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  1. #1
    cmassa's Avatar
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    Default RGB alignment issue in DSS - Confusing problem I can't figure out



    Here are three images from the same stack of subs. The first is 16 x 900s, iso800 lights with master darks, flats, bias (from other runs) of the Leo Triplet using my AT65EDQ and Canon 550D (Baader mod). RGB is very misaligned on the right as you can see. The left looks more or less OK.

    The second image is the same stack of lights without any dark, flat, bias. I don't notice the problem.

    The third is using the same light frames but now using the original darks, bias, flats subs (which were used in creating the masters for the first). The problem is there but the separation is much less.

    TRIPLET-1.jpg triplet-2.jpg triplet-3.jpg

    All images were treated the same in PS, a change to 16 bit, 2 levels stretch and output to JPG to post here. In DSS, all stacking done with Kappa-sigma clipping.

    If anyone can help me with this I would greatly appreciate it. I can't see why the colors would be so misaligned in only a small portion of the image and vary in restacking using the same files.

    If anyone wants the original autosave.tif files I've uploaded them in a zip to dropbox:
    https://www.dropbox.com/s/3pdlvvfrk5...20Autosave.zip

    The images were taken last night as the Triplet came up over the trees in the east until it crossed the meridian and the mount stopped - 4 hours total exposure.

    Here is one of the raw light frames:
    https://www.dropbox.com/s/l8inhqyb08...5m31s248ms.CR2

    Thanks for any help.
    Chris
    Last edited by cmassa; 01-25-2014 at 10:32 PM.
    OTA's: TPO 10" f/4 Newtonian, Celestron 8" EdgeHD SCT, 0.7x Edge reducer lens, Astro-Tech AT65EDQ, Astro-Tech 50mm guide scope
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  2. #2
    cmassa's Avatar
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    Default Re: RGB alignment issue in DSS - Confusing problem I can't figure out

    I've looked closely at the raw image and notice the stars are rounder on the left of the image. Is this a mechanical misalignment issue, possibly related to my Baader mod? Although if the sensor or the filter were not aligned correctly I would think the problem would affect the entire image.

    Could stray light from the side do this? I am set up on the back patio. There are a few neighbors who like to leave lights on. This image of M42 from a few days ago was taken with the same setup, just no light frames because I hadn't made any yet. All the star across the image are the same shape. There are some guiding issues but they are all trailing in the same direction, the same amount across the whole image. Could the light frames be a problem? I make them by stretching a white shirt across the scope and laying my iPad over it with a flashlight app on a white screen. Maybe I need to dim the iPad some? On the histogram, the red is about 2/3 from the left with green and blue around 1/2 or a little less.

    Sorry, just thinking out loud trying to figure this out.

    M42-wide-2.jpg
    OTA's: TPO 10" f/4 Newtonian, Celestron 8" EdgeHD SCT, 0.7x Edge reducer lens, Astro-Tech AT65EDQ, Astro-Tech 50mm guide scope
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  3. #3
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    Default Re: RGB alignment issue in DSS - Confusing problem I can't figure out

    I'm doing some more images of M31 tonight and was looking at one of my raw subs. I seem to have a problem with differential focus on the image. Stars on the right look like rings if zoomed in where on the right they are more solid. Here is a jpg (which has been stretched a bit):

    m31-sample.jpg

    and here is a raw image on Dropbox:

    https://www.dropbox.com/s/a1mvxklsd6...6m14s126ms.CR2

    Ideas? How do I go about working this out and how do I fix this? I bet this is causing the RGB alignment problem after stacking. The focus if off on the right side of the image. I've checked all my connections, everything is tight.

    Chris
    OTA's: TPO 10" f/4 Newtonian, Celestron 8" EdgeHD SCT, 0.7x Edge reducer lens, Astro-Tech AT65EDQ, Astro-Tech 50mm guide scope
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  4. #4
    Phil Leigh's Avatar
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    Default Re: RGB alignment issue in DSS - Confusing problem I can't figure out

    Chris - next time you are imaging, rotate just the camera 180 degrees and see if the problem swaps sides... if it does then I would say it has to be the sensor/filter not being at 90 degrees to the scope across its surface...

