Terrestrial telescope photography problem

Discussion in 'Photography' started by Chemiker, Sep 4, 2012.

  1. Chemiker

    Chemiker Guest

    I've noticed that my Phoenix 500mm mirror (Samyang) lacks contrast and
    color saturation both on film and on digital. So far, I've written it
    off to poor quality and the nature of the beast (the Matsukov mirror
    design). My wife has an 8" Meade mirror telescope with equatorial
    drive, and I decided to try it (rough equiv: 2000mm FL) and found the
    same problems with that instrument. Drive inactive, of course.

    Is there some technique or hardware I need to get this beast up to
    reasonable lens performance? Meade is not junk, I hear, so it must be
    me and my lack of knowledge. The main problems I see are poor
    contrast, lack of color saturation and poor definition (part of which
    may be due to heat shimmer, urban haze, or shutter vibration.)

    All comments appreciated, meaningful ones in proportion.

    Alex
     
    Chemiker, Sep 4, 2012
    #1
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  2. It is indeed the nature of the beast, and the effects
    you see are inherent in the design. The folded light
    path using mirrors results to a lighter weight and
    shorter lens for any given focal length, and those
    attributes are usually good. But the secondary mirror
    blocks the light path at the center... and that has
    several negative effects.

    Consider how most lens are not quite as sharp when used
    wide open, and get sharper as the aperture is made
    smaller, which is because light from the outer edges of
    the front element are progressively blocked and when
    only rays from the central portion are used the lens
    gets sharper (until diffraction sets in).

    Well, a mirror lens does exactly the opposite! The
    secondary mirror blocks the sharper focused center rays
    and only the outer portion of the lens passes light.

    The effects are not exactly what you might expect
    though. It affects low frequency spatial detail most,
    which causes the low contrast and the low color
    saturation. But that doesn't necessarily make the image
    that much less sharp at higher spatial frequencies where
    we see fine detail.

    The end result is that while even the best catadioptric
    lens will exhibit the low contrast and lack of color
    saturation inherent in the design, a good lens can still
    produce images that are sharp enough, and particularly
    with digital photography the contrast and saturation are
    relatively easy to work with in processing.

    If you shoot JPEG the camera should be configured for a
    "vivid" image. Higher contrast, brightness, saturation and
    sharpening. If you shoot RAW, just make those adjustment
    when converting to an image format, and expect that sharpening
    with Unsharp Mask will be very effective.

    At best though, there is still a trade between light
    weight and size against image characteristics. The
    donut shaped out of focus highlights are there. The
    lack of an adjustable aperture and the lack of Auto
    Focus are other drawbacks. But... you can easily carry
    and hand hold a 500mm or 800mm mirror lens!
     
    Floyd L. Davidson, Sep 4, 2012
    #2
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  3. Chemiker

    PeterN Guest

    Have you tried use of the levels or curves adjustments in your photo editor?

    I know the built in adjustments for contrast, saturation and vibrance
    may not give you what you are looking for.
     
    PeterN, Sep 4, 2012
    #3
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