Multicolour exposures and their calibration

Discussion in 'Demo User Tech Support' started by RFA, Oct 11, 2018.

  1. RFA

    RFA Standard User

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    Location:
    Orciatico, (PI) Italy
    First of all many thanks to the prompt and quick help to my questions.

    1. It is not clear for me, whether the calibration processing is done immediately after the image is aquired and the file saved to the hd is an already calibrated image, OR first all images have to be quired, and after that in a second step they are processed with calibrating (bias, dark, flat)?

    2. In the manual you speak about "groups" of calibrations files. I am using only one CCD, 5 filters (C, B, V, R, I) and maybe two temperatures -25K and -15K. So, I am right if my groups looks like that:
    group BIAS,
    group DARKS (15s, 30s, 45s, 60s, 90s, 120s, 150s, each 25 darks), 8x25=200 images!, or should the steps be max bigger e.g. 15s 45s 90s 150s =>100 images
    group FLATS (25 flats each filter with 30-40% of saturation level of the obsered stars)

    3. Are all these "groups" located in one directory, or do they have been stored each in a separate directory?

    4. During one night I am aquiring 400 or even more image e.g. during winter period. So I would prefer to have no double files raw and calibrated. That connect to my first question.

    5. Photometry, I could not find out what kind of photometry Maxim is doing. In Europe we are doing differential photometry, for which you use three stars: the variable star, a compare star and a so coalled check star. In your photometry section I could not find how to do it with Maxim. Can you tell me more?

    Thank you in advance. RFA
     
  2. Doug

    Doug Staff Member

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    1. You can do either, but we always recommend saving your raw, untouched data. That way if there's a problem with your calibration files you can correct it later.

    2. Depending on how your camera behaves, you can use one set of dark frames for each temperature, (use the maximum duration you'll use for light frames), and auto-scale them. Some cameras won't do well with this because of their hot pixel statistics, but it works well for most. (I usually recommend a little more exposure for flats than 30-40%; most cameras are very linear up past 90%, so 50-60% is safe and better for SNR.)

    3. No, you can just dump all your calibration files into one folder. The Set Calibration command will automatically identify the files and create the groups based on the FITS headers.

    4. Not quite sure what you mean, but you don't have to take calibration frames every session. I usually reuse my calibration frames for months, especially the bias and dark frames. The hot pixel characteristics of the sensor are stable for long periods of time; eventually you'll get some cosmic ray damage to the array so you'll want to shoot new ones periodically. As for flat-fields, it just depends on how stable they are. If you get new dust donuts you'll have to redo them. I often do my calibration frames on cloudy nights.

    5. Yes it is differential photometry. We developed our algorithms with assistance from Arne Hendon, so you can rely on their accuracy.
     
  3. RFA

    RFA Standard User

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    Hi Doug, many thanks for that help. Sorry but some more questions, still:
    • How many flats you are doing per filter?
    • Using my Johnson-Cousins filter set and trying to collect the flats during twilight in the evening (B - V - I - R - C) the brightness decreases faster than I can imaging. So you are doing it during daylight on a cloudy day? What about the inhomogen sky? Do you move the scope after every shot?
    • Darks: You suggested taking only say 30 darks with the maximum exposure and than auto scale it. I am using 150s (very seldom 180s). For images with 15s (that´s normally my minimum) auto scale still can down scale it?
    Two more questions:
    • Plate solve and telescope sync: So far I unpark and open my scope, open and sync the dome and take an image of 5s with filter clear, than I press astrometry button in my software and ask where is the center of the FOV, after I receive the answer with a few seconds, I send a sync command to the scope and I know exactly where I am pointing. How do I have to do in Maxim?
    • Is it possible with Maxim generating automated sessions?
    Many, many thanks for further help. RFA
     
  4. Doug

    Doug Staff Member

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    1. The number of flats to use is really an issue of data quality, and is more of a science question than a technology question. I don't have sufficient information to make a general recommendation. Really you're looking at how much noise the flat field adds to the image; I'd compare the flats and see what the difference is as a percentage, and compare that to your required tolerance (precision of magnitude measurement).
    2. Twilight flats can be tricky; you need to be constantly adjusting the exposure. John Winfield has a nice Sky Flats Assistant plug-in available at http://winfij.com/maximdl/skyflats.html - also on that page is some excellent advice on where in the sky to shoot, moving the telescope, etc.
    3. Auto scale can scale up/down, but I recommend down on the assumption that the noise will only get lower that way. If you are concerned about how much of a scaling is being done, then create a second set of darks with a shorter exposure. MaxIm DL will automatically pick the dark group with the closest exposure time, to minimize the amount of scaling required. A great way to test how well it's working is to scale your long dark master and subtract it from your short dark master. Then you'll know if it works correctly.
    4. You can do that the exact same way in MaxIm DL. PinPoint Astrometry has an all-sky search feature if you need that capability.
    5. You may have to write some scripts to do that. There are of course third party scripting systems that work with MaxIm DL; ACP is worth looking at.
     
  5. RFA

    RFA Standard User

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    Location:
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    Many thanks Doug, you are a very big help.
     

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