontal sides is typical.  This underlying dark pattern that you see here will also be in all your light frames, hence the need to calibrate properly (see below)!

 

Sounds to me that you've narrowed it down to good old sky gradient, so you'll definitely want to learn ways to remove it!  This is very much a normal step that many of us urban and suburban imagers have to take!  (And I suspect even dark sky folks need to do it from time to time.)  Don't let the LP dissuade you from ever using broadband filters!  You'll need to (1) spend sufficient integration time to boost your SNR, (2) perform careful calibration, (3) learn how to remove those gradients and (4) figure out the extent to which you can use (or can't use) the broadband filters when the moon is full or near full.  (Of course, when the moon is out, most of us will prefer to look for narrowband targets.)

 

Let me stress my strong opinion, based on a growing amount of experience, that you should calibrate your light frames, particularly when using sensors such as your KAF-16200 CCD.  Done properly, it can only improve your imaging.  If you are interested in discussing this further, please let me know.

 

As for background removal, I use PixInsight (PI), which is a powerful astronomy imaging software platform, but admittedly not simple and fast to learn.  It certainly takes time to figure out and master!  The two main processes for gradient removal in PI are AutomaticBackgroundExtraction (ABE) and DynamicBackgroundExtraction (DBE).

 

ABE can typically be used in a more automated fashion, as the name suggests, where a polynomial function model of the background gradient is measured from the image and then removed.  I usually like to start with this one, particularly when my image is mostly composed of dark sky background areas.  There are various control settings, but starting out I recommend using it with default values and just adjusting the degree of the polynomial.

 

DBE is, as the name suggests, dynamic in that you place points around the image that you decide are supposed to be dark background, and the background gradient model is then interpolated through these points.  This is often the better choice when the image is composed of widespread nebulosity, or I sometimes use it for tricky situations that ABE doesn't handle well.  DBE is a process that can easily be abused, and so practice is necessary to learn best how to use it with sparing number of points, taking advantage of symmetries when present, tweaking the many settings.  In the end, these processes are designed to take care of broad gradients like you seem to have here.

 

Undoubtedly, the image processing software of your choice should have some kind of background removal tool similar to the ones that I described.  It's been too long since I last used Nebulosity, so I don't remember if it has such a tool.  You'll have to let us know that.

 

Good luck!

 

Best Regards,

Ben

 

P.S.  I forgot to specify that gradient removal is typically done after you have combined (stacked, integrated) your frames into a final image.  You can remove the gradient from individual frames first if you wish - nothing prevents you from doing this - but that's not the usual way, and you might mess up the combination step.


Edited by BenKolt, 27 August 2019 - 06:01 PM.


#10 Manav

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Posted 28 August 2019 - 01:28 AM

Thanks for everything! I'm still learning so baby steps for now smile.gif

 

Next steps will be as follows:

 

1. I'll start with shooting straight up pointing at meridian once the weather clears up. This should equalise the gradient.

2. I will order the Aurora Flatfield Panel and start taking flats.

3. Finally, start reading up on calibration "how to" in nebulosity.

 

Thanks everyone once again and stay safe.

 

-Manav 




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