FW8G for STXL 6303 and calibration

Discussion in 'Filter Wheels' started by Bradley Walter, Aug 6, 2015.

  1. Bradley Walter

    Bradley Walter Cyanogen Customer

    Feb 9, 2015
    When I bought the camera with filter Wheel in June of 2013 (SN X13050236) The documentation contained no information on the guiding sensor in the filter wheel. I looked on the Website recently and it showed a KAI-0340 sensor. This sensor has 7.4 x 7.4 micron pixels. Can you confirm that this is the sensor used in my filter wheel? It seems to cover too large a patch of sky for this sized sensor but that is only an impression looking at star patterns I see on the chip. The fits header info is consistent with the KAI-0340 sensor.

    Second issue is that when I calibrate near the zenith (29 deg N) the motion in the Y direction is significantly larger than in the X direction. Also the speeds auto-filled into the the manual calibration speed boxes by the calibration process are typically something like X Speed = 8.50065 and Y speed = -8.7851 (these were with the telescope at 28N declination. Is the difference in speed due simply to the auto dec adjustment for guiding. The difference apparent motion from the calibration "trails" in the image seem larger than one can attribute to the approximately 3% difference in speed. This is not backlash or stiction issue because the beginning and ending points of the opposing pair of movements in each direction are identical. The camera is oriented almost exactly with the X axis in the RA direction. It is off by only 0.48 degrees. The pulse guide rate in the AP driver is set to 0.50X and the calibration times are 15 sec for both axes.

    On another note I found I can not calibrate the guider with the default 0.1 sec delay after correction and delay after command. My AP mount seems to require something around 0.5 sec.

    Thanks Brad Walter
  2. Bill

    Bill SBIG Service and Repairs Staff Member

    Oct 12, 2014

    I can confirm that it is the 340 guide chip, I'm unable to comment on the 2nd paragraph.

  3. Doug

    Doug Staff Member

    Sep 25, 2014
    There is a focal reducer lens built into the FW8G assembly; that is why the sky image covers a wider field than you might expect. The focal reducer gives much higher odds of finding a suitable guide star, and has no impact whatsoever on guiding accuracy.

    There is a 12.5% reduction in apparent RA speed at 29 degrees N. I gather you are using MaxIm DL for guiding. Auto Dec, if enabled, is taken into account when calibrating; it adjusts the X calibration number so that the effective equatorial rate is recorded. Your X Speed and Y Speed came out very close to each other; that suggests things are working as expected.

    The delay after correction / delay after command entirely depends on the response of your mount to guider commands. If there is a lag due to mechanical and/or electronic causes then you will have to increase the delay time. That's why the control is there!


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