7 Autoguiding
7.1 Introduction
All telescope mountings are imperfect, so to maintain accurate pointing for any but the briefest exposures it is necessary to monitor the telescope's drift, generate a correction signal, and feed this back into the telescope's fine motion controls. The autoguider PC accomplishes this using the video signal from the MIS guider camera. As explained in the previous chapter, the guider camera can either look directly into the telescope using an off-axis pickoff mirror or look through a microscope at the image reflected from a spectrograph's slit jaws. This chapter assumes that you are familiar with the operation of the Multiple Instrument System (MIS Section 6).
Either of two setups can be used for guiding:
It is not practical to switch from one mode to the other during the night. The two setups use different hardware -- the Seitzer guider uses a video-processing board in a PC, while the DTI guider uses the DTI box for this purpose -- so switching between them requires some non-obvious re-cabling which is best left to the staff. Also, the TV focus is different between the slit jaws and direct viewing with the pickoff optics, so it's not practical to switch quickly from one mode to another.
Several pieces of equipment work together to perform the autoguiding function:
7.2 Brief Remarks on the Seitzer Guider
The Seitzer guider is generally used with the offset guide stage configured to look directly into the telescope. In the past one had to search manually for guide stars after setting up on a field. This still works, but you can pre-select guide stars using a program on agung called gs24, which searches the USNO A2.0 catalog and presents a map of the guide field with candidate guide stars. The guide stars can be selected with a cursor, and "dialed in" ahead of time. In practice this can save a great deal of time.
To use gs24 from hiltner's console, do the following:7.3 The DTI Box
Note: You should only be reading this if you're in the minority using the DTI guider. (Of course, anyone is welcome to read this, but you're wasting your time if you're using the more standard Seitzer guider, which doesn't use the DTI.)
7.3.1 Basic Setup of the DTI
The DTI box takes video input and integrates it. It uses a "leaky memory", in which the older images are gradually forgotten, so the output image changes continuously, rather than in sudden jerks. The MODE SELECTION switch can be in DIRECT or DIGITAL position; in DIRECT position the output signal is the same as the input (although the signal is still integrated internally). The TIME CONSTANT selector controls how long the signal is integrated. The BLACK and WHITE controls select what level of signal corresponds to zero (black) and full intensity (white) in the output. The controls grouped under DISPLAY CONTROL (which are BRIGHTNESS and CONTRAST) are not particularly useful in practice; CONTRAST should be left at 1.
Basic Setting Procedure:
7.3.2 Using the DTI Hand Paddle
The DTI Hand Paddle (Figure 7.1b) is only functional in the DIGITAL mode. It is used to move cursors around the find/guide TV screen. The guide star candidate is centered within a BOX CURSOR. To activate the cursor the mode selector must be in DIGITAL and the camera selector, digital integrator, and the power supplies to the cameras located on the telescope must all be on.
The CURSOR/WINDOW button steps through a sequence of four states:
In the last three states, using the UP, DOWN, LEFT, or RIGHT buttons move the blinking cursor or (in the last state), both cursors. The coordinates of the active cursor are displayed in the lower right of the screen, together with the Z-value (pixel value). Note that the box cursor is the one used by the guider. The cross can be used as a fiducial marker.
The function of the GRAB/ERASE button is dependent on the current digital mode setting. In practice it is seldom used.
Both cursors may be reset to center by pressing the RESET button located above the AC power rocker switch on the front panel.
7.4 Autoguiding with Images from the DTI (PCGUIDER)
7.4.1 Introduction
The PCGUIDER program is used to process data from the DTI and generate guide pulses. It was developed by Mark Metzger of MIT and the following description is adapted from notes provided by him. John Thorstensen later modified the user interface and revised this writeup, and John Tonry of MIT made some further modifications.
PCGUIDER generates guide pulses using data from the DTI video integrator. It connects to the DTI through a parallel port and to the TCS through four guide lines (NSEW). The software reads a box of video data around the "box cursor" on the DTI 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 (generally three) 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.
7.4.2 Basic Startup and Operation
The guider PC is normally left on. The guider software is started by typing PCGUIDER at the MS-DOS prompt. If everything initializes properly, it will respond with Board configured. PCGuider Ready. and type a short menu.
Commands to the guider are given with single keystrokes, without typing return. The commands are given by the on-screen menu, which reads as follows.:
D : Select Default parameters for various setups ... S : START GUIDING (with current parameters) Q : Quit guiding (interrupt) R : Resume guiding (on original center) P : Set parameters individually M : Type one-page Mini-Manual ? : print this menu X : Exit
Here's how to get the guider going, and some basic information about it:
Type "D" to set default starting parameters for your setup. These won't be quite right but they're a start.
