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. The rest of the distance is in the nosepiece, filter wheel, CCD camera and any extensions you may use. Note also that, unlike the above example with the 27TVPH, the entire nosepiece length is a part of the light path with the CCDT67 since the nosepiece does not get buried inside another adapter relative to the telecompressor.
The CCDT67 has a clear aperture of 44 mm or 1.75”. At 0.67X compression, it will fully illuminate a 29 mm circle – 44 mm x 0.67 = 29 mm.
To give a real world example, let’s assume the following: We will set up a system with the CCDT67 @ 16 mm; our AP16T – 2”/T2 nosepiece @ 29 mm; a color filter wheel @ 25 mm; and a CCD camera @ 25 mm. 16+29+25+25 = 95 mm. (305 – 95) / 305 = 0.69X 44 mm x 0.69 = 30 mm – the fully illuminated circle.