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  1. #1

    Default Temperature focus correction

    Hello,

    I would be interested to know what solution people are using with ACP to correct real time the focus to take temperature variations into account ?

    I have recently upgraded our telescope with an FLI Atlas focuser (recording the Temp) and I would like to improve focus stability using Temp. corrections.

    or should this be done at the level of FocusMax ?

    Thank you,
    Emmanuel

  2. #2

    Default

    Hi Emmanuel

    I only focus once a night now and then just correct the focus throughout the night for temp changes. I connect MaxIm and FocusMax to my OpTec TCF focuser through POTH. I use the Temp comp feature in MaxIm to adjust focus for temp and ACP to adjust focus for filter changes. Basically I am only using FocusMax to connect ACP to POTH. Works for me. I had 100 clear nights this observing season and made about 45,000 photometric observations. I almost always only focused at the start of the night. Perhaps on 3 oar 4 nights I was forced to intervene for one reason or another and refocus at some point during the night. Usually I was just chasing poor seeing and the refocus didn't improve anything.

    I'm pretty sure that there is a simpler way to do it. But I don't fix things that aren't broken during the observing season. But I will follow up on a couple of things now that the big wet has arrived in the Pacific Northwest. First, I think that the OpTec driver may have the capability to implement temp coeff focusing directly and I could take MaxIm out of the loop. I might not need POTH either. Second, I *think* that FocusMax has the capability to directly implement temp comp focusing and I also have to check this possibility out.

    I hope this helps.


    Jim Jones

  3. #3
    Join Date
    Oct 2005
    Location
    Mesa, AZ
    Posts
    26,857

    Default

    James --

    Clever use of the available tools!! Please report on your efforts to take MaxIm and POTH out of the loop. The cleanest way would be to use the Optec temperature compensation, which I believe works while connected to focusing software (no more separate "computer" vs "temp" modes).
    Second, I *think* that FocusMax has the capability to directly implement temp comp focusing and I also have to check this possibility out.
    I believe this has been added. The FocusMax Yahoo! Group is the place to find out. I should have known, but I was buttonholed twice at AIC and didn't make it to Steve Brady's FocusMax workshops.
    -- Bob

  4. #4

    Default

    Hello Jim,

    With the TCFS it should be much easier indeed as you can record several runs of focus with temperature, it then makes a linear regression to get the best relation focus/Temp and this is directly played back on the focuser (no need for a loop with the computer).
    I have now the Atlas, but such a feature does not exist, at least to my knowlege (I see nothing about it in the FLI forum, I'll ask Greg Terrance).
    I don't think FocusMax is able to compensate for Temp, there is nothing about it in the help , and nearly nothing on the FocusMAx forum about it. I put a thread , nobody replied.
    I guess most people have usually seeing too bad and telescope flexures that are more severe than the change of image quality due to the variations of the ambiant Temperature...

    Emmanuel

  5. #5

    Default

    Hi Emmanuel,
    I came across this post when trying to set up temp comp for our LX200 and ATLAS. I've got all new versions of ACP and FocusMax and I read online that the ATLAS doesn't work with the temperature compensation in FocusMax. I was wondering if you found a solution for automatic temperature compensation?

    Is it possible to connect to FocusMax via the ACP hub? I have an AG script that speaks to the telescope every 20-30s and adding temp comp logic in there seems sensible if possible. The nightly drifts in focus is the main thing that keeps me awake with with the telescope

    Cheers
    James

  6. #6

    Default

    Hello James,

    sorry for the late reply,

    We are not using the Atlas focuser anymore and I had not found a way yet to correct for temperature unfortunately. We are now using a focuser at M2 based on a TCFS, it is faster and the load is not so heavy. We have a temperature probe with it but not connected yet.

    I understand your concern about the focus drift during the night
    best
    ej

  7. #7

    Default

    FocusMax wil provide temperature compensation for any focuser that has a temperature sensor. BUT, the senor in FLI Atlas is buried in the metal focuser housing with electronics and would have my doubts that it will successfully track ambient temperature. You may want to test this yourself by tracking the delta temp between FLI and an external thermometer.
    The RoboFocus controller has the sensor built into the controller box - I have never found a strong correlation
    between temp & position. My Optec TCF has an external sensor and find a strong statistical correlation.

    See Menu/Open/Temperature Compensation for various options that are available.
    Steve Brady

  8. #8

    Default

    Hi guys,
    Thanks for the info. I've got about 30 measurements now and I'm just waiting on covering more temperature range before I do some fitting. I've noticed that the ATLAS temp does follow the ambient to some degree, but running a focus run always bumps the temp up by around 1C from the temp value that it started at (Tend = Tstart + 1C). This systematic jump is not an issue as it can be characterised.

    I was warned by Jim from FLI about temp comp while observing (although when I went looking for the post explaining the reasons why, the Yahoo Group wouldn't load any of the messages so I'm still in the dark as to why FLI advise against temp comp during an observing run). To me this seems strange, surely everyone wants to have good focus as much of the time as possible, right? Especially if you splash out on the flagship focuser.

    I can easily add some PT100 probes to the mirror cell and OTA to get a good measurement of the actual temperature, but then getting those values from my Raspberry Pi to FocusMax/Maxim/ACP etc might be tricky (I've only dabbled briefly with scripting in Windows/ACP etc, but I am keen to make this work). I'm curious to know what you think?

    As I mentioned before, solving this issue will allow for a lot more sleep during the week

    Cheers
    James

  9. #9

    Default

    Hi James--
    Once you have your data (temp vs position), then run a regression analysis in say Excel which wil provide the slope coefficient and goodness of fit r^2.

    I run FocusMax Temperature Compensation Wizard and collect a least 2 nights of data and compare the slope coefficients - I *DO NOT* combine the data from multiple nights to avoid introducing data bias (perhaps due to environmental influences). For my system, the slopes are the same on different nights but the regression lines are physically offset - see FocusMax Help/ Tips/Temperature compensation for more. FocusMax V4 will provide the data analysis from the data collection when the TC Wizard is run.
    Keep us posted on your progress!

    Steve Brady

  10. #10

    Default

    Hi Steve,
    I had been assuming I could use measurements from multiple nights. I should devote a few nights to model this correctly.

    I've not tried yet, but are you saying I can use the TC wizard to do the measurements. I assumed that as the ATLAS cannot do it that it wouldn't work. It's too windy tonight to try which is a shame.

    I did find this script:

    http://www.russels-astro.com/ftp/focus.vbs

    on Russel's blog (http://www.russels-astro.com/blog/?p=178) which seems to do exactly what I want. He suggests running this automatically at the end of each image acquired by Maxim. Bob, could ACP support a similar script?

    To do this accurately I'll need to get some external sensors. Another job for the projects list

 

 

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