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Table of Contents

Guider Program Functions

Commands for automatic guiding are built into MOVE. These commands are enabled upon startup when MOVE is initialized. The user is asked to choose N for no guider, V for video guiding only, or G for the Lowell guider.

The G option requires that the Lowell guider be on the telescope and turned on. An error message is generated if the guider is off or disconnected and the user is asked to choose another option. The guider at the Perkins telecope has a pick off mirror feeding a ScanCo CCD imager. The pick off mirror may be moved around in the focal plane in order to locate guide stars while the guider at the Hall telescope has a ScanCo CCD imager which looks at a mirror with a central hole. Selecting the G option enables all of the guider commands.

Guiding may also be performed with or without the Lowell guiders in place by simply providing a video RS170 signal to the frame grabber board in the MOVE PC and selecting the V option. This signal can come from any video device which looks at the focal plane. Selecting the V option enables a subset of the guider commands.

Finally, MOVE may be run with guiding disabled which, in turn, disables all of the guider commands except for GAF (Guider Alternate Focus) at the 72" and GFW (Guider Filter Wheel) at the 42" and 72" (the N option).

All guider commands consist of three letters, the first of which is always G. Commands which require a single number may be entered as command-space-value (eg GBL 125 or GSI 1.2). If a required value is omitted, the program will prompt for the value.

While not actually part of the guider, there is a shutter attached to the guider, just above the filter wheel at the 42." This shutter must be opened to get light to the guider (and the instrument).

Many of the guider commands (and some general MOVE commands) use the numeric keypad. In MOVE, the keyboard is set up such that the four arrow keys will give the same result as the numeric keypad keys which have arrows on them. That is, the numeric 8 key is identical to the up arrow key. However, the six other numeric keys which have secondary marks (eg, the keypad 9 key is marked Page Up) are not identical in function to the six corresponding keys on the keyboard. Thus numeric keypad 9 is different from the Page Up key even though it is also marked Page Up.

Instrument and Pickoff Fields

Using the Lowell guider (G option), it is possible (at the 72") for the pickoff mirror to block the instrument. A warning is given if the circles defined by the pickoff mirror and the instrument's field of view overlap. The size of the pickoff is fixed at 5.2 arcmin in diameter, but the instrument field is variable and may be set by the user with the GIF (Guider Instrument Field) command. On entry to the program, the previously set instrument field size is read from the Guider Parameter file (see below). At the 42", there is no pickoff mirror. However, the guide camera can see the hole in the mirror. A warning is given if this occurs. Since these fields are fixed, the GIF command is disabled at the 42"; the ``pickoff'' field is set to zero and the instrument field is fixed at 5.5 arcmin in diameter, the size of the mirror hole.

Guider Parameter File

If the V option is chosen, the program attempts to read a parameter file (GONSTV.DAT) which contains the orientation and scale of the sky plane relative to the video image. These parameters are set by the GOS (Guider Orientation and Scale) command. If the G option is chosen, the program attempts to read a parameter file (GCONST.DAT) which contains the X,Y offset of the guider home position, values set by the GSO (Guider Set Offsets) command; the orientation and scale of the sky plane and X,Y stage axes relative to the video image determined by the GOS and/or GEG commands; the size of the pickoff field which is fixed at each telescope; and the size of the instrument field which is fixed at the 42" and set by the GIF command at the 72." The appropriate file is updated whenever a parameter it contains is changed (with the GOS, GEG, GSO, or GIF commands in G mode -- GCONST.DAT and with the GOS command in V mode -- GCONSTV.DAT) and is saved from one run of MOVE to the next (see Guider Initialization, below).

Guider Positions File

If the G option is chosen, the program also attempts to read a file (GUDPOS.DAT), written upon normal exit from MOVE. This file contains the positions of the guider axes at the previous exit from MOVE -- the stage axes (X,Y) as well as the focus axis (Z). If this file is not found, the guider does not know where the stages currently are and must do an automatic reinitialization of all of the axes. The initialization process takes about a minute. Upon completion of initialization, the XY stage is left in the stow position (see below); the focus is left at its mid point (defined as zero). Appropriate messages are printed. If the file is found, the guider is assumed to be at the saved positions contained in the file which is then deleted (to be rewritten at the next normal exit of MOVE).

Guider Orientation and Scale

Many commands require the parameters defined by the GOS command and will not execute if these parameters are undefined. From one night to the next, these values will be the unchanged since the scales and orientations are unchanged. However, when instruments are changed, it is possible that the guider may not be put back in the same orientation as previously, or that the tailpiece of the telescope may be rotated from the last time a GOS was done. In that case, even though the GOS parameters are not undefined, the constants which were read from the guider parameter file may no longer be appropriate and the guider may behave improperly. Thus, it is always necessary to perform a GOS after any instrument change. If the guider parameter file is missing on startup (which should never happen), a GOS must be done in order to execute commands which require GOS parameters.

