Guider Documentation

Guider Manual for the Perkins and Hall Telescopes

20-Oct-07


Table of Contents

Introduction
Instrument and Pickoff Fields
Guider Parameter File
Guider Positions File
Guider Orientation and Scale
Guider Initialization
Guider Catalog
Guider Coordinates and Guide "Home"
Setting and Clearing Offsets
Guider Stow Positions
The StellaCam II Camera
Guider Hard Limits and Re-Initialization
Guider Duty Cycle
Dragging the Guide Box
The Special Setup Command for the 42" -- GFS"
Initial Setup -- 72"
Initial Setup -- 42"
Guiding
 

Guider Commands


Introduction

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. At the Planet Search telescope, this question is not asked as only the V option is available and it is always automatically selected.

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 StellaCam II 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 StellaCam IIS 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 (the N option) which, in turn, disables all of the guider commands except for GAF (Guider Alternate Focus) and GAS (Guider Alternate focus Status) at the 72" and GFW (Guider Filter Wheel) at the 42" and 72"..

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 there is some vignetting, it is difficult to precisely set the size of the hole. For this reason, the user can change the size of the instrument field with the GIF command, but the new value is not saved in the Guider Parameter File and is reset back to its default value of 5.5 arcmin diameter each time MOVE is restarted. The "pickoff" field is always zero. 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 current 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 different 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 can be changed via the GIF command. " 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 or 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 any command which requires 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 is 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, 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 Camera

The guider camera is a Watec 120N camera from Adirondack Video astronomy. They sell it undeer the name Astrovid -- StellaCam II. Currently the camera has to be set manually. There is an integration time setting which can be set with a switch on the camera control box (in multiples of 2) from 1/60, 1/30,...,8.5s The light on the camera flashes at the exposure rate. The gain can also be set via a second knob on the camera, but this control is best left near the center. Integration times between 1/7.5 and 1 second work well. Currently MOVE does not talk to the camera, so the integration time and/or gain cannot be set via MOVE. However, in order to synchronize guide updates with camera updates, MOVE needs to be informed of the actual camera integration time. Use the GSI command to do this.

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 reinitialized (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.

Guider Duty Cycle

On entry to guiding, the guider duty cycle (the interval between telescope updates) is set to about one second by setting the number of frames between guider updates to 1 second divided by the integration time. This number is displayed at the bottom of the screen and may be changed at any time by using the keypad + and - keys to increase or decrease the number of frames between guider updates. Note that there are two items at the bottom of the screen of which one is highlighted. If the other is highlighted (drag step size), use the F5 key to switch the highlighting to the number of frames before using the keypad keys.

Dragging the Guide Box

While guiding, the position of the guide box, and with it the telescope and your target object, may be adjusted using 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. The step size of the dragging is displayed at the bottom of the screen and may be changed at any time by using the keypad + and - keys to increase or decrease the number of pixels of each step. Note that there are two items at the bottom of the screen of which one is highlighted. If the other is highlighted (number of frames between guider updates), use the F5 key to switch the highlighting to the step size before using the keypad + and - keys. The direction keys which actully move the box can be used at any time independent of which item is currently highlighted.

The Special Setup Command for the 42" -- GFS

Since it is difficult to find a initial star at the 42", due to the hole in the mirror, a special startup command is provided -- GFS (Guider Find initial Star). To run this command, you must first put a bright (1st or 2nd mag, preferably) star at the center of your instrument and then run GFS.

1) You are asked if you did put a bright star in the instrument. Answer Y to proceed. Anything else will exit the routine.

2) The guide head is moved to home.

3) The telescope is moved 900 arcsec north.

4) "Default" orientation and scale parameters are loaded; the offsets are set to zero. Since the 42" guider is never rotated, the orientation and scale parameters do not change much from run to run and "default" values should work fairly well.

5) The "default" orientation/scale values are used to move the stage to the position of the bright star.

6) The GSO (set offset) routine is entered so that you can move the stage to center the bright star in the field. Since the star is very bright it should be easy to fish around and find it, even if it is not actually in the field. On exit of GSO, the program remembers the offsets you just used to center the star.

7) The GFO (focus) routine is entered so that you can focus the guider. It is impossible to get an accurate focus on such a bright star, but if it is way off, you should be able to get a "reasonable" focus.

8) The program next does an "internal" GGS to find all stars currently in the guider field (they are not shown on the screen as in a "normal" GGS).

9) The stage is then moved to the first star found which is fainter than 9th mag using the "default" GOS parameters and the offsets set in (5). The user is asked if this star is acceptable for a GOS. If the user answers Y, the routine exits. If N, the stage moves to the next faintest star in the list and the process repeats. Note that the guider parameters may be adjusted during this process using the hotkeys.

10) Upon exiting the GFS, the guider scale and offset parameters (as well as the offsets) are restored to their values before you called the GFS. At this point, you will need to tweak up the focus using GFO and do a careful GOS. It may also be necessary to reset the offsets with GSO since the offsets change when the GOS changes.

