ACF 10″ and CCDT67 telecompressor

It has already been some time from first light of Meade 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.Β 

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 very little. It is designed to work with instruments of low focal ratio – from f/9 to f/18.Β 

AP CCDT67 telecompression graph
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
Astro Physics CCDT67 telecompressor

Good thing is, that CCDT67 is just a 2″ barrel that can be inserted 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
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 30x2 + 3x10x2 minutes
NGC1528 open cluster. LRGB composite 30×2 + 3x10x2 minutes
NGC1528 open cluster. 30x2 minutes inverted
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. 

Meade ACF f/10 seems to work fine with Astro Physics CCDT67 telecompressor. 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, only slight curvature is visible at QHY163M sensor. However it needs to be verified, when seeing will be good.Β 

And few more pictures images on December, 13th:

M15 globular cluster in Pegasus. 8x2 minutes of luminance
M15 globular cluster in Pegasus. 8×2 minutes of luminance
M52 open cluster in Cassiopeia. 120x5 seconds of luminance - full frame
M52 open cluster in Cassiopeia. 120×5 seconds of luminance – full frame
M52 open cluster in Cassiopeia. 120x5 seconds of luminance - frame center crop. Image scale 0.45"/px
M52 open cluster in Cassiopeia. 120×5 seconds of luminance – frame center crop. Image scale 0.45″/px
NGC7635 Bubble nebula. 8x5 minutes with Ha filter
NGC7635 Bubble nebula. 8×5 minutes with Ha filter

Clear skies!

45 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


  8. 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

  9. 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

  10. 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 !

  11. 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

  12. 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 πŸ™ You can still adapt some regular SCT corrector/reducer to this scope (like the one from Starizona).

      Clear skies!

  13. Hello Lucas , yesterday night i found on NGC 6939 a new result for forming the starshapes .
    Some of my starshapes like eggs in the edges . But i found that die distance between rear flange of the reducer and the CCD plane determine only the F ratio . fe. yesterday F7.3 on d= 98 mm . if the image strain is deep in the focuser i got radial starshapeabberations , if i went back out of focusertube i got banana shapes, so i tried to search the point between and – the starshapes got better . Its nessecary to optimise the shapes , but i think that here i found a clear relation between the deep of the imagetrain in the focusertube and the starhape in the edges . To find the best location i will try sharpcapture in live mode and will measure the fwhm of stars in the edges while moving the imagetrain in or outwards to get the lowest FWHM value on theese stars . I hope so i solved a problem that eats many hours of my livetime and so evererybody can find the optimal location for the imagetrain at any F ratio . ; )))
    Best regards Rainer

    1. Hi Rainer, good to hear that! Idea with SharpCap liveview measurements looks good, but probably you need to review all four corners, just to eliminate possible tilt in image train. When train is tilted then image can be a little bit different in each corner. But that is cosmetics I think πŸ™‚
      Clear skies, Lucas

  14. Hi , I’m Michele and I have a 12″ ACF and apccd67. I have star shape problems at the corner of my Sbig ST8XME.
    Collimation and flexure are the firsts suspects. But Is important also the position of the telecompressor inside the tube?

    1. Hello Michele,

      As Rainer wrote below – it affects two factors. First is effective focal length. Larger backfocus means longer focal length. I have my APCCD67 deep inside focuser (front lens is close to the ACF inner baffle tube), and telescope effective FL is 2600mm at this position (1800mm with reduction at 115mm corrector backfocus). Second factor is that as far as I know SCT telescope image is optimal at its optimal FL, so for 10″ f/10 it is at 2540mm. If the SCT telescope is set to work at different FL, then image may degrade.
      In my configuration I have stars at frame corners little bit larger than in the centre (like 10-15% larger), but when all is well collimated and not tilted, then stars are round.