Additionally, if you expect to do large remote moves (over 150 arcsec) while guiding or to guide on moving objects over large distances (over 150 arcsec), then it is necessary to do an extended GOS via the GEG command following the completion of the GOS. The GEG uses the inital GOS to determine the scale and orientation more accurately than the GOS.

Guider Initialization

If the N option is chosen, MOVE simply continues with its normal initialization. If the V option is chosen, the frame grabber is initialized. MOVE then attempts to read the video guider parameter file (GCONSTV.DAT). If the file is found, the user is warned that the GOS parameters come from a previous night's run and may not apply to the current instrument. If it is not found, the user is warned that it will be necessary to execute a GOS command before guiding. If the G option is chosen, the frame grabber and ScanCo camera are initialized. MOVE then attempts to read the Lowell guider parameter file (GCONST.DAT). If the file is found, the user is warned that the parameters come from a previous night's run and may not apply to the current instrument. If it is not found. the user is warned that it will be necessary to execute a GOS before guiding. In addition, the user may also want to reset offsets via the GSO command, redefine the instrument field with the GIF command (at the 72"), and run GEG to get an extra-accurate scale and orientation. MOVE next attempts to read the guider positions file, GUDPOS.DAT. If found, it is read in and deleted (a new version will be written on exit (QU) from MOVE with the positions of the axes on exit. If it is not found, the axes are initialized as discussed above.

Guider Catalog

The on-line guider catalog is a concatenation of all stars brighter than 13th mag north of -50 dec in the guide star catalog & the USNO A1.0 catalogs (about 7,700,000 stars).

Guider Coordinates and Guider ``Home''

At the Perkins, home is the position where the pickoff mirror is nominally centered on the optical axis of the telescope (and instrument). At the Hall, home is the position where the CCD is looking at the center of the hole in the mirror (and where the optical axis would be if the hole was absent). The software sets the stage position of home to be (0,0); positions of the guider camera head are measured relative to the home position. These positions are displayed in microns of stage motion and in arcsec (E and N) relative to the home position. The display also gives the RA and DEC of the current position of the guider box. This position is the RA/DEC of the telescope plus the offset to the guide head plus the offset of the guider box within the guider field (relative to the center of the guider field at pixel position (256,256)). On startup, the guider box is placed at the center of the guider field.

Setting and Clearing Offsets

Since the home position is often not precisely the optical axis of the instrument, the GSO (Guider Set Offsets) command is provided to reset that mechanical position which is to be defined as (0,0) and, as a result, define where in the guider field the home position is located. If the optical axis of the instrument is far from the nominal position, the GGS command may set guide stars near the edges of the guider field. The GSO command can be used to take out this error. GSO works identically to GXY except that on exit, the net motion of the stage (ending position minus starting position) is added to the exisisting offsets. Thus successive GSO's are cumulative. To set the offsets back to zero, use GZO (Guider Zero Offsets).

Lowell Guider Stow Positions

Once initialized, the Lowell Guider has two special positions defined - home as discussed above, and the stow position in the corner of the guider. On the Perkins telescope you can move the XY stage here (via the GST -- Guider STow command) to be sure that the pickoff mirror is out of the way of any instrument. At the Hall telescope, this is not as critical, as there is no pickoff mirror to block the instrument.

The ScanCo Camera

The ScanCo camera has several parameters which may be adjusted by the user. There is a remote/local mode switch on the front of the camera electronics box. In local mode these parameters are controlled via switches; in remote mode they are controlled by commands from the MOVE PC. The camera must be in remote mode. The adjustable parameters are the integration time (set with the GSI command), scan mode (slow scan or TV -- GSS/GTV commands), black (background) level (GBL command), and gain (GGA command). Before guiding on a star, these parameters must be adjusted to yield a star image which has a reasonable contrast with the background without saturating in the center. In most cases, the scan mode should be left in the slow scan mode as this gives better image quality. The integration time is quantized in units of a single frame time -- 1/30 or 0.033 seconds for TV mode and 1/7.5 or 0.133 seconds for slow scan mode. The actual integration time is chosen to be the nearest whole multiple of a single frame time of the integration time which is requested. There are four gain levels: 1, 2, 5, and 30. Only these values will be accepted by the program. Note the distinction between the black DAC value and the black value. The black DAC value is the ``knob'' setting. The black value is the actual value of the background as read from the image. One adjusts the black DAC with the GBL command to yield an appropriate actual black value in the image. Valid values for the black DAC range from 0 to 255; the actual values also range from 0 to 255. Increasing the DAC value decreases the black value. At higher gains, the black value is extremely sensitive to the black DAC; the entire range of black values may be spanned in only 5--10 values of the black DAC. The GAB command (Guider Auto Black) can be used to automatically set the DAC so the the black value is adjusted to a preset value previously given in the GSA (Guider Set Autoblack value) command. A GAB command takes 10-15 seconds to complete. On entry to the program, the initialization puts the camera in Slow Scan mode with a gain of 2 and integration time of 0.27 sec. The GSA value is set to 50 and the black level is set to 128.

