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Coma Correctors

Discussion in 'Astrophotography' started by Jamie Mustoe, Aug 6, 2017.

  1. Jamie Mustoe

    Jamie Mustoe CAS Treasurer CAS Member CAS Committee

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    Hi All, I'm looking for some advice/personal experiences......I'm banging my head against the wall with the GSO coma corrector I have, my spacing from CC shoulder to the DSLR sensor is right to specifications and forum consensus (75mm exactly) but I still get a slight amount of coma in my frames that I'm not happy with.

    I have wondered if some of it could be induced by sensor tilt because to get the right focus I have to pull my setup out of the focuser by about 5mm....e.g. the CC and DLSR not fitting properly in the focus tube, maybe a fraction off center/alignment because I don't have a focuser with compression rings just thumb screws....I've rotated the camera, yes I could make the image worse by doing this but regardless and no matter how good I got things I can still see a small amount of coma in my frames.

    I believe I have my collimation pretty dam good, I used the collimation cap to position the secondary, then a laser to align the secondary and primary.....then I have refined that using the star test method to the point where as mentioned I'm pretty sure my collimation is spot on.

    I hear of people getting perfectly round stars to the edge using coma correctors and I'm at that next phase of refining my astropohotgraphy where I want round stars to the edge of my frames. I'm at the point of purchasing a Baader MPCC III and was wondering what you may have used or may recommend? And I'll go from there.

    Also for CAS members, do you yourself have or know anyone who might have a decent coma corrector that I may be able to borrow to try out for a comparison?

    My scope is the Orion 8" Astrograph f/3.9, 203mm aperture 800mm focal length. I'm using a Canon 700D for imaging.
     
    Last edited: Aug 6, 2017
  2. Simon Lewis

    Simon Lewis Well-Known Member CAS Member

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    Jamie my skywatcher CC would work as its for an f/4

    I'm starting to see you get what you pay for in CC's

    the image through the SW one was v different that the GSO .... but they are different beasts

    the SW has a 48mm thread on it .. u'd need an adapter if you have a canon ... I use Nikon :)

    if you can find a 48mm adapter ur welcome to try it
     
  3. Jamie Mustoe

    Jamie Mustoe CAS Treasurer CAS Member CAS Committee

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    Hi Simon,
    That would be awesome thanks, I have a 48mm female to T2 (42mm) male adapter, this would work but it would add 5mm to the spacing. The canon 700D is 44mm from sensor to mount face, then the canon t-ring adapter adds 10mm then the 48mm to T2 adapter adds 5mm to that a total of 59mm from the sensor the CC would that work? Also if needed I have a 20mm 48mm female to T2 (42mm) male adapter.

    upload_2017-8-6_13-57-25.png
     
  4. Simon Lewis

    Simon Lewis Well-Known Member CAS Member

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    ok heres the images of the CC and threads

    we can at last try and see if it makes it any better

    upload_2017-8-6_16-19-37.png



    front of the CC - the tube drops inside the drawtube

    upload_2017-8-6_16-20-28.png


    The adapter takes 48 to the Nikon Bayonet

    its 11mm thick about half screws on the front of the CC above
     
  5. Euan Mason

    Euan Mason Active Member CAS Member

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    Coma correctors are very sensitive to positioning and overall telescope design. It's best to try before you buy, and read as many reviews as you can. I've used a couple with Celestron F10 CATs, and the F6.3 works well while the F3.3 is almost unusable because its positioning is so sensitive.

    I have a GSO that's supposed to work with ETXs, but so far no dice with that one. OTOH, a less harsh model built by a Meade associate does work well with an ETX.

    So the lesson so far is don't be too ambitious with F reduction.
     
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  6. Jamie Mustoe

    Jamie Mustoe CAS Treasurer CAS Member CAS Committee

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    Hi @Euan Mason , yeah it did become apparent very quickly that positioning in the focuser draw tube is crucial. As I have to pull the camera/CC setup out by a few millimeters to reach focus I think that might be having an effect on the quality. I've ordered a couple of 2" parafocal rings that I will use as spacers so I can get the distance out I need and also keep a true snug fit on top of the focuser draw tube.

    @Simon Lewis has kindly offered me a try with his SW CC, and from my reading this is a good CC. The more I can try before I buy is the key here.
     
  7. Euan Mason

    Euan Mason Active Member CAS Member

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    @Jamie Mustoe, a bit more info: The GSO I have is a 1.25" 0.5 focal reducer and it came with a small extension tube. The other one that works well is a Scopetronix that screws onto the back of an ETX and takes the F ratio from 15 to 7.
     
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  8. Simon Lewis

    Simon Lewis Well-Known Member CAS Member

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    that sounds like a different coma corrector to the ones we have

    upload_2017-8-8_10-32-28.png


    these are 2" ones

    the forums are full of AP's who experienced the extended focus position

    I suspect the mechanics are upsetting Jamie's light path as its not that tight and the DSLR sticks further out due to the extension

    We will try the SW one which fits down inside the drawtube and reduces the mechanical flex

    Cheers

    Simon
     
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  9. Jamie Mustoe

    Jamie Mustoe CAS Treasurer CAS Member CAS Committee

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    @Simon Lewis I messaged that guy with the Baader he's got an interested buyer but will contact me if it falls through.
     
  10. Simon Lewis

    Simon Lewis Well-Known Member CAS Member

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    meh ... yeah was ok price so prob quick to sell

    they sometimes come up on ebay and on CN classifieds

    occasionally see them come on .. sell quick!
     

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