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Back Focus Help.... again!!! Different scope same issue!

Discussion in 'Astronomy Gear' started by Simon Lewis, Jul 26, 2017.

  1. Simon Lewis

    Simon Lewis Well-Known Member CAS Member

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    Doing my imaging last night on the new SW Quattro 8 and I found it really hard to get good focus

    The only way I could get focus with the ASI224MC was to juggle a combination of camera, barlow and a coma corrector. Now as rightly pointed out I should not need the coma corrector.

    But ...

    OTA -> GSO Coma Corrector > X2 Barlow > ASI224

    was the only way to obtain focus ...

    Without the barlow .. .no focus
    Without the CC .. no focus

    I suspect that the barlow was extending the light path enough to reach focus on the ASI224 and as the Quattro is a true Astrograph thats prob not good for the ASI camera

    So my question now is .. how big an extension do I need on the SW focuser to reach focus

    Without buying a bucket of extensions and adapters its a bit hit and miss!!!
    I suspect something in the region of 80mm ....

    I've no idea if Skywatcher have the same info as the WO refractor has!

    Cheers
    Simon
     
  2. Andrei Cotiga

    Andrei Cotiga CAS Observatory Director CAS Member CAS Committee

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    instead ob buying extensions which are quite ""bendy" I would suggest you look for a flip mirror. In this way you may look at your framing before flipping the mirror and taking you shot. Way more sturdy.

    Or you can look at a normal long off-axis guider. This way you can use your ASI to either guide your DSLR or take planetary images.

    To check approx distance needed f out focus, you can take you camera out and slowly take it outwards until it looks like it may reach focus (harder to do with cameras than with eyepieces).
     
  3. Simon Lewis

    Simon Lewis Well-Known Member CAS Member

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    thanks Andrei

    I seem to really struggle finding the right bits in the optical path !

    hmm I guess a flip mirror would add some length into the imaging too

    maybe with the flip and the nose on the camera with the tube it would reach focus

    OAG .. not sure I can add barlows and things then which is really needed

    yes _ guess I can try pulling the camera out of the focuser and see how far I need to go!
     
  4. Andrei Cotiga

    Andrei Cotiga CAS Observatory Director CAS Member CAS Committee

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    you can add 1.25 barlows to an orion OAG. Next time at the obs I'll show you the model we have and you may try it. The flip mirror ads about 60mm (maybe slightly more). I had a Vixen I was really happy with.
     
  5. Simon Lewis

    Simon Lewis Well-Known Member CAS Member

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  6. Andrei Cotiga

    Andrei Cotiga CAS Observatory Director CAS Member CAS Committee

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    Exactly like that.
     
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  7. Simon Lewis

    Simon Lewis Well-Known Member CAS Member

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    ok some daylight and some heading scratching and i think I am at the bottom of the issue

    i must have been lucky using the coma corrector, the extension tube and barlow on the ASI224 does focus but the camera chip must be someway out of the focuser - i assume you can find multiple focus points?

    i stuck my DSLR on and with the hotech 2" adapater its just focusing with the focus tube all the way in - add the CC and the focus is out

    so I need a shorter nose for the DSLR

    the hotech has quite a deep nose

    [​IMG]

    the distance from the silver adapter to the DSLR body is too long and JUST makes focus

    this should be better

    Adapter T2 to 2"

    upload_2017-7-30_13-50-57.png

    ok to swapping to the CCD.....

    I again struggled but taking all the adapters out and just using the short nose on the camera I was able to pull the camera out and find a focus with about 45mm of distance from 2" screw adapter on the focuser

    the tube that rob loaned as 80mm - too long so I need a short tube of say 45mm

    I was able to find focus using that distance

    honestly this is like Lego!!!!
     
  8. CAS Admin

    CAS Admin Administrator CAS Member CAS Committee

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    This is what I had with the 12 last night. The dslr looked like it needed to go closer to focus, but worked with a Barlow. Then you had issues with your camera with the same set up
     
  9. Simon Lewis

    Simon Lewis Well-Known Member CAS Member

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    now i found this

    "The Quattro's tube is lined with a set of nine internal baffle rings. Their main purpose is to cut down on stray light entering the telescope tube, helping to improve image contrast, but they also lend a bit of extra stability to the tube structure....The Quattro-8S is fitted with a Crayford-style focuser that performed well. It has tension adjustment, focus-lock and a 10:1 dual-speed fine focusing option, and also comes with a 2-inch fitting that has a T-thread on its external side. This allows a good connection to be made between scope and camera with the right adaptor ring....Visually, the Pleiades looked like diamonds hanging in space, with a hint of nebulosity visible around the brightest star Merope. The stars were generally crisp and sharp, with good colour on show. The scope is superb for imaging too, delivering shots that appeared to have higher contrast than the other scopes on test."

    while that might be ok for the DSLR it won't work with the coma corrector unless one comes prethreaded

    So think I need a short nose for the dslr and short adapter for the ccd

    of course .. not the bits I have!!!
     
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  10. Simon Lewis

    Simon Lewis Well-Known Member CAS Member

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  11. Simon Lewis

    Simon Lewis Well-Known Member CAS Member

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    OK solved
    at least for the DSLR

    I bought the skywatcher coma corrector this has a very thin nose and mounts inside the drawtube

    http://www.skywatcherusa.com/wp-content/uploads/2015/04/s20204_quattrocomacorrector-600x533.jpg

    with this I now have a healthy 30mm or more back focus available

    with say 15mm for a TOAG I still have a good 15-20mm of back focus with the DSLR in place

    I am satisfied this will work - the SW CC is also a 4 element aplanatic ED glass - I will try it tonight but the mountain view was stunning colour so I am convinced this is a good solution

    Skywatcher f4 Aplanatic Coma Corrector | First Light Optics

    now I need to get a screw extension to extend the drawtube out to suit the ASI

    but thats somewhat easier!

    :)
     

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