Gradient Problems

Discussion in 'Image Capture' started by JoshuaHufford, Mar 8, 2018.

  1. JoshuaHufford

    JoshuaHufford Cyanogen Customer

    Joined:
    Oct 10, 2014
    Messages:
    318
    My setup is fixed in an observatory and has not changed in several years, I rarely even need to remove the camera and filter wheel from the scope. It seems over the last 2 years or so I keep having problems with gradients in my images, sometimes I don't see the gradient until I stack the images, or sometimes they are very apparent in a sub frame. Previously I would only take new flats when I started seeing dust spots in the images, now it seems I need to take flats more often because gradients start showing up out of nowhere, sometimes new flats fix the gradients, sometimes they don't. I don't image when the moon is up, light pollution is minimal at my location, NELM is usually 6.1-6.2 according to my meter. So I have a few questions, maybe someone has an idea of what might be going on.


    If nothing is changing in my optical train, what could cause gradients to just start showing up in my images?


    Could something possibly be wrong with my camera causing this?


    Is it possibly thin clouds that I'm not noticing?


    My equipment is this.


    Intes Micro MN-65 6.5" F5.5 Mak Newt

    Sbig ST-8300M

    FW8-8300 filter wheel. Astrodon LRGB, Baader Ha, SII, OIII


    I've uploaded an example so you can see what I'm talking about.

    https://www.dropbox....005Lum.fit?dl=0

    This is a calibrated image, as you can see there is a pretty strong gradient on the side of the image. Interestingly the next night I imaged, which was about 2 weeks later because of weather, using the same calibration files has the gradient there, but isn't nearly as strong. Here is that example.

    https://www.dropbox....006Lum.fit?dl=0

    When I started seeing this I took new flats and that seemed to fix the problem. My system has pretty strong vignetting, which flats always takes care of, but as expected the image is bright at the center and falls off at the edges. I didn't have this problem until a few years ago.

    The other interesting thing is every image from the night of the first example pretty much looks the same. I would think if it was clouds it would at least vary somewhat.

    Thanks for your help.
     
  2. Doug

    Doug Staff Member

    Joined:
    Sep 25, 2014
    Messages:
    4,421
    The most obvious cause of gradients would be... light.

    Is your observatory in a truly dark sky site? Do the gradients appear when you're imaging closer to the horizon?

    Any possibility of light leaks into the system from equipment in the observatory? One thing to possibly watch out for - some webcams have a "night vision mode" that use infrared illuminators. You can't really see them, but they're blindingly bright to a CCD.

    Is the gradient the same in all filter bands?

    If you divide your night up into four segments, and stack images from each of the four separately, do you see the same gradient in each? If it changes that would suggest an external light source, as the telescope pointing changes through the night.
     
  3. JoshuaHufford

    JoshuaHufford Cyanogen Customer

    Joined:
    Oct 10, 2014
    Messages:
    318
    Truly dark? No, but still pretty good.

    http://www.cleardarksky.com/lp/RtlSkObMOlp.html?Mn=CCD

    I can't think of anything in the observatory that would be putting off any light IR or otherwise, there are 3 IP cameras, but none of them have IR.

    I see the gradient in all broadband filters, but I haven't seen it in narrowband.

    Thanks for the suggestions, I'll have to do some more investigating. Intermittent problems are so much fun.
     

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