ACF 10″ and CCDT67 telecompressor

It has already been some time from first light of ACF telescope and I was looking forward a clear night. And it happened, a few days after supermoon 🙂 Main goals for this another test night were: to set distance between Astro Physics CCDT67 telecompressor and sensor and to check guiding. 

CCDT67 telecompressor is simple two element optic device, that reduces instrument focal length. Reduction factor depends on distance to sensor, and for 85mm the factor is 0.67x with illuminated circle of 29mm diameter. Increasing distance lowers both reduction factor and illuminated circle, as presented at the graph below. CCDT67 also flattens curvature, but only a little. It is designed to work with instrumets of low focal ratio – from f/9 to f/18. 

AP CCDT67 telecompression graph.

I started with a distance of 85mm, so 0.67x compression factor. And that combination gave effective focal length about 1900mm. It is much more than 2540 x 0.67. The reason for that is well known for SCT users. SCT focal length is not constant, it depends on focal plane position. Once you move focal plane away from the tube, the focal length increases. In my setup there are two factors that affect it. One is additional focuser that adds some distance. And second one is telecompressor itself, that “eats” some backfocus, so it is able to compress focal length.

Astro Physics CCDT67 telecompressor

Good thing is, that CCDT67 is just a 2″ barrel that can be insterted deep into the focuser. If it would need to be mounted at focuser tube far end, then focal plane would be moved even more far away. 1900mm was a little bit higher than I expected, so I added two more 2″ extenders between telecompressor and camera. Then the total distance was 115mm. And in this configuration I achieved effective focal length about 1700mm. I aim for 1600mm and ideally for 1500mm, but at this moment I do not have more 2″ extenders, so I leave it at 1700mm that gives pixel scale of 0.45″/px with QHY163M camera.

M103 open cluster. 3x10x2 minutes RGB composition

Then I shot some test images. Atmosphere transparency was good, but it was the first and last good thing this night. There were bright Moon shining in the sky, strong wind and the seeing was horrible. Stars diameter was in the range of 3.5-4.5 arc seconds. Such conditions happens about twice a year at my location. Usually seeing is in the range 2-2.5″, such it was when I made ACF first light. But I just started guiding and shot some open clusters around Cassiopeia to check how mount tracks such heavy load and if there is any vignetting or field curvature in that setup composition.

NGC663 open cluster. 10×2 minutes of luminance
NGC1528 open cluster. LRGB composite 30×2 + 3x10x2 minutes
NGC1528 open cluster. 30×2 minutes inverted

All components seem to work fine. Mount does not look like overloaded and tracking was ok, and also field curvature does not seem to affect image much. However it may be more visible, when seeing will be better and stars will shrink. 

CCDT67 telecompressor seems to work fine with ACF f/10 telescope. Effective focal length is affected by focal plane position (it is SCT telescopes feature), so it will be longer one may think basing on telescope specifications. But image quality is good, and no curvature is visible at QHY163M sensor. However it needs to be verfied, when seeing will be good. 

And few more pictures images on December, 13th:

M15 globular cluster in Pegasus. 8×2 minutes of luminance.
M52 open cluster in Cassiopeia. 120×5 seconds of luminance – full frame
M52 open cluster in Cassiopeia. 120×5 seconds of luminance – frame center crop. Image scale 0.45″/px
NGC7635 Bubble nebula. 8×5 minutes with Ha filter

Clear skies!

27 thoughts on “ACF 10″ and CCDT67 telecompressor”

  1. Hello Lucas, thanks for your answer, what i need is the distance – how much longer
    is the image train front flange of CCD T 67, than the telescopside flange of the focuser . This is the way inside
    the telecop tube in the position 15 mm focuser ?
    Thank you very much ; ))
    Best regards

  2. Hello Lucas ,
    yesterday i measured the distance at 15 mm Focuser position – the front flange position of CCD T 67 that is inside the first baffle section its : 13 mm , at position 0 mm its 28 mm . The distance rear flange of the Reducer CCD T 67 to CCD Chip is 115 mm . Today i will collimate after installation of bobs knobs the scope again and then i hope i get round stars in all 4 image corners ??
    Best regards

  3. Hello Lucas ,
    did you found that 120 mm is the better distance ? wich F ratio that is ?
    I will make the same Imagetrain to compare the results . As i saw the CCDT 67 has 35 mm itsself
    and the distance is from the CCD Chip to Focuse screws is 120 mm . Any vignette ? How are the stars in the corners , can you show an example ? You wrote “AP CCDT67 extends another 35mm to the left.” In the image
    i saw 120 mm distance from CCD Chip left , to Screw of the Focuser right . I think that is the rear flange of the CCD T 67 and than the Reducer to the right with 35 mm istself. Wrong or right ?
    Best wishes !

  4. Hi Rainer,

    Can you upload one frame and send me URL to download? You can use contact form of this site to contact with me. I think stars should not be like bananas or comet like. ACF has coma eliminated, and only field curvature is left. And field curvature increases star size when they are far away from center. It can be something else, maybe some tilt? There is no really optimal distance for CCDT67 – it is not field flattener or any corrector. It just reduces focal length (first picture in this post).

    1. Hello Lucas,
      today evening i will test another distance 112 mm, 110 mm, 108 mm to get better stars,
      then i can send you a gif animation of 5 images various distances 104 – 114 to see the differences of star shape . The best and the ugly image i can send together as zip file with , so i need your emailadress
      to do so.
      Best wishes Rainer

  5. Hello Lucas,
    thank you very much for your help, problem solved . With my 8 inch Meade real !!! ACF , no problems with the starshapees in the edges , our old public observatory scope had a LX 200 ACF mount, but a normal no ACF OTA .
    We did not found ACF on the schmidtplate ring, clearly non ACF . Now i work at d= 105 mm as f 7.3 and the focus is 1458 mm instead 2000 mm without reducer .
    Best regards

    1. Hi Rainer,

      Glad to hear you figured it out, and sorry to hear that your big scope is not ACF 🙁 You can still adapt some regular SCT corrector/reducer to this scope (like the one from Starizona).

      Clear skies!

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