Adaptive optics useful on FSQ-106 @f/5?

Discussion in 'Guiding and Adaptive Optics - StarChaser and AO' started by Chris Anderson, Jul 30, 2016.

  1. Chris Anderson

    Chris Anderson Cyanogen Customer

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    I am in a bit of a pickle. Quick background. I just purchased an STT-8300M+FW8G-8300 to mount on my Takahashi FSQ106EDXIII with FLI Atlas focuser. I am running this setup at f/3.5 (using the QE .73 focal reducer.

    Problem is, I don't have enough back focus for this image train. (Face slap). So, I am losing the .73 focal reducer to gain significant back focus to use the new toys. In another thread in another forum on an unrelated topic, someone suggested that at the fast focal lengths of the FSQ, I'd see little significant increase in image quality using the AO-8T.

    Additional background (and please forgive my lack of knowledge of adaptive optics). I live in the Ohio Valley, where nighttime temperatures of 80+degrees are matched only by the percentage of humidity. Lots of twinkling stars duringthe summer months from March to November.

    Also, I am going to have to span a gap of significant back focus that the AO-8T would come close to filling. It's either that or a super long adapter ring from PreciseParts. Sure, that ring would be significantly cheaper than an AO-8T, but if it would help at all, I'd be willing to invest. It's all about toys, right?

    I'd be interested in thoughts on whether the AO is worthwhile with my FSQ106.

    Thanks Forum!

    CG
     
    Chris Anderson, Jul 30, 2016
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  2. Colin Haig

    Colin Haig Staff Member

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    Hi CG, others will have to weigh in, AO is almost always advantageous, but that said, at short focal lengths, with those pixels, not sure how much better it would get.
    Am curious about your back focus problem - do you have some dimensions to share - am wondering if there is a way to move the reducer to the other side of the focuser or something like that - e.g. another configuration.
     
    Colin Haig, Jul 30, 2016
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  3. Colin Haig

    Colin Haig Staff Member

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    Actually a photo or sketch might help, illustrating the setup.
     
    Colin Haig, Jul 30, 2016
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  4. Chris Anderson

    Chris Anderson Cyanogen Customer

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    Colin, thanks. I don't have any photos as I have yet to receive all my adapter rings (which are all now on hold). But what I can say is my current rig goes theoretically something like this, from the "end" of the QE .73 reducer on my FSQ:

    FLI Atlas (with 6.6mm of adapter rings/focus in all the way= 38.3mm) + SBIG STT-8300+FW8G (54.05mm). This, I think, puts me at 92.35mm beyond the reducer.

    I only have 72mm+\- of back focus with the reducer. So I think I am pretty much out of the park with this setup, being at 92mm+.

    I'm still doing the math, but I think the AO would neatly fit in the train @f5 native. Lose some wide field, but not a deal breaker, considering the alternative is to send back the STT-8300M/FW8G kit I just bought and try and scrape together a non-self guided wheel and their Remote Guide Head. Really looking forward to guiding in the filter wheel.

    Have no idea what moving the reducer would entail.

    Does this help?

    CG
     
    Chris Anderson, Jul 30, 2016
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  5. Colin Haig

    Colin Haig Staff Member

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    Chris, I spent some time on this, for fun. (Just to verify if there were discrepancies between your calculations and mine.)

    I couldn't figure out how much actual back focus you would have from the FSQ with or without the reducer - seems like a critical dimension, and not specified anywhere.
    Do you know what it is (have you measured?)

    With the QE reducer, looks like your system would be f/3.64 not f/3.5, with a focal length of 387mm.

    STT Camera+FW8G-STT+2mm filters = 52.8mm (if you have 3mm filters, the backfocus table says 52.5), without whatever adapter plate.
    About 25mm to 34mm for the FLI focuser if I read that right, plus 6.6mm of adapters.
    52.8+31.6 =85.4, which is still too much.
    Any chance you could put the focuser AHEAD of the reducer? Not sure how much that would buy you, but it might be just enough.
     