Guider Hard Limits and Re-Initialization

The mechanical guider has hard limits on all three axes. Normally, the software soft limits should not allow the stages to reach the hard limits at any time. However, if a limit is seen in any axis, the current task is aborted and a reinitialization is done. If the limit is seen in X or Y, the XY stage is reinitialized (leaving the focus untouched). If the limit is seen in the focus (Z) axis, it is einitialized (leaving the stage untouched). If the user suspects that the stage positions are corrupted, one may run GIN (Guider INitialization) which will intitialize the XY and Z axes.

Setting Guider Orientation and Scale Parameters

The procedures to do a GOS at the 72" and 42" are slightly different due to the differences between the two guiders. At the 72", first find a star and center it in your instrument (any star between 11th and 5th or 6th mag should do). Then do a GHO to bring the pickoff mirror to the home position. The star should now be visible in the guider camera. If is is not, either the guider coordinates are lost, and/or one or both of the available offsets are bad. To reset the coordinates, do a GIN (note that this command will leave the stages in the stow position and the focus at zero). To reset the offsets from a GSO command, use the GZO command. Once the star is found, put it in the SE quadrant of the guider and run GOS (GOS turns off the tracking and then moves the telescope to the south). At the 42", things are complicated by the fact that one cannot see the same star as the instrument because of the hole in the mirror. To find a star to do a GOS use one of the following:

1) If there was a previous file (GCONST/GCONSTV as appropriate), do a GGS to find a guide star. If the previous GOS is not wildly different from the present setup, the GGS will probably find a star which can then be used for a new GOS.

2) Try to find a star by moving the stages with the GXY command.

3) If the tailpiece is at the zero rotation position, do a GIN to be sure that the stages are initialized. Use GZO to zero any offsets. Then, find an appropriate star in your instrument. Move the stages to home (GHO). Next, move the telescope 900 arcsec (15 arcmin) north. Finally, use the GXY command to move the stages such that the X stage position is -37500 and theY stage position is +500. The same star should now be in the guider field and can be used for the GOS.

4) If all else fails, do a GIN and GZO to be sure that the stages are initialized and the offsets are set to zero. Then move the heads at least 20000 steps from (0,0) so that you are not looking at the hole. Set the gain to 5 and set an appropriate black level. Be sure the telescope is looking through the dome slit and turn off the tracking. Wait for a star to drift through the guider field. When one does, turn on the tracking and use it do the GOS.

Guiding

Guiding is entered from the command level via a GLK (Guider LocK) command, or directly from the GGS command after finding a guide star. If the telescope is in sidereal lock when guiding is initiated, the guider will lock to a fixed position (guider stages not moving). If the telescope is in rate track lock when guiding is initiated, the guider will lock to the current rate (guider stages moving).

Initiating guiding initially performs a GBX (Guider BoX -- see below), and, if the telescope is in rate track lock, turns on the stage motion to match the current telescope motion. Once the box is centered on the star and has a ``reasonable'' size, press F2 or F3 to begin guiding. F2 guides by first determining the star's position and then moving the box to that position. This option is useful if you do not want the telescope to be moved as it would if the star were not initially centered in the box. F3 will guide to the center used the last time GLK was called. If this is the first time it is called, the guide will be to the center of the box. Note that the size of the box is not critical. The box should easily contain the star image and extend several pixels beyond. The program dynamically determines the size of the star image. Once guiding begins, the position of the box (and the target object) may be adjusted 1/3 of a pixel at a time via the keypad keys (left, right, up, down and diagonally). This may be used, for example, to adjust a star image onto a slit after guiding begins.

In GLK mode with G (Lowell Guider) selected, the stage is moved at a rate such that the telescope will follow the star. If the guider mode is V (Video guiding), then the box will be moved across the screen such that the telescope will follow the star.