Focal Reducer Notes
  In these notes I'm going to try and clear up some misconceptions about using the 6.3 focal reducer.
                                        Also the design philosophy of the UNIVERSAL ADAPTER TUBE

    To start with, I think we have to look at the original design concept and application.  This is not an easy thing to figure out because there are no notes available as to what the designer was trying to do. The only thing we have are some typical applications to start with. In the Meade catalogue we see the Focal Reducer attached to a 35 mm camera via. a short extension tube ( 50 mm long ) with the focal reducer attached directly to the telescope using the 2" sct threads. Taking this information, we find that the spacing from the lens to film plane to be about 106 mm not the much used 85 mm that seems to be commonly thought. The extension tube is 50 mm long and the 35 mm camera with t-thread adapter is 56 mm thus 55 mm plus 56 mm = 106 mm. The F/ ratio for this setup is about F/6.45

  So for the rest of these notes we will consider 106 mm as the optimum spacing and all tests were run using a 10" Meade SCT .

  At this point we added a 2" ( JMI type ) focuser, sense the original spacing was based on the focal reducer directly attached to the telescope, we now move via. 2 " SCT adapter the focal reducer to the end of the JMI focuser, we must move the mirror forward to come to focus. Two things happen when we refocus ( A ) we move the mirror forward, from it's optimum design position changing the optical systems effective focal length, and ( B ) we also change the effective focal ratio of the focal reducer,  to something like F/7 .
  If we use the Optic TCF-s focuser ( 3" from the back plain ) we to move the mirror further forward to come to focus.
We are changing the effective focal length even further, and also the effective F/ratio of the focal reducer. We are also further degrading our image due to the extreme departure from the design point of the original optical system. At this point we could run out of focus range. ( The back focus issue )
 This degradation can be further exacerbated by the fact that not all SCT have the exact same optical design, such as the C-11 the C14 or the Meade 12" SCT. In some cases running out of back focus is a real problem.

  The conclusion that we make from this is that in order to get the best performance from the Focal Reducer we must have the right spacing and keep the Focal Reducer as close to the back plane of the telescope as possible.

The AO-7 Problem

 In the regular setup with the AO-7 - ST-8 - CFW-8 filter wheel the chip to front of the AO-7 is about 115 mm, it was quite evident that trying to add the focal reducer to this imaging train was a daunting task. Adding this to the end of a Optic TCF-s focuser you ended up with the imaging camera in another time zone. One solution would be to machine the reducer barrel to 2" o.d. so that it would fit into the focuser. The drawback is that you still needed a SCT thread to t-thread adapter to couple the reducer to the AO-7, this add several more mm to the spacing.
 The solution was to design an adapter with a T-Thread on one end and cell inside to hold the optics from a focal reducer mounted as close to the T-Thread end as possible. The AO-7 Adapter Tube solves this problem and adds only 3 mm to the imaging train. Allowing full insertion into the focuser it reduces overhang to a minimum. Depending on which focuser you are using it also shortens the distance to the back plane thereby improving the image quality and reducing the effects of vigneting.     See Fig. 1
Fig.1
Regular CCD Imaging Users

  At first I thought that a tube with a fixed spacing ( around 110 mm ) would be the next step for non AO-7 users, but I soon realized that the 110 mm spacing was not the best for all telescopes systems, so the design of the UNIVERSAL ADAPTER TUBE evolved. This allowed the user to custom taylor the focal reducer spacing to the telescope and focuser used.   See Fig. 2

Fig.2

Results and Conclusions

  Now with the design completed it was time to test this system to find out what the useful range of spacing and focuser combinations worked the best. Fig. 2 gives a good indication of how different spacing effects the focal ratio and if you go back to fig. 1 you can see how with a fixed spacing, the focal ratio is also effected by the back plain spacing
( different focuser combinations ).
 The next step was a series of images using the variable spacing to check on image quality ( vigneting, optical distortion, and elongation at the field edges )
  The test setup --- 10 " Meade SCT -- Optic TCF-S focuser -- Universal 6.3 optics Adapter Tube -- ST-8 camera  with CFW-8 filter wheel.  Results see Fig. 3

Fig.3

   A series of images were taken at short exposures ( to minimize tracking errors ) of the M-13 star field and these images were examined to determine vigneting and edge distortion at various spacing. At this point the results were quite suppressing, it showed that the usable range was much larger than thought and that the correctable (flat fielding)
was also much larger than is commonly considered. The main problem is that the analysis is very subjective and is only my view.
  What I would call the SWEET SPOT for the 6.3 focal reducer in this setup would be around 112-115 mm for the 10" SCT. Other focusers, such as the JMI would yield better results because of the closer back plane spacing.  This could also very with scopes like the C-11, Meade 12" and the C-14 because the optical design could be slightly different between models.
 The new focuser from FLI the DF-2 would yield the best results because there would be almost zero back plane issues.
Note : The DF-2 is in redesign so that the throat will take 2" accessories, this should be a winner and most useful to the AO-7 users.
 The key information learned from these tests is that not only the focal reducer spacing, but the spacing from the backplane effects the final focal reduction and the image quality in any SCT imaging system.

Ted Agos
Acorn Hollow Observatory

link to my Universal adapter Page   http://www.ziplink.net/~lester/FLR.html