CCDT67 reducer visual use

[casstony]

Astro Physics doesn't recommend the ccdt67 for visual use but I'd noted a couple of guys on CN saying that it worked ok with schmidt cassegrains; one said it worked attached directly to a Nagler 31mm and another guy used it in front of a diagonal.

Theoretically the spacing is a bit excessive attached to the diagonal but I tried it with a 10" Meade ACF and it seems to work well. An ES30mm 82 deg eyepiece with the reducer gave a wider field of view than a ES40mm 68 deg eyepiece without. Alpha and Beta Muscae fit into the field of the 30mm making the fov about 1.3 degrees (I think those stars are about 1.25 deg apart?)

Getting a low power, wide field of view is harder in larger sized SCT's and this seems like a decent option.

[allan gould]

Tony
I recently used the CCDT67 with my 10"SCT and found it gave an excellent flat field, far better than the Meade f6.3 reducer. The spacing was exactly 111mm as recommended from the reducer to ccd chip.

[casstony]

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_support/accessories/photo/Telecompresssor-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.

[allan gould]

Tony
That 85mm is correct as you have to add 16mm for the internals of the flattener/reducer and that gets to the 101mm stated in the download. I'm sorry I stated 111mm whereas I should have stated 101mm. My mistake.
I found when I used it on my 10" SCT, 10" GSO RC and 5" refractor that it was perfect for all of them. Thus I'll have a few reducers to sell.
They say you can vary the distance but I have not experimented to that degree but I trust what they say. David Courtney has experimented to get to f5.5 or f5.4 with his GSO RC and the results were impressive.

[allan gould]

On further reading of his blog he eventually got the f ratio down to f5.1 with excellent results. See http://www.davidcortner.com/slowblog/20120124.php
That represents a distance (total) of 149.5mm to get to that f ratio. His stars were very good.
He finally stopped at a f ratio of f4.8. See http://www.davidcortner.com/slowblog/20120225.php
Hope this helps
Allan

[casstony]

Thanks for the info and link Allan. That matches my visual observations. I had about 145mm total separation representing a 0.52 reduction of ~f/11.5 SCT, ending up with about f/6 effectively. It should work well behind the 12" too but I haven't had a chance to try it yet.

[casstony]

After reading again Allan, the maximum separation he used was 120mm, giving 0.6x reduction of f/8 = f4.8.

110mm total separation on an f/8 scope would give f/5.1 (195/305 x 8).

For imaging you should be able to use a greater separation on a SCT due to the slower native f ratio.

[allan gould]

Sorry you're correct As I was working on my SCT f ratio, for the f8 of the GSO RC then your distances are correct.

[bert]

One thing that has not been mentioned is that the ccd67 is a smaller reducer and vignettes on larger chips. Works great on 8300 sized chips and smaller, aps size... Not so much. Ymmv.

I have a 67 reducer and I use the larger ap27tvph a lot more. On my ap 130, I get some curvature of field natively, when the tvph is installed it flattens the field neatly and reduces it to f4.8, this is with a 6303 sized sensor.

I did try the 67 on a 150mm f11 refractor, I pulled it out when I starting using a 35mm sensor and installed the tvph. After reading this thread, I might have to revisit experimentation with the ccd67.

Brett



Brett

[Logieberra]

I am sorry to dig up this old post - but I’m about to do a “visual only” test of both the Tak 0.8x Mewlon reducer/corrector and AP CCDT67 (0.67x) reducer on a baby Mewlon 180. Thanks Hans for the AP loaner! If of interest, I’ll report findings/photos here… eyepieces will include TV Naglers and WO UWANs both at 82deg FOVs.