Last edited: Dec. 7, 2012 - AB Please send comments or suggestions to: Alaina at APO

ARC 3.5m | General Info

Telescope Limits

The telescope is an Alt-Az design so you can observe below the pole with no problem, you just cannot observe at the zenith. The telescope CANNOT track or be slewed above 85 degrees altitude. Beware of this limitation as your target may track into this "zone of avoidance "! The mirror becomes unstable in the mirror cell above this altitude and it has been hard coded into the software to prevent you from tracking (or slewing) above this limit. The telescope WILL stop moving if you hit the 85 degree limit.

A simple rule of thumb is if your targets declination is between 27 & 37 degrees declination you may run the risk of moving into the "zone of avoidance". The telescope will be at it's highest altitude when the RA equals the LMST. This doesn't mean that you can't observe objects between 27-37 degrees declination, just that there is the possibility that you may track (or slew) into the "zone of avoidance". Always ask the Observing Specialist for help if you are unsure if you will slew into, or your target will track into, this zone BEFORE you slew. The Observing Specialist can advise you if and when the limitiation will occur.

If you need to slew the above 80 degrees altitude, Please get the permission from the Observing Specialist BEFORE you slew. We need to more closely monitor the mirror load sensors when the telescope is above this altitude.

The telescope also cannot track/slew below about 7 degrees altitude. There are hard stops at about 4 degrees.



Control room - This is the room where the 3.5m telescope observing specialists work and operate the telescope from. It is located in the main building.

Computer room - This room is across the halway from the Control room. It houses several of the instrument control computers (ICC) as well as the telescope control computer (TCC).

Ground level - This is the base level of the telescope. It is located on the ground level of the 3.5m telescope enlosure (building), which is adjacent to the main building with a tunel (arcade) connecting the two. It is mostly used for storage of non-essential 3.5m equipment and a stairwell that leads up to the intermediate and observing levels of the 3.5m enclosure.

Intermediate level - This level houses the main drive motors for the building as well as the azimuth drive motors for the telescope. Also on this level are the cooling systems for several instruments, several ICCs, Altitude/Azimuth drive motor control boxes, and the controls (manual) for balancing the telescope during instrument changes.

Observing level - This is the level on which the 3.5m telescope is located and also houses the 3.5m instruments. Low level telescope control systems are also located on this level.

Telescope Control Computer (TCC) - This is the computer that controls the low level operations of the telescope.

Master Control (MC) - This is an intermediate computer that recieves commands from Remark or TUI and parses (or acts upon) them to control the telecope at an intermediate stage. Telescope and instrument control commands can be sent directly, via a command line, to this computer if it is not possible to use the gui interface TUI or Remark.

Telescope User Interface (TUI) - This is the front end control system for using the telescope and most of its instruments. It does not control the telescope directly but instead user commands are sent to the MC from this level and that computer parses the commands and takes the appropriate action. TUI can be run on a variety of system that can run Python (Unix/Linux, Macintosh, Windows).

Enclosure doors (Observing specialist control) - These are the main enclosure doors that open the telescope to the night sky. These doors must be manually operated and require the observing specialist to physically go up to the observing level to open/close them.

Mirror covers (Observing specialist control) and Mirror cover flats (user control) - The mirror covers give secondary protection to the mirror to help keep dust off the mirror surface. The mirror covers also act as flat field screens. It is not necessary to point the 3.5m telescope at a flat field sceeen to take flats, we just need to close the mirror covers. The telescope drive motors MUST BE TURNED OFF when the mirror covers are closed. See Truss Lamps below.

Tertiary (Observing specialist control) - The 3.5m telescope does not have a cassigriain port on which an instrument can be mounted, instead there is a 3rd (tertiary) mirror located at this position which allows light to be directed at a right angle (out the side) of the telescope through various selectable ports around the telescope.

Ports & switching between instruments (Observing specialist control) - The instruments of the 3.5m telescope can be mounted onto several ports on the side of the telescope, depending upon the instrument and it's requirements. This unique feature of the 3.5m telescope can allow for very rapid (1-2 minute) changeovers between instruments. Other instruments may require the use of the same port (Naysmyth 2 instruments, see below) and actually require the observing specialist to physically unmount/mount the required instruments (typically a 15-25 minute process) as well as adjust the balance of the telescope (on the intermediate level).

