5  Telescope Control System

5.1) Introduction
5.2) TCS Computer Rack
5.3) Telescope Controller and Monitor
5.4) Telescope Control System
5.4.1) Introduction
5.4.2) Initializing the TCS and tcsBridge
5.4.3) Entering coordinates from the keyboard
5.4.4) Entering coordinates from a file
5.4.5) Setting Other Encoders
5.4.6) Automatic Focus Temperature Compensation
5.4.7) Track Rate
5.5) Telescope Hand Paddle
5.6) The Dome
5.6.1) Introduction
5.6.2) Dome controller
5.6.3) Back-up procedure for closing a disabled dome
5.6.4) Occultation of the telescope by the dome near the zenith
5.6.5) Obtaining dome flat fields with the white screen
5.7) Moving the Telescope Out of the Hard Limits
5.8) WWV Time
5.9) Observing Room / Dome Intercom
5.10) Moving Platform
5.11) What To Do When Things Don't Work

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 5-11 shows the various components in the complete control system. The observer commands the TCS through the Control System Software.

This chapter assumes that you are familiar with the operation of all workstations and the X-window system as described in the previous section.

Figure 5-1: Telescope and Control System Overview

Figure 5-2: Telescope Control System Interface

2.4m TCS Display

*While this still presents a good overview, this diagram is not current.  Some items have been removed, added, or upgraded.  A new flow control diagram will be added when available.

5.2  TCS Computer Rack

While telescope operations are controlled from the Control Room, the "MDC" control electronics and TCS PC reside in the left-most column of the rack in the computer room.  Various components can be controlled through the MDC, although typically cycling the drives is the only thing necessary at the start and conclusion of observations.

5.3  Telescope Controller and Monitor

The DFM Engineering, Inc. Control System Interface (Figure 5-2) is located in the observing room. It, along with the mdm24ws1 work station, comprise the area where telescope control and data acquisition will take place.  Beyond that, the MDC rack (Figure 5-3), located in the computer room, contains an array of switches for various subsystems to the telescope.  Theae switches are described below.  Note that typically, the only switch that requires interaction is the Drives switch.  This DFM System is the culmination of upgrades started in October of 2017 and running through August of 2018. 

Figure 5-3: The MDC Rack

MDC Console

Toggle Switches - Typically, these can all be left in the UP position, although the DRIVES switch can be cycled bracketing observations:

HALT MOTORS:  Disengages power to the RA & Dec motor systems in the event of a major failure.  Typically, this will not need to be pressed ever.

TRACK/AUX TRACK:  Allows the user to toggle between standard (typically sidereal) rates and some other user-defined rate.  For the typical program, this is set (up) to TRACK.

DRIVES:  Allows power to the various motor systems (RA, Dec).  For observations, this is set (up) ON.  Although not necessary, once the telescope is stowed and observations are concluded, this can be switched (down) OFF.

MIRROR COVERS:  This is a 3-position switch, allowing physical override of the software-control typically used for cover actuation.  The switch typically resides in the center position.  If flipped up, the mirror covers will open.  Flipped down will close them.  In the normal, central position, the previously mentioned software controls will operate actuation.  Under normal circumstances, there is no reason to adjust this switch.

AUTO DOME:  Enables automatic tracking of the dome.  The dome cannot be controlled manually when it is in the automatic state.  Unless there is issue with dome positioning, or dome motion is desired without moving the telescope, this switch is typically left (up) ON.  See section 5.6 for more details about the dome.

DOME TRACK/HOME:  Works only when AUTO DOME is on.  When set to HOME, the dome is sent to position angle ~320 degrees which allows the shutter to be closed.  When set to TRACK, the dome automatically follows the telescope.  Typically, this switch is left in the (up) TRACK position.  See section 5.6 for more details about the dome. 

MTR DRIVER CHASSIS:  Power switch for the MDC.  Unless performing a lightning shutdown, there is no reason to turn this switch (down) OFF.

