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Thread: Focusing Question for Astrophotography

  1. #1
    Join Date
    Mar 2004
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    Focusing Question for Astrophotography

    The basic question here is: How do you focus in your camera?

    Originally, I would focus as best I could by looking through the viewfinder of my SLR camera when mounted on the telescope, and then snap an image and preview it on the camera, looking to see if it was "acceptable." Note that the camera body was used as the "eyepiece" (the eyepiece was taken out and an adaptor was put on the camera to allow it to function as the eyepiece would).

    But now I actually looked at the software that came with my camera and I found that I can feed directly into my computer. So, what I did was image a bright star (I think I used Capella) and looked at the images on my computer. I then adjusted the focus in one direction, by pressing the focusing button on the telescope I was using for just long enough for the motor to kick in to move it, and then I re-imaged.

    I kept this up until I got as small a stellar disk as I could (signified by when the disk started to get larger). However, this corresponded to only about 10 arcsec, which to be honest, just sucks. It's about half the diameter of Saturn's disk these days.

    I know the seeing was better than that, for when I looked at Saturn with an eyepiece instead of my camera, I was just able to make out the Cassini Division, and everything was very crisp about the rings and Saturn's disk. But my camera showed fuzz for Saturn.

    I also know that the camera is very capable of producing higher quality images since I've used it for many months in the day and the focus is always very crisp when shooting in the same file resolution (I use highest quality JPEG when I shoot, though I'm just starting to experiment with camera RAW, though the focusing was no better in this mode).

    So after that long-winded explanation of what I'm doing, can people suggest ways to improve this poor focusing, or possibly suggest another culprit to look into?

    Thanks in advance.
    Last edited by stu; 2006-Jan-16 at 08:08 AM.

  2. #2
    Join Date
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    I never tried focusing with a computer, so I can't help you there.

    I think that a bright star, Jupiter or the Moon's edge will be helpful if you can find your object again after focusing (not a certainty if you are into "faint fuzzies". Alternatively, this supplier in Germany (which I can recommend) offers a special eyepiece which instead of the usual 1 1/4" barrel has a T2 thread at the front. You can thread the eyepiece to your T2 ring, look through the eyepiece, focus, thread off the eyepiece thread on the camera, and shoot. The supplier garuantees that the eyepiece will be exactly parfocal with your camera. I own the piece and found it very helpful, event though threading T2's (no, I won't go into that discussion ) in the dark can be a pain.

    (Remark to moderators - I hope this is not seen as advertising. I just bought most of my equipment from this supplier and I liked the way they treat customers: Competently, friendly, and individually. No call center "how can I be of help to you" stuff. They have English pages and deal in English, too).

    Another possibility is to buy a viewfinder extension that works as a magnifying glass. I've been prowling ebay for one fitting my old Canon A-1 equipment, but no luck so far.

    Then there is the Scheiner disc, something I haven't tried. Sounds like quite some fuss to me, and as far as I've looked, they aren't cheap if you don't build them yourself. Knife-edge focussing is undoubtedly at the high end - it requires a lot of patience.

    If you have a SLR camera, exchanging the matte screen in the viewfinder for a clear one will be helpful at least if a bright object is around.

  3. #3
    Join Date
    Mar 2004
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    Quote Originally Posted by Arneb
    I think that a bright star, Jupiter or the Moon's edge will be helpful if you can find your object again after focusing (not a certainty if you are into "faint fuzzies".
    I really don't want to spend money on this, especially since the telescope I'm using isn't mine (it's a University's) and it has some finiky settings and sizes (e.g. 2-inch eyepieces).

    I'll try your suggestion about focusing on other objects. Perhaps I'll do a star first, and then I'll re-focus on the particular object that I'm viewing, as long as it has something bright and point-like in it (like the Orion Nebula has those bright trapesium stars, or the moon, or Saturn ... but not a galaxy).

    Anyone else ever have this happen and can suggest a way out of this problem that doesn't involve extra equipment?

  4. #4
    Quote Originally Posted by stu
    I really don't want to spend money on this, especially since the telescope I'm using isn't mine (it's a University's) and it has some finiky settings and sizes (e.g. 2-inch eyepieces).

    I'll try your suggestion about focusing on other objects. Perhaps I'll do a star first, and then I'll re-focus on the particular object that I'm viewing, as long as it has something bright and point-like in it (like the Orion Nebula has those bright trapesium stars, or the moon, or Saturn ... but not a galaxy).

    Anyone else ever have this happen and can suggest a way out of this problem that doesn't involve extra equipment?
    Try googling for a Hartmann Mask (or click here )

    You can make one at home out of cardboard and it helps a lot.

  5. #5
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    Hmmm. That looks interesting. I'll talk with the telescope administrator about rigging one up. Thanks.

  6. #6

    Vibrations

    How quiet is the University. When I was Loyola in the Big City a bus two blocks away would register on the siesmograph in the Science building. Many vibrations around a telescope will effect it's performance. The longer the exposure, the more vibrations collected. (Just a thought.)

  7. #7
    Join Date
    Mar 2004
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    427
    Quote Originally Posted by Fr. Wayne
    How quiet is the University. When I was Loyola in the Big City a bus two blocks away would register on the siesmograph in the Science building. Many vibrations around a telescope will effect it's performance. The longer the exposure, the more vibrations collected. (Just a thought.)
    I took the photos during Winter Break when nearly no one was on campus. Also, the "focusing" pictures I took of Capella were taken at 1/20 sec, so I don't think vibrations were an issue for that. Same with my Saturn pictures -- taken between 1/30 and 1 sec long, and all the same focusing quality.

    Vibrations could be for long exposures, though.

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