Why does the SBIG auto dark function take the Dark Frame first?

Discussion in 'CCDOPS and SBIG Universal Driver' started by gmdruga, Oct 25, 2015.

  1. gmdruga

    gmdruga Standard User

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    I own a number of cameras but am only a casual and occasional imager. I have always wondered why the auto dark function takes the dark frame first, and can you change it somehow to take the light frame first?

    The reason .....as a casual user who only does this when time permits I don't have dark frame libraries and image with relatively quick setups near urban areas with imperfect sky's. Sometimes in the dark frame capture timeframe I experience image drift from center, declining sky conditions like clouds, vehicle lights etc......when I am set to take a frame I really wish it took the light frame first.

    Thoughts or solutions?


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  2. Joseph Zeglinski

    Joseph Zeglinski Standard User

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    Hi,

    As many will probably tell you, the reason for the Dark Frame First is that long exposures heat up the sensor, and the electronics driving it. In that case, the temperature effects on the Dark, for example a 15 minute Dark, will still have the same temperature for the second sub-exposure, the LIGHT frame. Now both are taken at the same warmer temperature. The reverse way, the Light would have been taken at a slightly cooler temperature than its corresponding slightly heated sensor Dark.

    Besides, you are not suppose to waste precious sky time taking Darks. Take only Light frames. The Darks can be taken as a full series while you are packing up, or even during the trip back home. The slight chip temperature difference, or minor change of ambient night temps, I describe shouldn't be that big a problem.

    Joe
     
  3. Doug

    Doug Staff Member

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    Turn off calibration during imaging, and take your dark frame exposures later at your leisure. You really should be averaging multiple dark frames, otherwise you're adding about 41% extra noise to the images. So this is something you should be doing anyway. Just make sure you take them at the same CCD temperature.

    Auto-dark is handy for quick pointing exposures, focusing, etc. I don't recommend it for actual imaging.
     
  4. gmdruga

    gmdruga Standard User

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    Thanks Doug. I assume the exposure time and temp both need to be the same? BTW regardless I still don't understand the logic in auto it being taken first...my DSLR with noise reduction on certainly does not do that. Is Joe right? Seems to me the temp issue he talks about is minor (with cooling) and exists either way. And if you are using it for the reasons you state (i.e. Focus,quick pointing etc) wouldn't it still make more sense to take the light first...like my DSLR?


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  5. Doug

    Doug Staff Member

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    Yes temperature needs to be the same. Exposure time can vary if you take bias frames.

    Dark is taken first because it is used for ALL subsequent exposures with the same settings. DSLR does it every time (no cooling / regulation).
     

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