In addition to the status of these switches the Control System is also used to display the following information: Figure 5-4: 2.4m Control System
Control Interface

5.4  Telescope Control System

5.4.1  Introduction

The various aspects of the telescope are controlled via software located on the Win10-based system, as seen in Figure 5-2 above.  Information is shared between the telescope, instrumentation and JSkyCalc via a background process called tcsBridge.

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 tcsBridge

If necessary, load the program tcsBridge (Applications>Telescope Control>xtcsBridge).  However, this is usually already running, on workspace 4 of mdm24ws1.

5.4.3  Entering coordinates from the keyboard

Coordinates can be manually entered by opening the dialog box at Telescope>Movement>Slew Position tab on the control PC.  Better yet, load them up through JSkyCalc on mdm24ws1.

Figure 5-5: Manually Entering Coordinates

manual coordinates

5.4.4  Entering coordinates from a file

If you have a large number of objects or standard stars in your observing program it might be more efficient to construct a database file.  This can be done in advance and then loaded onto the TCS PC locally via thumb drive.  Object lists can be loaded into the Control System by going to Telescope>Movement>Mark/Move Table tab.  File type is expected to be filename.mrk.  These can be edited using any standard text editor.  A blank form (blank.mrk) as well as a sample form (one.mrk) can be downloaded here.  Using Figure 5-6 as an example, make note of the following:  Count should equal the number of objects being added to the list; Namen, RAn, Decn & Epochn correspond to the listing reference, n.  Of greatest note, RA & Dec are not input as hh mm ss & deg mm ss as typical, but instead as decimal values.  This requires conversion prior to creating object lists.  A blank conversion spreadsheet (coord_conv.xlsx) as well as an example version (coord_conv_samp.xlsx) can be downloaded here in case assistance is required in converting coordinates.  Properly formatted .mrk files can be uploaded via USB to the TCS Control PC  and placed into a directory, located on the desktop, labelled Target-Lists.  Once the .mrk file is uploaded, click on the 'Load Mark File' button and load your desired .mrk list file.  You should see each object appear, one per row, on the list.  Note that RA and Dec follow standard hh mm ss & deg mm ss convention in the list.

Figure 5-6: A Sample .mrk File

.mrk file

5.4.5  Setting other encoders

RA, declination and focus readouts are all achieved through absolute encoders.  Therefore, there will be no need to ever set these values.  The dome azimuth and rotator position however use incremental encoders.  In certain situations, such as cycling the Control System software with equipment not at home positions, it may be necessary to set these two encoder values.  This is done by going to Telescope>Initialization>Other Positions tab.

TCS Equinox
Displayed equinox can be changed through Telescope>Misc>Display Equinox/Dome Shutter tab.  There may be a bug in the code where the equinox value written to data headers is incorrect unless it is changed to the desired value, typically 2000 (default is to current epoch).  It is recommended that the equinox value be manually changed to whatever reference standard is currently being used.
 5.4.6  Automatic Focus Temperature Compensation

As the night proceeds, typically temperatures drop, causing the telescope truss to cool, ultimately resulting in a shift in optimal focus, typically at a rate of roughly 45 microns per 1 degree C.  The telescope control software takes this into account and compensates the positioning of the M2 within its enclosure to maintain proper displacement between the primary and secondary mirrors.  Once the optimal focus position is found manually and observations begin, the system will maintain proper focus without required input from the user.  It is worth noting that while the focusing motor will be moving to adjust secondary mirror positioning, the focus display as seen on the TCS software will not be changing.  This is normal.  This routine can be found through the  Telescope>Misc>Focus tab (Figure 5-7 below).  The default reference temperature is "Center Section".  This can be changed by the user although this is not recommended.  This is continually running as long as the telescope is powered on.  Therefore, even as temperatures warm back up during the day, the control system is compensating.  As a result, focus should be correct when returning on a second night.  Still, it is good practice to verify focus position at the start of the night before taking data.

Figure 5-7: Automatic Focus Temperature Compensation
Focus Comp

5.4.7  Track Rate

Track rates are set from Telescope>Rates>Track Rates tab.  To initiate tracking, simply click the "Set Sidereal Rate" button, then click "Apply".  This will set the HA track rate to 14.451 Arcsec/sec (and Dec to 0).  Users are able to also set an auxiliary track rate.  To switch between the two defined track rates, flip the switch between Track and Aux Track on the MDC.

