Allan, can you vary the distance significantly and still get good imaging results (with different effective f ratios)?
It seems to be a very flexible reducer with the distance not being particularly important with visual use.
The AP info suggests 85mm separation from the shoulder of the reducer to the chip for 0.67 reduction.
http://www.astro-physics.com/tech_su...r-techdata.pdf
The CCD Telecompressor (CCDT67)
The focal length of the CCDT67 CCD Telecompressor is 305 mm or 12.0” Again, using the formula introduced above, a distance of
101 mm or 4.0 inches gives the stated 0.67X compression. (305 - 101) / 305 = 0.67 Because the CCDT67’s focal length is much
shorter than the 27TVPH’s, an equal change in distance will have a greater effect on the compression. If we shorten the distance by
35 mm as we did above for the 27TVPH, the compression factor becomes 0.78X. (305 - 66) / 305 = 0.78 Changing from 0.67X to
0.78X is certainly more drastic than changing from 0.75X to 0.80X as was the case for the 27TVPH. Please note also that the
CCDT67’s more aggressive compression makes it less suitable for faster instruments.
We strongly recommend using the CCDT67
only on instruments with focal ratios of f/9 and higher.
The CCDT67 takes up 16 mm of the distance in your calculations.