Resolved Arc of light in all photos

Discussion in 'STT Series' started by blireina, Sep 27, 2016.

  1. blireina

    blireina Guest

    I am recently, in the last two weeks, getting an arc of white light in the left lower quadrant of the photo. I have an SBIG STT8300, which is close on 3 years old. I thought it might be the filter wheel sticking, as when I moved the HDMI cables, I heard a clicking sound. I cleaned them with air, and replaced them. It worked fine for one evening, and then the whitish quadrant is back again. I enclose a FITS file.
    Any help or advice appreciated.
    Philip
     
  2. Doug

    Doug Staff Member

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    Please post a raw FITS frame showing the issue.
     
  3. blireina

    blireina Guest

    Here is the picture Philip
     
  4. blireina

    blireina Guest

    Doug
    I keep getting an error posting the FITS file. I have tried 3 - 4 times, but am clearly doing something wrong.
    Philip
     
  5. blireina

    blireina Guest

  6. William B

    William B Cyanogen Customer

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    Try editing the file name to remove all the extra (.) characters, something like " example.fit " should work.
     
  7. blireina

    blireina Guest

    Doug
    I tried editing to example.fit, but I got the same error message as before. I did send the file to:
    'Diffraction Limited - SBIG / Cyanogen Forum', and it went through to somewhere. That was a reply from me to you, as you had sent mail to my MS Outlook email.

    Philip
     
  8. Doug

    Doug Staff Member

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    You can upload it to ftp.cyanogen.com, using a utility like FileZilla.
     
  9. blireina

    blireina Guest

    Doug

    I have tried the FTP method, but it is the same as trying to transfer it on this site. I get an immediate error, the same as the other ones, and it will not send. I had reamed the file a couple of different names, but they had the FIT extension.I can open these files in TSXP and Maxim. I do all my developing with Images Plus.

    The file in question is a FIT file produced by SkyXPro, latest version. I enclose a screen shot of the FITS file, which shows the light band bottom left, and another lighter band to the right of it. This usually occurs during the first dozen or so 5 minute exposures, and then the pictures are more normal. It started a week or two ago, and is variable, for no obvious reason that I know.

    This picture is of SH2140. I rarely go over 5 minutes exposure time, as I live within 3 miles of an airport. I cool the camera to -30Centigrade, and they are guided.

    I would very much appreciate any advice.

    Philip
     

    Attached Files:

  10. Doug

    Doug Staff Member

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    Hmm... that looks optical. Maybe that's a pupil ghost. They tend to happen when you have concave optical elements close to the focal plane. Typically it's a reflection off the sensor (they're not 100% quantum efficient after all), that is re-imaged by an element that just happens to have the wrong curvature in the wrong place. They tend to produce an out-of-focus image of the telescope's front aperture.

    What optical system are you using? If you're using a focal reducer, try removing it from the system temporarily.
     
  11. blireina

    blireina Guest

    Thanks Doug,

    I think I understand what you are saying. But does your suggestion explain why the ghost is not always there. For what it is worth, my recent objects have tended to be North East, which I rarely pointed to over the last few years, as my house was in the way, until I moved the observatory a year or so ago. I took about 30 lights of 5 mins each of this object, but ONLY the first 7 - 12 pictures show this phenomenon - the rest are fine.

    The 'scope is an AP 155 mm F7, with a four inch AP focuser, and a Flip/flat device. There is a field flattener in there made by APM. I was going to use the AP flattener, but the APM was half the cost. I do have a reducer by APM, which I have not used with this scope.

    So perhaps the APM flattener does not like my system. It is easy enough to remove and try without it, and I am not sure I need it anyway with my camera sensor not being very big. But I have had it for over one year without this problem being noticed.

    I will let you know how things go, without the flattener.

    Philip
     
  12. Doug

    Doug Staff Member

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    The ghost would only appear when there is a bright star in or near the FOV.
     
  13. blireina

    blireina Guest

    Hi Doug,

    I took off the flattener, but the white part was still there in the pictures. I noticed my stars at the edges were a little deformed, due to the loss of the flattener.

    So, I can conclude that the flattener lens is not causing the problem. Correct?

    But, let me repeat that this phenomenon only occurs in the first few pictures, and then it goes.

    Your comment "The ghost would only appear when there is a bright star in or near the FOV" I am taking a piture, and so any bright stars in that picture would always be in the FOV.

    I will carry on using the camera, and I do allow it to cool to around -30 degrees centigrade. Any other ideas as to the cause of the problem. Should I send it in for a check up?
    Other than this intermittent loss of about the first 30-45 minutes of use, it seems to be fine. I use 5 minute exposures because of aircraft overhead from a near by airport.

