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20316Re: Need focal reducer help!

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  • uwpf23
    Dec 1, 2003
      Hi Roger,
      Thanks very much for your help on this. That is the same thing that
      I (being a CCD novice) had concluded--too far away from the CCD chip.
      I'm considering one other option--I have a Celestron ST-80 F/5
      refractor mounted on the main 14 inch OTA. If I can find the proper
      T-Ring to adapt it's focuser tube to the captain's wheel of the AO-7,
      (even though the AO-7 wouldn't be needed here--just don't want to
      mess with taking it apart every time!), I will try imaging and
      manually focusing with an aperature mask through the ST-80 for the
      very large sky objects (M42, etc...), then just go with the setup you
      recommended below for the fainter, smaller objects. I may wind up
      having a similar distance problem with the ST-80 though, even without
      the reducer and JMI focuser: focus may be too far away from the CCD
      chip to rack the drawtube in far enough to get there. I may not have
      any other choice but to take the AO-7 off when using the refractor.
      Any comments on this setup with the ST-80 refractor would be greatly
      appreciated, as well.

      Thanks also for the information on the 14 inch OTA F-values with the
      .63 compressor. This will aid greatly in fine-tuning the proper
      binning mode to get a more reasonable arc-sec/pixel value with such
      long focal lengths! Looks like I'll be using 3x3 binning most of the
      time with this long focal length.

      Clear skies,
      Wade Van Arsdale


      --- In ccd-newastro@yahoogroups.com, "Roger Hamlett" <roger@t...>
      > > 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
      > > 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
      > > 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
      > > 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
      > > 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
      > > focal reducer is incompatible, anybody have any suggestions for
      > > 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
      > > with such a long main focal length to work with!). Would like to
      > > 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
      > 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
      > 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
      > giving you a final ratio close to F/6.3. You will not get much more
      > than the ratio you are allready using, without severe problems
      > (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
      > 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
      > lengths.
      > Best Wishes
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