Ritchey-Chrétien Telescope Considerations

updated: 07/14/2005


Native operation

Native operation refers to operating an RC scope without any reducers.  For optimal performance, it is necessary to have the spacing between the primary and secondary mirror close to their design center.  Since this is spacing can not be measured easily, the manufacturer usually specifies a back focus distance (BFD) for each telescope.  It is then up to the user to achieve this BFD through a combination of draw tube extension, fixed spacers, etc.  Fine focusing is then best done by slight movement of the secondary focuser, usually electronically.

I have seen BFD's from 8.9" to 10.3" and the longer the better is generally the rule.  A longer BFD generally gives the user more flexibility as will be seen below.

One of the challenges in developing one's instrument stack-up that contributes to the BFD is determining the actual spacing and distances of various pieces and parts.  Below is a list of measurements I and others have made for various instruments, adapters, etc.  If you have any to add, or find any errors, please let me know and I will keep this updated as a service to other RC users

Item * Inches     Item * Inches
SBIG ST-8/9/10 3 0.66     SBIG T-thread - SCT adapter   0.71
SBIG CFW-8A 2 1.00     AP ADA204 2.7" - SCT adapter   0.73
SBIG AO-7 (SBIG spec)   3.50     AP ADA2003 2.7" - 2" nosepiece   0.75
Muscle Plate 4 0.19     AP ADA2013 2.7" - 2" nosepiece   0.38
SBIG AO-7 D-block + 2xT-thread 1 0.34     Van Slyke ZPTA 2" nosepiece   0.00
SBIG AO-7 Mirror box 1 2.52     Optec 2" nosepiece   0.25
SBIG AO7 captain's wheel 1 0.75     Optec Intelligent Filter Wheel   0.92
RCOS FIA base 6 0.25     RCOS T-thread - 2.7" adapter   0.38
RCOS Instrument Rotator (PIR) 5 2.81     AP AP16T 2" nosepiece   0.13
RCOS Extension tube   2.50     Baader Planetarium BP16   0.08
RCOS Extension tube   2.00     SBIG CFW-8/AO7 adapter plate   0.31
RCOS Extension tube   1.75          
RCOS Extension tube   1.25          
RCOS Extension tube   0.75          


  1. These are actual measurements and for some reason, do not add up to the SBIG spec for the assembly.  I cannot explain why this is so.
  2. This dimension includes the impact of the output window
  3. This dimension includes the impact of the filter glass.
  4. This dimension represents the increased thickness over the standard CFW-8 cover plate.
  5. The actual dimension from the front of the PIR to the back of the mounting flange is 2.94".  The number reported here assumes a .13" back plate counter-bore i.e., how much the mounting surface for the PIR is inset into the back plate.  If your counter-bore is deeper, for example .38", then the corresponding table entry would be 2.56".
  6. The actual dimension from the front of the FIA base to the back of the mounting flange is 0.38".  The number reported here assumes a .13" back plate counter-bore i.e., how much the mounting surface for the PIR is inset into the back plate.  If your counter-bore is deeper, for example .38", then the corresponding table entry would be 0".


As an example, my setup consists of an ST-8E/CFW-8A, AO-7 hard mounted to the Muscle Plate, SBIG captain's wheel attached to an AP ADA204 and using an RCOS PIR gives:

0.66 + 1.00 + .19 + 2.30 + 0.75 + 0.73 + 2.81 = 8.44".

The 10RC truss I currently am using has a BFD of 9.24".  So my required spacer to achieve the correct BFD is 9.24 - 8.44 or 0.80".  I use the standard RCOS 0.75" Extension Tube, which gets me within .05".


Focal Reducer Considerations

To goal of a focal reducer is to achieve a wider field of view without compromising the optical performance of the telescope.  This means one must maintain the mirror spacing to its design point.  When a focal reducer is added to the imaging chain, it is possible to unintentionally move considerably away from the design point, compromising performance.  Russ Croman has done a very nice analysis of a way to optimize the focal reducer location and is linked here.  While this analysis was done with an eye toward the Astro-Physics .75x reducer, the concepts are extensible to other reducers and designs.  I put together an Excel (2000) spreadsheet that steps a user through the spacing considerations that is located here.


By knowing your BFD, the focal length of the reducer and the specifics of your image chain as shown in the previous section, you may be able to insert a focal reducer and maintain your BFD but this is not always possible.  Please consult the spreadsheet for your specifics.  It will depend on your working distance, 'D' in the above drawing.  The AP .75x reducer has a focal length of 700 mm.  Here are some examples using my optical chain as above, with a BFD of 9.24".  These are defaults in the spreadsheet but can be changed to accommodate your system.

Arrangement Spacer1 Spacer2 Reduction
ST8/CFW8/MP/AO7/Captains wheel/AP SCT adapter/PIR N/A N/A N/A
ST8/CFW8/MP/AO7/Captains wheel/AP SCT adapter/FIA N/A N/A N/A
ST8/CFW8/MP/AO7/ZPTA nosepiece/AP ADA2003/PIR N/A N/A N/A
ST8/CFW8/MP/AO7/ZPTA nosepiece/AP ADA2003/FIA 0.00 0.99 .750
ST8/CFW8/MP/SBIG T-thread/AP SCT adapter/PIR 0.72 0.00 .782
ST8/CFW8/MP/SBIG T-thread/AP SCT adapter/FIA 1.75 0.73 .745

Spacer1 is a spacer located between the camera and the reducer.
Spacer2 is a spacer located between the reducer and the OTA.
Reduction is the calculated value with the specified spacers in place.
N/A means the proposed arrangement can not be used and still maintain BFD.

