5 Telescope Control System
5.1 Introduction
This chapter describes the operation of the telescope and dome through the Telescope Control System (TCS). The heart of the control system is the TCS computer rack. Figure 5.1 shows the various components in the complete control system. The observer commands the TCS with xtcs (an X-windows package developed by John Tonry, Mark Metzger of M.I.T. and wih some modification by Gary Bernstein of Michigan in 1996) and the current status of the telescope is displayed on the telescope monitor.
This chapter assumes that you are familiar with the operation of all the Sparc and Linux workstations and the X Window System as described in the previous section.
5.2 TCS Computer Rack
The Telescope Console is normally turned OFF when observing stops. The Telescope Control System (TCS), a series of four electronics racks located in the computer room, is normally left continually ON. These racks do the actual moving of the telescope and dome and keep track of the RA, Dec, focus, instrument rotator and dome encoders. Output from these encoders, duly corrected for precession, refraction and flexure, and additional information is displayed on the Telescope Monitor which is located above the Telescope Console. This information is also accessed by the Sparc computers.
5.3 Telescope Controller and Monitor
The DFM Engineering, Inc., model 70 TELESCOPE CONTROLLER (Figure 5.2) is located in the observing room. It comprises a bank of illuminated push buttons and toggle switches, the status of which are displayed on the TELESCOPE MONITOR. If the monitor is turned off, power it on and adjust the brightness and contrast. This DFM System has since been replaced by a new system developed by Robb Lovell, Bob Barr and John Thorstensen at MDM, in 1995.
Push Buttons
Toggle Switches
Note: unless otherwise noted, set all toggle switches to OFF (down) at the end of the night.
In addition to the status of these switches the TELESCOPE MONITOR is also used to display the following information:
These parameters are accessed through the Telescope Control Software window, xtcs.
5.4 Telescope Control Software: xtcs
5.4.1 Introduction
The telescope and dome are controlled through an X-windows program called xtcs (Fig 5-3) which is run and displayed by hiltner. Startup procedures for the software are described in section 4.3.2; this chapter assumes you are familiar with all of Chapter 4.
5.4.2 Initializing the TCS and xtcs
When the software is loaded a large red box with the word INITIALIZE appears. Click on this with mouse. This initializes communications with the TCS computer by clearing the communications link and setting the Universal Time. These two functions (Clear Link and Set UT) are also available at any time in the xtcs SETUP menu (section 5.4.6). The Set UT should be done once a day.
5.4.3 Entering coordinates from the keyboard
Coordinates can be entered at the keyboard or recalled from a file. To make an entry from the keyboard place the mouse anywhere inside the RA field. The cursor changes from a pointer arrow to an I-beam (I). Enter the Right Ascension (hh mm ss.s). Then either move the mouse into the Dec field or press the TAB key to enter the declination (dd mm ss.s). Repeat this procedure to get access into the Epoch field.
You can perform editing inside all of the input fields. The current position of the cursor is marked by a ^ symbol. Click the SELECT (left) mouse button to change the position of the cursor. The BACKSPACE or DELETE key both delete the character to the left of the cursor. To delete the entire field press CTRL-U. You can also ``cut & paste'' entries using the mouse buttons in the usual way.
5.4.4 Entering coordinates from a file
As we have just seen, you can enter the coordinates of each object at the keyboard as you need them. However, if you have a large number of objects or standard stars in your observing program it will probably be more efficient to construct a database file.
The program expects items in the order Object name, R.A., Dec., Epoch. Embedded spaces are not allowed in the object name (maximum 10 characters). A sample file follows:
HILTNER102 1 5 53.8 62 31 32.0 1950.0
My_star 1 46 28.5 -13 18 17.0 1992.6
HD60778 7 33 39.0 -0 1 49.0 1950.0
2207+8201 22 07 13.2 82 01 13.9 1975.0
Note that the software expects the RA and Declination coordinates in (hh mm ss.s dd mm ss.s) respectively. Some objects, such as bright galaxies and clusters, are listed in the Astronomical Almanac as (hh mm.m dd mm.m). Convert these coordinates to the above format because xtcs interprets (12 18.50 +14 27.2) as (12 18 00.5 +14 27 00.2).
You can construct this file before arriving at the observatory. The file should be placed in the visitor directory on hiltner. Please backup the file(s) before you leave if you want them to stay around as as they will be deleted by subsequent observers.
The following input fields and selection boxes deal with coordinate files:
5.4.5 Down-loading coordinates to the TCS and commanding the telescope to move
The coordinates are initially checked by xtcs for format errors. The TCS computer further checks that the coordinates you have entered are valid and within the observing limits of the telescope. There are six selection boxes to send the information displayed in the RA DEC and Epoch fields: Send Coords; Prev Coords; Send Offset; Stop; and Go.
If the coordinates you sent were not valid one of the following messages will appear in the xtcs message area:
TCS Error on last command
TCS Error (Coordinate out of range).
