Using Maxim DL for polar alignment

Accurate polar alignment is critical for accurate guiding of astronomical objects for long exposure imaging. Here I describe a simple and fast method to polar align your scope using Maxim DL. I can usually get an accurate polar alignment within 10 - 15 minutes using this technique. My camera is a QSI 660WSG monochrome camera and the mount I currently have is a Takahashi EM200 USD III german equatorial mount. My guidescope is an SBIG ST-i.

 

Below is a description of the procedure that I use:

 

First, level the tripod. I use the polar alignment scope on the Takahashi EM200 to get a rough (it's actually pretty accurate) polar alignment. Then disconnect the autoguider cable that runs from the SBIG ST-i autoguider to the mount. The ST-i autoguide camera (or whichever camera you are using to autoguide) must be aligned east-west. You can do this by taking taking an exposure with the ST-i, then right click on the image and enable the crosshairs. Then with the mount tracking turned off, take multiple exposures separated by several seconds with the ST-i autoguider and note the direction that the stars move. Rotate the autoguider until the stars drift parallel to the horizontal crosshair line. The autoguider is now aligned east-west.

Note that the ST-i is mounted in a cradle so that the ST-i can be rotated and then tightened. With this arrangement, and storing the telescope with the ST-i attached to the scope, makes aligning the ST-i a one-time only procedure-- very handy for portable set-ups.

Now point the scope at the celestial meridian toward the equator. Go to the "Guide" tab in Maxim and take an exposure with the autoguider. Then start tracking, turn on the graph display and watch the drift in declination only (Y error)(see Figure below).

Notice if the drift is up or down. You may need to adjust the vertical scale to see the drift. I usually start with a vertical scale of 5 pixels with the SBIG ST-i autoguider, then go to 2 pixels for fine tuning.

If the star drifts North of the central line, adjust the azimuth to the East. If the star drifts South, adjust the azimuth to the West. It is best to make a cheat sheet so you can remember which azimuth knobs to tighten depending on the drift you see on the graph.

Adjust the azimuth knobs until there is virtually no drift above or below the "Y" axis line. You may overshoot or undershoot the correction, but that is OK, you can make slight adjustments until the running average deviation is only a couple of pixels.

Usually, the corrections needed are VERY small due to the accuracy of the Takahashi polar alignment scope. When you have the azimuth deviation corrected, stop the autoguider. Next, point to a star near the East horizon, close to the equator. Make sure the star is at least 20 degrees above the horizon, to avoid effects of refraction.

Again, take an exposure, and start "Tracking", and watch the north/south drift on the "Y" axis as before. If the star drifts North, move the polar axis down. If the star drifts South, move the polar axis up using the mount elevation control. Adjust the elevation until the drift is small.

You can point the scope at the meridian near the equator and repeat the process for azimuth and elevation adjustment to get an even better polar alignment. Remember to attach the autoguider cable to the mount when you are done with the polar alignment. procedure.