dark band seen on light frames

Discussion in 'Image Processing' started by Peter Brack, Nov 2, 2018.

  1. Peter Brack

    Peter Brack Cyanogen Customer

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    I have noticed for some time a dark band on one edge of my light frames. Sometimes it is on the left side of the image, some times at the top of the image. It is meaning I have to crop the images a lot
    I am using SBIG STX 16803 with FW7-STX filter wheel and STX off-axis guider.
    The band is right across so didnt think it would be anything to do with the guider.

    The flat images don't seem to have this on them

    Have linked 2 light and a flat below:

    https://www.dropbox.com/s/deuflwd7500w6qp/NGC 2070-0004Ha.fit?dl=0

    https://www.dropbox.com/s/rbag18d0ikfukss/NGC 2070-0004SII.fit?dl=0

    https://www.dropbox.com/s/yn4vq939o72fjpu/AutoFlat-PA000-Halpha-Bin1-001-East.fts?dl=0

    Peter
     
  2. Colin Haig

    Colin Haig Staff Member

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    Hi Peter - first off -these are going to be great when done!
    In the SII image, is the banding around Y=828 ? I'm not entirely sure I'm seeing what you're talkng about, but it looks like there is a ribbon of slightly darker image across there horizontally. Maybe you could give us coordinates and screen stretch settings to better see it.
    What's the optical train - eg what scope, any reducer/flattener etc in front of the FW and 16803 ?
    Am I correct in assuming you acquired in MaxIm 6.18 and then brought into PixInsight 1.8.5 ? Am just wondering if somehow its a procesing artifact.
     
  3. Peter Brack

    Peter Brack Cyanogen Customer

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    Hi Colin. Good to hear from you. Yes well I was hoping would be a good image!. Having a few issues , this being one of them.
    The band is at the top of the image, but unsure on how you measure the Y= 828- Yes have taken image in Maxim 6.18 and have processed in PixInsight 1.8.5
    I also had master lights which did not look like they had the flat divided out as you could still see the dust motes:

    https://www.dropbox.com/s/o286ymb7fo9dsvd/Image81_DBE.xisf?dl=0
    This is not fully processed, so very rough, but does show the dust motes and the band at the top of the image
    Scope is Alluna RC-18 , has field flattener, no reducer.
    Peter
     
  4. Colin Haig

    Colin Haig Staff Member

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    In Maxim, the mouse pointer will give you coordinates. Make sure you have the info window open.
     
  5. Doug

    Doug Staff Member

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    Looks optical, i.e. vignetting.

    It's really not possible for the same problem to cause this on the top and on the side, intermittently. Unless you rotate the camera.

    Possibilities:

    Something in the image train blocking light on one side - perhaps the off-axis pickoff mirror? Check for that.

    Shutter misbehavior? Dismount the camera and check that the shutter works reliably on repeated long and short exposures.

    Problems with flat-field light source not properly illuminating the full aperture?
     
  6. Peter Brack

    Peter Brack Cyanogen Customer

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    Thanks Doug

    Would the off-axis mirror go right across one side, or just a part of one side?
    Seems to be consistent across filters. I don't see the band on the sky flats- unsure what that means as far as cause.
    Will have to take things apart and see if cameras shutter is a problem, and also try adjusting the off-axis mirror
    I seem to have consistent dust motes in the sky flats across various filters- would this mean the dust is more likely on the camera chip? What is best way to clean that ?

    Peter
     
  7. Colin Haig

    Colin Haig Staff Member

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    To me, the dust looks like its on the filters or the field flattener. They would be smaller if on the chip/chamber window.
     
  8. Peter Brack

    Peter Brack Cyanogen Customer

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    Yes thats true. The field flattener would make more sense and the dust gives a very similar pattern on the three narrowband filters that I made the flats for ,which would go against the filters I would've thought. And the flattener is more likely to be getting the dust as it is in front of the filter and Off Axis AG
    Will have to check that.
    For some reason my flats on this image didnt go away from the lights on calibration which i am trying to figure out, using PixInsight. Will try with MaxIm and see if any difference. Haven't used with maxIm as have to log into my remote computer to do it, and that is slower.
     
  9. Peter Brack

    Peter Brack Cyanogen Customer

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    how would you remove the dust?
     
  10. Colin Haig

    Colin Haig Staff Member

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    Doug will probably have good suggestions on this.

    Generally I've found the Giotto Rocket Blower really helpful for lens and photographic equipment - you can get a good puff of air where you want it.
    This will get rid of a lot of surface dust that isn't stuck on.
    Start with blowing the dust off the front and back of your reducer, and then see how clean a flat is.
    If you have to do filters, then you'll need to open the camera in a relatively dust-free environment (I use large zip-loc plastic freezer bags) and remove the filter carousel, to do both sides.
    If there is crud blowing doesnt remove, then it's time for Kimwipes (dustless lintfree wipes) and a cleaning solution of 100% distilled water and possibly a drop of dawn dish detergent to 1L water. There are probably a lot of better suggestions
    You'll want to wear gloves (non-powdered, antistatic - nitrile or nylon. Cotton in a pinch) .
    If you need to get rid of something particularly nasty, then you need a bit of lab-grade 100% pure ethanol in with the distilled water.
    Don't use benzene or any other cleaning solutions.
    If the CCD window or chip itself need cleaning, you migh want to get the professionally done.
     
  11. Doug

    Doug Staff Member

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    Those donuts are so large they must be well in front of the camera, so probably your reducer is really dirty. Try blowing the dust off first. Use a can of "dust-off" compressed air, follow the instructions (i.e. keep vertical and don't shake, or liquid propellant might come out and leave a residue).

    Colin's other suggestions are good. One drop of dish soap in a quantity of water can work, just make sure you rinse well. Only wipe the glass surface when it is wet, and very gently - NO pressure. Use a very soft paper towel and use a new surface on every wipe (keep folding it). Rinse and then sit it vertical and let the water drain off. If you don't use distilled water then pick off the last few drops with the edge of a paper towel, or you'll get drying spots.
     
  12. Peter Brack

    Peter Brack Cyanogen Customer

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    Should’ve re read yours and Colin’s repies as I just used the isopropyl alcohol on everything after using the dust off spray and I think I may have sprayed residue on things although hopefully the alcohol and wipes helped get rid of that . I think I will have to go back and re-clean the field flattener again
     

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