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 πŸ™‚ 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 ,
    a very fine website !!!!, i have a similar system ( Meade 10 ” ACF from 2009 without fix of the main mirror position ) and CCD T 67 , but with a few starshapeproblems Coma ??
    With 85 mm from rear flange CCD T 67 to CCD i found a focal lenght of 1900 mm, my optical train starts directly
    from the SCT backside .
    My question : how much is the distance between back of the ota and the Reducer front flange of the reducer ?. I have no additional focuser .
    Where did you buy the Acun focuser and how much was it ?
    i hope you can help me !?

    1. Thanks Rainer πŸ™‚
      It was the same for me – at 85mm distance I got about 1900mm, and when I increased distance to 115mm with additional 30mm extender I got about 1700mm focal. I will measure the exact distance for you, but at this moment I think that telecompressor front is about 1-2cm inside the OTA. There are baffles in the inner tube, and telecompressor fits a little into them.
      I bought my focuser here–UNC-V-Power-Crayford-Focuser-for-2–and-3–SC-Thread.html and I am pretty happy about this device.

      Clear skies,

  2. Hello Lucas,
    thanks for the quick reply and your informations . I thought you have the reducer inside
    the focuser . But if in the baffles tube – how you fix it ? with the focuser frontside ?
    Images welcome if you can send ?
    I wish you and your family a happy christmas and a happy new year 2018 .
    Best regards

    1. You are totally right – it is inside focuser. Starting from the filter wheel I have about 40mm 2″ nosepiece, then 60mm of extenders and then telecompressor. That’s why compressor itself already peeks out of focuser and it is inside the inner tube of ACF telescope πŸ™‚ You can see it in the small picture in this blog entry – there is only one 30mm extender, now I have 60mm of them, and that long pipe is inserted into focuser.
      I wish you also happy Christmas and many clear nights in 2018!


      1. Hello Lucas ,
        i canΒ΄t see the the small picture in this blog entry ,
        can you link the image ? please ? Last question : what position of the primary mirror
        i must take . The focuser original knob counterclockwise to the end , or focuser knob clockwise up to the end ?
        Remember i have no knob to fix the primary mirror, only one knob to focuse the image . Older Model of Meade 10 ” ACF OTA .

        Best regards Rainer

  3. Hello Lucas , i found your image above ,thanks !
    First i will take your distance 115 mm CCD T 67 –2–CCD–Chip of my ASI 1600 MC cooled, similar chip size as your ccd .
    From rear flange of the reducer . Second attach the focuser as you described to the OTA.
    Third i insert the reducer into the focuser – how deep into the baffle tube you send me
    And at least i must define the position of the main mirror inside the OTA, in the backside, frontside direction ? Or bettween booth positions .
    BTW : What Kind of thread M 48 ? Is the focuser telescope side ?
    Sorry about so much questions , but i need your help to get my setup to work properly .

    1. Hi Rainer,
      I put a picture with marked distance here . This is a distance between sensor and telescope back in the marked point. So there is 15cm, and also at this point 15cm away from sensor there is end of telecompressor. It is actually not yet inside tube then πŸ™‚ But I plan to add 2-3cm more of extenders to see how compression factor will change and if there will be any vignette then.
      The focuser at telescope side has two threads. One is standard SCT small thread, but I used big SCT 3.3 inch thread. You can unscrew this adapter from Meade ACF back and then you get large thread. And focuser also have it. Each Meade 10″ and larger have this thread.


      1. Hello Lucas, thanks the answer and the link
        Today i have ordered the focuser you have to ,
        My lighttrain is 150 mm from the rear flange of CCD T 67 to
        CCD of ASI 1600 MC and i will place this chain inside the focuser up to
        the end of focusertube, as you have i understood.
        Thank you for your effort to help me . I will try it as soon as possible
        and will report my results. But i forgot where is your Main mirror inside the
        tube. Near the back of ota or near the schmidt plate, or in the middle between
        booth .
        Best wishes

        1. Right – I forgot. But actually there is only one position where the mirror can be. I set this additional focuser to the middle (it is 15mm, because focuser has 30mm travel) and then using ACF focusing knob I adjusted focus on camera. Then I locked mirror, collimated telescope on star, and starting from that I use only this TS focuser to adjust focus. I motorized it with stepper motor and AstroLink controller. The mirror is more less in the same position when I use it visually with 2″ diagonal and 34mm eyepiece.