Be sure the silver AUTO GUIDER toggle switch on the TCS panel (black box) is up (ON).
In the xtcs window on hiltner, under "Track", set "Cos Dec" to ON. This will appear on the TCS monitor on the line labeled "Guide".
While you're at it, set the Guide rate to 2.0 in the same window.
Type "P" to set guide parameters (see below). The default orientation parameters (rotation and parity) for your setup should be correct (unless the instrument rotator is not at zero). However, since the TCS was updated in 1995, the default guide rates have been much too low (corresponding to too-aggressive guiding). For spectroscopy, good default N/S and E/W rates are around 8, when the guide rate on the TCS is 2.
Find a guide star, switch to digital mode, and put the box cursor on the star using buttons on the DTI hand-paddle box. In ideal circumstances you should adjust the black level until the background just turns dark, and the white level to give at least a level of 100 (shown as Z on the screen). If the Z is 255, the star is saturated: the guider will still work, but perhaps not as well as it might otherwise. (These ideal numbers are often not achievable in practice; just do what you can.)
Type "S" to start guiding.
To test the guider (before starting an exposure!), try pushing the star to the edge of the guide box using the telescope hand paddle, and watching it come back. If the guider corrects in the wrong direction, the orientation or parity parameters are incorrect. If the star begins to oscillate, the rate parameter is too small. In a few minutes you should have good parameters which you can use for the rest of your run.
Each time a guide pulse is generated, a status line is printed on the screen, and a label line is printed periodically. A typical line might read:
x +- y +- Counts Sky FWHM **E guide N** RMS 7.02 0.05 8.02 0.07 12345 0 4.2 -0.73 0.43 0.62
The first two numbers (and errors) indicate the centroid position of the star in
the box, in pixels. The box is 16x16 pixels; note that this is twice as
large on a side as the cursor box appears on the screen, and the guider
does see data slightly outside the screen on the box.
The "Counts" field is the integrated flux of the guide star above
sky, in arbitrary units, and the "Sky" level is the counts per pixel
in the sky. The numbers under
Two diagnostic tests are applied on each cycle of guiding. If the Counts reading drops below 10% of its original value, the guider will sound an alarm and will stop guiding (the N and E motion will be zero). Also, if the FWHM of the guide star becomes so large that it's indistinguishable from background, the guider will beep, complain, and stop guiding until it can figure out what it's looking at.
Avoid using the large brightness knob on the DTI, since the PC gets its data before the brightness is applied.
7.4.3 Setting the Guider Parameters
Several guider parameters can be configured using the P command. When you type "P", the program prompts you with the parameter and its present value in square brackets; if you like it, just hit ENTER, to change it, just give the new value and hit ENTER.
If you notice that the guide star is visibly oscillating, then the guider's rate parameter is probably off (perhaps the installed lens is not what you thought, etc.). This can be compensated for by artificially increasing the telescope guide rate parameter in the pcguider. This will damp out overshoot of the guider, at the cost of perhaps not correcting as much at each interval as is required.
7.4.4 More Guider Notes
If the seeing is exceptionally variable or poor, or if there is a lot of wind shake, the best configuration is to increase the guider samples per update to 5-10. This will use an average of many positions, and discourage the guider from moving the telescope to a position when the stellar image may have already moved away.
If you need to guide on very faint stars, increase the guider sample interval parameter to 2 sec and increase the DTI leak time to 2 sec for better performance.
The guide star's centroid is found by a simple first-moment algorithm. There is no attempt to fit a sloping background. Because of vignetting in the optics, a bright background at high gain often shows a strong slope, which affects the centroid calculation. The guider can quite happily "chase" a sloping background far off target. For this reason it is good to make the background black, if that's possible.
7.5 Field Orientation Maps
Figure 7.2 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 7.4.3):
Direct imaging (Off-axis guiding)
Guider angle (deg) = 90 + (Instrument Rotator Angle) E/W flip: 0
Spectroscopy (Slit-plate guiding)
(Autoguider PA) = 270 - (Instrument Rotator Angle) E/W flip: 1
Notes:
7.6 DTI Technical Reference Section
If you are a first time user, you will certainly want to skip this section, which is adapted from the DTI-20C Operator's Manual. It is included for completeness only.
To undertake a rigorous setup of the DVI:
The Digital Video Integrator uses a "LEAKY MEMORY" algorithm,
Xi = Xi-1 + (xi - Xi-1)/60T
where:
T = time constant in seconds xi = ADC sample value Xi = accumulated ADC average
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