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!

38 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
    Rainer

  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!
      Lucas

  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
    Rainer

    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 🙁

      Lucas

  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

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.