Eyelids (Observing specialist control) - Each port has a corresponding eyelid (in the mirror cover) that needs to be opened to allow for light to enter the instrument.

Naysmyth (NA) ports - The Naysmyth ports are the two ports located on the Altitude axis of the telescope. The Echelle Spectrograph is permanently mounted to the Naysmyth 1 (NA1) port of the telescope. The Naysmyth 2 port has an available field de-rotator (see rotator below) that compensates for the field rotation inherent in all Alt-Az designed telescopes. DIS, SPICam and NIC-FPS are instruments that mount to the NA2 port.

(Field De-)Rotator - There is a large field (de-)rotator mounted permanently to the NA2 port. This counters field rotation for imaging type instruments, or to allow for certain rotation angles to be set (Paralactic angle observations or along/perpendicular galaxies). Field rotation is inherint to all Alt-Az mounted telescopes. Field (de-)rotation is not necessary for every instrument (ie. Echelle spectra observations).

On Axis guiding (NA1 port) (User control) - The Echelle Spectrograph (permanently mounted to the NA1 port) has an on-axis guider. This is a separate camera (with separate controls) that images a mirror located in the focal plane of the Echelle light path. There is a slit in the center of the mirror (size selectable by the observing specialist per observer setup request) through which light actually enters the spectrograph. It is important to note that the adjustment (exposure, etc.) of this camera DOES NOT affect the light entering the spectrograph. Guiding is performed by looking at the light spillage from the star, from th guide camera images, surrounding the slit in the mirror and then moving the telescope such that the light in the vertical and horizontal axis is roughly equivalent. Therefore IT IS IMPORTANT that the exposure of the guidecamera is long enough to produce a slightly over-exposed image of the star on the guidecamera with some light spilling over the slit (ie. if the light from the star on the guider image goes completely into the slit, no spillover, the guider CAN'T work properly). Again, this spillover seen on the guidecamera DOES NOT affect the light actually entering the Echelle. Since the mirror/slit is at the focal plane of the Echelle, focusing the star image with the NA1 slitview camera also focuses the Echelle. NA1 guider images can be downloaded to your computer (via FTP or scp) and are located in the /export/images/ecam directory on Tycho. They are overwritten every night so they should be copied over during or at the end of the night.

Off axis guiding (NA2 port) (User control) - The NA2 port of the telescope has a 2 degree off axis guider. This also is a separate camera but it is NOT at the focal plane of the instrument, therfore the main instrument MUST be focused first (by the observing specialist) and then the guide camera can be focused (by the observing specialist). The NA2 guider uses the whole guide image and automatically chooses an appropriate star. Most of the time the star it will choose will be fine for guiding, but if it is too close to the edge of the field the guider can become confused. The user can manually select another, more suitable, star and override the automatic selection. If you are doing paralactic angle observations the guide field will also move the guidestar out of the field (as the rotator keeps the paralactic angle) and the observing specialist MUST be aware of this to help choose different guidestars as they move out of the frame.

Focus (Telescope & Off-axis guider) (Observing specialist control) - The main focus of the 3.5m telescope is controlled by very small motions of the secondary mirror. Depending upon the guider used, it will either be focused simultaneously with the main telescope (Echelle guide camera) or separately AFTER the main telescope is focused. The NA2 guide camera focus is accomplished by moving a optical stage that has 2 mirrors on it which increase/decrease the optical light path to the guide camera. If using the NA2 guider the 3.5m telescope (Instrument) MUST BE focused first, then the guide camera can be focused. Typicaly the observing specialist on duty will do the initial focus for both the instrument and the guider, although the user can also tweak the focus the instrument in use at anytime. Since the guider focus is relative to the main telescopes (Instruments) focus, when you adjust the focus on the main instrument you also change (hopefully improve, but this is not always the case) the focus on the guider. The observing specialists use this information to inversely infer when the guider focus is poor, it is likely that the main instrument focus is also poor (or needs adjusting). The Observing specialist will focus the guide camera and this typically only takes a couple of minutes after the main telescope focus is set.