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  5. #5
    Phil Leigh's Avatar
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    Default Re: RGB alignment issue in DSS - Confusing problem I can't figure out

    If you are using the refractor...

    The other possible causes of differential focus are:
    1) focuser droop - the focuser is racked so far out that the weight of the camera is causing the focuser drawtube to sag under gravity (this is fixed using a T2 extension tube between camera t-ring and nosepiece)
    2) camera not square in focuser - always insert the camera into the focuser with the scope vertical and tighten the screws a little at a time moving from screw to screw - never do one screw up tight and then move to the next one.

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  6. #6
    Phil Leigh's Avatar
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    Default Re: RGB alignment issue in DSS - Confusing problem I can't figure out

    On step 2 perhaps I should say "with the scope vertical facing downwards so the camera is inserted from the top"


    I'm not exactly clear why you are seeing different results when stacking with different calibration files.
    What is strange on the first image is that the distance between the red and blue versions of the stars appears to related to the brightness of the star... and that they are pretty circular which they aren't in the raw frame. It's almost like DSS is getting confused...

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  7. #7
    cmassa's Avatar
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    Default Re: RGB alignment issue in DSS - Confusing problem I can't figure out

    I think you are right about the focuser. I'm used to the SCT which is an easy focuser to deal with. I'm using a Hotech camera adapter so it keeps it centered. I was more careful last night to check the focus when changing targets as it seemed to need tweaking quite a bit each time - not really needed with the SCT. I let it run on the Leo Triplet again last night and just my initial check looks OK. Have to head to work this morning so I'll do a better check later.

    I went with just the refractor and the guide scope to try and improve guiding. The load on the mount was low (10 lbs or less). The numbers on PHD looked good but there are still oval stars. Grrr! The way it comes, there is no way to balance the refractor in DEC, probably part of the problem. The dovetail it comes with is short so I can't move it in the saddle. Being a short scope, the rings are already back as far as they can go.

    Chris
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  8. #8
    Phil Leigh's Avatar
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    Default Re: RGB alignment issue in DSS - Confusing problem I can't figure out

    I use an ADM dovetail adapter (female to male) on my scope (which has a very short vixen dovetail) - this gives me a lot of wiggle room for balancing.

    The extension tube trick works great. My drawtube is only 10mm out of the scope at focus! There is no drawtube sag.

    The Hotech SCA is very good.

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    Default Re: RGB alignment issue in DSS - Confusing problem I can't figure out

    When I got the scope I asked at the shop and they recommended an extension tube which I got (35mm). Unfortunately, its too long. I can't focus in when using it. The focuser is only out 30 mm with the camera attached.

    I'm not getting it on the dovetail adapter, how that would help. I guess I'm not finding the one you are talking about.

    Thanks,
    Chris
    OTA's: TPO 10" f/4 Newtonian, Celestron 8" EdgeHD SCT, 0.7x Edge reducer lens, Astro-Tech AT65EDQ, Astro-Tech 50mm guide scope
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  10. #10
    Phil Leigh's Avatar
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    Default Re: RGB alignment issue in DSS - Confusing problem I can't figure out

    Quote Originally Posted by cmassa View Post
    When I got the scope I asked at the shop and they recommended an extension tube which I got (35mm). Unfortunately, its too long. I can't focus in when using it. The focuser is only out 30 mm with the camera attached.

    I'm not getting it on the dovetail adapter, how that would help. I guess I'm not finding the one you are talking about.

    Thanks,
    Chris
    You're probably OK with 30mm but I'd put a 20mm extension tube in there personally. It's just one less thing to worry about / go wrong...

    The ADM adapter I use for my little 3 inch vixen foot is this one: ADM Dual Saddle Upgrade for HEQ5, NEQ6 & AVX - First Light Optics

    Note that an AVX version is available...

    This thing is so strong you can grip the little foot with just an inch of footplate.

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