    I will repeat that this is a recent thing (2-3 weeks). The only thing I found wrong was the HDMI cables seemed to not have a good connection, as when I touched them while the power was on, there would be a clicking sound, which I confirmed today.

    Your best advice much appreciated.

    Regards

    Philip.
     
  14. William B

    William B Cyanogen Customer

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    Couple of extra elimination ideas you could try:

    When the bright quadrant artefact is present take a dark frame, is the bright quadrant still visible in the dark frame? if so suspect uneven CCD cooling, if not then suspect optical path issue, out of position shutter or filters would cause a dark artefact rather than light so an internal light leak or reflection would be the obvious conclusion but from your comments above seems unlikely.

    When the artefact is present run a quick series through each filter in turn, is the artefact still present with each filter selected?

    With the camera at ambient temperature remove the camera from the telescope place it on a bench and set a continuous series of exposures of a minute or so using the luminance filter and turn on cooling just before you start the exposure run, keep the series running until the full target temperature is reached and the cooling is stable, look into the camera, use a torch or desk lamp to get a good view of the CCD chip through the CCD chamber window, does dew or frost form on the surface of the sensor as the CCD drops below freezing? if so reactivate the desiccant plug ( if present with your camera). Note: you are not assessing the images here, they will be saturated, you are just looking for dew or frost on the CCD chip as the temperature drops.

    Speaking as a QSI heretic I only cool my QSI 683 to -15 and do this in -10 deg steps over thirty to forty minutes, if I cool in one step straight from ambient to -15 then dew and frost forms on the surface of the CCD which gradually sublimates if left alone over the next hour but going down in steps no dew forms.

    William.
     
  15. Doug

    Doug Staff Member

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    In that case, it sounds like the humidity level in the chamber is borderline. The streaks must be caused by a faint trace of frost on the sensor.

    When you are close to the "dew point" inside the chamber, moisture tends to settle out on the front of the sensor when you first turn it on. But the structures at the back of the sensor are a little colder than the front glass, and moisture in the chamber gradually accumulates back there - and the humidity drops. Once the humidity is low enough, the front of the sensor dries out.

    I recommend baking the desiccant plug. Instructions on how to do that are in the manual.
     
  16. blireina

    blireina Guest

    Hi Doug and William

    Talking about icing and frost reminded me that I used to get that when I was cooling to -30 C. The camera no longer does that anymore, and it makes sense to me that this temporary white arc is likely dew or frost. I will recook the dessicant as you suggest, and the other things mentioned. I redo the dessicant yearly - is that enough?

    Also, do I need to go to -30C. It seems to me that the lower the better.

    Many thanks

    Philip
     
  17. Doug

    Doug Staff Member

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    Annual refresh is probably reasonable, but your mileage may vary depending on operating conditions.

    More is not always better. The dark current for the KAF-8300 is already pretty negligible at -20C. In general RBI characteristics tend to be worse at lower temperatures, though I've not quantified that for the 8300. (ON Semi/Kodak does not specify RBI performance for their sensors.)
     
  18. blireina

    blireina Guest

    Hi Doug

    Re doing the dessicant did the trick, I think. I also reduced my temperature setting to -20 instead of -30 C. Should I see any difference in picture quality?

    I did not think of this cause initially, as when the camera was new, I saw what I thought was ice in the picture, which took about 20 mins to go away. This new problem did not look like the ice I used to see. Anyway, it has gone!

    I want to thank you for helping me deal with this irritating problem.

    Philip
     
  19. blireina

    blireina Guest


    Hi Doug

    I thought I would follow up on the "arc of light" issue I was having. I had said that regenerating the dessicant had sorted it. That proved not to be the case.

    The key was that it was only on certain occasions. I found out that it invariably occurred when the telescope was pointing to the north. I suddenly realised yesterday night that I have a baby monitor video camera with 8 LED's around the lens, in the observatory, so it can "see" in the dark. The camera has always pointed north, and it clicked that infra red light would be picked up by the camera. So I disconnected the video, and the light has now gone. I have confirmed it comes back when I switch on.

    I am going to modify light, and reduce it so that I can still see inside the obs. It is nice to check the shutter closes when CCDAP sends the command.

    Regards

    Philip
     
  20. Doug

    Doug Staff Member

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    LOL yep, that will do it! Infrared is particularly nasty because we can't see it, but CCD cameras are extremely sensitive to it.

    Good catch.
     

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