Precise CCD Drift Alignment in the Southern Hemisphere

This method makes use of a CCD camera. I will only reference this using a SBIG camera and a software package called CCDSoft V5. However its principles can be applied to other software packages.

In terms of the level of accuracy the Meade 9mm IR eyepiece tiny square cross hair is 5.23 x 5.23 arc-seconds in size at F10 on a ten inch telescope. This method we will be measuring drift at the 1-2 arc-second level which wouldn’t appear as movement in the 9mm IR eyepiece.

I will assume you know how to use CCDSoft and Microsoft Excel.

  1. Align the camera so the X and Y axis of the camera are close to the telescope X (RA Axis) and Y (DEC Axis) axis. Calibrating the Autoguider will confirm this in the settings button under “Drive Calibration Results”.

  2. Point the telescope to the east somewhere on the celestial equator as described in the previous section.

  3. Calibrate the Autoguider using the Primary Imager and not or the Autoguider CCD (otherwise your wedge adjustment corrections will be to be in reverse) as the SBIG inbuilt Autoguider CCD is reflected off a mirror.

  4. Under Setting/Drive Settings/Advanced turn on “Log Auto guiding Data” and un-check the enable box on the X-Axis and Y-Axis this disables the corrections. If X errors get bad during auto guiding which is caused by mount tracking speed or periodic error turn X-Axis corrections back on.

  5. Set the Autoguider for a 5 second exposure this will average out any bad seeing.

  6. Move the telescope in the DEC axis and then move it back to centre and note which way north and south drift is. This will correspond to Y+ or Y- errors when auto guiding.

  7. Select a suitable guide star and select Autoguide.

  8. Wait 5 minutes and stop the Autoguide. Open up the log file “C:Program FilesSoftware BisqueCCDSoft Version 5Autoguider.log” with Microsoft Excel and import the data into the spreadsheet. The data you are interested in is the “GuideErrY” column. Select this data and cut and paste it into this spreadsheet into the “(Y) DEC Error Pixels” column. Make sure you set the arc-second per pixel size in the spreadsheet. This can be found by using Ron Wodaski’s CCD calculator.

    Drift Latitude Alignment - Up/Down (5 minutes)

    Download drift alignment spreadsheet example.

  9. If the graph shows:

    1. Positive Result = Your Wedge is to Low, Raise It

    2. Negative Result = Your Wedge is to High, Lower It

  10. Make small adjustments only and repeat the procedure from step 7 and delete the Autoguide log file.

  11. You will notice the line gets shallower as you get closer to the SCP. Once you have drift under control for 5 minutes go to 10, then 20 and finally 30 minutes. You will also notice the graph fluctuating around +/- 1 to 2 arc-second mark this is normal due to seeing. You can also add a trend line to the graph just to be sure that average drift doesn’t exceed 1 arc second.

    Note: Make sure your guide star doesn’t get too high.

    TIP: The adjustments become very small, almost hairline moves of the knobs. You can also take an exposure before making the adjustment noting the X, Y position of the star, this works best where the meridian and celestial equator meet. Then make the adjustments, take an exposure then subtract the difference to find out how much the mount moved (we are talking arc-seconds of movement) this gives you better control over your adjustments. Adjustments will get smaller and smaller as you get closer to the pole.

  12. Slew the telescope to the celestial equator and the meridian and select a suitable guide star and select Autoguide and repeat the procedure above.

    Drift Azimuth Alignment - East/West (5 minutes)

    Download drift alignment spreadsheet example

  13. If the graph shows:

    1. Positive Result = Your Wedge is too far East, Rotate it West (Right)

    2. Negative Result = Your Wedge is too far West, Rotate it East (Left)

  14. Once movement is under control for 30 minutes go back to the east part of the sky and check its alignment and make any additional adjustments. This procedure put me under 1 arc-minute of the south celestial pole.

    Drift Latitude Alignment - Up/Down (30 minutes) Drift Azimuth Alignment - East/West (30 minutes) Tpoint Polar Alignment Info

Step 4 of 4 – Confirming your alignment with Tpoint

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