ST-i as guider with ST-i Kit -- image scale

Discussion in 'Specialty Cameras SG-4, AllSky, ST-I' started by Pieter Strauss, May 7, 2020.

  1. Pieter Strauss

    Pieter Strauss Cyanogen Customer

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    Nov 3, 2014
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    I have been working some guiding issues with the PHD2 folks. Their software reports the image scale of the ST-i when guiding through the kit's cctv 100mm lens as "15.26 arc-sec/px".
    It is their stated opinion that this scale is too rough and is "ridiculous".
    I have not been dissatisfied with the performance of this guiding solution (I attach the kit to a TMB80 SS piggybacked on a Meade LX200). However, the ST-i probably won't last forever. I would like to know SBIG/Diffraction Limited's recommendation for a proper imaging scale for a guiding camera.
    --Pieter Strauss

    PS when imaging through the LX200 itself, I guide with the ST-i mounted in the 8300 OAG.
     
  2. Colin Haig

    Colin Haig Staff Member

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    What's the main imaging scope optical parameters?
    What mount do you have? (LX200 classic? LX200GPS? or a GEM?)
    It's not "ridiculous" if your seeing conditions are 1 to 2 arcsecond/pixel, as the guiding software should be using a centroid calculation.
    If you are doing high magnification stuff (e.g. 3000mm focal length), then it's not ideal, we'd recommend going off-axis with an SBIG StarChaser SC-2.
     
  3. Doug

    Doug Staff Member

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    You can easily guide at 20X lower resolution than the main camera. The guider algorithms use a centroid that can measure to much better than 1/10th a pixel.
     
  4. Pieter Strauss

    Pieter Strauss Cyanogen Customer

    Joined:
    Nov 3, 2014
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    Sorry, forgot I needed to supply more info. In this case, the ST-i kit is hard mounted on a TMB80SS (80 mm, f6.3, focal length 504mm), and the 8300M with filterwheel is mounted on the TMB80SS. There is an Astrotech field flattener, then a spacer, then the 8300M. The TMB80SS is ring mounted (Losmandy rings) on a Losmandy plate bolted to the top of the LX200, which is the 10 inch ACF GPS model. So the ST-i is sending guiding images to PHD2 on a PC via USB, and PHD2 is sending the move information to the LX200 mount via a USB adapter and a serial cable. PHD2 now prefers to send guiding commands to the LX200, rather than have them go through the ST4 port. In PHD2 the ST-i is set to take 3 second images.

    For your information, PHD2 reports the following
    RMS
    RA 3.49" (0.23 px)
    DEC 2.55" (0.17 px)
    Total 4.32" (0.28 px)

    Also FYI the problem I am working on is the fact that the TMB80SS produces star images which are flat on one side (the top, I believe). I have been working on this for 10 years, and think it is time for a new telescope!
     

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