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Back Focus Help

Discussion in 'Astronomy Gear' started by Simon Lewis, Jul 11, 2017.

  1. Simon Lewis

    Simon Lewis Well-Known Member CAS Member

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    I think I kind of understand this but some advice would be useful

    So my little Willam Optics Zenithstar 71 has a field flattener which I have attached an M42 nikon adapter to.

    This focuses fine on the D5300 and I can achieve focus easily using it

    Now I added a ZWO ASI274MC which has a 1 1/4" adapter on it ... inserting this into the 2-1.25" adapter on the ZS71 does not give focus.

    I suspect the CCD chip is too close on the ASI - so I asked WO for a spec of the back focus positions and they sent me this:

    upload_2017-7-11_13-41-50.png

    I suspect I will need to add an 80mm extension to the tube to achieve focus vs the flattener

    The question also comes ... with the CCD ... do you still use / need the field flattener ... or just on the DSLR

    Half the hobby seems to be lego bricks :) Buy this
    to connect this to this!

    :)

    Advice appreciated!

    Cheers

    and of course the weathers packed a sad now I pretty much ready to do some AP runs!
     
  2. Andrei Cotiga

    Andrei Cotiga CAS Observatory Director CAS Member CAS Committee

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    yes, you may need to extend it. In that case, you may experience some focus tube bending, depending on your weights.

    Field flattener should be used with CCD as well as DSLR as it corrects an optical problem.
     
  3. Simon Lewis

    Simon Lewis Well-Known Member CAS Member

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    I think I just overlooked something

    the FF has an M42 screw on it.

    The ASI224MC has an adapter that looks lie it takes M42 - 1.25"

    It should be a simple matter of unscrewing the 1.25" and screwing it on the FF.

    I'll have a look at that later!

    upload_2017-7-11_14-42-58.png
     
  4. Simon Lewis

    Simon Lewis Well-Known Member CAS Member

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    nope still won't focus pulling the FF out as far out of the focuser i am just about there - I think I need something like a 40mm T2 extension

    anyone have I can borrow to test my theory??
     
  5. Gary Steel

    Gary Steel CAS President CAS Member CAS Committee

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    I just happen to have a set of M42 thread extension tubes, total length about 60mm. You can borrow those.
     
  6. Rob Glassey

    Rob Glassey CAS Vice President CAS Member CAS Committee

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    Yes, you'll need an extension of some kind.

    My ASI120MC has a CS-mount adaptor that is pretty much flush with the M42x0.75 outer face. A standard CS mount has the flange 12.5mm from the focal plane. So the M42x0.75 on the camera is also 12.5mm from the focal plane. A standard M42x0.75 T-mount is 55mm from flange to focal plane. (or maybe 54.8mm?)

    In your drawing the focuser is shown extended 35mm out. In that position the focal plane is 68.32mm from the first M42 and 54.8mm from the second M42 with the adaptor.

    So with the adaptor, the focuser would need to move out 54.8-12.5 = 42.3mm to achieve focus. I suspect the focuser cannot move out 35+42.3 = 77.3mm. It looks like about 50mm of M42x0.75 extension would work.

    When ordering, don't confuse M42x1 Pentax screw mount with M42x0.75 T-mount!

    Not sure about the 1.25 adaptor. I guess none of the the M42 parts are there when an eyepiece is used? A 2" to 1.25" adaptor may be about 10mm thick? The the 1.25" adaptor on the camera adds 2 or 3mm. So maybe 70-80mm 1.25" extension?

    A diagonal has roughly this much extension. An eyepiece may need the diagonal to come to focus - this is pretty common with refractors. If you can focus a 1.25" eyepiece, you will need about 15mm of inward focus to focus the ASI camera with a 1.25" nose piece. A diagonal isn't the perfect solution, but it may well work!

    It's also worth noting that telescopes don't like focusing close, so if you're testing using an object only 50m away you will need even more back focus.
     
  7. Simon Lewis

    Simon Lewis Well-Known Member CAS Member

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    yeah I did think I should stick a diagonal in the way but maybe better with a set of T extension tubes

    its does say on the manual to use a diagonal for an EP

    sure .. def using detail father away!

    next time we catch up if I can borrow them I'll confirm and get a set ordered :)

    Cheers from snowy greendale!
     
  8. Rob Glassey

    Rob Glassey CAS Vice President CAS Member CAS Committee

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    I see 2" long T-mount extension tubes on e-bay for very little.
     
  9. Grant

    Grant CAS Webmaster CAS Member CAS Committee

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    Is this the same as a nose piece? I'm looking for one from my Nikon T mount to 2 inch focuser but they seem to be called different things.
     
  10. Rob Glassey

    Rob Glassey CAS Vice President CAS Member CAS Committee

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    No. The extension tube has M42x0.75 threads at each end (inside and outside threads). The tube add 2" extra length to the optical path.

    A nose piece has an M42x0.75 thread for the camera and a 2" eyepiece barrel to adapt the camera to a 2" focuser.

    It is possible that some nose pieces may have an M42x0.75 thread on the inside where it fits into the focuser, however most nose pieces should have an M48x0.75 thread at the focuser end for 2" filters.
     
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  11. Grant

    Grant CAS Webmaster CAS Member CAS Committee

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    Thank you. That's amazingly helpful. (y)
     
  12. Simon Lewis

    Simon Lewis Well-Known Member CAS Member

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    Rob whats the link to those tubes?
     
  13. Simon Lewis

    Simon Lewis Well-Known Member CAS Member

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    bearing in mind i need about 45-50mm of tube!
     
  14. Rob Glassey

    Rob Glassey CAS Vice President CAS Member CAS Committee

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    Here's a 40mm long one, very cheap, about $NZ12 delivered! Probably long enough.

    M42x0.75 40mm T2 Thread Extension Tube T-mount Extension Ring on Both Sides new | eBay

    Another option is this: (also $NZ 12 delivered)

    1.25" Extension Tube T Mount Adapter For Digital Camera Telescope Eyepiece DC636

    It's a longer tube, intended for eyepiece or barlow projection, but the 1.25" nosepiece can be removed and I strongly suspect the internal thread in the tube is also a M42x0.75 T-thread. Can be used as just a tube with nothing inside.

    I've actually got one of these (GSO version), and mine provides a 52mm extension with T mounts each end and it fits my ASI120MC camera. Astronz sells the GSO version for $19. They are nicely made and will surely work! Probably faster delivery too.

    Camera Adapter 1.25"
     
  15. Simon Lewis

    Simon Lewis Well-Known Member CAS Member

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    ohh thought you meant on TM :)

    actually that astronz might do the job

    indeed faster!

    nice!
     
  16. Rob Glassey

    Rob Glassey CAS Vice President CAS Member CAS Committee

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    I see the 40mm tube has internal threads the full length of the tube to reduce reflections. The other adapters are just black inside, and may have reflections that might reduce contrast. Might as well try the 40mm tube.

    I can lend you my adapter if you like.
     
  17. Simon Lewis

    Simon Lewis Well-Known Member CAS Member

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    ok that be great :)

    - sat night looks best at CAS if your about

    CHeers!

    Simon
     
  18. Rob Glassey

    Rob Glassey CAS Vice President CAS Member CAS Committee

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    I'll bring it along.
     

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