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  1. #1

    Default Focus Max and ACP Scheduler

    I intend to install a MoonLite motorized focuser on at least one scope up here. Currently the stepper motor/controller unit is indoors for testing, and so I can learn about it, and Focus Max.

    I have a tough task for this focuser.

    On many nights a scope may cover hundreds of photometry targets, and use at least two filters per target (e.g. V and B, or B V R I). I would like the motorized MoonLite focuser to utilize ACP Scheduler's filter offsets (so that each filter has similar focus/FWHM) for every filter change (up to 500 on long winter nights), and at the same time use temperature compensation through the night so that I don't have to stop and refocus.

    Is this possible with ACP Scheduler and Focus Max? (The MoonLite motor controller has a temperature sensor.)

    Thanks in advance.
    --
    -------------------------------------------
    Tom Krajci
    Cloudcroft, New Mexico
    http://picasaweb.google.com/tom.krajci

    Center for Backyard Astrophysics (CBA)
    http://cbastro.org/ CBA New Mexico

    American Association of Variable Star
    Observers (AAVSO): KTC http://www.aavso.org/
    -------------------------------------------

  2. #2
    Join Date
    Dec 2006
    Location
    San Jose, CA, USA
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    898

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    I agree - it'd be great to see temperature compensation integrated into ACP so it had an option to only adjust the focuser between exposures, rather than the standard ASCOM driver behavior which could affect image quality by causing vibration when adjusting focus during a long exposure.

    John

  3. #3
    Join Date
    Oct 2005
    Location
    Mesa, AZ
    Posts
    28,216

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    Just for clarity, it already uses the focus offsets... you're asking for it to also allow TempComp during the night? The problem is, per the ASCOM Focuser interface:

    If the TempCompAvailable property is True, then setting TempComp to True puts the focuser into temperature tracking mode. While in temperature tracking mode, Move commands will be rejected by the focuser. Set to False to turn off temperature tracking. An exception will be raised if TempCompAvailable is False and an attempt is made to set TempComp to true.
    So AFAIK there's not a way to do both from a client like ACP or MaxIm. Perhaps the Moonlite driver has a way to turn on TempComp and still allow focus position changes for AutoFocus and filter offsets?

    The bigger picture, though, is that it takes maybe 2 minutes to focus. Out of an 8 hour night with a focus every hour, that's 97% efficiency. I know, the temperature changes rapidly in the early evening, etc. OK, for a focus every 30 min, that's 94% effficiency. Maybe I'm missing someting?
    -- Bob

  4. #4
    Join Date
    Oct 2005
    Location
    Mesa, AZ
    Posts
    28,216

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    John --

    Are you saying ACP should implement the temperature compensation servo? My gut feel is that's a bit "low level" for ACP to do. It seems more reasonable to make that a feature of the focuser/driver so that whenever the client changes the focus, additional temperature compensation would be applied as well. This keeps all that part of the focuser system itself. Plus it works within the existing ASCOM interface.
    -- Bob

  5. #5

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    Quote Originally Posted by Bob Denny View Post
    Just for clarity, it already uses the focus offsets... you're asking for it to also allow TempComp during the night? The problem is, per the ASCOM Focuser interface:...
    Yes, turning on ASCOM temp. compensation means the focuser ignores move commands.

    But there are other ways to skin the cat, yet allow for temp. compensation in the focusing...if you do it when you do filter changes/filter offsets.

    At the start of the night the focuser position can be read. And the ASCOM driver has info on the temp compensation 'slope' of the focuser/optical system. And, read the temperature at the start of the night (if the focuser has a temperature probe...and that's becoming more widely available.)

    At each future focus change...read the temperature, and compare it to the start of the night. Delta-T multiplied by focus slope tells you how much 'temperature offset' is needed at that moment. And, add the filter offfset...and that's the desired focus position to move to.

    If you use really long individual exposures,(and/or your system changes focus a large amount for a given temperature change), then you may need the ASCOM temp. compensation to do its magic in the middle of the exposure (if it doesn't shake the OTA too much when the focuser motor runs).

    But for most folks that use shorter exposures...calculating and implementing a 'temperature offset' at each focus move (along with filter offset)...should do pretty well.

    Quote Originally Posted by Bob Denny View Post
    So AFAIK there's not a way to do both from a client like ACP or MaxIm. Perhaps the Moonlite driver has a way to turn on TempComp and still allow focus position changes for AutoFocus and filter offsets?
    I think the OPTEC TCF focusers allow something like that, but don't quote me on that.

    Indoor testing shows that the MoonLite focuser will not respond to a move command when the ASCOM temp. compensation mode is active.

    Quote Originally Posted by Bob Denny View Post
    The bigger picture, though, is that it takes maybe 2 minutes to focus. Out of an 8 hour night with a focus every hour, that's 97% efficiency. I know, the temperature changes rapidly in the early evening, etc. OK, for a focus every 30 min, that's 94% effficiency. Maybe I'm missing someting?
    Some of the mounts up here point 'in the blind' well enough...that I check alignment in evening twilight...and press the go button. No more plate solve exposures every so often. You are asking for a passing cloud to invoke Murphy's Law and be there when you take the plate solve exposure...and then another...

    Some of the mounts up here track well enough that I don't need to use an autoguider for the exposure durations I typically use. Therefore I don't use the autguider. That's another way of tempting passing clouds to give you grief.

    I'm doing indoors/bench testing of the Moonlite Focuser...repeatability...creep/position loss, etc. I think it'll do well for my not-heavy CCD/filter wheel.

    If that's the case, then I'd prefer to do temp. compensated focusing all night in the blind...and not tempt fate by focusing on an acutal star every so often.

    In the next year or two, expect more and more motorized/robotic focusers to incorporate linear encoders on the focuser drawtube. Those focusers will be even better in terms of long term repeatability/precision. They will be eminently suited to focusing all night 'in the blind'.
    --
    -------------------------------------------
    Tom Krajci
    Cloudcroft, New Mexico
    http://picasaweb.google.com/tom.krajci

    Center for Backyard Astrophysics (CBA)
    http://cbastro.org/ CBA New Mexico

    American Association of Variable Star
    Observers (AAVSO): KTC http://www.aavso.org/
    -------------------------------------------

  6. #6

    Default

    Quote Originally Posted by Bob Denny View Post
    John --

    Are you saying ACP should implement the temperature compensation servo? My gut feel is that's a bit "low level" for ACP to do. It seems more reasonable to make that a feature of the focuser/driver so that whenever the client changes the focus, additional temperature compensation would be applied as well. This keeps all that part of the focuser system itself. Plus it works within the existing ASCOM interface.
    If someone can show me how to implement temperature compensated focusing 'in the blind'...that also incorporates filter offsets...then it's not too important to me exactly how it's done.

    ACP has auto-centering and a pointing model. For some of the scopes up here I don't use that. Instead I use TheSky/TPoint. Six of one, half a dozen of the other.

    I'm still learning about robotic focusers, but currently I don't see an easy way to incorporate temperature offsets along with filter offsets. With the ASCOM temp. compensation method...it's one, or the other, but not both.

    Or am I missing something?
    --
    -------------------------------------------
    Tom Krajci
    Cloudcroft, New Mexico
    http://picasaweb.google.com/tom.krajci

    Center for Backyard Astrophysics (CBA)
    http://cbastro.org/ CBA New Mexico

    American Association of Variable Star
    Observers (AAVSO): KTC http://www.aavso.org/
    -------------------------------------------

 

 

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