A0-8T backfocus spacing

Discussion in 'Adaptive Optics (AO)' started by Wannaberocker, Nov 20, 2016.

  1. Wannaberocker

    Wannaberocker Cyanogen Customer

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    Hello, I am a bit confused here.....Do you measure the back focus to the CCD chip or to the A0-8T lens? Does the AO-8T have its own backfocus from the lens paddle to the CCD chip? thanks
     
  2. William B

    William B Cyanogen Customer

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    Backfocus is measured from the CCD chip to the e.g. flattener, or primary objective, primary mirror etc and not the AO-8 tilt paddle which being a plain, flat glass disk has no "focussing power" for want of a better expression, of its own.

    But, as with any plain glass element in a converging beam path (light cone) such as a glass filter, or in the AO-8, the tilt plate, refraction of the light beam as it passes through the plate pushes the focal point further out and the general rule is to add ~1/3 the thickness of the glass element to the backfocus length.

    For the AO-8 the backfocus used up when installed in the focal path is the mechanical thickness of the body plus adapter plates etc minus 2mm, i.e. when working out your BF distances the body of the AO-8 will appear optically 2mm thinner than it's mechanical thickness.

    Hope that has not confused you more...See page 9 of the AO-8 manual where the actual dimensions are stated:

    https://diffractionlimited.com/wp-content/uploads/2016/04/ao8manual.pdf

    William.
     
  3. Doug

    Doug Staff Member

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  4. Wannaberocker

    Wannaberocker Cyanogen Customer

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    Ok so does this mean I cannot use a reducer with the AO unit? The guys at OPT told me I basically cannot. I have the Celestron Edge 9.25 with Astro-Physics 27TVPH .75reducer and get a messed up star field or quite a bit of optical distortion. I was also informed that you can us ethe AO device with the EdgeHD 11" with reducer. I'd like to be able to use the AO because it is alot like magic when guiding. SO can I use this with a reducer on my 9.25 or do I need to get a different telescope such as the EdgeHD 11"" with reducer?
     
  5. Doug

    Doug Staff Member

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    The spacing from the reducer to the focal plane is critical. If you can arrange some adapters to put the AO in front of your reducer then you might be able to get it to work.
     
  6. Wannaberocker

    Wannaberocker Cyanogen Customer

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    In front of my reducer? That just sounds ridiculous and impractical. How am I supposed to do that? So what kind of telescope exactly is this AO unit supposed to work with? Does it work with any kind of practical setup? When it was designed there must be some kind of optical system that works with this, what is it? Otherwise this thing is just a really expensive paper weight.
     
  7. Doug

    Doug Staff Member

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    You can mount it directly on just about any SCT, RC, CDK, etc.

    Focal reducers are always an issue for imaging. It's inherent in the way focal reducers work - they always greatly reduce your back focus, and their position relative to the focal plane is critical.

    Generally speaking, AO is more important at longer focal lengths. Shorter focal lengths require less critical guiding.
     
  8. JoshuaHufford

    JoshuaHufford Cyanogen Customer

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    I'm just curious as to why you want to use an AO unit to hopefully increase your resolution then use a focal reducer to reduce your resolution? Perhaps I'm missing something.
     
  9. Wannaberocker

    Wannaberocker Cyanogen Customer

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    Joshua my plan was just to use the optical guiding aspect of the AO to help with random guiding errors and not to increase resolution. The point of the focal reducer is to get to the F7.5 speed and have a wider FOV and then use the AO to help "cancel out" random guiding errors that my guidescope can't account for. I have to use the guidescope because MaxIm DL has a major glitch when trying to calibrate the drive in the AO tab, therefore MaxIm DL cannot activate the mount properly when the AO reaches it's tip tilt limit so I just use a guide scope with PHD2 and use the AO to cancel out any flexure, mirror shift etc..., I am not trying to increase resolution. That can only be done by changing the image scale which the AO doesn't do, infact this type of AO is basically only a high speed optical guider and as a "true" AO that will distort the mirror based on a diffraction sample generated by a false star projection like "real" AO devices do. It sounds like you haven't used one of these before based on your question but then again I could be missing something here as I have about 3 months experience with an SCT.
     
  10. Doug

    Doug Staff Member

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    FYI, MaxIm DL's AO control is being overhauled right now. Owen expects to have it available for beta test in about a week.
     
  11. Wannaberocker

    Wannaberocker Cyanogen Customer

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    Good to hear Doug! Hopfully the guidestar faded issue gets fixed too!
     
  12. Doug

    Doug Staff Member

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    We have eliminated at least one glitch that can cause a guide star fade.

    The thing is, "guide star fade" as a symptom is kinda like a sneeze. It could mean you have a common cold, or the flu, or an allergy, or an infection, or... etc. A fade could be caused by certain types of software glitch, hardware glitches of all kinds, clouds, user error, you name it.

    What I can say is the new AO control will be much more robust than its predecessor. Owen has refactored major parts of it, simplifying its design and eliminating potential timing loopholes.
     
  13. Wannaberocker

    Wannaberocker Cyanogen Customer

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    I see what you are saying, however there is a repeatable condition where I'll start guiding on a good star and as soon as I start the sequence the star will fade even if I still see it in the track box. I have to stop he sequence and restart guiding and usually this fixes it but it would be nice to not have to do all that.
     
  14. Wannaberocker

    Wannaberocker Cyanogen Customer

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    Doug...any word on that beta release? will the update be free to use MaxIm users?
     
  15. Doug

    Doug Staff Member

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    "Soon." It's getting there... but I won't make any prediction of when it will be released.

    It will be available to anyone who has 6.13.
     

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