C40 Telescope Control Program

Ian Thompson

Version V1.07, 4 September 2000

C40 Command Summary

C40 Command Syntax

Starting the Program

How to Set the Coordinates

Slewing the Telescope

Bob's Box

Coordinate Catalogs

How to Take Date to Check or Update the Pointing Model

Acknowledgements

1. C40 Command Summary

Panic Commands:
<Esc>	Immediately abort any dome or telescope move.
<F1>	Display help screen.

Pointing Commands:
MP n.n	Set the coordinate display to show mean place for the year n.n.
AP	Set the coordinate display to show apparent place.
OP	Set the coordinate display to show observed place.
TP	Set the coordinate display to show corrected values according to the telescope pointing model.

Time and Coordinate Commands:
UT n n n.n	Set the UT to n n n.n.
RA n n n.n	Enter the RA coordinate n n n.n.
DEC n n n.n	Enter the Dec. coordinate (+/-)n n n.n.
ROFF n.n	Set the RA offset to n.n sec of time.
DOFF n.n	Set the Dec. offset to n.n arcsec.
CSET	Change the RA and Dec. offsets so that the coordinate display agrees with the coordinate inputs.

Guiding Commands:
GUIDE	Activate telescope guiding.
IDLE	Turn off telescope guiding.

Slewing Commands:
SLEW	Slew the telescope to the position entered with the commands RA and DEC.
COVER	Park the telescope at the south pole to allow easy removal of the dust cover.
ZENITH	Slew the telescope to the zenith.
DOMEFLAT	Slew the telescope (and move the dome) to dome flat position.

Bob's Box Commands:
RBB n.n	Set the RA increment to n.n arcsec.
DBB n.n	Set the Dec. increment to n.n arcsec.
NN	Move telescope north. Also EE, SS, WW.
NE	Move telescope north and east. Also SE, SW, NW.
ZBB	Set the home position (zero the cumulative values).
HBB	Move telescope to the home position.

Dome Commands:
DOME AUTO	Turn on automatic dome tracking.
DOME IDLE	Turn off automatic dome tracking. Also DOME STOP.
DOME PARK	Park the dome to the south.
DOME n.n	Set dome mode to manual and move to n.n degrees azimuth.

Catalog Commands:
LOAD filename	Load the catalog "filename".
OBJ n	Load the coordinates of catalog object n into the commanded position.
CAT n	Store the commanded RA and DEC to catalog object n.
LIST	List the currently loaded catalog to the system log.
SAVE filename	Save the current catalog to "filename" on the floppy disk.
CLEAR n/ALL	Clear catalog entry n, or clear entire catalog.

Other Commands:
Q	Exit from the program.
EAST	Set telescope mode for east side of mount.
WEST	Set telescope mode for west side of mount..

Diagnostic and Engineering Commands:
TERM n	Open a terminal on COM port n (1-3).
VER n	Set message log verbose level to n (0-2)..
<F2>	Dump system log to file TEMP.LOG.
<F3>	Dump screen to file SCREEN.BMP.
<F5>	Append telescope positions and sidereal time to POINT.LOG.
<F6>	Toggle motion logging (encoders).
<F7>	Dump motion log to MOTION.LOG.
<SCROLL LOCK>	Pause log output. Scroll log with up, down, left, right keys and PageUp, PageDown, Home and End keys.

All C40 commands begin with a command code followed by up to three numeric arguments. The numeric arguments (if there are more than one) are separated by spaces; spaces are permitted but not required between the command and the first numeric argument. The numeric arguments (including zero) may be negative, and the last may include a decimal point. For example, the commands

DEC-0 25 47.2

DEC -0 25 47.20

will both set the Dec. coordinate input to -00° 25' 47.2". The backspace key moves the cursor back one character at time. All commands are terminated by a <CR>.

3. Starting the Program

To start the C40 program, push the reset button (the black rocker switch just below the red power switch) on the computer, or start the computer if it is powered off. The program will load automatically from the floppy disk. Resetting the computer assures that the system will start each time in the proper state.

The program will begin displaying telescope data as long as the computer is connected to the various serial lines, independent of the power switch on the telescope.

Note that the UT date is initialized from the internal computer clock, which MUST be set to the correct UT date and UT time. The user is prompted to enter the correct UT time when the program starts. If the UT time displayed is correct then just press . The UT can be updated at any time with the UT command.

