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  1. #1
    FrankMcC's Avatar
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    Default Newbie question: focal ratio and "flats" for webcam photos



    Hi, guys. I am venturing into astrophotography, slowly getting my "feet wet". I have ordered an Orion StarShoot Solar System Camera IV and I will be using a Mac computer. The camera hasn't arrived yet, but I am studying up on processes and techniques. I was reviewing the Lynkeos Tutorials and found a couple of pieces of new information which confused me a bit.

    First, focal ratio. They make the statement when addressing limitations in collecting the raw data with a webcam: "To get the information you expect from your telescope, the detector shall have a sampling frequency at least twice the highest frequency in the image (Shannon's theorem/Nyquist's theorem in the spatial domain). This means that the pixel size must be less than one half of the Airy disk on the focal plane. In other words, you must use a focal ratio higher than 3.44 x p, where p is the pixel size in your webcam in micrometers..."

    I am using a Celestron NexStar 8" SE with a native focal ratio = 10. The Orion camera has a pixel size of 3.6 x 3.6 Microns. So the above says I need a focal ratio of at least 12.4 (3.44 x 3.6) which I can get either by using a 2x Barlow in front of the camera, or by using the 6.3 Focal reducer/corrector and the same 2x Barlow. The former would give f/=20 with a dimmer image, while the latter would give f/=12.6 for a brighter image, but pushing the minimum requirement.

    It looks like I cannot simply attach the camera for prime focus with the 'scope's f/=10 without getting poor picture quality.

    Am I understanding this correctly?

    Second, shooting "flats" for calibration. I understand "darks", but the explanation of "flats" says, "...a stack of many images recorded with the telescope looking at an evenly illuminated target with the same optical set up as the images to be processed, at a shutter speed giving a bright enough but not saturated image..."

    This makes no sense to me. Am I supposed to point the telescope at the moon, at a distant hillside the following day, a nearby subdivision with streetlights? What is an "evenly illuminated target" when it comes to the night sky?

    Thanks for any help.
    Frank
    Telescopes: NexStar 8" SE; ES ED80 on Celestron AVX mount
    Accessories: Celestron lense kit, f/6.3 reducer/corrector, Power Tank, UHC filter; ES Field Flattner
    Cameras: Orion StarShoot Solar System Camera IV; Canon 600D(Rebel T3i)

  2. #2
    FrankMcC's Avatar
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    Default Newbie question: focal ratio and "flats" for webcam photos

    Hi, guys. I am venturing into astrophotography, slowly getting my "feet wet". I have ordered an Orion StarShoot Solar System Camera IV and I will be using a Mac computer. The camera hasn't arrived yet, but I am studying up on processes and techniques. I was reviewing the Lynkeos Tutorials and found a couple of pieces of new information which confused me a bit.

    First, focal ratio. They make the statement when addressing limitations in collecting the raw data with a webcam: "To get the information you expect from your telescope, the detector shall have a sampling frequency at least twice the highest frequency in the image (Shannon's theorem/Nyquist's theorem in the spatial domain). This means that the pixel size must be less than one half of the Airy disk on the focal plane. In other words, you must use a focal ratio higher than 3.44 x p, where p is the pixel size in your webcam in micrometers..."

    I am using a Celestron NexStar 8" SE with a native focal ratio = 10. The Orion camera has a pixel size of 3.6 x 3.6 Microns. So the above says I need a focal ratio of at least 12.4 (3.44 x 3.6) which I can get either by using a 2x Barlow in front of the camera, or by using the 6.3 Focal reducer/corrector and the same 2x Barlow. The former would give f/=20 with a dimmer image, while the latter would give f/=12.6 for a brighter image, but pushing the minimum requirement.

    It looks like I cannot simply attach the camera for prime focus with the 'scope's f/=10 without getting poor picture quality.


    Am I understanding this correctly?

