| How to suppress coma using a focal reducer with an SBIG AO7* |
* When using Peterson Eye Opener on a Meade 10" LX200 with f6.3 focal reducer mounted in
Agos 2" adapter slid back to threads and screwed into T-thread of AO7, mounted on Mandel/Smith Muscle plate
mounted on SBIG CFW8, attached to SBIG ST10XME :-)
Figure 1 - equipment. See Observatory
for full-size image.
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References
Ted Agos: 6.3 UNIVERSAL Focal Reducer Solution
Ted Agos: Focal Reducer Technical Notes
Peterson Engineering Eye Opener
Steve Mandel and John Smith's Muscle Plate assembly
Chris Vedeler: F3.3 Focal reducers compared
Chris Vedeler: Schmidt-Cassegrain Mirror Stabilizer
| ___ Bottom Line First ___
Replace the SCT thread with a Peterson Eye Opener, insert focal reducer in Agos adapter, bolt AO7 directly
to CFW8 with Mandel/Smith Muscle plate, on SCT without mirror lock use Vedeler solution to reduce mirror flop,
and use an in-line focuser behind the rear mirror cell, or extend the 3.375" Agos 2" tube away from rear
cell of the scope by at least 1.875". Re-Collimate your system using out of focus stars in an open star
cluster, carefully centering the patterns to the center of the chip.
The use of an in-line focuser behind the rear mirror of an SCT, as delivered on new
Meade LX200 systems, improves the issue discussed below. For imagers who do NOT use an in-line focuser,
this article discusses a focal reducer problem that can occur when using an AO7 bolted to a color wheel using the Mandel/Smith
Muscle Plate, CFW8, and SBIG camera assembly. If you wish to use an f6.3 focal reducer, you are
already out of the designed focal length of the focal reducer. By increasing the position of the
focal reducer with respect to the rear of the SCT mirror cell, you effectively increase the main telescope's
focal ratio (from f/10 to f/11 or greater). This in turn REDUCES the negative affects of being out of
tolerance with the focal reducer, since the convergence of the light rays from your main optical system is now reduced.
Figure 1 - comparison of diffraction rings as taken at f10 (no reducer), f6.3 where
optics are set back 2" from rear mirror of SCT, and f6.3 where optics are flush with back of 10" LX200.
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Figure 2 - demonstration of the change in collimation/coma resulting from sliding the 3.375" long
Agos Universal Adapter (2") tube 1.875" out of the Peterson Eye Opener, so that the f6.3 focal reducer and
AO7/CFW8/ST10XME assembly are all now 1.875" farther AWAY from the rear mirror cell. (Target is M15)
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Problem Statement
For those of us unable to purchase a Ritchey-Chretien' design telescope, we often
purchase the popular Meade LX200 or the Celestron Schmidt-Cassegrain Telescopes (SCT). This optical
design can be used to good advantage, especially with smaller imaging areas, if proper care is taken.
When using a small chip, coma is rarely an issue. As we expand our hobby, the size of our
imaging chip increases. In upgrading from an SBIG ST-237 to an SBIG ST-8e, and then to an SBIG ST-10xme,
I quickly understood I had a problem when attempting to apply a focal reducer.
An internet search of available solutions provided four (references above):
1) The Agos UNIVERSAL focal reducer allows placement of
the focal reducer lens assembly as close to the AO7 as is possible.
2) The Peterson Eye Opener reduces telescopes vignetting by replacing the SCT threads with a
larger 2" receiver.
3) The Mandel/Smith Muscle Plate allows direct mounting the AO7 onto the CFW8 assembly, reducing the
back-focus by 0.14".
4) Solution by Chris Vedeler to reduce mirror flop.
Armed with my new solutions, I proceeded to image. What I found was that when using the focal reducer,
sometimes I had coma, and sometimes I did not.
After reading Ted Agos' technical notes (see reference above), I had been left with the impression
that the optimal position for the Agos adapter was flush against the Peterson Eye Opener. Indeed, this
reduces vignetting and produces the fastest optical system, faster than f6.3. I imaged with this configuration
for 2 years. Sometimes I had coma free images, and sometimes not. It turns out that this _IS_ the optimal
position as long as you are not introducing the AO7 into your optical path.
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Analysis
Reference the chart posted on Ted's page, showing
the relationship between distance behind the primary mirror cell (the "design" configuration for the
Meade/Celestron f6.3 focal reducer). When attaching the AO7, the optical path can be 115mm, versus a
focal reducer focal length of about 90mm. This longer light path results in a faster focal reducer, per
Ted's chart. This also exacerbates coma on images taken with a larger chip (i.e. ST8e, 10xme).
Figure 2 - Comparison of full fields from ST10XME, using 3x3 binning.
AO7/Agos tube flush with Peterson
AO7/Agos tube pulled out 1.875" from Peterson
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Note in particular the rounder stars in the image on the right.
The optical system should be RE-COLLIMATED
after determining the desired optical config. This was NOT done here.
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Solution
As already stated in the Bottom Line above, it is highly advantageous to always use a behind the rear cell
in-line focuser. Then you can use Chris Vedeler's solution to reduce image shift, use Ted Agos' solution to
mount the focal reducer inside a 2" tube slid back very close to the AO7 T-thread, use Steve Mandel and
John Smith's solution to direct mount the AO7 to the CFW8 to reduce the light path, and use Peterson's Eye
Opener to prevent vignetting in the system. If you only have the Agos solution with the Peterson Eye opener,
then when using an f6.3 focal reducer, carefully pull the Agos tube further out in the Peterson Eye opener.
Shim-ing: another issue is that the Peterson Eye opener is manufactured with a slightly larger receiver
than is needed by just the Agos reducer. It may be useful to shim the Agos tube. This can be done with scotch
tape or metal shim material available at a hobby shop. The heavy AO7/CFW8/ST10xme assembly should not
rock in the Peterson receiver, or else the CCD will not be perpendicular to the light path.
CAUTION:
On my version of the Peterson Eye Opener, I replaced the nylon bolts with large metal
bolts and large plastic knobs. It is always advisable to attach a small loop of wire between your $$ expensive
camera assembly and your telescope, to prevent accidents where the assembly falls!
Michael Good October 2005
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