  15. Hi Lucuas , Hello Michele
    last week i tested my theoretical hypotheisis, that the only the Deep of the imagetrain
    inside the focuser tube determne the starshapes in the edges , but the measurement didin`t show that . ( CCD Inspector ) and PI 1.8 .
    But the position of the imagetraine chnage the focal lenght , f.e. 10 mm outwards mean 20 mm + to the focal lenght . The starshapes seem to be better if the imagetrain in deeper inside the focuser tube . But my test was at 98 mm between reaf Flange of the AP CCD T67-CCD Plane . I think i will test today again at 88 mm Distance and Imagetrain completely inside the Focuser tube .
    At my last test i found a horrible isssue of my polaraxis of > 30 Γ rcmin, now i have fixit to 20 arcsec . The mean Collimation seems to be 1,5 arcscec . ( CCD I ) So i will looking forward today evening to test with New polar aligned , new collimated equipement at 88 mm distance between Reducer – CCD .
    I will report the results soon . test object is NGC 6039 near zenit point .
    Best Regards

  16. Hello Lucas, at monday night i testet my new PA , now the polaralignement PA is 24 arcsec much better
    Collimation is about 1,5 arcsec , good, the imagetrain was totally inside the focustube . The Distance Reducer-CCD
    was 88 mm. The resulting focal lenght was 1480 mm like F 7.2 . The stars in the upper field were round but in the right bottom corner i saw bananas big in bin 1 x 1 , not so big in 2×2 and only little bananas at 3 x 3 . In the left bottom corner the similar issue but not so clearly like in the right. In my next test i will test the new distance RED – CCD of 85 mm, at bin 1 x 1 / 2x2x/3×3 . The exposure times were from 10 ” to 300 ” . The edge issue not depend from the lenght of the exposure , guiding or not guiding . CCD Inspector told me a minimal tilt of 0.1% .
    What you think were i must search the problem ? Any good ideas ?
    Best Wishes

  17. Hello Lucas,
    yesterday i tested with distance 85 mm reducer rear flange to CCD Chip .
    Bin 1 x 1 / 2 x 2 and 3 x 3 with FWHM 2.5 to 3.5 on NGC 6939 .
    Exposures times were from 7 sec – to 300 sec. Guided and unguided .
    My results much better no eggstars in the edges in Bin 3 x3, with the FWHM above
    a little bit oversampled, Pixelsize 11,5 at focallengh of 1499 mm . Scaling about 1,5 arcsec/pixel .
    The field in the first images up to 120 sec was super flat, Colliimation 1,5 and curvatur < 15.
    The aspectratio for the stars were about 10 % near ideally round !
    With increasing the exposuretime the flatness of the field decrease a little bit with curvatur max. 25 %
    But with bin 2 x 2 and bin 1 x 1 the image quality decreased, bad values at bin 1 x 1 ( undersampled ++ )
    and bin 2 x 2 ( undersampled not so much . ) The guidung with MGEN II was really good .

    My conclusions :
    at Distance 85 mm , Bin 3 x 3 may be somtimes with super FWHM 2.0 2 x 2, ( next test with F6.3 for 2 x 2 )
    exposure times not so much, multiple exposures +, stacking, Dithering .
    Alternativ i tried 1048 Exposures at 2 sec exposure time from the OC NGC 6939 .
    I will develope the image soon .
    Best wishes Rainer

    1. That was a lot of work – thanks Rainer for sharing this information. I am curious about the results at f/6.3 – I found it too much for my setup. Curvature at level of 15 is perfectly ok in my opinion.
      I keep my fingers crossed!

  18. Good Morning Lucas,
    i am again ; ) . I found last night the really reason of starabberations like bananas or
    trails in the image edges . It is not so much the RED-CCD distance, not the reducer , and not the image train !
    It is …. The Focuser istself ! The tilt after insert and clamping the image train is so important that the tilt
    measured with CCD Inspector was up to 26 % and the abberations in the edges showed not a uniform type of abberations . f.e. right bootom bananas, left bottom correct starshapes and left upper edge trails a.s.o.
    My recent question is :
    How you can manage the imagetrain you have inserted in the focusertube don`t have
    any tilt ?
    My solution was insert the image train close in contact ! with the buffle tube and tight the 3 screws very strong, but it is not a good solution i think .
    What do you think ?
    After this i found round stars in all edges ; )) , but i am not lucky with this type of focuser, any suggestions ?
    Did you found any possibility`s to prevent tilt inside the focusertube ?
    Best regards