    Second, shooting "flats" for calibration. I understand "darks", but the explanation of "flats" says, "...a stack of many images recorded with the telescope looking at an evenly illuminated target with the same optical set up as the images to be processed, at a shutter speed giving a bright enough but not saturated image..."

    This makes no sense to me. Am I supposed to point the telescope at the moon, at a distant hillside the following day, a nearby subdivision with streetlights? What is an "evenly illuminated target" when it comes to the night sky?

    Sorry for the length. Thanks for any help
    Last edited by FrankMcC; 03-12-2012 at 09:49 PM. Reason: Duplicated message
    Frank
    Telescopes: NexStar 8" SE; ES ED80 on Celestron AVX mount
    Accessories: Celestron lense kit, f/6.3 reducer/corrector, Power Tank, UHC filter; ES Field Flattner
    Cameras: Orion StarShoot Solar System Camera IV; Canon 600D(Rebel T3i)

  3. #3
    FrankMcC's Avatar
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    Default

    Sorry; I posted the same message twice.

    If someone knows how to delete this one, they can.
    Frank
    Telescopes: NexStar 8" SE; ES ED80 on Celestron AVX mount
    Accessories: Celestron lense kit, f/6.3 reducer/corrector, Power Tank, UHC filter; ES Field Flattner
    Cameras: Orion StarShoot Solar System Camera IV; Canon 600D(Rebel T3i)

  4. #4
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    Default

    Flats are often done by draping a white cloth (Tshirt) over the front of the scope and aiming at the dawn sky, after a night of AP imaging.
    The idea is to get an image of any slight shadows from dust in your image train or vignetting from a camera sensor not fully illuminated by the objective. Lower ISO is used to keep from saturating any part of the flat image, as is a moderate exposure time of only a few seconds.
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  6. #5
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    Default

    I haven't had a chance to do any real photography yet, but have done a lot of reading on it. Not sure how to answer the first question (but would be interested in the answer myself.) As for flats, the general consensus seems to be using a white cotton t-shirt lit by a flashlight. Hope this helps.
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  8. #6
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    Default

    If you use a flashlight to illuminate the stretched tshirt, you need to be careful to use one that has a wide and even illumination pattern - something that most do not (just think of the bright central spot and the 1-more rings where the reflector focuses the beam).
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    Default

    AN EL panel has proven to be the best (but not cheapest) way of doing flats for me. No problem with uneven or slow/noisy exposures and no waiting for sunrise :-)

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    You can make a fake one by blurring the image you want to process (duplicating it first, of course ), smudging out any nebula or bright star remnants, and then subtracting it from the original image. Works like a charm!
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    Default

    Quote Originally Posted by FrankMcC View Post
    Second, shooting "flats" for calibration. I understand "darks", but the explanation of "flats" says, "...a stack of many images recorded with the telescope looking at an evenly illuminated target with the same optical set up as the images to be processed, at a shutter speed giving a bright enough but not saturated image..."

    This makes no sense to me. Am I supposed to point the telescope at the moon, at a distant hillside the following day, a nearby subdivision with streetlights? What is an "evenly illuminated target" when it comes to the night sky?
    For flats you don't point the scope at anything that has detail. It needs to be a totally featureless surface. And it must be evenly illuminated - you can't have one part brighter than another.

    In a perfect optical system, your flat image would be a totally featureless, even, light gray frame. You want to aim for as close to that as you can get. Any imperfections in the optical system will then show up as irregularities in the evenness of the frame. Specks of dust will show up as bark blobs. Vignetting will show up as uneven illumination. The objective is to have the only irregularities be the result of optical imperfections. The stacking software then uses this information to remove those irregularities from the light frames.

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  14. #10
    FrankMcC's Avatar
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    OK, I think I understand "flats" now.

    Anyone want to take a shot at question #1 on focal ratio?
    Frank
    Telescopes: NexStar 8" SE; ES ED80 on Celestron AVX mount
    Accessories: Celestron lense kit, f/6.3 reducer/corrector, Power Tank, UHC filter; ES Field Flattner
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