    Colin Haig, Jul 31, 2016
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  6. Colin Haig

    Colin Haig Staff Member

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    After posting I realized Ihad already used your back focus numbers, but am curious where they came from. Basically, looks like you can't make this work with the reducer ahead of the focuser, but maybe after the focuser with the right adapters.
     
    Colin Haig, Jul 31, 2016
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  7. Chris Anderson

    Chris Anderson Cyanogen Customer

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    Colin: It is "documented" that the back focus with the Reducer QE is 72mm +/-2mm. Without it, the back focus is 178mm. Those numbers are floating around, and I also got them from Fred at TakAm. My problem, is I'm not sure where this is measured from. I plan on actually measuring it today using the sun and perhaps a Hartmann mask. You are correct. With the reducer the EF is 387mm @f3.64~. My mistake.

    As far as mounting the reducer someplace else, have no idea. Most of the TAk stuff seems to be very placement-specific (hence the gazillion adapter rings they make!). I'd have to ask Fred about thinking this far out of the Tak box. Still, I'm actually OK with operating at f5, as it's still pretty damned wide field.

    I'll post back some real numbers.

    Still can't find much info on running AO with short refractors like mine... CG
     
    Chris Anderson, Jul 31, 2016
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  8. Colin Haig

    Colin Haig Staff Member

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    Yeah, I had the same issue, trying to figure out the reference datum for the back focus distance. e.g. is it from the back of the end of the OTA, past the captains wheel, etc.
    Maybe Fred will have ideas.
     
    Colin Haig, Jul 31, 2016
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  9. Chris Anderson

    Chris Anderson Cyanogen Customer

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    OK. Spent several hours, caliper in hand, trying to get some numbers. The @f5 back focus is measured from the very end of the OTA. This is what I have so far:

    STT8300M + FW8G-STT - threaded adapter plate = 52.5mm
    AO-8T bolted flush to FW8G = 48.0mm
    PrecideParts AO8T to FLI Atlas bolted adapter = 5.10mm
    FLI Atlas set at mid point of focus travel* = 37.1mm
    PreciseParts FLI Atlas to TAK CAA adapter ring = 8.7mm
    Camera Angle Adjuster (CAA) including #5 ring = 42.4mm
    ____________
    TOTAL 193.8mm
    WORKING BACK FOCUS 178.0mm
    OVER 15.8mm
    It won't fit.

    (*) I prefer to use the FLI Atlas to control all focus and leave the FSQ focuser set all the way in to minimize any flexure.

    Now, I could lose the Camera Angle Adjuster and have plenty of room, just have PreciseParts machine an additional ring to span the gap from the rear of the OTA to the FLI Atlas. I seldom ever use the CAA to rotate the camera, as I like keeping the avix of the camera parallel with the scope axis.

    So, the question still begs: Is it worth going with AO in an f5 ratio scope?

    Chris
     
    Chris Anderson, Jul 31, 2016
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  10. Doug

    Doug Staff Member

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    The f/ratio isn't the important factor. Image scale is. That depends on focal length and pixel size.

    I should also point out that AO can be used to make a crappy mount work like a nearly perfect one.
     
    Doug, Jul 31, 2016
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  11. Chris Anderson

    Chris Anderson Cyanogen Customer

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    Learning! I'm at f.l. 530mm and 2.1 arcsec/pixel. My mount is an AP900 GTO, so pretty good. Maybe not dead nuts polar aligned.... CG
     
    Chris Anderson, Jul 31, 2016
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  12. Doug

    Doug Staff Member

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    Probably you would want sub arc-second pixel image scale for AO to have significant benefits.
     
    Doug, Jul 31, 2016
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  13. Chris Anderson

    Chris Anderson Cyanogen Customer

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    I'm tending to agree....
     
    Chris Anderson, Jul 31, 2016
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