Russell Croman Astrophotography  



Finding a good focal reducer for my 14" f/10 RC Optical Systems Ritchey-Chrétien telescope has been a long quest. I tried the AP 0.75x reducer, which works but produces soft images for whatever reason. Others have tried other reducers, but either find that they introduce similar degradation, or cannot be made to work in conjunction with SBIG's AO-7 unit due to the extra working distance it adds.

Takahashi recently came out with their own RC telescope, and it was noted that they offered a focal reducer for it as an optional accessory. Thinking that the optics of this would have been designed specifically with an RC in mind, I decided to give it a try. It is a pricey unit (list is $600), and it only fits their telescope. But the focal length of the optics was workable (barely) for AO-7 use, and after some thought and design work (and a couple of revisions), I was able to come up with an adapter that I had a local machinist make that satisfied my requirements:

  1. Uses the original Takahashi optical cell.

  2. Places the optics close enough to the correct distance with the AO-7 in the train for good results.

  3. Allows for setting the camera angle arbitrarily.

  4. Allows for adding spacers in front of and behind the optics, to accommodate a variety of configurations.

Here is the adapter, disassembled:

As you can see, the adapter consists of three parts: A T-mount adapter to fit on the AO-7 (or SBIG camera body if no AO-7), a slip ring for camera angle adjustment, and a male-male AP 2.7" threaded adapter.

Here is how it goes together. The T-Mount adapter is threaded to accept the optical cell:

The T-Mount adapter and optics assembly fits into the slip ring:

Finally, the male-male adapter threads into the front of the slip ring:

The T-Mount threads exposed at the rear go directly into the AO-7 body, replacing the "captain's wheel:"

The slip ring can be loosened, very similarly to the way the "captain's wheel" works, to allow the camera to be rotated to any angle. As you can see, it is knurled to make this easier. When it is tightened, the T-Mount adapter is squeezed between the slip ring and the male-male adapter, holding it firmly in place and square to the optical axis.

The male AP 2.7" threads at the front thread directly into the RC Optical Systems Fixed-Instruments Adapter (or appropriate spacer ring):

As far as optical performance goes, it seems to be quite good. I get only a slight degradation in star FWHM with the reducer in place, as compared to the native performance of the telescope. For example, on a good night here in Austin, I get native FWHM values of about 1.7". With the reducer in place, I get about 1.8". As a comparison, the AP 0.75x reducer degrades the FWHM to about 2.2"-2.4". (This isn't to say that the AP reducer is bad, just not particularly optimized, apparently, for use with an RC.)

Here are some recent images I took with the reducer. Click on the thumbnails to go to the images: