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54527RE: [ccd-newastro] I'm about to smash my ST2k........Please stop me!

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  • George Sallit
    Oct 1, 2006
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      Frank,

      I had a look at your images and they do look a little noisier than my ST2K.
      Mine normally end up with just a few dark/bright pixels although I use a 10
      minute exposure time due to my closeness to a major airport. My initial
      thoughts are that your images look like mine when I need to refresh the dark
      library.

      A couple of comments, are you certain there is no light leak? It should not
      give a problem with hot pixels but may make things worse.

      I compared the average noise levels in your camera and the standard
      deviation and they were not too dissimilar. Mine were (for 600 secs) 1000
      +/- 188 @-20C and yours (for 1200) 2040 +/- 431 @ -25C. Your gain is 0.64
      mine 0.55 so the numbers are not too far out.

      Have you tried taking the darks at the same time as the lights, there can
      sometimes be a difference if the outside air temp is higher for the darks
      than the lights?

      The other way of really reducing noise it to use a random shift between
      images so that the noise in the images does not coincide. It is possible to
      do this automatically with Maxim and auto dither or manually with the hand
      set (a bit more of a chore).

      Using blink comparison is normally not too helpful, particularly if the
      programme out-ranges. I also get a noisy looking dark frame if I subtract
      two images. If you have two sets of dark frames then median combine the
      first and then the second and then subtract them, that may give a better
      clue.

      It may be worth (and I suspect you may have already) try starting from
      scratch in a methodical way. Start by using Maxim (I know a change but it
      may be something subtle).

      Firstly take a set (10) of bias frames and then a set (10) of dark frames of
      about 2 mins.
      Now take a set (10) of light frames of 2mins.

      Now median combine the bias and darks and use them to calibrate the light
      frames. The median combine the calibrated light frames and see what results.
      If the result has few hot/dark pixels then OK. If not measure all the frames
      and see if there is anything odd about them. I can give you comparison data
      on my measurements if that helps so we can try and track down the issue.

      Now try extending the times of the dark and lights and see what happens. You
      should look to see when things get worse (cliff-edge effect) or whether the
      gradually get worse. This should help track down the issue(s).

      Sorry I do not have the magic bullet but your calibration approach is right
      and those images are more noisier than mine.

      -----Original Message-----
      From: ccd-newastro@yahoogroups.com
      [mailto:ccd-newastro@yahoogroups.com]On Behalf Of Photon Collector
      Sent: 01 October 2006 07:14
      To: ccd-newastro@yahoogroups.com
      Subject: [ccd-newastro] I'm about to smash my ST2k........Please stop
      me!


      People, I'm at wit's end.
      I have been imaging with my ST2k imager for a couple years now. I have
      always had big problems with random hot (or bright) pixels. My images and
      darks have the usual consistent hot pixels that subtract out as they should.
      But both the images and the darks also have a large quantity of random hot
      pixels that differ from image to image and dark to dark. These random hot
      pixels do not subtract out because they pop up at random throughout the
      images and darks. The result is an image with many bright and dark pixels.
      Bright pixels remain in the image because the dark frame doesn't contain a
      corresponding bright pixel to subtract out. Dark pixels appear in the
      reduced image because the dark frame has bright pixels where there are none
      on the image.
      To prove to myself that the problem is real, I subtracted one dark frame
      from another dark frame shot in sequence. The resultant image should be very
      smooth. Instead it has bright and dark pixels, just like my images. As
      another test, I shot 10 darks in a row, of course all with the same temp and
      time settings. Then I opened them up in both Maxim and CCDSoft and ran a
      blink comparison. During the blink comparison, you can see that about 1/2 of
      the hot pixels are fixed in their position while the rest jump around all
      over the image. The same is true when doing a blink comparison of a series
      of raw images shot in sequence.
      I even went as far as upgrading my imaging chip from the older 2001 chip to
      the new and (supposedly) less noisy 2020 chip at great cost to me. The
      results are exactly the same.
      Here is an example of a completed image:
      http://publicmissiles.com/franku/2020_-25_f55_processed_NGC_7331.jpg
      It is a combination of 8, 20 minute exposures median combined after
      reduction using 10 median combined dark frames and the appropriate flat
      fields.
      If someone out there has a broadband connection (12 meg file!) and is
      willing to help, I have up-loaded a zip file containing a raw image, a flat
      field, and 3 dark frames. Try processing the image with the data I provided
      to see if you can get a clean image......I sure can't. Here is the zip file
      with everything you need: http://publicmissiles.com/franku/Raw_stuff.zip
      In the past I would spend an hour or two with the clone tool cleaning up
      every bad pixel. But I just can't believe this is what everyone else goes
      through. I never hear about problems similar to what I'm having. I hope it's
      just something I'm doing wrong but I can't imagine what it is. Is it
      possible that something in the observatory environment is causing the random
      hot pixels?



      ---Frank Rocketman Uroda

      [Non-text portions of this message have been removed]





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