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20315Re: Need focal reducer help!

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  • Roger Hamlett
    Dec 1, 2003
      > Hi,
      > I cannot achieve focus with a Meade F/3.3 focal reducer used with
      > the following:
      > 1) Celestron CGE-1400 OTA. 3910mm focal length @ F-11 when no focal
      > reducer used.
      > 2) CCD setup is an ST-7/CFW-8/AO-7 all attached together.
      > 3) JMI SmartFocus elecric micro-focuser.
      >
      > I've tried the following already:
      > 1) Mounted the reducer directly onto the visual back of the OTA first,
      > then attached the JMI, then the CCD gear. Racked the Celestron
      > main focus knob all the way clockwise (mirror moved all the way
      > back toward the rear). The image was moving toward focus, but
      > could not get all the way focused.
      > 2) Mounted JMI focuser first, then the reducer, then the CCD gear.
      > Same result.
      >
      > Note: I CAN get focus with the F/3.3 reducer through the eyepiece and
      > a star diagonal visually (NO CCD attached).
      >
      > Is my problem that the focal reducer lens is too far away from the
      > CCD chip with all the gear attached? Or, is it too close to the CCD
      > chip? It seems from the visual eyepiece check that the reducer is
      > too far away from the CCD chip. I can't get it any closer without
      > removing some of the CCD attachments, which is not feasible for me
      > (can't do without the color wheel or adaptive optics).
      Too far from the CCD chip.

      > Anybody have any ideas on what the cure for this is? If this specific
      > focal reducer is incompatible, anybody have any suggestions for brands
      > that will work and are faster than F/6.3 (already have a .63 from
      > Celestron that works fine--just need shorter focal lengths than this
      > with such a long main focal length to work with!). Would like to get
      > down to at least F/4 or 4.5 if possible, as F/6.3 just is not
      > desirable with some objects on the CGE-1400.
      >
      > Thanks for any help you all can give me on this!
      You will allready be using a compressor that is more extreme than F*.63. The
      reduction acheived by a compressor, is dependant on it's own focal length,
      and the distance to the target. Assuming you have the F*.63, mounted with
      the normal "captain's wheel" on the AO7, the compressor will be running at
      about F*.5. The final focal ratio achieved though, is dependant on the scope
      itself. As the back focus increases (which with the compressor and the
      distances involved with the focusser, will be at perhaps 300mm), the scope
      itself give higher than design focal ratios. It's design ratio is F/11, and
      with the extended backfocus, it will probably be working more like F/12.5,
      giving you a final ratio close to F/6.3. You will not get much more extreme
      than the ratio you are allready using, without severe problems elsewhere
      (you will be getting some shading of the main CCD, by the 'pick off' for the
      guide CCD, vignetting from the filters and compressor etc. etc..).
      The 'key' of the AO7, is that it allows detailed imaging at quite long focal
      lengths, by giving fast accurate guiding. You have two basic choices:
      1) Stick with the high focal ratio, and use the AO7.
      2) Remove the AO7 and image at perhaps F/4 (which should be fine with normal
      guiding).
      You _can_ 'do without' the adaptive optics, when running at shorter focal
      lengths.

      Best Wishes
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