  4. Hello Lucas, thanks for your reply , i understand now
    conditions inside the ota . Today i will get the focuser, The CCD T 67 to ASI 1600 MC chain is ready for use , weather i will see whats going on in the sky ?
    Astrolink is interresting for me to , after the other devices work well.
    Now many thanks to you for your help and happy days with your family.
    Best wishes

  5. Hello Lucas,
    i hope you are well and the new year started very good for you ?
    i have got first better results with the new image train, new focuser same as yours, d = 115 mm and CCD T 67 from AP.
    Th reducer is inside the focuser up to a stop line bevor the end of the focuser.
    Have you a solution to insert the reducer train deeper in the focuser / baffle tube without the limitation of the end of focuser ?
    I hope to get closer to the main mirror, may be usefull for the image quality . Have you results with CCD Inspector Curvatur 3 D ?
    I hope you have very good ideas again ?
    Best wishes

    1. Hi Rainer,

      Have you got the same focuser as mine? If so, then you can unscrew this plate at the focuser end – there is large 3.3 inch thread. Then you can unscrew 3.3 inch adapter from ACF back and fit focuser via this large SCT thread. Then you can put reducer train into the baffle tube. Check this image
      I do not have CCD inspector results yet.


    1. I think at 115mm distance telecompressor is just touches first baffle πŸ™‚ I have now two more 2″ extenders so maybe I will try to use them. But I also purchased screwed adapter, so optical train is no more hold by clamping ring. This long optical train was quite sensitive for any tilt in focuser – watch if it is locked without tilt πŸ™‚
      Clear skies

  6. Hello Lucas,
    if the weather conditions today are good,
    i will test it .
    Did you mean the beginning rear flange of the baffle tube ? ( “touches first baffle..” )
    A tilt adpater is a part of my image train today ; )
    Thank you !!!

    1. Yes, exactly – when focuser is in the middle position (15mm) telecompressor is almost starting to move into first baffle. When I move focuser more into the tube, then optical trains dives into baffles πŸ™‚

  7. Hello Lucas,
    the test was not so as i hoped ; (, The moon was to bright and the seeing mmmh , than my collimation was wrong . What your best and shortest method to collimate your OTA exactly ?
    Did you mean , that the front of your reducer is in the focuser position of 15mm
    inside the first bafflesection / ring ? and than can dive in more than the first baffle section with move the focuser inwards ?
    Best wishes Rainer

    1. Yes, when focuser shows 15mm position then reducer front is around first baffle. When I move focuser to position 0 then reducer goes inside – at least that is what I calculated. I cleaned filters yesterday and will mount camera back to telescope tomorrow, so I will check it again and let you know.
      Fastest way for me to collimate is using 150-250x eyepiece and high altitude star 5-6mag sligthly defocused, so you can see airy discs. They need to be centric. Star needs to be in the center of FOV. Also make sure you collimate and image in the same mirror tilt position. For example when I adjust focus with main focus knob, I always end this process with turning this knob in the same direction (CCW in my case) and then lock mirror. Then I do final adjustment with new focuser.


      1. Hello Lucas ,
        yesterday i took images from M 37 after fine collimation with two distances 114 mm and 104 mm . The position of the front flange CCD T 67 was as yours at 120 mm ( 114 mm d )
        And the position at 104 mm was the front end of the focuser ( near the start of the baffletube ) .
        The star shapes depend very clear from two distances .

        a – CCD to rear flange of the reducer CCD T 67 and
        b – the position of the front flange of the reducer inside the focuser tube .

        At d= 114 mm starshapes where cometlike with radial distribution to the image centre
        At d= 104 mm starshapes where like bananas with concentric distribution to the image centre .

        What do you think , what is the best distance CCD T 67 inside the focuser tube, and how this position influence the starshapes ?

        It seems that in my setup the best distance CCD to CCDT67 rear is between 114 and 104 mm

        To get round stars i think it is neccessary to understand how the two distances
        ( CCD – rear – CCD T 67) and the position of CCD T 67 font side inside the reducer works .

        What do you think abaout this ?

        Best regards


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