The command prompt is a "*" in the lower left box on the screen. All system commands are entered in this box. The cursor is invisible, but it is always at the end of the command string.

4. How to Set the Coordinates

At the beginning of the night, you will usually point the telescope to a bright star to check the coordinates. It is a good idea to choose a star which is in a location where the residuals of the pointing model are small (the zenith is a good place). Or, you may want to choose a position very close to the place you will start observing. If the star is from the ephemeris, then the coordinates are mean place for the middle of the ephemeris year (e.g. J2000.5). The coordinate display should be set to mean place for the ephemeris date. If you will be using the telescope pointing model, make sure that TP is enabled. A green "TP" message will appear in the coordinate input box. Toggle the pointing model off by typing "TP" again, and the green "TP" message will turn black. When the star is centered on the crosshairs, adjust the offsets so that the green coordinate display agrees with the position of the star using the commands ROFF and DOFF. Or, enter the coordinates with the RA and DC commands and type CSET. Once the coordinates have been set, the display can be changed to mean place for any convenient year, or to apparent, and the telescope should continue to point accurately. The ROFF and DOFF commands are useful if you want to set the offsets to exactly the same values used for a particular object on a previous night, for example.

Set the coordinate display to show mean place coordinates for a given year by using the MP command. The coordinate system is the FK5 (IAU 1976) system (e.g. J2000.), which is very slightly different from the FK4 (e.g. B1950.) system. However, the difference between the two systems (for example between J1950. and B1950.) is negligible. The coordinate system in use is shown just above the green RA and Dec. display.

Type "AP" to set the coordinate display to show apparent place. Apparent place includes the effect of nutation (amplitude 10 arcsec) and aberration (amplitude 20 arcsec), so apparent place coordinates are NOT the same as mean place coordinates for the time of observation. DON'T use apparent place unless you are setting to a position which has been precessed to apparent place. DON'T use coordinates which include the effect of atmospheric refraction, since this effect has already been taken out by the control program.

5. Slewing the Telescope

The telescope can be slewed to a given position under computer control. ALL TELESCOPE SLEWS MUST BE MADE WITH THE MOVABLE PLATFORM AT ITS DOWN POSITION. First enter the target coordinates using the RA and DEC commands, these coordinates will appear in the RA and DEC display in the system command box. The SLEW command will then slew the telescope to this position. If the target is at an aimass greater than 3 then the SLEW command will not work, and the telescope has to be set with the control consol. If the dome is in AUTO mode then it will move to the target location on the sky while the telescope is slewing.

The telescope can be set to the zenith with the ZENITH command. The observer is asked if the movable platform is parked before the telescope is moved. The command COVER will slew the telescope to the south pole to allow easy access to the dust cover. Be sure to return the platform to its down position immediately after removing or replacing the dust cover. The command DOMEFLAT will move the telescope (and the dome) to point to the flat field screen to allow the observer to take dome flats.

A system message will appear when the slew has been successfully completed. At the moment, the north slew relay occasionally sticks closed after a long slew to the north, and the telescope will want to continue to move in that direction for the next slew. If this happens, a system message will appear, and the slew will be halted. The observer should wait a minute or so for this condition to clear before attempting the slew again. Note that this can occur at the end of a slew as well, and it will seem as if the completion of the move is delayed (the telescope does not converge on the requested position, and the "Slew Complete" message is delayed).

Any telescope motion can be immediately aborted by hitting the <Esc> key.

6. Bob's Box

The Bob's Box function can be used to move the telescope by a programmed increment in either RA, Dec., or both. Enter the increment in RA using the command RBB, and in Dec. using DBB. The increment is in arcseconds on the sky, can be either positive or negative, and can include a decimal fraction of an arcsecond. The program will automatically derive the appropriate correction for the RA coordinates, which depends upon cos(d). If the increment is positive, the NN command will move the telescope north, and the EE command will move the telescope east.

The Bob's Box function keeps track of the cumulative motion in RA and Dec. Use the ZBB command to zero the cumulative values. This establishes a "home" position. The HBB command will move the telescope back to the home position. The screen display shows the size of the RA and Dec increments and the cumulative motion. Moves generated by external serial line control will be shown next to the Ext: label.

Note that the encoder that is presently being used on the declination axis has a resolution that is less than 1 arcsec, and so the requested move may not be accurately indicated by the new declination position. The declination stepper motor does have a resolution of better than 1 arcsec.

