1. Mr Grumpysaurus
    Mr Grumpysaurus
    I have a canon 450D, I have had it mounted on my telescope but want to try some astrophotography from my tripod, can anyone help with what would be the best settings and exposure times.
  2. TCampbell
    This is probably better asked in the regular forums because we are limited to 1000 characters per message here. I'll be brief.

    You can apply the "Rule of 600". The catch is that rule was designed for "full frame" cameras and you are using an APS-C crop-frame camera. That involves a slight tweak of the rule.

    The basic rule: Divide 600 by the focal length of your lens. That results in the number of seconds you can expose and still have pinpoint stars (they won't elongate due to the rotation of the Earth).

    Since you have an APS-C crop-frame sensor, you have to multiply the focal length by 1.6 (the crop-factor for YOUR sensor -- that never changes) BEFORE you divide it into 600.

    E.g. if you have a 10mm ultra-wide angle lens, it would be:

    600 ÷ 16 = 37.5

    With a 10mm lens you can expose for 37.5 seconds and still have pinpoint stars.

    The general formula is: 600 ÷ (focal-length X crop-factor) = max-exposure-length
  3. Mr Grumpysaurus
    Mr Grumpysaurus
    Thanks for that, I will take my calculator out with me next time. I will let you know how I get on.
  4. Carl D
    Carl D
    Mr. Campbell - I have heard of the Rule of 600 before and the several folks I asked to explain as good as you did looked at me like I was talking a foreign language. But, I do have a question for you and would appreciate it if you could explain this to me. I use a Canon T3i (600D) for both Prime Focus and A-focal shots. I have never been able to accurately figure how many seconds I have before trailing starts. I have a Celestron 8SE mostly with 9mm, 15mm and 25mm Plossl EP's. Since I have no idea what a crop factor is, I am lost. Also, would the f ratio of the scope matter? Mainly because I also use a Celestron 6.3 FR at times. Thanks for your help.
  5. watson1
    Your Canon 600D is also an APS-C sensor.
    i.e. the same 1.6 factor applies.

    take your 600 rule and divide by that 1.6 to get 375
    That's your new exposure time rule for your 600D.

    Prime focus using a 750mm focal length telescope:
    take 375 and divide by 750mm and you get a maximum exposure time of 0.5seconds
    You need a very bright target, or a tracking drive.

    Eyepiece exposures:
    If you use your eyepiece to focus on the camera's focal plane and you have chosen the front focus distance to equal the back focus distance, then the image size at the telescope focal plane equals the image at the camera focal plane. This means you have a magnification that is the same as the prime focus telescope.
  6. watson1
    If you move the EP closer to the telescope FP, then the back focus distance beomes bigger and the image at the camera focal plane becomes bigger. You end up with a magnified image at the camera.
    Small changes bring about big changes in magnification. With shorter EP lengths you can easily get 10x the prime focus image. Don't go too big !

    Now find your EP projection distance and determine the magnification you have/want.
    If your prime focus allowed 0.5seconds for no visible trails, then at 2x magnification your need to divide that 0.5seconds by 2x to get 1/4second.

    I did a set of example calculations for my EPs.
    A 16mm eyepiece with front and back focus distances of 32mm, I would get 1x the prime focus magnification.
    Changing to 25.6mm front focus and 42.67mm back focus gives 1.67x magnification.
    22.4mm & 56mm gives 2.5x
    20.8mm & 69.3mm gives 3.33x
    19.2mm & 96mm gives 5x
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