1.0) Introduction

This document presents the top level science driven performance requirements for the SST Observatory. A description of the baseline design along with science drivers and performance estimates is given in SST TR930703 and a discussion of sample science programs is given by SST TR930731. Justification for the below requirements are found in these documents. The organization of this document follows to the extent possible the work breakdown structure of the project. Thus, the major telescope elements are optical system, primary mirror, telescope structure, tracker, enclosure and facility and system software and instrument package.

2.0) Summary of Science Driven Requirements on Telescope

The SST will most frequently be used for spectroscopy. For low resolution prime focus spectroscopy and spectrophotometry, the requirements are 1) good image quality over a wide well corrected field, 2) accurate tracking including field rotation, 3) low scattered and stray light, 4) atmospheric dispersion correction, 5) ability to quickly acquire and center objects on instrumentation entrance slit in both the visible and NIR, 6) offset guiding capability, and 7) high throughput from 0.35 to 2.5 microns. Moderate and high resolution spectroscopy place equally high demands on the SST. The throughput resolution product is inversely proportional to the entrance aperture of the spectrograph, which can be expressed as arc-seconds on the sky. Thus, the major performance requirement for the SST is image quality. Additionally, many exciting moderate and high resolution science programs utilize the light gathering power of the SST to the limit where sky background becomes important. Thus low scattered and stray light is important. High throughput from 0.35 to 2.5 microns, calibration source(s) in focal plane, excellent tracking and atmospheric dispersion compensation, are all key telescope factors also.

The SST prime focus will have an image scale (~12 arc-sec/mm) similar to that for a Cassegrain focus on more moderate aperture (~3 meter) telescopes. This fast focus will allow sky background to be reached in relatively short times so that the exposure time limitation of the SST is not an issue. The objects of greatest interest for imaging, such as very faint galaxies, QSO's and low surface brightness nebulae are near or below the background sky brightness. Thus it is imperative that the SST not degrade the contrast in the focal plane. The key requirements for imaging with the SST in addition to good image quality over a wide well corrected field are low scattered and stray light and low emissivity for K band imaging.

3.0) SST System Requirements

3.1) Environmental Requirement:

3.1.1) Normal Operating Conditions

The telescope will meet all performance specifications throughout the following range of normal environmental conditions:

3.1.2) Marginal Operating Conditions

The telescope will be expected to operate, but with reduced performance specifications throughout the following range of marginal conditions:

3.1.3) Survival Conditions

The facility will withstand the following extreme environmental conditions in a non-operating mode over a 20 year operational lifetime:

3.2) Optical Performance Requirement for SST System

3.2.1) Image quality

The SST optical system, which in the baseline concept applies to primary array and two-element corrector, operating in the enclosure and tracking an object should not degrade the enclosed energy diameter of a point source on a flat focal surface beyond the following specifications in the absence of atmospheric seeing (" == arc second):

Requirement:

    Field angle         EE(50%)               EE(80%)
   (arc minutes)         diam.                diam.
       <0.5'              0.6"                 0.9"
       <1.5'              0.7"                 1.0"
       <2.0'              0.8"                 1.1"

Goal:
   Field angle          EE(50%)               EE(80%)
   (arc minutes)         diam.                diam.
       <1.0'              0.5"                 0.8"
       <2.0'              0.7"                 1.0"

The requirement should be met over the range of normal operational environmental conditions described below, for any given night, at least 90% (goal 99%) of the time. The dead time for mirror realignment must be <10% In addition, the image quality will be maintained for a minimum of 60 minutes which may include azimuth rotations. The image quality should not degrade more than 5% from the above specifications during a 1.5 hour track on a single object. A goal shall be to keep image degradation at or below 10% for up to 2.5 hour; the maximum SST tracking time at northerly declinations. Image quality is to be assessed independently of tracking errors.

3.2.2) Focal surface

The focal surface will be no faster than f/1.6. The focal surface that meets imaging requirements must be flat and at a fixed distance above the instrument mounting plate. This distance must be at least 75 mm. to allow adequate clearance for guiding optics.

3.2.3) Throughput requirements

The telescope will transmit light to the focal surface with minimal losses. We define the optical transmittance as the ratio between photon flux in the focal surface, integrated over the stellar image, and the photon flux from the object in a 9-meter diameter column incident on the top of the SST along its central axis. The primary array will have a total effective area greater than 77 m2. The below table states transmittance requirement in three pass bands. This includes surface reflectivity as well as pupil obscuration.

   Wavelength       Transmittance
      (nm)
    350-400             > 46%
    400-500             > 62%
    500-2500            > 76%*

A capability, such as CO2 cleaning, to keep the reflecting surfaces free of dust shall be available at first light.

3.2.4) Pupil requirements

The focal plane will have a pupil that projects to 9-m diameter (64 m2) on the primary (goal is 10 m). Obscuration of the pupil by optics, bevels and gaps in the primary, tracker, cables, focal plane instrument, telescope structure and dome will be kept at or below 16% on the central axis. At a maximum off axis angle of 8.5^o, the obstruction due to the tracker, cables and focal plane instrumentation shall be less than 16%.

