How to zero the telescope pointing in TCS

How to zero the telescope pointing in TCS

Ideally, the telescope pointing should be properly zeroed before you start your run. After slewing to your target, you should be able to see it in the guide camera. If you are unable to see your target in the guide camera after slewing, you probably need to reset the zero of the pointing.

Below we detail the steps you need to take to reset the zero of the pointing:

TCS software does not properly take into account proper motions for any stars besides those in the FK5 catalog.  Do not use any other catalogs stored in TCS when selecting a star to zero the telescope pointing on!

Question for telescope staff: (Does picking a guide star at the celestial equator Dec ~ 0 improve the pointing? Or is picking a star near the zenith Dec ~ +30 preferable?)

Select the next target for the telescope as a bright star only one of the following ways:

1. Select Next -> Star -> FK5 and enter the star number in the FK5 catalog.

2. Select Next -> FK5 Stars Near Zenith then click on a star near the zenith.

3. Enter RA & Dec. of a star with the current epoch (NOT J2000) so that you can be sure to be pointing at a star, even if it has high proper motion. These coordinates can be taken from The Astronomical Almanac for the current year

      Alternatively, you can use any other bright star with low proper motion and known coordiantes. Ensure that the coordinate and epoch are consistant or you may induce a larger offset than you     intended to remove. 

Slew telescope to target star.

Center the star in your guider camera. If you are unable to see the star, dither the telescope around in increments of 1 or 2 arcminutes until you find the star.

On TCS, if CZero is not =0 in RA and Dec., select the following: CZero -> New HA/DEC cos(dec) Zero Points. Warning: Do not select Default HA/Dec cos(Dec) zero points, this will not update CZero but set the value in TCS to 0, and you will continue have trouble pointing the telescope.

On TCS select the following: Zero -> New HA/Dec Constant Zero Points

Now the telescope pointing should be properly zeroed and the targets for the rest of your run should appear properly in your guide camera after the telescope slews to a target. If the pointing is still off a little bit, repeat the steps above.

If pointing is still bad you might want to try resetting TCS, as detailed in this manual.