When stowing the telescope, it is necessary to reset track rates to 0 prior to slewing to zenith.  To do so, simply enter '0'  in the "H.A. Rate" box and click "Apply".  You can verify tracking has stopped because the HA value should stop clocking (and RA should start).

Figure 5-8: Setting Track Rates
track rates

Cos Dec & Rate Correction

Normally this, as well as Rate Correction, are on.  To modify this setting, point to Telescope>Misc>Switches tab.  Be sure to click "Apply" to accept any changes you make to settings.  WhenCos Dec is enabled, the R.A. guide and set rates will increase with declination to provide constant rates on the sky.  When Rate Correction is enabled, track rates are adjusted for refraction, flexure and telescope misalignment.  The magnitude of the corrections  can be seen  on the Control System  GUI main panel.


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 as well as rotate the dome.

Figure 5-9: Telescope Hand Paddle

Hand Paddle

The following features are available:

N, S, E, W:

Moves the telescope at the guide speed in the cardinal directions.


When simultaneously pressed with N, S, E or W, causes telescope motors to be ramped up to the set speed. Releasing the SET button will ramp the motors back to guide speed if the direction button remains depressed. Momentarily releasing just one of the buttons will keep the telescope moving at that instantaneous ramp speed.


Works the same as SET except much faster, ~2 degrees per second.


Focus buttons for the secondary mirror. IN decreases the mirror separation and hence lowers the focal plane of the telescope (focus encoder value increases).

Pressing IN (or OUT) together with SET moves the focus ram at a much faster rate.

An absolute encoder displays the position in microns of the secondary ram on the Telescope Monitor.

The TCS software compensates secondary mirror positioning with reference to changes in temperature. The focus value decreases as the temperature falls at a rate of roughly 47 microns per 1 degree C.  A routine is provided within the Control System that maintains proper displacement between the primary and secondary mirrors once optimal focus is manually determined.  While the reference temperature for the compensating routine can be user-set, it is strongly recommended that the default selection of "Center Section" be left as is.  See section 5.4.6 above for more information on this.

Focus the telescope at the start of your first night.  Once focused, the TCS will maintain focus throughout the night.  Note that the focus readout in the TCS display does not change--this is normal and to be expected.  Consider this to be more an indication of secondary mirror position referenced to the primary as opposed to referencing to its enclosure.  While the compensation will run closed-loop all night and day, theoretically maintaining proper focal positioning, it is still worthwhile to verify your focus at the start of each subsequent night (although don't be surprised if it is already correct!).  It is also worth noting that this system does not take into consideration any focal changes due to filter changes that may not be parfocal.

DOME L and R:
The dome can be moved manually either to the left or right, as long as Auto Dome is OFF on the MDC.

Virtual Hand Paddle
A virtual hand paddle is also available under Options>Hand Paddle.  This hand paddle offers similar options to the physical hand paddles, although speeds can be user-defined (Telescope>Rates>Handpaddle Rates tab).  Dome control is not available through the virtual hand paddle.

Figure 5-10: Virtual Hand Paddle.
virtual paddle

5.6  The Dome

Figure 5-10: Dome Control Panel

Shutter Control

5.6.1  Introduction

The observatory dome is a single skin aluminum hemisphere 10.8 m (425") in diameter, manufactured by Ash Dome. 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. This is achieved by either selecting AUTO DOME OFF or ON, respectively.

To open the dome:

  1. Ensure the mirror covers and the MIS dark hatch are closed.
  2. Ensure dome contactor boxes are aligned for good electrical contact.  This is at a dome azimuth of roughly 320.5 degrees.
  3. Open MAIN shutter; it will automatically stop when fully open.
  4. Open DROP-OUT shutter 10 seconds after main shutter started; keep finger on button until fully open.  You know it is open when the sound of the hydraulics audibly changes.