Ventilation of the enclosure (Observing specialist control) - The 3.5m telescope does not have active cooling. Typically temperatures in the Sacramento mountains do not get hot enough to warrant this. We cool the primary mirror by using ventilation fans which draw filtered outsde air in and move it through the mirror and supporting structure. Typically cooling of the primary to equalization takes about an hour (after sunset). Focus changes should be expected during the twilight as the mirror and structure comes to equilization. During the daytime (non-operational) hours, we employ a pressurization fan (filtered) to help keep dust out of the enclosure (and also moderate the inside temperature).

Calibration lamps, Integrating sphere, and Truss lamps (User control) - The Calibration lamps for the Echelle are self-contained inside the Echelle itself. These are under user control and can be used without affecting other instruments. We also have a set of lamps that are located on the upper truss structure of the telescope. The available lamps ar He, Ne, Ar, Bright Quartz, and Dim Quartz. We can, upon request, replace any of the He, Ne, or Ar tubes with another element (pending availability of tube, ask the observing specialist what is currently available on site). We can also place an integrating sphere (currently a HgNe lamp) that can be mounted to DIS for daytime calibration use. Calibration lamps are typically reflected off the 3.5m mirror covers (recommended), effectively our flat field screen, or off the primary mirror itself. If the mirror covers are used the telescope drives MUST BE TURNED OFF while the covers are closed.

Base instruments and mounting locations - The 3.5m telescope has 4 main instruments that are typically available every night. The Echelle is permanently mounted to the NA1 port of the telescope. DIS , TripleSpec , and SPIcam are NA2 mounted instruments (only one can be mounted at a time). There are instruments also mounted to other ports on the telescope, but these are typically engineering instruments or program specific instruments. If you have a specialty instrument that you would like to mount to the 3.5m telescope, please contact the APO engineering staff for our requirements and specifications for mounting visiting instruments.

Boresight -

Offset types - RA/DEC, X/Y, Computed/Uncomputed -

Keeping Offsets - Object, Boresight, Guider Correction, Calibration -

Coordinate systems - The 3.5m telescope uses a NON-CONVENTIONAL coordinate system. It is defined as due South as 0 degrees, due East as 90 degrees, due North as 180 degrees, and due West as 270 degrees. These are the coordinates that the low level telescope control computer uses (TCC) and are required when manually slewing the telescope via the master control (MC) level. Typically the conversion is taken care of automatically for the user when using the TUI interface using the normal RA/DEC coordinate inputs.

Slew & Wrap limits - Alt/Az Axis, Rotator (individual instrument specific wraps limits) - The altitude of the telescope can slew anwhere between 7 to 85 degrees. For altitudes larger than 80 degrees the observing specialist MUST be notified BEFORE you slew. Be aware that the telescope may track, if on the east side of the meridian, to this altitude during observations. If you are unsure if you will be entering this region let the observing specialist on duty know and they can better advise you. The telescope also has an azimuth wrap limit. Typically the user does not need to be concerned with these, as the software will position the telescope in the best position for tracking, but in certain circumstances the user should be aware of the limits. The telescope has a wrap limit at due South (0 degrees, see Coordinate systems above) rotating Eastward for 540 (360 + 180) degrees to due North (180 degrees). The rotator on the NA2 port  also has rotation limits. This is dependant upon the instrument that is mounted on that port. The observing specialist can advise you on the instruments rotator limits. This typically will also be taken care of automatically when using the TUI interface.

Authority of the telescope during night operations

The observing specialist on duty has the ultimate AND final authority on the operations of the ARC 3.5m telescope and its instruments. The judgement call regarding the operations of the telescope and its instruments, for whatever reason, is the sole responsibility of the observing specialist on duty and CANNOT be overruled during the night time operation of the telescope.

If the user has a complaint on the decision of the observing specialist to open or stay closed, they may address it (in e-mail only) and to the site director and it will be investigated for any merit at a later time (ie. the following day). The site director WILL NOT overrule the decision of the observing specialist during the night in question. Please see Closure Conditions for more on ARC's policy and for reasons the observing specialist may have to keep the telescope closed during the night.