3.2.5) Scattered requirements

The pupil must be baffled to prevent scattered and stray light at the focal surface from being greater than the sky background at any wavelength in the 0.35 to 2.5 micron range.

3.3) Telescope Structure

The telescope structure shall be designed so as to provide a central axis zenith distance of 35^o. This structure shall provide a clear aperture at the top of 10 meters inscribed diameter on the central axis. Below the top ring, no part of the structure shall obscure the primary mirror array as viewed by the corrector over the required tracking range. The structure shall also provide access for mirror installation and cleaning of primary mirrors so as to maintain high transmittance. In addition it should be designed so as to support trackers that have an instrument package up to 400 kg and have adequate dynamic stiffness to meet the pointing and tracking requirements at first light with a 200 kg instrument package. A high priority goal is to have adequate dynamic stiffness to meet the pointing and tracking requirements with a 400 kg instrument package. (Note: The instrument package is but one part of the tracker payload which includes the corrector, guider mechanism, and finding telescope in addition to the instrument package.)

3.4) Acquisition and tracking

The tracker components should be consistent with the obscuration specification in section 3.2.3 and minimize scattered light and IR emissivity. (They should also avoid obvious roosting sites to maintain a clean mirror.)

A region of diameter 2.5 m and height 2 m above the mount plate should be free of obstructions for all possible positions of the tracker. The length of optical fiber from the focal plane to the main instrument room shall be minimized, in no case to exceed 30 m (goal 25 m) with bend radii less than 0.4 meter. The fiber routing and handling shall be such so as to avoid transmitting any compressive force or tension to the optical fibers through the protective cabling.

3.4.1) Range

This tracker will have access to declinations -10o 20' < DEC < 71o 40' with a 180^o azimuth rotation. The tracker will be able to follow any object along a 12^o arc in the Right Ascension anywhere in the required declination range. The telescope will have access to azimuth positions and an unobstructed view of the sky from 0 to 390^o to allow for scheduling versatility and observing efficiency (goal is 540^o to enhance observing efficiency).

3.4.2) Absolute Pointing

The telescope will have an absolute pointing accuracy of <30 arc second peak to peak. (goal <10 arc second peak to peak.) to any accessible point in the sky after an azimuth move.

3.4.3) Offset Pointing accuracy (open loop)

The offsetting accuracy is defined as the ability to place a given point in the sky on the bore sight once the telescope has acquired another object in the FoV. The tracker will be able to execute offsets from any object within a 3 arc minute diameter field to 0.25 arc seconds p-p. (Goal of 0.1 arc seconds p-p.)

3.4.4) Field acquisition time

The telescope will complete an azimuth slew from any point in the allowable range to any other accessible point to the absolute pointing accuracy within 5 minutes 90% of the time and 10 minutes 99% of the time. When an azimuth slew is not required, the acquisition time will be < 2 min. 80% of the time (goal: <1 min. 90% of the time).

3.4.5) Target acquisition time

A Target is defined as a point in the sky. If the target is not visible to the acquisition imager, then the target is defined as an offset from a visible star that is within the focal plane field of view. Acquisition time is defined as the length of time required to put the target at a defined position (a bore sight), within the offset pointing requirement, from end-of-slew, until start of the integration.

Acquisition time will be < 60 seconds. (goal < 30 seconds.) for <95% of potential targets. Another goal will be to allow very rapid re-acquisition of objects (< 2 min.) after an azimuth move to extend tracking at the southern and northern limits of the declination range.

3.4.6) Tracking (sidereal rate)

3.4.6.1) Open loop tracking

Open loop tracking, i.e. without feedback from a guider, shall be sufficient to achieve pointing accuracy as specified in 3.4.2 and acquire a guide star within the target acquisition requirement in 3.4.5.

3.4.6.2) Closed loop tracking

Once acquired, the bore sight will track the target such that the error vector between target centroid and bore sight will be <0.1 arc second rms. over the entire tracking range. Field rotation compensation shall not degrade images at the periphery of the field of view by more than 0.25 arc second rms. over the entire tracking range.

3.4.7) Tracking (non-sidereal rate)

The tracker shall follow the motions of objects that deviate from the nominal sidereal track rate by as much as 4 arc seconds per second in any direction, meeting the requirement in 3.4.6.2.

3.4.8) Field Rotation

The tracker system shall provide a rotating mount plate for scientific instruments with absolute positioning accuracy of +/-1 arc minute RMS. in sky coordinates and range of 230 degrees.

3.4.9) Heat Removal

Means shall be provided to remove additional heat generated by scientific payloads, up to 1 kW., as well as acquisition and guiding equipment

3.5) Site, Facility and Enclosure

3.5.1) Site

The SST site must allow unlimited access to the sky area provide by the SST design.