Experimenting with this spreadsheet can give some interesting insights.  Increasing the BFD gives much greater flexibility of instrument arrangements with reducers.  Using lower profile attachments between the camera and OTA makes AO-7 usage with my BFD possible that is not possible otherwise.  To use the PIR requires considerable more BFD than the 9.24" the 10RC has.  And having more BFD can permit achieving reductions closer to .75x with a judicious selection of spacers.


Celestron .63x reducer/corrector

7/24/2003 Update: Between the monsoon storms, I was able to check out a Celestron .63x focal reducer with my RC.  Using the above techniques plus an estimation of the reducer's focal length, I calculated that with the proper spacing, I would get a reduction of .65x.  I determined I needed a space of around 1.13" between the reducer and the ST-10XME/CFW-8/Muscle Plate combination.  The T-thread-SCT adapters I had were .75" long (Captain's wheel for AO7) 1.95" (Celestron #93363A) and .71" SBIG T-thread adapter.  It turns out that a Pentax camera T-ring has male and female T-threads and is .38" thick!  I put this combination together and checked out some image scales to determine the reduction ratio.  Here is what the chain looks like.


From left to right: 2.7" extensions, ADA-204 AP-SCT adapter, Celestron reducer, Captain's wheel SCT-T Thread adapter, Pentax T-ring, Muscle Plate/CFW-8/ST-10XME.  (The shiny spacer at the left is a .13" stainless steel ring made by Michael Grady to provide fine increment spacing adjustments.)

For my RC, the back focus was 8.95".  With the PIR inset by .25", the calculated spacer2 was .43" and spacer1 was 1.13".  If you are looking at the spreadsheet, some modification was needed to work with this reducer.  The revised one for this reducer is here.  Note that by changing the number of T-ring adapters from 0 to 2, you can change the reduction ratio from .69x to .61x.  Of course, appropriate changes must be made to spacer2.  For my testing, I used Case 2.  Case 3 does not allow a precise match since the ADA-204's length of .75" is part of the spacer2 requirement but it may be close enough.

Using this arrangement, I first focused a star.  I found that with the above choice of spacers, I was within .25" of the native focus position.  An image link in TheSky reported an image scale of 0.75 arc-sec./pixel.  Since the native resolution was .49 arc-sec./pixel, the reduction calculated by the ratio of these image scales was in fact .65x!

I was curious on the field flatness.  I took some light box flats.  The intensity variation across the field was an acceptable 5% or so and would easily be eliminated by flat fields.  Incidentally, the flat field exposure time had to be reduced by the ratio of .652 or .42 to maintain approximately the same flat field level of 20,000 ADU.

The seeing was pretty terrible with clouds overhead and some thunderstorms in the distance.  Star sizes were all over the place.  I took a quick image of M39 before I shut down.  While nothing definitive can be determined from the image, there was nothing bad obvious from a star size viewpoint.  I am anxious to check out the star sizes on a night of good seeing.  With those caveats, here is a 1-minute exposure of the vicinity of M39.

I realize nothing definitive can be drawn from this image but it sure is encouraging!

I believe this reducer has some potential that needs some careful testing under decent stars.  Based on setting up a FOV indicator in TheSky, it certainly will allow for better framing on shorter exposures of targets.  I am looking forward to experimenting with it further.



Aberrations can impact the focus and sharpness of the stellar images.  These aberrations generally arise from mis-alignments, which can include collimation errors (coma), improper back focus or mirror spacing (spherical aberration), improper indexing of indexed optics (astigmatism), tube currents, etc.  Morgan Wilson put me on to a really nice little program that allows one to see the impact of these kinds of errors.  Here is a link to Aberrator.  Don't be overwhelmed by the opening screen but play with it a little bit to see what can happen.

First, input your mirror size, obstruction and F/number.  The "Star" and "Intra/Extra" windows are most interesting to me.  Add coma and astigmatism to see what it does.  Peter Ceravolo did an analysis that was reported in an email on a private mail list.  In it, he analytically reduced the mirror spacing 0.3 inch from the design center, corresponding to a back focus change of around 2.3", and reported the wave front aberration ballooned to 1/2 wave.  Assuming the error is linear, and a nominal mirror spec is 0.2 wave peak-to-valley or so, then you should add as little distortion as possible due to spacing errors.  If we shoot for a 0.05 wave contribution due to spacing error, then we should get the spacing under 0.25".  Put in 0.5 error for coma and see what happens to the star image.  Add some astigmatism that could arise from index errors and see how the star pattern changes.  Of course

This program can give you a good indication of what can go wrong with improper alignment.  Of course, the better your seeing, the more important alignment is and the easier it is to see these problems.  Aberrator predicts what you would see with a definable FL eyepiece.



Here is a link to a collimation write-up I did Ritchey-Chrétien telescopes.  This procedure requires a center dot on the secondary, as supplied by RC Optical Systems, and the Takahashi Collimating Telescope.  The entire process is very easy to do and achieves good results.  I have used this procedure on at least 6 RC's with excellent results.


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