## The drift method

I recommend that you confirm your polar alignment using the drift method, i.e., by measuring how well the telescope actually tracks a star. This can be done with the drive turned off, simply moving the telescope by hand around

Table 9.1 How to adjust your polar axis using the drift method. If an autoguider is used, alignment to within 1° of the pole is sufficient; if guiding corrections are not made, greater accuracy is desirable.

Drift method alignment

 If star drifts NORTH 2" per minute, move polar axis 1/8° RIGHT. NORTH 4" 1/4° RIGHT. NORTH 8" 1/2° RIGHT. NORTH 16" 1° RIGHT. If star drifts SOUTH do the opposite. Star at 40° N, low in EAST, > 60° from meridian In S. Hemisphere, star at 40° S, low in WEST, > 60° from meridian If star drifts NORTH 2" per minute, move polar axis 1/8° DOWN. NORTH 4" 1/4° DOWN. NORTH 8" 1/2° DOWN. NORTH 16" 1° DOWN. If star drifts SOUTH do the opposite.

the right-ascension axis, or with the drive turned on if you are sure it is only tracking on one axis.

You'll need an eyepiece with double crosshairs whose separation corresponds to a known distance in the sky. To measure the distance, compare the crosshairs to a double star of known separation, or point the telescope at a star near the celestial equator and time its motion with the drive turned off. Such a star will move 15" per second of time.

Then turn on the drive motor and track a couple of stars. First track a star near declination 0°, high in the south. Measure how fast it seems to drift north or south, and the upper part of Table 9.1 will tell you how to adjust your tripod.

Only northward or southward drift is significant. If the star drifts east or west, bring it back into the center of the field (by moving east or west only) before making the measurement. In fact, you can perform this whole test with the drive turned off, tracking the star by moving the telescope by hand in right ascension only.

Once you've adjusted the polar axis in the left-right direction (azimuth), track a star that is low in the east, around declination +40°, and use the lower half of Table 9.1 to adjust the polar axis in altitude.

Table 9.1 also gives directions for drift method alignment in the Southern Hemisphere, where the drift method is more necessary because there is no bright pole star.

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