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 buttons on the hand paddle (with AUTO DOME set to OFF). Often, MDM staff will do this during the day if needed.

To use the automatic dome controller:

  1. Set AUTO DOME to ON on the MDC front panel. 
  2. Open the dome manually from inside the dome.
  3. Make sure that DOME is set to TRACK on the MDC front panel (there is no reason really to ever switch it to the alternate, HOME position).
  4. Under Telescope>Misc>witches tab, set Dome=On, Dome Mode=Track, ome Target=Telescope.  Click "Apply" to accept changes.  This should cause the dome to rotate into alignment with telescope azimuth. 

The dome should automatically follow the telescope. The TELESCOPE MONITOR should display Auto Dome ON and DOME TRACK.  Dome Aximuth (along with Telescope Azimuth) can be found near the bottom-left of the Control System GUI main page.  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.

To close the dome:

  1. Under Telescope>Misc>Switches tab, Dome=On, Dome Mode=Home, Dome Target=Telescope.  Click "Apply" to accept changes.  This should cause the dome to rotate to the home position, ~320.5 degrees dome azimuth.  Homing the dome only moves the dome in the CW direction.  If the dome is at an azimuth not appreciably greater than 320, it might be easier to simply disable AUTO DOME on the MDC and stow it manually.
  2. Shut off tracking and move telescope to zenith position.
  3. Close the mirror covers and MIS dark hatch.
  4. Ensure the dome contactor boxes are centered for good electrical contact.
  5. Close the DROP-OUT shutter (keep finger on button until closed).
  6. Close the MAIN shutter (will stop automatically).
  7. When the dome closes the exhaust fans in the chimney vent are activated.

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 rotation or contactor box fails:

  1. If possible rotate the dome to a position where it is safest to climb up and retrieve the power cord.  
  2. Climb ladder and retrieve power cord from motor unit near the bottom of the shutter.
  3. Verify that the contactor plates are not "homed".  If they are and the cord is plugged in, a short will occur.
  4. Plug cord into the REMOTE socket (Figure 5.10).
  5. Open or close the shutters by climbing the ladder and using the green paddle located near the motor.  Be sure to close the droppleaf prior to the shutter.
  6. Disconnect the remote cable and STOW THE POWER CORD.
  7. Make note via trouble report and email so that MDM staff can correct any issues.

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 (41.3"). 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.

Table 5-1: Percentage of Light Occulted by Dome Near Zenith

+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.  The projectors are no longer in use however.  Methods are available for taking twilight sky flats.  These are detailed here if interested.  MDM may possibly revisit dome flats if demand is great enough, however the engineering required would be extensive as lamps would have to be implemented from scratch.


The TCS has positional limits to keep the telescope out of unsafe positions.  While most programs require observations near the meridian, there are times when observations near the horizon are desired. 

There are several levels of limits:
Recovery From Limits:
The telescope can be backed out of Approaching Limit, Limit Reached, and the directional limits via the hand paddle.  The directions that put you deeper into the limits should be disabled, but use caution to only go in the appropriate direction anyways.  Once out of the limits, thetelescope should behave normally.  It is advisable to use the hand paddle within the dome to perform this so that telescope movement can be monitored.

Getting out of the HIL is potentially dangerous as the limits have to be physically overridden.  If it is late at night, consider this an indication that observations for the night are lost.  If early enough, contact MDM staff and there's a chance we can walk you through the procedure to get out of the limits.  Thankfully, it should be extremely difficult to get to this point.


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 better, 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:

Top (black) box: Controls DOME microphone

MIC 1 6 - 8
MIC 2 not used
PHONE/AUX not used

Bottom (silver) box: Controls OBSERVING ROOM microphone

AUX not used

During windy weather you can silence the speaker by turning the black box MIC 1 down.

5.10  Moving Platform

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.  It is absolutely critical that the platform be lowered all the way prior to moving the telescope or severe damage could occur.  Always double-check that the platform is down at the start of the night.  

5.11  What to do When Things Don't Work.

Please contact the staff.

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Updated: 2019Jan16 (Galayda/MDM)