Initial Setup -- 72"

This is the setup procedure for a "cold" start at the 72". The assumption is that you are on the first night of a run -- the guider rotation, scale, offsets and focus have all changed because of an instrument change.

1) Focus the telescope on your instrument. Check that the camera is set to a reasonable integration time (0.1-1.0 sec). Be sure to use the GSI command to set the same value within MOVE.. If you have an idea of where the guider focus should be, put it there (most instruments have the approximate guider focus on the white board)

2) Move the telescope to a star using GS (7th-8th mag), center it up on your instrument, and update the coordinates with UC (be sure the guider is not blocking the instrument).

3) Do a GHO to put the guider on the same star. If it is not in the field of view, do a GXY (or GSN) to find the star. Note that there is no point in using a GSO to find the star as the offsets will change when you later do a GOS.

4) If you cannot find the star at this point, it is possible that the guider coordinates are bad. Do a GIN and then repeat from (1). If that doesn't work, zero the offsets with GZO and repeat from (1). If that doesn't work, be sure the focus is about right. If that fails, call me.

5) Focus the guider on this star.

6) Using this star, do a GOS. Since the saved offsets from the previous instrument are likely meaninless, you can reply Y when you are asked if you want to zero the offsets. If you are NOT going to do a GEG, then do an GHO again and then use a GSO to return the star to the center of the field, thus setting the offsets.

7) If you are doing a GEG, you can use the same star. After the GEG completes, do a PM to return the star to the instrument and do a GHO to return the guider to home. Then do a GSO to center up the star and set the offsets.

Initial Setup -- 42"

This is the setup procedure for a "cold" start at the 42". The assumption is that you are on the first night of a run -- the guider rotation, scale, offsets and focus have all changed because of an instrument change.

1)Focus the telescope on your instrument.heck that the camera is set to a reasonable integration time (0.1-1.0 sec). Be sure to use the GSI command to set the same value within MOVE. If you have an idea of where the guider focus should be, put it there (most instruments have the approximate guider focus on the white board)

2) Move the telescope to a star using GS (7th-8th mag), center it up on your instrument, and update the coordinates with UC.

3) Using PP, move the telescope to a bright (2.5 or brighter) star. Then run GFS. This command will allow you to focus the guider and also to find a fainter star for doing a GOS.

4) If you cannot find the star at this point, it is possible that the guider coordinates are bad. Do a GIN and then repeat from (1). If that doesn't work, zero the offsets with GZO and repeat from (1). If that doesn't work, be sure the focus is about right. If that fails, call me.

5) Using the star you selected from the GFS, do a GOS. Since the saved offsets from the previous instrument are likely meaninless, you can reply Y when you are asked if you want to zero the offsets. If you are NOT going to do a GEG, then set the offsets with GSO (see below).

6) If you are doing a GEG, you can use the same star. After the GEG completes set the offsets with GSO (see below).

Note: Finding a star to do a GSO at the 42": Choose one of the following (all of which require that at least a "reasonable" GOS has been done) to move the guider head to a star. Then use GSO to center it up and set the offsets. Since you may need to "fish" around to find the star, it is best if you do this with a brighter star.

a) Move the telescope to a star; use RM to move at least 800-900 arcsec away (to be sure the star is now out of the hole); use GGS to select the same star and move the guider to the star.

b) Move the telescoe to a star; use GXY to move the guider at least 800-900 arcsec away (the offsets in RA/Dec are displayed on the guider screen); then use GIG to move the star from the instrument to the guider. You could also use RM (as in (a)) and then use GIG.

c) Use a GGS directly. This doesn't always work as there is no guarantee that there will be a bright enough star in the guider field of view. With methods (1) and (2) above, you can select the starting star.

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. On each update cycle, the guider tests to see if there really is a star within the guide box. If the guider cannot find a star in the box (usually due to clouds), it will automatically maintain the appropriate coordinate lock. An appropriate warning message is displayed.

The guider may be toggled in and out of pause mode via the F1 key. While paused, the telescope will be locked at sidereal rate if on a fixed object or at ephemeris rate if on a moving object. While paused on a moving object, the stage will continue to move. Note that on exiting a manual pause, if the star has drifted out of the box, the guider will behave as though the star were lost due to clouds. Use the paddle to move the star back into the box and normal guiding will resume. Alternatively, one may use the F3 key while still paused to momentarily display the guide box on screen (the box is not continually displayed in pause mode). Using the paddle and the F3 key, one can move the star to a location on the screen that will be inside the box when you exit pause mode.

If a remote move is issued while guiding, the telescope will first move and then the stage will move. If everything is working properly, the star will end up back in the box and guiding will automatically resume. If the star should miss the box, the guider will behave as though the star were lost to clouds. Use the paddle to move the star back into the box. Note that if the remote move is too large to move the stage or would put the stage into the blocked region, the telescope is moved, but the guider will go into pause mode and the stage will not be moved (if ephemeris guiding, the stage will continue to move at the ephemeris rate). The same will happen if the guider is already paused at the time the remote command is issued.

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.

 

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