    1. Hello Rainer,

      I have screwed connection – custom ring that is screwed on the focuser drawtube, and there is 2″ filter thread inside, where I put image train (it is not visible in this blog entry images though). But I still have some tilt in the train, and the source of it is poor “rotator” of the focuser. It is a place locked with 5 small grub screws, where you can loosen them and rotate focuser against its mount to the tube. It is very easy to lock it tilted, and I try not to touch this at all πŸ™


  19. Hello Lucas,
    thanks for your answer, maximum tilt in my setup reaches from 35 % to minimum 3 %
    measured in CCD Inspector 2.5.3 . This produced a lot of ugly starshpes in the edges and only when the imagetrain in deep inside the focusertube close in contact to the buffletube and the 3 looking screws are very strong tighten, i found a minimum tilt from about 3 % , nearly round stars in all edges .
    So i ordered a new focuser to test it , named MonorailSC from TS , i will report my results ; )
    Best wishes Rainer

  20. Hello Lucas,
    the tested Focuser above is not really useful for astrophotography, thatswhy i ordered
    the baader diamond steeltrack with screwed ! connection between imagingtrain and focusertube
    than i hope i can get tilt values of about 1-2 % . My recent distance RCCD is 74 mm , F 7.5 F= 1518 mm .
    Best wishes

  21. Hello,

    I am struggling with this compressor on my C11 edgeHD. I am trying to insert an OAG but can’t seem to get focus in my guidecam. Unguided pictures show slight oblong stars in the corners. Where am I failing in my setup? Should I insert the compressor even further in the scope? (I was told to insert it up to a certain point)
    Spacing from compressor glass to camera sensor is about 100 mm.

    1. Hi David,
      The 100mm distance seems to be ok. What main camera do you use?
      Getting guide camera to focus with OAG may be a little bit tricky. You need to focus stars at main camera, and then move guide camera to reach the focus. If you could upload somewhere a photo of your imaging train and link it here in comment, then I could tell more.
      With Edge HD you should not have elongated stars in the corners. You may try also to upload full res single image and link it, so I can take a look.

      1. Single 30 second exposure, unguided:
        Main camera is the asi 071mc pro (osc cooled)
        Image train is the compressor-oag-filterwheel-camera. Compressor was inserted into the 2″ sct ring, but not completely.
        When focused on the main camera, The guidecam should move further inward than it allows me. So I guess I should add more space between the prism and the main camera, as that would also make the guide camera focus point shift outward?
        I guess you can see my email, I can send you more info if needed.

        1. For the OAG focusing – exactly as you wrote. You need to put more distance between OAG and main camera.
          As for the image you linked I think the stars are also little bit elongated in the frame center. This may be due to tracking error and/or to miscollimation. Once these two factors will be eliminated, then corners may be examined more accurate. But I agree that corners are also affected – it may be imaging train tilt and/or collimation.

  22. Hi Lucas,
    How did you work out the achieved focal length with the CCDT67? How do you work out the effects in terms of FL of the additional focuser on the Rear Flange? I have the ACF 14” and just got a CCDT67 and buying a 3.25 focuser. Trying to work out what FL I will get.
    Regards, Stephen

    1. Hello Stephen,
      As you may notice at the second image the telecompressor is mounted with extending rings so it goes as deep into focuser as possible. This way focus plane is not shifted much outwards. But still with additional focuser at rear FL without compressor is about 2630mm (as I checked with plate solving software). With telecompressor I work currently at effective FL 1780mm, so the compression factor is 0.68.
      I think with additional focuser you may expect effective FL 5-10% larger than standard, so maybe in the range 3700-3800mm. With 0.67 compression ratio you may get something around 2500mm I would estimate.

      Clear skies,
      BTW – congratulations on your ACF 14″ beast πŸ™‚

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