LWS Non-sidereal Observing

Background

Observing targets with non-sidereal motions (e.g., planets and asteroids) is possible with LWS, but certain caveats apply. This page describes considerations of which the observer should be aware regarding guiding and tracking of objects.

Non-sidereal guiding

The ability to take guided exposures of non-sidereal targets with LWS depends on the speed of your targets. The major problem is that the choice of guide stars is severely constrained by the placement of the fixed guider available for the instrument in the Keck I forward cassegrain module. If your target is slow-moving, then it should be possible either to select a guide star in advance using the SKY program, or to have the observing assistant (OA) scan for a guide star once you acquire the target. However, near-earth targets may move so fast that no guide star can be maintained for more than a few minutes before it moves out the guider's field of view. In this case, you may be forced to take observations unguided.

Starlist Files for Non-Sidereal Targets

Starlist files are electronic text files containing the coordinates of your target object. Such files can be read in to the Sky program which controls the telescope, enabling the Observing Assistant to select your target at the click of a button without having to re-type your coordinates. Generally, the starlist file contains one line for each target, giving the R.A. and Dec. for a given equinox.

The SKY software has been pre-programmed with orbital information for certain solar system targets, including the Moon, the 8 other planets, and the asteroids Ceres, Pallas, Juno, and Vesta. For other targets, you must include in your starlist file the apparent position in the form of instantaneous R.A.,Dec. coordinates and corresponding differential tracking rates (see below).

A commonly used strategy by observers of non-sidereal targets is to generate a separate starlist for each object. Each contains the coordinates of a given target at standard intervals of time (perhaps every 15 minutes for a near-earth target, or at longer intervals for more distant objects). Each line begins with a unique "name" formed by combining the target name with the time (either sidereal, UT, or HST). For example, the following excerpt from a starlist for an asteroid WQ89JZ shows listings for every 10 minutes:

WQ89JZ 04:30     9 24 27.610 +15 20 46.40 2000.0 dra=7.749 ddec=-50.960
WQ89JZ 04:40     9 24 28.900 +15 20 37.90 2000.0 dra=7.727 ddec=-51.010
WQ89JZ 04:50     9 24 30.180 +15 20 29.40 2000.0 dra=7.707 ddec=-51.070
WQ89JZ 05:00     9 24 31.460 +15 20 20.90 2000.0 dra=7.687 ddec=-51.130
WQ89JZ 05:10     9 24 32.740 +15 20 12.40 2000.0 dra=7.668 ddec=-51.190
WQ89JZ 05:20     9 24 34.020 +15 20  3.80 2000.0 dra=7.650 ddec=-51.260
WQ89JZ 05:30     9 24 35.290 +15 19 55.30 2000.0 dra=7.632 ddec=-51.330
WQ89JZ 05:40     9 24 36.560 +15 19 46.70 2000.0 dra=7.615 ddec=-51.400
WQ89JZ 05:50     9 24 37.830 +15 19 38.20 2000.0 dra=7.600 ddec=-51.470
WQ89JZ 06:00     9 24 39.090 +15 19 29.60 2000.0 dra=7.584 ddec=-51.550
WQ89JZ 06:10     9 24 40.360 +15 19 21.00 2000.0 dra=7.570 ddec=-51.630
WQ89JZ 06:20     9 24 41.610 +15 19 12.40 2000.0 dra=7.557 ddec=-51.710
WQ89JZ 06:30     9 24 42.870 +15 19  3.80 2000.0 dra=7.545 ddec=-51.790
WQ89JZ 06:40     9 24 44.130 +15 18 55.10 2000.0 dra=7.534 ddec=-51.870

Note that each line contains the object name, followed by R.A., Dec., equinox, and differential tracking rates. When the observer wants to point the telescope to this target at UT=5:00, she checks the current time, directs the OA to the appropriate starlist, and requests the target 'WQ89JZ 05:00'.

Computing the Differential Tracking Rates

Bring with you tracking rates computed in the following two sets of units to ensure that things are set up correctly:

Input units

These are units that you should include in your starlist file to define the differential tracking rates. The units are:

R.A.: arcsec/hr divided by 15
Dec.: arcsec/hr

Enter these as the dra and ddec values in your starlist, as shown in the example above.

Note: The factor of 15 noted above requires some explanation. The original intent was for the R.A. tracking rates to be input as seconds of time per hour; however, the computer instructions neglect to apply the required factor of cos(Dec) to the resulting value in order to convert to the desired units of arcsec/hr. Hence, you can derive the appropriate value either by computing seconds of time per hour and multiplying by the factor cos(Dec), or by computing the R.A. tracking rates in arcsec per hour and dividing by 15.

Output units

These are units that the computer returns when you issue the command show -s dcs dra ddec to display the current differential tracking rates. The units are:

R.A.: arcsec/sec divided by 15
Dec.: arcsec/sec

Note: the output units of R.A. are mislabeled! Although they claim to be units of seconds/sec, they are actually arcsec/sec/15; i.e., if you multiply these values by 15., then they will be arcsec/sec.

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Last modified: Tue Jun 20 15:16:29 HST 2000