Local Pointing Check Stars

John Thorstensen, Dartmouth College

2012 September

Why and What: The MDM telescopes point pretty well, but not well enough to put your object dead in the slit if you're doing slit spectroscopy with CCDS, Modspec, or Mark III. The guide-star based strategy outlined here is very useful, but it's still a great help to be able to hop over to a nearby bright star (with known coordinates) to see where the object will fall in the slit-viewing camera when you go back to it.

The very simplest (but not the best!) way to do this with the 2.4m is to go to a star from the Bright Star Catalogue. The xtcs lets you load the "Nearest Bright" star coordinate automatically; it's an option in the "Coords" menu. But this is less than ideal (unless you're totally lost and need something unmistakable). The nearest bright star is typically several degrees away, so (a)it takes significant time to get there, and (b) this far from the target, the telescope's pointing offsets may be significantly different.

Here is a better way, which takes only a few minutes to set up for your whole target list. It uses the Tycho-2 astrometric catalog, which goes to about 11th magnitude and hence is much denser than the bright star catalogue.

Here's how to do it:

Pointing stars in action: The ".pt1" format is designed to be handy with the xtcs window on the 2.4m. After loading your coordinate list, you call up your object's coordinates by typing its name and hitting Enter. To get the pointing star all you need to do is add ".pt1" to the name and type Enter again. To get another, just delete the last digit and replace it, then again hit Enter. [Note that the GUI requires the DELETE key, not backspace.] By calling up the coordinates and hitting Send and Go as needed, you can quickly hop between the pointing stars and the target, so you can be certain you're looking at the right star.

Note that if your target is a relatively bright star, like 10th magnitude, it will probably be in the Tycho catalog, in which case the ".pt1" star will actually be your target.

Why not just re-zero the coordinates? Well, you could, and you should if your pointing starts to drift. However:

So, it's best to wait until the pointing gets a little ragged, and then reset the coordinates very carefully not too far from the zenith. As always, when you do reset the pointing, do not move away until you've checked that the coordinates are correct, or you're lost.