3.5.2) Enclosure

The enclosure and dome shall:

• Be consistent with the SST system image requirement.
• Should provide heat protection during the day and ventilation at night so as to allow observations to begin at astronomical twilight with images that meet the requirements.
• Be capable of slew rates commensurate with telescope azimuth drive.
• Have an azimuth range commensurate with the telescope.
• Not vignette the pupil during any track.
• Must be tightly sealed against dust and liquid or frozen precipitation in winds up to 120 mph.
• Should provide for lightning protection and provide a low impedance path to an independent earth ground.
• Be designed such that wind induced vibration is not coupled through the ground to the telescope pier or fiber fed instrument room during observing.
• Will allow routine access to the tracker and instrument package for instrument maintenance. This access will be available to two people simultaneously and independently, i.e. with no additional personnel required. Provisions for supply dewars, test equipment, including power, data and control lines will be made.

3.5.3) Facility

The SST facility includes any and all buildings and structures associated with the SST. The facility shall meet the following requirements.

• Shall not degrade the image quality beyond the specifications above through dissipation of heat from the control building or other structures during normal operating conditions.
• No part of the facility shall be allowed to partly obscure a declination zone permanently. For example a CCAS tower cannot be placed near the north point.
• Provide lightning protection.
• Provide a permanent fixture for mirror removal.
• Electric supplies must all be well grounded and surge protected. Ground-loops must be eliminated.
• Provide at least 400 sq. foot control room with computer power, and a rest room with toilet, sink, hot and cold water.
• Provide at least 600 sq. ft instrument work area with instrument and computer power, compressed air, a sink and cold water.
• Provide a Main Instrument Room that shall be at least 45 feet diameter and 12 feet high internally, and shall have access doors a minimum of 8-feet wide and 12 feet high. This instrument room must be
  1. light tight, and free of fluorescent and phosphorescent materials and indicator lamps,
  2. thermally stable within 0.5 ^oC per week and 2 ^oC per year,
  3. not be actively air-conditioned,
  4. avoid materials that produce dust when scratched or degraded by age,
  5. have instrument grade power, compressed air cleaned with an oil filter , and water.
  It shall also be capable of being divided into light-tight subsections so as to allow instruments to be worked on while not disturbing the others. LN2 must be available for detector cooling. vi) radioactivity and EMI background during normal operations should not exceed that currently extant in the 2.7 m coude room.

3.6) Control System, Software, and Communications

3.6.1) Control System

The basic control system shall meet the acquisition and tracking specifications. Basic environmental information on conditions inside and outside the dome such as temperature, humidity, wind speed and direction, barometric pressure and dew point shall be available to the control system.

3.6.1.1) Observer/Operator Interfaces

System software shall provide a visual display, and computer-readable output of:

• The telescope position in right ascension and declination (with precession to an arbitrary epoch); Local, Universal and Sidereal times; the zenith distance and air mass.
• The current environmental data.
• The instrument plate rotation.
• The last measured seeing and transparency.

• A visual display of the acquisition field for the operator, in standard North-up, East-left orientation with any field rotation removed.
• The ability for the operator to acquire objects through simple mouse-controlled commands.
• The operation and control computer with all the information necessary to acquire and observe the object: coordinates, magnitude of object, offset star coordinates, exposure time, and instrument configuration.

3.6.2) Communication

Telephone, FaX and computer network links should be provided to outside world, as well as to the rest of the McDonald observatory complex. Remote login or dial in capability must be available to all SST partner institutions to allow the input of observing programs/parameters or to ascertain the status of on-going programs.

3.7)Maintenance Safety and Reliability

3.7.1) Reliability

The amount of down time due to system failure and/or scheduled maintenance of telescope systems should not exceed 2% of nighttime hours. Tracker shall be capable of up to 70 traverses per night over a lifetime of 20 years consistent with the above reliability requirement.

3.7.2) Safety, fail-safe modes

The capability shall exist to install a protective cover over the primary mirror during servicing of the tracker and focal plane instruments. The dome shutter must close in less than 3 minutes, and must be manually closable in the event of electrical power or shutter motor failure.

3.7.3) Maintenance

The maintenance downtime at night should not exceed 2%. (Goal is to carry out all scheduled maintenance during daylight hours.)

3.8) First light instrument package

3.8.1) Atmospheric dispersion compensation

Atmospheric dispersion compensation shall be provided. The compensator shall have secondary dispersion less than 0.15", and 95% external transmission over the wavelength range from 350 to 1100 nm. The atmospheric dispersion compensator will introduce no more than a 5% degradation of the image at 500 nm over the requirements in 3.2.1. The atmospheric dispersion compensator must be readily removable so as to be compatible with a queue scheduled mode of operation.

3.8.2) Acquisition

An acquisition camera shall provide imaging of the entire 4-arc minute field of view. It shall be capable of obtaining images suitable for identification of objects with brightness between 0 and 21st visual magnitude in 5 seconds or less integration over the entire tracking area.

3.8.3) Guiding

A guiding camera shall be capable of imaging 2 sub fields anywhere within the 4-arc minute field of view. It shall be capable of obtaining images suitable for guiding anywhere in the SST sky access range in 10 seconds or less including readout.

3.8.4) Spectroscopic capability

The first light instrument package must have the capability of conducting spectroscopic observations consistent with a major subset of the science goals. The baseline plan for achieving this is to have fiber coupled medium (up to 20000) and low (2000) resolution spectrograph. The specific requirements for these instruments will be addressed in a separate document.

SST TR930621