What would cause this in an image

Discussion in 'Digital Photography' started by Craig M. Bobchin, Mar 3, 2006.

  1. Hi all,

    I just posted an image I had taken last weekend while up in Big Bear.
    It was of the flame nebula/Horeshead area in Orion. This was with my
    LX200 @ f10 and my Canon 20d @ prime focus.

    This is a 2 min, ISO 3200 Exposure. You can just see the flame
    starting to peek out. But i'm concerned about the blue Swirly laserium
    effect and the blue circle around the star.

    What caused them? I'm leaning towards some sort of reflection possibly
    off the sensor, but I'm not positive. I've seen this in other astro
    images I've tried such as M45.

    Any ideas would be appreciated.

    You can see the image at:


    Craig M. Bobchin, Mar 3, 2006
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  2. Craig M. Bobchin

    ShibbyShane Guest

    Cool photo, what kind of lens did you use?
    ShibbyShane, Mar 4, 2006
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  3. Craig M. Bobchin

    Rich Guest

    You've got a centred halo reflection and an off-axis astigmatic
    reflection, cause by
    some optical element in the train. Obviously, the overexposed star
    image is the cause,
    and its possible your secondary mirror is slightly out of alignment
    which may have caused
    the off-axis reflection. Did you use any kind of compressor lens?
    Rich, Mar 4, 2006
  4. Craig M. Bobchin

    Ian Anderson Guest


    I am going to stick my neck out here. The purple/blue colour suggests
    that the artifacts are devived from coated refracting optical surfaces,
    viz the corrector plate. Also, next time you have your telescope set up,
    point it at a bright star (mag 1), remove the eyepiece and look up through
    the optical path and check for any internal reflections off the baffle

    On another aspect of your image, it is apparent from the trailed star
    images that you are not auto-tracking/guiding. Could I suggest that you
    drop your ISO to 1600 and take multiple images of not more that 15 seconds
    duration and preferably shoot raw. Select the best of the images and add
    them using a programme like Registax. You will of course need to have 16
    good images to yield the same depth as the one 2 minute expoure at
    ISO 3200. However you will greatly improve the signal to noise ratio,
    improve the resolution and reduce the effect of bright stars burning into
    big blobs.

    Ian Anderson
    Ian Anderson, Mar 4, 2006
  5. Craig M. Bobchin

    223rem Guest

    Very interesting approach. I suppose Registax computes pixel correspondence
    between two images.
    223rem, Mar 4, 2006
  6. Craig M. Bobchin

    Craig Guest

    No Lens, just a 10" LX200GPS telescope (Focal Length: 2500mm)
    Craig, Mar 4, 2006
  7. Craig M. Bobchin

    Craig Guest

    I know the star is the cause of the halo, is it also the cause of the
    off axis reflection? No Compressor, and I checked collimation before
    shooting and all was in order. Star tested perfectly.
    Craig, Mar 4, 2006
  8. Craig M. Bobchin

    Craig Guest

    I'ltry that check next time I have the scope out. It's been raining here
    so no go.
    You are correct no autoguiding was going on. I did a polar alignment and
    let 'er rip. I wasn't too concerned about trailing this time. I do have
    registax and use it regularly.
    Craig, Mar 4, 2006
  9. Craig M. Bobchin

    Ian Anderson Guest

    Ok, I'm glad I am guilty of preaching to the converted but I did notice
    from other replies to this thread that not everyone is aware of
    registax and its application, nor that a LX200 is basically a mirror lens
    on steroids but with less compromise.

    Ian Anderson, Mar 4, 2006
  10. Craig M. Bobchin

    Mike Thomas Guest

    Yup, a reflection artifact. Also, your mount (if you had one) isn't
    correctly polar aligned.
    Star trails galore. The pic isn't any better than what one would have
    gotten with film.
    Mike Thomas, Mar 4, 2006
  11. Craig M. Bobchin

    Craig Guest

    What is the cause of the reflection to create the swirly artifact? I'm
    aware of the trailing, I'm in the process of learning how to get good
    polar alignment.
    Craig, Mar 4, 2006
  12. Craig M. Bobchin

    Ray Fischer Guest

    You'll notice that the same shape is in each of the star images. My
    guess is that it's a periodic tracking error and that the large blue
    swirly is the tracking error and the bright start reflected
    Ray Fischer, Mar 4, 2006
  13. Craig M. Bobchin

    Mike Thomas Guest

    Nope. Poor polar alignment. Also, I am confounded as to why they use such a
    fast ISO.
    If they have good polar alignment, they shouldn't need anything faster than
    ISO 1600 and maybe a 3-5 minute
    exposure. My lowly CG5 can even muster 5 minutes with little star trailing.

    You should do more homework on astro-photography.
    Mike Thomas, Mar 4, 2006
  14. Craig M. Bobchin

    Phil Wheeler Guest

    Likely many of us could benefit from that. Astrophotog is likely the
    most demanding and time-consuming aspect of this hobby .. until it
    becomes second nature (e.g., folks like Pete Lawrence seem only need
    trip a shutter to get excellent images, but I know from my own limited
    results it is not that easy).

    Phil Wheeler, Mar 4, 2006
  15. There is no such thing as a perfect polar alignment. The actual point
    you need to align on depends on where you are imaging in the sky. If you
    do a "normal" polar alignment, which essentially places you at the
    refracted pole, there will be parts of the sky where you can't image for
    5 minutes at a constant tracking rate. High end systems deal with this
    by dynamically changing the tracking rate (on both axes) and/or by using
    active guiding.

    No doubt, there is some room here for improvement in the alignment-
    Craig admits as much. But very few systems are capable of producing
    pinpoint stars when operating unguided with 2000mm FL optics.

    The trailing in the image is not simply the result of polar
    misalignment. There is a movement pattern showing up that is constant
    across all the stars. I suspect what we are seeing is a combination of
    drift, vibration, and periodic error.


    Chris L Peterson
    Cloudbait Observatory
    Chris L Peterson, Mar 4, 2006
  16. Craig M. Bobchin

    Mike Thomas Guest

    I said 3-5 minutes..it's known as a range.
    Mike Thomas, Mar 5, 2006
  17. Craig M. Bobchin

    Ian Anderson Guest


    Incorrect polar alignment presents as slight drifting in Dec. This
    image shows trailing in RA. The trailing also shows the effect of periodic
    error in the drive train.

    Ian Anderson, Mar 5, 2006
  18. Mike,

    I used a fast ISO, because I was seeing hints of the blueness in slower
    ISO images, and I wanted to see what it could be. I thought at 1st it
    might be some reflection nebulosity, but I knew that there was no such
    animal where I was imaging. So I got curious and decided to see what I
    could see... So to speak.

    Craig M. Bobchin, Mar 6, 2006
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