Telescope Alignment Eyepiece For Polar Alignment

This is the Agena 1.25″ Wireless Illuminated Reticle Eyepiece with Focusable Dual Crosshair – 20mm. I bought it for $100. The illuminator can go from very dim and gradually to very bright if needed – it works well.

An eyepiece with dual crosshairs is very helpful when setting up your telescope for polar alignment. It’s better to do it with a 20mm eyepiece than a lot of the 12.5mm on the market because the 20mm has a wider field of view to find your stars.

Here are some tips for Polar Alignment when using a dual crosshair eyepiece:

Step 1: Perform a quick alignment using the procedure you normally would for Polar Alignment and make adjustments to the equatorial mount’s altitude and azimuth settings. If you don’t have an equatorial mount – you will need a wedge.

Step 2: Next, choose a bright star within twenty degrees of the Celestial Equator and within an hour in RA of the Meridian. Put the star at the center of the eyepiece cross hairs. Slowly adjust the telescope slow motion control in declination and watch which way the star moves in the eyepiece. You must rotate the eyepiece so that the cross hairs are aligned with the north-south motion of the telescope. This may take several iterations between slow motion adjustments and eyepiece rotation.

The eyepiece cross hairs are now oriented with the north-south and east-west motions of the telescope and star. Put the star on the cross hairs and let the telescope track for five minutes without making any corrections. Ignore any drift in the east-west direction. If the star drifts north with respect to the cross hairs, the telescope azimuth is pointing west of north. Use the azimuth adjustment on the mount to turn it a small amount to the East. If the star drifts south with respect to the cross hairs, the telescope azimuth is pointing east of north. Use the azimuth adjustment on the mount to turn it a small amount to the west.

After making a small azimuth correction, use the telescope RA and Dec controls to put the star back on the cross hairs and wait another five minutes. Once again, note any drift north or south of the cross hairs and make correction to the telescope azimuth as described above. Repeat this process until the is no (or very little) drift in the north-south direction. When this is accomplished, the azimuth of the telescope is now pointing to true north.

Step 3: Now it’s time to test and correct the altitude of the telescope’s polar axis. Choose a bright star within twenty degrees of the celestial Equator, and within twenty degrees of the eastern horizon. Put the star on the cross hairs and let the telescope track for five minutes without making any corrections. Ignore any drift in the east-west direction. If the star drifts north with respect to the cross hairs, the altitude of the telescope’s polar axis is too high. Use the altitude adjustment on the mount to lower the polar axis altitude a small amount. If the star drifts south with respect to the cross hairs, the altitude of the telescope’s polar axis is too LOW. Use the altitude adjustment on the mount to turn raise the polar axis altitude a small amount. Repeat the drift test for altitude several times until the drift is eliminated (or very small). The telescope is now polar aligned.

Here is a chart to simplify:

Meridian: Star drifts NORTH, rotate telescope azimuth EAST. Star drifts SOUTH, rotate telescope azimuth WEST.

Eastern Horizon: Star drifts NORTH, adjust telescope altitude LOWER. Star drifts SOUTH, adjust telescope altitude HIGHER.

Western Horizon: Star drifts NORTH, adjust telescope altitude HIGHER. Star drifts SOUTH, adjust telescope altitude LOWER.

Comments

Paul Rodgers says:

Thank you for showing us, it is good to see what it looks like before you buy.
As I am thinking of buying this one.

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