In the later case the message TARGET OUT OF RANGE will appear on the TELESCOPE MONITOR.
Following either of these messages select Stop.
5.4.6 The Setup menu
The following functions are available in the Setup menu:
If the TCS freezes and you cannot set the U.T. then click the mouse on the STOP button in the xtcs window.
Section 5.6.2 contains more details on the dome control.
5.4.7 Manually restoring coordinates using the tilt meters
When the TCS is powered up from a cold start and initialized, it assumes that the telescope was left pointing at the zenith. The encoders are automatically set to 00h 00m 00.0s Hour Angle and the declination equal to the latitude of the observatory (31o 57.0').
If the TCS crashes and the telescope positions are lost a manual recovery must be made. The new TCS computer stores the most recent info in a file usually, so turning it off and back on may be less painfull.
The tilt switches are accurate enough to allow you to acquire a bright star from the Astronomical Almanac. You should now move to such a bright star, as close to zenith as possible, center the image on the detector, and reset the encoders.
5.4.8 TCS display: setting other encoders
Clicking on the TCS display button produces a pop-up window with the following functions:
(Technical note: Entering -1 in the display epoch field will coerce the TCS to display apparent coordinates without correcting for refraction and flexure. This feature should only be used for engineering purposes.)
If absolute values of focus are required run the focus all the way OUT (the display will stop changing when the limit is reached) and then set the encoder to zero. Now focus the telescope.
The focus of the telescope is a function of the truss temperature, decreasing by 10 focus units for every 1oC fall in the truss temperature (section 5.5).
5.4.9 Track Rate
When the TCS and telescope control windows are initialized (section 5.4.2) the track rates are set to 15.001 arcseconds per second in R.A. and zero in declination. The track rates are displayed in the RATES window of the TELESCOPE MONITOR. The track, guide and set rates can be customized as follows:
Default 15.001 Maximum 25
Default 0 Maximum 25
Default 2 Maximum 10
Default 60 Maximum 100
5.4.10 Trail Rate
TCS has a facility for trailing the telescope back and forth to broaden spectra. Three variables must be set:
5.5 TELESCOPE HAND PADDLE
Two identical paddles are connected to the telescope. One is in the observing room, the other is normally hanging to the west side of the polar axle in the dome. They allow the observer to manually guide, set, slew and focus the telescope.
The following features are available (see Figure 5.4):
Pressing IN or OUT together with SET moves the focus ram at a much
faster rate.
An incremental encoder displays the position of the secondary ram on
the Telescope Monitor. Under good seeing conditions the telescope can
be focussed to better than 3 focus units. The value of the focus
readout can be arbitrarily set through the xtcs Epoch,
etc window (section 5.4.8).
The telescope has to be re-focussed as the temperature of the
telescope truss structure changes. The focus decreases as the
temperature falls such that:
Focus the telescope at the start of the night and note the truss
temperature from the air-conditioning computer. It is advisable to
change the focus according to the above formula during long exposures.
Use the FOCUS OUT button if the temperature is falling. Unless there is
a large temperature change there is no need to change the focus of the
guider camera for the autoguider while the exposure is in progress.
5.6 The Dome
5.6.1 Introduction
The observatory dome is a single skin aluminum hemisphere 10.8 m (36
ft) in diameter, manufactured by Ash Domes. The outside surface is
covered with 3M-ScotchR 425 aluminium foil tape to stop
over-cooling by radiation at night. The main shutter opening is 2.92 m
(115 inches) wide. A drop-out shutter allows the telescope access down
to the horizon although this is limited in practice by the telescope
hard limits (Section 5.7).
5.6.2 Dome controller
The dome can be operated manually or under computer control. The
dome controller is the bottom unit in the TCS rack. It has only two
illuminated push button controls, a red one for power and a yellow one
toggling between manual and automatic control. When the yellow button
is illuminated the dome cannot be turned manually.
To open the dome:
Special attention must be paid to ice build up during the winter
months. Do not attempt to open the shutter doors if they are iced over.
Try turning the icy area towards the sun using the left- right switch.
The automatic dome controller must in manual mode for this switch to
work.
To use the automatic dome controller:
The dome should automatically follow the telescope. The TELESCOPE
MONITOR should display DOME ON and report both the current DOME AZIMUTH
and the DOME AZ ERROR, which is the difference between the current
position of the dome and the position calculated by the TCS for the
telescope to be pointing through the center of the slit. The dead band
that the TCS uses to determine when to move the dome is a function of
the zenith distance of the telescope. At the zenith this dead band is
360o. The TELESCOPE MONITOR displays the message DOME OK
when it believes the dome is not occulting the telescope. When the TCS
moves the dome this message disappears until the dome stops.
