7 The Autoguider
The autoguider uses the video signal from the MIS guider camera. It can therefore be used for both off-axis guiding and slit plate reflection guiding. This chapter assumes that you are familiar with the operation of the Multiple Instrument System (MIS - Chapter 6). This chapter is mainly concerned with off-axis guiding. In addition, a Digital Video Integrator (DVI) made by Digital Television Imagery (DTI), similar to the one at the 2.4-m, is available for guiding on faint objects off a spectroscopic slit plate. An important application of the DVI is to provide a "leaky" integrated image so that fainter objects can be seen and placed in the spectroscopic slit. Instructions for the use of the DVI are contained in Chapter 7 of the 2.4m Telescope Manual, and will not be repeated here. In general, you should specifically request the DVI to be installed when doing spectroscopy.
The observer needs to interact with the Autoguider PC, located at the far right end of the observer's desk.
7.2 The Pentium PC-based Autoguider (TVGUIDER)
7.2.1 Introduction
The current version of the PC guider was developed by Pat Seitzer at Michigan, and the following description is from notes provided by him. This chapter only gives a brief overview. For more information, see the TVGUIDER Manual.
The PC Guider is a standalone system to generate guide pulses using data from a video integrator board. It connects to the through a parallel printer port and to the TCS through 4 guide lines (NSEW). The software reads a box of video data around the "box cursor" on the display, computes a star centroid, and attempts to maintain the star at the same centroid position by moving the telescope.
The guiding algorithm works by computing star centroids from several sequential images (the default is 3), taken at a fixed interval, and computing a mean. An offset is computed between this mean position and a fiducial position (the position of the star when guiding was started), and used to compute a motion for the telescope that will move the centroid to the fiducial position. The guide pulses are sent to the telescope and the process is repeated.
7.2.2 Basic Startup and Operation
The PC is normally left on. The guider software is started by cd tvguider and then starting the program by typing tvguider -i when the pc is booted and then selecting the correct telescope.. If everything initializes properly, it will respond with a MENU as show below:
Command Keys
The following keys are supported - other keys are ignored.
# Guiding control
s start guiding
q quit guiding
r set rotator angle
a go to average mode
# Cursor control - all guiding is done with square cursor
c toggle between cursors (active one has lines or 'hair')
+ increase cursor size (maximum 41x41 pixels)
- decrease cursor size (minimum 9x9 pixels)
# Cursor movement via keypad (NUM LOCK must be OFF!) -
# diagonal motion can be done with the 1 3 7 9 keys
5 toggle between steps of 1 and 10
i move selected cursor to center of field
# video sampling
z set values for bright stars (min gain, max range)
Z set values for faint stars (max gain, min range)
b adjust sky to 30-60 units.
1 2 3 4 select gain state 1 (low, max range)
through 4 (high, min range)
5 decrease background
6 increase background
space bar quick sky and centroid of star in guider
box using data from last TV frame
# display brightness and contrast
h j adjust black
k l adjust white
f transpose one frame and write to display.
m move telescope specified direction and time.
w write full TV frame to disk as binary file of 8 bit
integers: 512 columns by 486 rows. Useful to see
just how awful this data really is.
p set guider parameters (stops guiding)
? type this help page
x exit the TVGUIDER program. *All* guider parameters
and cursor positions and gain/offset values are
saved to the file 'tvguider.ini' in the current directory.
This file is read when the program is next started.
More information can be found in the TVGUIDER Manual.
The default orientation parameters are good for direct imaging. If you are using a spectrograph see Section 7.4 below. If you're using a guider lens of 85 mm (the 135 mm lens is recommended for direct imaging), you will need to modify the parameters on the TVGUIDER.
Note that the autoguider and the hand paddle send pulses to the same telescope motors and relays. Therefore, you shouldn't press buttons on the handle paddle while the autoguider is guiding, or the signals may clash. For example, pressing the set button on the hand paddle at the same time that the autoguider sends a guide pulse will cause the telescope to make a large jump. If you need to use the hand paddle to make an adjustment, first quit guiding by using the q command. If you suspect that the autoguider is not working, see Section 7.5 on troubleshooting.
7.3 Marking the Field Centers with the Guider
You can mark the positions of field centers and spectrograph slits on the guide monitor with the cursors, and create digital reference coordinates.
If you plan on returning to the same objects at some later date it is useful to know the positions of the guide stars you used. These are relative to the field centers, which change slightly between observing runs.
To mark the field centers:
determine the guider field center
Figure 7-1 shows the FINDER/GUIDER field orientation for three instrument rotator angles.
The orientation of the GUIDER field displayed on the FIND/GUIDE MONITOR depends upon the internal MIS optics.
Use the following formulae to determine the orientation parameters which are entered in the 486-PC using the p command (Section 7.2.2):
Direct imaging (Off-axis guiding)
(Autoguider PA) = 90 + (Instrument Rotator) E/W flip = 1
Spectroscopy (Slit-plate guiding)
(Autoguider PA) = 270 - (Instrument Rotator) E/W flip = 0
Notes:
The following are known problems that directly or indirectly affect the autoguider occasionally. The symptoms and diagnoses are described, followed by the solution.
7.5.1 Autoguider is unstable or oscillating
Normally, if the telescope is guiding and you push the star away from the center of the box, the guider should re-center it fairly quickly. If the guider motion is oscillating and does not settle down after a minute or so, then it is possible that the guiding parameters need to be adjusted. If a guider parameter has been set far from its default value, and especially if the position angle or the parity (E/W flip) has been set incorrectly (see Section 7.4), then positive feedback can quickly drive the telescope in the wrong direction. See the TVGUIDER Manual for suggested values of parameters and hints about how they might be optimized.
7.5.2 Autoguider is taking a long time
If the guider just seems to take a long time to re-center the star, or can't do it at all, then there may be something wrong with the telescope drive guide motion. For example, a clutch may have loosened or a pre-load may need adjustment. This can happen in either the E-W or the N-S directions. The existence of a drive problem can usually be diagnosed when it affects motions from the hand paddle as well as from the autoguider. (The autoguider and the hand paddle send signals to the same telescope inputs). The telescope may be slow to respond in a particular direction at guide or set speed, or may need to be nudged in the opposite direction first. The observer can't fix this problem, but must work around it if possible. Issuing a Trouble Report will usually enable the staff to locate the problem and make the needed adjustment in the morning.
7.5.3 Autoguider is not sending
Occasionally the star will slowly and continuously drift out of the guide box, toward the west. Even though the PC guider program is running normally and calculating corrections, guide pulses are not being sent to the telescope, and the calculated offsets are increasing. If the hand paddle is still working normally, then almost certainly it is the 5-volt power supply on the autoguider board that has turned off. This can be checked by going to the right-hand rack in the computer room (Figure 7-2) where a digital volt-meter (DVM) is attached to the autoguider power supply. Turn the voltmeter on (it is kept off to conserve its battery). If it is reading 0 volts, then the autoguider power supply has turned off. It can be turned on again by toggling the switch for the autoguider power (Figure 7-3) off and then on. The DVM should then read 5 volts, and the guider will start guiding again. Turn off the voltmeter when you are finished. (Seasoned users will remember that before the switch was installed, you had to go to the back of the rack and unplug the power cord that was marked with a piece of red tape. The red tape is probably still there.)
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