SG-4 crop-factor and ZWOASI120MC-s comparision?

Discussion in 'My Astrophotos' started by ROBERT T SCHAEFER JR, Jul 20, 2018.

  1. ROBERT T SCHAEFER JR

    ROBERT T SCHAEFER JR Cyanogen Customer

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    The question is : "What is the crop-factor of the SG-4 Stand-Alone Autoguide ?"

    These two comparison images today thru a TeleVue TV 60is Scope I use for guiding while there was some
    light outside and I did both cameras in the TV 60is . When I 'open' them into MaximDL Pro the SG-4's image
    is 1/2 the size of the ZWOASI120 MC-S image with both at a 25% magnified view . The SG-4 is 640x480 and
    the ZWO is 1280x960 pixels . It looks like the ZWO's 'chip' is larger and the image has a little more image area
    ... and when I magnify the SG-4 image to 50% they look like they are about the same size and the leaves in the
    images that look alike are the same size . If I knew the 'crop-factor' of the SG-4 I could 'solve' a star image and realize the magnification and then do the same with the ZWO WebCam ... because they don't know the 'crop-factor' of it . ?? Then if I used a TeleVue 4XPowerMate with the ZWO WebCam I might know the magnification power of it because that's the PowerMate that does the best on Jupiter and Saturn .. I didn't try Mars with it yet but Mars is almost at a 24"arc-sec size .x
    These images were a Bin3 size and now at a "Double Size" .x
     
  2. Colin Haig

    Colin Haig Staff Member

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    Robert, you need to look at the data sheet / specifications for the sensors in each camera.
    This will tell you the size of the pixels, in addition to the number in each direction.
    Then, work out the field of view, based on the optics and the diagonal dimension of the chip.
    Then you'll know how many arcseconds per pixel the image scale is.

    You can use the Analyse - PinPoint function to work it all out for you, if you use the All Sky, and then refine it.
     
  3. Colin Haig

    Colin Haig Staff Member

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    Taking that further for you:
    "Crop factor" is used by photographers to tell the apparent impact of the chip size on a final image for a given lens eg 1.4x for APS-C vs full frame 35mm. It doesnt make sense to use that term in astroimaging.
    So toss that idea out of your head for the moment ;-)

    The pixels are 7.4um in the SG-4.
    You can use these tools to work out the math easily:
    https://astronomy.tools/calculators/ccd

    The focal length of the TV60is is 360mm. So that 4.24 arcseconds / pixel on the SG4, so your image will be 4.24 x 640 along the wide axis = 2713 arcseconds = 45 arc minutes wide.

    You multiplied it by 4x with the powermate barlow = 4 x 360mm = 1440mm focal length, so 1.06 arcseconds / pixel = 678 arcseconds = 11 arcminutes wide

    Repeat the calculation for the ZWO, and you'll see the difference. The ZWO chip at 1280x960 with 3.75um.
    The chip is almost the same physical size, but the pixels are half the size.
     
  4. JoshuaHufford

    JoshuaHufford Cyanogen Customer

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    Colin pretty much summed up what I was thinking when I read your post.

    When you are doing astrophtogrophy you need to forget terms like magnification and crop factor, they really don't apply. Learn what arc seconds per pixel are, and the area of the sky the FOV of your equipment gives you.

    This little program called CCD Calculator has always been a very useful tool for me.

    http://www.newastro.com/book_new/camera_app.html
     
  5. Colin Haig

    Colin Haig Staff Member

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    Joshua - that is a great book and handy utility! Would be good for Robert to add to his collection.
     

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