7. Coordinate Catalogs

The program can read, write, and list catalogs of coordinates, load the coordinates of individual objects from a loaded catalog, and write the coordinates of objects to that catalog. The catalogs must be ascii files on a 2.5-inch floppy disk. The format is

nnn hr min ss.ss (+/-)deg min sec.sec

with one star per line. Each catalog can contain up to 999 objects. To use a catalog, first remove the boot floppy disk in the computer and load your catalog disk. The command LOAD filename will read the catalog "filename" from the floppy disk into the program. The coordinates for object number n in that catalog can be loaded into the commanded RA and DEC in the coordinate input box with the command OBJ n. The user can then, for example, slew to that position with the SLEW command. The commanded coordinates (note, not the position of the telescope) can be saved to object n in the catalog with the command CAT n. The contents of the loaded catalog can be listed to the system log with the command LIST. The command SAVE filename will save the contents of the currently loaded catalog to the file "filename" on the floppy disk.

8. How to Take Data to Check or Update the Pointing Model

A good check of the pointing model requires 30-50 observations, and a good set of data for deriving a new pointing model requires 100 or so. Each setting will require about 3 minutes, so the best thing to do is to get organized in a systematic way. Plan to start an hour or two late, so that the telescope will have time to come to thermal equilibrium after the dome is opened.

While you are waiting (or during the afternoon), login as "obs40" on the Sun computer (Henrietta) in the console room, and start IRAF in an xgterm. Now type "pointing" to load a special pointing task. In order to run a pointing model, it is necessary to have a file containing a grid of fairly evenly spaced alt/az positions. In the directory "/export/data/obs40/pointing" you should find three files called "grid42", "grid49", and "grid61" which have coordinates for 42, 49, and 61 grid points, respectively. Alternatively, you can generate your own grid file by typing "makegrid" in IRAF. You will be asked to enter the following parameters:

Number of azimuth bands
Number of altitude bands
Lowest altitude (in degrees)
Highest altitude (in degrees)
Amount of randomness (in degrees)
Output file name

A grid of alt/az positions will then be written to the output file. This file may will be used as input to the "get_pointing_stars" task (see below).

When you start observing, slew to a star in the bright star catalog. Choose a focus for an expected temperature and DON'T CHANGE IT, since the star image sometimes moves around when the secondary focus is adjusted. Zoom the guider camera lens in or out, and adjust the position of the star until it doesn't move when zooming the lens. This defines the optical axis. Do a "cset" on the TCS computer at this position, and record the offsets. Finally, remove the boot floppy disk and load your own floppy (make sure it has plenty of space available on it!).

Now type "get_pointing_stars" in the IRAF window on Henrietta. This task will ask for the name of the file with the grid points that you wish to use. It will then print the coordinates of the nearest bright (~9th mag) star in the PPM catalog corresponding to the first alt/az position. The task will then pause in order for you to set the telescope on the coordinates of the star. Be sure to set the coordinate display to show the epoch (to 2 decimal places) of the star coordinates that have been printed out by the "get_pointing_stars" task. (Do this by typing "MP 200X.XX" before slewing to the first star.) Once you have slewed to the position of the star, center it on the optical axis. (Keep the guide camera zoom at the widest setting.) Now turn off the pointing model by typing "TP" on the TCS computer, and set the RA and Dec offset to zero ("ROFF 0" and "DOFF 0"). Next, press the <F5> key. This writes the true coordinates, the present coordinates and the present sidereal time to the file POINT.LOG on the floppy disk. Now toggle back on the pointing model ("TP") and reenter the original RA and Dec offsets. You are now ready to slew to the next star. To get the coordinates of this star, hit "return" in the "IRAF" window where the "get_pointing_stars" script is running. Repeat this procedure until the "get_pointing_stars" script finishes.

The file POINT.LOG can be used as input to the analysis program TPOINT. The output model from TPOINT is used as the pointing model for the telscope. Make sure to record the outside temperature, barometric pressure (in millibars), and humidity at the time that the pointing data are taken, as these will also be required by tpoint.

9. Acknowledgements

The C40 program makes extensive use of the SLALIB and TPOINT program libraries, which were developed and graciously provided by Pat Wallace of STARLINK. The C40 program was written by Greg Bredthauer.

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