To close the dome:
5.6.3 Back-up procedure for closing a disabled dome.
The dome shutter doors can only be operated when the contactor box
located at encoder position zero makes positive contact with the feet
attached to the dome. In the unlikely event that the contactor box
fails it will be impossible to open or close the dome from the dome
control panel, as described in the previous section.
To open or close the dome when the contactor box fails:
5.6.4 Occultation
of the telescope by the dome near the zenith
The distance from the zenith to the back dome bulkhead (where the
shutter motor resides) is only 1.05 m (41.5 inches). As a result the
dome occults when the telescope is pointing within 1.25o of
the zenith. The percentage of light lost as a function of Hour Angle
and declination is shown in Table 5.1 below.
dF/dT(oC) = 10
| dDec | Dec. | ||||||
| +75' | 0.00 | 0.00 | 33o 12' | ||||
| +60' | 0.17 | 0.14 | 0.00 | 32o 57' | |||
| +45' | 0.48 | 0.41 | 0.14 | 0.00 | 32o 42' | ||
| +30' | 0.87 | 0.77 | 0.41 | 0.14 | 0.00 | 32o 27' | |
| +15' | 1.34 | 1.14 | 0.77 | 0.41 | 0.14 | 0.00 | 32o 12' |
| +00' | 1.87 | 1.34 | 0.87 | 0.48 | 0.17 | 0.00 | 31o 57' |
| -15' | 1.34 | 1.14 | 0.77 | 0.41 | 0.14 | 0.00 | 31o 42' |
| -30' | 0.87 | 0.77 | 0.41 | 0.14 | 0.00 | 31o 27' | |
| -45' | 0.48 | 0.41 | 0.14 | 0.00 | 31o 12' | ||
| -60' | 0.17 | 0.14 | 0.00 | 30o 57' | |||
| -75' | 0.00 | 0.00 | 30o 42' | ||||
| H.A. | +0m | +1m | +2m | +3m | +4m | +5m |
5.6.5 Obtaining dome flat fields with the white screen.
The dome is equipped with a large white screen for obtaining flat fields.
5.7 MOVING THE TELESCOPE OUT OF THE HARD LIMITS
The telescope is equipped with mercury limit switches which are activated when the telescope approaches its safe horizon. When this occurs the TELESCOPE MONITOR displays the message LIMIT REACHED, the brakes are applied to the telescope and the TELESCOPE CONTROLLER is turned off. Approximately 3 to 5 minutes before this fail-safe condition is reached the TELESCOPE MONITOR displays the message APPROACHING LIMITS. Be wary of this condition if you are working far over in the west.
To bring the telescope out of the hard limits first go into the dome and check that the telescope is really where you think it is! Then hold down the START button on the TELESCOPE CONTROLLER with one hand while pressing SET plus the appropriate N S E W button on the hand paddle to move the telescope towards the zenith. Once the LIMIT REACHED message has vanished from the TELESCOPE MONITOR you can release the START button.
5.8 WWV TIME
The WWV TIME is used to synchronize the mountain-top computers. The NTP (Network Time Protocol) program, uses the U.S. Naval Observatory and a couple of reliable alternative time servers to synchronize all the computers. In practice, sychronization should be to the nearest second or so, depending on the external network connection.
5.9 Observing Room / Dome Intercom
A simple intercom system is used for communications between the observing room and the dome. It is controlled by two independent amplifiers located under the computer bench in the observing room.
Use the following settings to minimize feedback:
| MIC 1 | 6 - 8 |
| MIC 2 | not used |
| PHONE/AUX | not used |
| TONE | 3 |
| MASTER VOLUME | 15 |
| MIC | 2 |
| AUX | not used |
| TREBLE | 3 |
During windy weather you can silence the speaker by turning the black box MIC 1 down.
5.10 Moving Platform and the Locking Pin
The moving platform inside the dome is used for instrument changes and other engineering tasks. It can also be used when filling the CCD dewar when you are not observing.
When the platform is moved or raised the power to telescope drives is removed and the brakes are applied to the telescope. This is the equivalent of pressing the EMERGENCY STOP [POWER] button on the TELESCOPE CONTROLLER. The START button is then disabled for safety reasons. If you accidentally raise the platform while observing, simply lower the platform again and then press the START button on the TELESCOPE CONTROLLER. The coordinates will be correct, but your object will have drifted to the west because the telescope was not tracking. Use the xtcs software to send the coordinates again or manually set the telescope with the hand paddle.
The telescope employs friction drives and must be locked whenever it is out of balance, such as during instrument changes. A locking pin is inserted in a hole through the R.A. drive, just above the nameplate on the telescope. This disables the START button on the TELESCOPE CONTROLLER. Remove the pin if inserted and check for and remove the declination brace if it is also present. There are pins at either end of the declination brace which attaches to the south side of the mirror cell and the cradle of the fork arms.
5.11 What to do When Things Don't Work.
Please call Bob or Steven their phone numbers are on the cork board in the control room.
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