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!

45 thoughts on “ACF 10″ and CCDT67 telecompressor”

  1. 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

  2. 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!

  3. 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 🙁


  4. 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

  5. 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

  6. 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.

  7. 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 🙂

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