The Dome Shadow Program

Note:

Introduction

• whether the sun shines on any part of the dish at the current telescope position at the present time or
• whether the sun would shine on the dish if you slewed the telescope to a certain new position or
• whether the sun would shine on the dish if you slewed the telescope to any position you specify at any time you specify

It is meant to facilitate some daytime observing without getting the sun on any part of the telescope (in normal observing conditions this would usually be the back of the secondary mirror assembly). Unless you are already observing and know that you are at a point where the sun doesn't shine on the antenna structure, run it up before you open the shutters! It will tell you whether you can safely open the shutters for the position you are currently pointing at. The dome shadow program will always provide you with the percentage you have to open the shutter to in order that the dish is fully illuminated by the source if that is possible for the current position (or source you want to observe) and it will also inform you how far you can open the shutter so that the sun doesn't shine on the secondary, the front or the back of the dish. The program does not control either the dome shutters or the antenna. The only communication with the antenna computer is in reading the time and telescope position out of it.

How to use it

Command description

• [c] -- new coordinate system (AZ or RA)
• [d] -- percentage you can open the dome to for the current position of the telescope
• [m] -- middle part of telescope illuminated only (this is a flag that gets set and has never been implemented to be unset - quit the program and run it up again to go back to full illumination). This command was implemented to accommodate the Texas focal plane reducer - which is almost exclusively used during the day.
• [n] -- new source (in the coordinate system set up)
• [p] -- predict, i.e. the program will prompt you not only for the source coordinates but also for the time and date you want to check its observability for
• [q] -- quit
• [r (or <cr>)] -- repeat current telescope position
• [s] -- repeat source already specified (if no source was specified, it will use 0.0 0.0 in the chosen coordinate system)
• [t] -- track source (prompts for coordinates but not coordinate system). Tracking will stop when the sun sets, when the source sets, or when the sun starts shining on the dish if you stay on the source
• [x] -- experimental position, mainly for use by crew and engineers, to find out how far the dome can be opened at a given position (in contrast to the 'repeat' command this one will only give you the dome percentage advisable, and it won't beep)

Constraints and Deficiencies

It will throw you out when the sun sets (or when it thinks the sun sets).

SHADOW uses the time in the VAX. To ensure some reasonable level of accuracy it is necessary that the time in the VAX is fairly accurate (close to the time in the antenna computer).

Coordinates have to be typed in, they cannot be input from source catalogs.

Coordinate systems are restricted to altaz and RA/DEC.

Not all input is trapped for input errors.

The coordinates for the sun are taken from the usual sun-ephemeris file in CAT_DIR and interpolated linear between the preceding and following day (i.e. only two values are used). For this reason the position you get from the shadow program will be slightly different than the position you get from the UIP using VER/ALTAZ SUN.

Again:
The program has no control of the dome shutters or of the telescope to prevent you from pointing it into the sun.

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