20316Re: Need focal reducer help!
- Dec 1, 2003Hi 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.
Wade Van Arsdale
--- In email@example.com, "Roger Hamlett" <roger@t...>
> > Hi,focal
> > 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.first,
> > 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 Celestronand
> > 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).CCD
> > 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 isspecific
> > 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 forbrands
> > that will work and are faster than F/6.3 (already have a .63 fromthis
> > Celestron that works fine--just need shorter focal lengths than
> > with such a long main focal length to work with!). Would like toget
> > down to at least F/4 or 4.5 if possible, as F/6.3 just is notF*.63. The
> > 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
> reduction acheived by a compressor, is dependant on it's own focallength,
> 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 berunning at
> about F*.5. The final focal ratio achieved though, is dependant onthe scope
> itself. As the back focus increases (which with the compressor andthe
> distances involved with the focusser, will be at perhaps 300mm),the scope
> itself give higher than design focal ratios. It's design ratio isF/11, and
> with the extended backfocus, it will probably be working more likeF/12.5,
> giving you a final ratio close to F/6.3. You will not get much moreextreme
> than the ratio you are allready using, without severe problemselsewhere
> (you will be getting some shading of the main CCD, by the 'pickoff' for the
> guide CCD, vignetting from the filters and compressor etc. etc..).long focal
> The 'key' of the AO7, is that it allows detailed imaging at quite
> lengths, by giving fast accurate guiding. You have two basicchoices:
> 1) Stick with the high focal ratio, and use the AO7.with normal
> 2) Remove the AO7 and image at perhaps F/4 (which should be fine
> You _can_ 'do without' the adaptive optics, when running at shorter
> Best Wishes
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