MILT Description

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MILT Description

The Multiple Instrument Large Telescope will be a 3.5 meter Cassegrain Nasmyth telescope on an altitude over azimuth mounting. It will have an f/1.75 primary, fastest yet attempted for a telescope its size. The two Nasmyth foci will be the principle instrument locations.

All telescope operational functions will be computer controlled. Typical operation will be controlled entirely from a standard computer terminal (VDU if you're British). Computers will also provide extensive background and off-line engineering and maintenance functions to assist in diagnosing and repairing system failures.

A variety of instruments will be installed simultaneously at the Nasmyth foci. All will remain operational at all times and can be selected and deployed rapidly by the operator using terminal keyboard commands. In this way, the system can readily be configured to take optimal advantage of transient events, perfect seeing, etc.

Software and hardware will be designed at the outset to support observing sessions where the astronomer remains at his home institution, communicating with an assistant at the telescope. Batch observing will also be possible for programs which do not require observer participation.

The telescope site is one kilometer south of Sacramento Peak Observatory (SPO) and about 20 km southeast of Alamogordo, New Mexico. The site is 2797 meters (9175 feet) above sea level.

Justification of Requirements

User Needs

Slew Rates and Ramp Time

Electronic array detectors are very efficient compared to photographic plates. Since exposure times will be correspondingly shorter, slew time between objects becomes a larger fraction of telescope 'dead' time. We will provide slew rates (including dome) nearly an order of magnitude higher than is currently available on large telescopes, allowing the instrument to fill a new niche in the large telescope community. This is possible at modest cost due to the low moments of inertia of the telescope and enclosure.

Multiple Instruments

The scheduling flexibility offered by multiple instruments deployable in minutes, along with remote observing facilities will allow astronomers to schedule their research, teaching, and personal responsibilities with greater efficiency and lower cost. Also, important scientific goals can only be realized with this type of system.

Pointing Accuracy

High accuracy blind pointing is mandatory for infrared work on objects with insufficient visible brightness to be detected for acquisition against the sky, daytime or nighttime. Raster photometry also requires high pointing precision. In general high accuracy pointing improves operating efficiency and observer confidence. Finder charts will be needed only for objects with poorly known coordinates.

Current Performance Standards

It will be apparent throughout this document that the Multiple Mirror Telescope (MMT) currently represents the performance standard to be met or exceeded by all new telescopes.

Conditions

The performance requirements are applicable for a mean wind speed of 5 m/s (11 mph). 5 m/s is the mean wind speed on clear nights measured at Sacramento Peak and meeting the requirement described below guarantees exquisite telescope performance roughly half of all nights (nights with winds less than 5 m/s) with no use of wind screens. This same performance will be achieved in higher winds when the telescope enclosure is facing away from the wind, or by using wind screens.

Functional Requirements

Aperture

A lower limit of 3.0 meters is considered the criterion for a world class telescope capable of dealing with modern astrophysical research topics. ARC has selected 3.5 meters as a goal.

Remote Operation

The telescope system (MILT) will be remotely operable with an on-site operator's assistance by an astronomer located at one of the ARC member institutions. A further goal will be to provide generalized access via standard commercial communications utilities; satellite, switched telephone, etc. As a first step, operation via a single terminal communicating with a MILT dial-up port will be provided.

Reliability The heavy use of standard components is to be emphasized especially those subject to most frequent failure or whose failure is most crucial to continued system functioning. Care will be taken to avoid using components or subsystems which are extremely expensive at the replacement level. (Example: 24-bit absolute encoders at $50000). If spares are low cost, a better backup stock can be maintained.

All mechanical designs are to be as maintenance-free as is feasible. A carefully designed preventive maintenance program will be in effect routinely. Access to system equipment by users will be restricted.

Performance Requirements

Conditions

Mean Wind Speed: 5 m/s
Temperature: -20 to +30 deg C
Humidity: TBD, non-precipitating
Altitude: 2800 meters nominal
Mains Power: +10% -10% of nominal voltage
Zenith angle: 0.5 > Z > 80 degrees

The following proposed rms image quality budget applies under the above conditions. It is to be assumed that the enclosure provides no wind shielding for the upper end of the telescope optical support structure. It is also assumed that the enclosure reduces the wind loading on the rest of the telescope by a factor of 10 as compared with the unshielded condition.

Primary         0.25"
Other optics    0.08"
Local seeing    0.08"
Tracking        0.08"
Focus           0.04"
Alignment       0.04"

total           0.29"

Tracking components include image motion due to wind induced deformation of the tube, deflection of the fork, deflection of the pier and its foundation, and coupling of wind loading between the enclosure and the pier. This item should also include control system errors caused by wind loading and abrupt changes in bearing friction.

Pointing accuracy: Offsets > 10 deg: 1 arc sec rms max; Offsets < 10 deg: 0.5 arc sec rms max
Open Loop Tracking: 0.2 arc sec rms max for 10 min of time; 1 arc sec rms max all night.
Response Time Acquisition time 1 min max, tracking to tracking, any two objects above 10 degrees altitude. Slew rate 9 deg/sec Ramp time 3 seconds 1 degree offset 5 sec max, tracking to tracking Instrument reconfig. 2 minutes max to operating status Operating wind speed 15 m/s (34 mph) max
Zenith Blind Spot: 1 degree max dimension

Survival Requirements

Environmental Conditions

Wind: 60 m/s (118 knots) peak gust
Humidity: 100%, monsoon type rains
Temperature: -40 to +40 deg C
Snow: 1.2 meters max on horizontal surface
Earthquakes: TBD g, any direction
Electric Power: 1 week continuous outage

We require that the telescope and enclosure withstand or at least ameliorate the effects of the following:

Lightning
Insects including Miller Moths and ladybugs
High altitude UV radiation and ozone
Blowing dust and sand