MIRLIN Sky Position Angles

The offset guider for the f/40 Visitor's Port provides a 50 arcsec square field of view centered 84 arcsec from the rotator axis. Finding guide stars for use with MIRLIN is complicated by the concern over spilling cryo. If the dewar is tilted too far, cryogen can drain out of the dewar through the fill tube, causing the detector to start warming prematurely and possibly seriously compromising your data. To avoid this we use a somewhat elaborate scheme for selecting the best rotator angle for the dewar.

The current document helps you find guide stars. The optimal dewar angle varies with elevation and declination, and the plot below shows how that value maps onto the normal astronomical position angle (North = 0°, East = 90°).

To use the plot below, find the declination of your target on the x-axis. Then, if you have some idea of when in the night you will observe the target, select the curve corresponding to the hour angle of the target at that time. The intersection of the declination and hour angle curves gives, on the y-axis, the sky p.a. of the optimal rotation of the dewar. The range in angles available is more or less 70° in either direction from the optimal p.a. (It varies slightly with elevation, but that can be ignored for the most part.

An example: you want to observe a star at declination -15°, when the star is roughly 2 hours East of meridian (HA = -2 H). The -2 H curve intersects -15° declination at p.a. +7°. Hence you could use guide stars anywhere in the range -63° to +77°.

Note, too, that the guider field covers an rotator angle of 17°, so it is even feasible to use a guide star more than 70° away from the optimal angle, if it is placed on the edge of the guide field.

Figure 1. Sky p.a. for the optimum dewar angle as a function of elevation and declination.

Using SKY

If you would like to find guide stars in real time at the telescope, you might like to use the SKY program. For instance, you might not have a good idea of when during the night you will be able to observe a target, hence which HA curve in the plot above is relevant.

Start SKY by finding the "Keck-menu" item on the pull-down menu on your computer, then selecting "Solaris apps" and finally "SKY (K2)". This will bring up SKY in planning mode, which means it will not read the time and telescope information from the telescope control system, DCS. It will also default to LRIS as the instrument, but this is easily changed.

Get your field by choosing "File," then "Open," and locating your starlist. Click on the desired target in your list, which will transfer the RA and Dec. to the fields towards the top of the main SKY screen. Now click the "LIST" button and a second window, "Sky Chart" will pop up. At this point select "F40" at the left of this screen and click "LIST" on the main screen again to load the Visitor's Port setup and reload the star information. To bring up an image of the Digitized Sky Survey click the "DSS" button near top right of the "Sky Chart" window. You should be able to guide on any of the stars visible on the DSS image, although the very very faintest ones might prove difficult.

When you get ready to observe the target, read the appropriate angle off the graph. You can either type this into the PA field on SKY's main screen, or you can use the rotation buttons in the upper right of the "Sky Chart" to rotate the image of the guide camera. The arrows between the rotation buttons increase or decrease the step size of the rotation. The current p.a. is shown in the small box near lower left.

Now the box representing the guider field is at the optimal angle. You can see whether there is a good guide star within that field, or you can look to either side roughly 70°. Find a good guide star, keeping in mind that the rotator will rotate during the integration, and you may have a limited time using that guide star. (I.e. choose a guide star close to the optimal angle if possible, especially on targets that will cross meridian close to zenith.)

Bob Goodrich
Last modified: 23 July 1999.