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SDSS Engineering

2.5-m telescope test plan

Sloan Digital Sky Survey Telescope Technical Note 19961009

Walter Siegmund

Contents

Introduction

The systems that comprise the 2.5-m Sloan Digital Sky Survey (SDSS) telescope must be tested as installation and integration occurs so that problems are discovered at the component and subsystem level rather than at the system level where debugging is more expensive and time consuming. Component testing allows problems to be discovered earlier when they are less likely to cause major project delays.

This document complements the telescope tests dependency chart that is available as a postscript file. The chart provides guidance as to the order of the tests and the state of telescope assembly required for each test, whereas this document describes each test.

Definitions

These are definitions of common parameters that are used in the description of the tests.

a       Telescope altitude angle
        A       Telescope Azimuth angle (south = 0°, east = 90°)
        W       Mean wind speed
        Z       Mean wind direction (south = 0°, east = 90°,
                prevailing wind is 215° < Z < 360°)
        X1,Y1,  Telescope enclosure coordinate system. 
        Z1      +X1 == Z = 60°, i.e., toward east side of the building.
                +Y1 == Z = 330°, i.e., toward north side of the building.
                +Z1 == azimuth axis (zenith direction). 
                +Z1 = 0 at floor level.
        X2,Y2,  Telescope azimuth coordinate system. 
        Z2      +X2 == right altitude axis (wind baffle drive side).
                +Y2 == telescope boresight at a = 0°.
                +Z2 == azimuth axis (zenith direction).
        X3,Y3,  Telescope altitude coordinate system. 
        Z3      +X3 == right altitude axis (wind baffle drive side).
                +Y3 == front of primary support structure.
                +Z3 == telescope boresight at a = 0°.
         

Measure natural frequencies: I

Stability of pier

Purpose of test

  • Measure the rocking stiffness of the pier so that (hopefully) it can be eliminated as a significant source of wind-induced tracking error.
  • These parameters are inputs to the design of the axis servos.
  • Note that the torsional stiffness is not measured. It is not likely to be important, can be estimated, and is hard to measure.

Criterion

  • The pier should contribute negligibly to wind-induced tracking error, i.e., 5 mas 2D RMS at W = 10 m/s.

Conditions

  • W > 5 m/s; 215° < Z < 360° a = 0°; A = 240°, 150° enclosure open and closed Special tools
  • 3 ea accelerometers 1 ea LabVIEW 1 ea A/D card 1 ea cup anemometer 6 ea wind dynamic pressure sensors

Record

  • +x1 and +y1 acceleration at the top of the pier dynamic pressure near the primary mirror platform (PMP)

Calculate

  • Power spectral densities
  • Transfer function

Mount stiffness

Purpose of test

  • Measure the response of the mount to the wind.
  • Determine the locked rotor resonance frequencies and other low frequency resonances that are needed for the design of the axis servos.
  • Verify that no excessively compliant components are present.

Criteria

  • Natural frequencies are to be above 5 Hz.
  • The secondary should contribute negligibly to wind-induced tracking error, i.e., 5 mas 2D RMS at W = 10 m/s, with the wind-baffle present. Since the wind-baffle is expected to attenuate the dynamic pressure of the wind by a factor of 7, 35 mas 2D RMS is allowable at W = 10 m/s for this test.

Conditions

  • W > 5 m/s; 215° < Z < 360° a = 30°, 80°; Z - A = 0°, 90°
  • enclosure open and closed
  • motor shafts locked Special tools
  • 3 ea accelerometers 1 ea LabVIEW 1 ea A/D card 1 ea cup anemometer 6 ea wind dynamic pressure sensors

Record

  • +x2 and +y2 acceleration of the fork base
  • angular acceleration of the fork base about the azimuth axis
  • angular acceleration of the PMP about the altitude axis
  • angular acceleration of the 2ry about the x3 axis
  • angular acceleration of the 2ry about the y3 axis
  • dynamic pressure near the PMP
  • dynamic pressure near the 2ry

Calculate

  • Power spectral densities
  • Transfer function

Measure internal baffles

Purpose of test

  • The criteria are for the worst case tolerances and flexure over 30° < a < 90°. They refer to departures from the pupil obstruction in the baffle design.
  • The baffles must be installed so that they block all direct rays to the focal surface.
  • They must not vignette rays in the photometric field of view.
  • Obstruction of the edge of the entrance pupil at the edge of the astrometric field of view must be less 2%. The RMS image diameter at the edge of field 18 is 1.4 arc sec. If 2% obstruction causes a 2% shift in centroid, this would result in a 28 mas centroid error. This is small enough to be acceptable.

Criteria

  • Parts must satisfy tolerances on shop drawings.
  • The primary and secondary baffles are to be located to 2 mm true position referred to their respective optical surfaces.
  • The conical baffle is to be located to 3 mm true position at 30° < a < 90°.

Conditions

  • W < 5 m/s; Z don't care
  • a = 0°, 90°; A don't care
  • enclosure open

Special tools

  • piano wire
  • protective cover for the primary mirror
  • planks Technique
  • Use a tape measure, piano wire and scale to measure positions.

Record

  • X3,Y3,Z3 of points on primary, secondary, and conical baffles.

Comments

  • Measurement at a = 90 degrees will require a protective cover for the primary mirror and planks supported by the telescope to walk on.

Measure spectrographic field distortion

Purpose of test

  • The coordinate transformation from the sky to the focal plane for each spectrographic target requires that the spectrographic field distortion be measured.

Criterion

  • In the fiber position error budget, the budget for errors in the field distortion is 4 µm RMS. Note that this is a change from the earlier value of 1 µm RMS (Table 4 of "Results of the 3/93 Drilling Tests" Sloan Digital Sky Survey Telescope Technical Note 19930430) that neglected systematic errors and flexure. The residual error from independent measurements fit to the same distortion model should satisfy this criterion.

Conditions

  • W don't care; Z don't care
  • Sky condition; clear, good seeing
  • a = 30°, 90°; A don't care
  • enclosure open

Special tools

  • Spectrographic distortion plug-plates
  • Spectrographic distortion coherent fiber bundle
  • Guide camera with centroid software

Technique

  • Feed guide camera with coherent fibers plugged into spectrographic distortion plug-plate.
  • Make multiple observations of an astrometric field of stars using several rotator angles.

Record

  • Use guide camera to measure centroids of stars.
  • Fit centroids to distortion model to calculate xd, yd, a1, a3, a5, a7, i.e., the center of the distortion pattern and the coefficients of the first four terms of an odd power series expansion.
  • Verify that xd and yd are inconsequential as they should be for collimated and aligned optics.
  • Note that a1 is the scale which can be adjusted for each plate.
  • Compare measured coefficients to those from modeling the as-built optical system.
  • Verify stability of distortion with elevation, rotator angle and time.

Comments

  • Another means of performing this task is to use the science fibers and to offset the telescope one fiber radius (1.5 arc seconds) in four directions. This requires special purpose software to analyze the resultant data from the spectrograph and requires that the PSF be known.

Scattered light

Purpose of test

  • Scattered light affects the photometric accuracy of the imager and the spectrophotometric accuracy of the spectrographs. This test will discover design errors, positioning errors, excessive flexure, and flaws in the baffle coatings.

Criteria

  • The point source normal incidence transmittance (PSNIT, the ratio of flux in the focal surface to incident flux) should be less than 2x10^-6 for sources more than 30° off axis.
  • For sources less than 30° from the field center, the PSNIT can be higher, but it is desirable that the focal surface illumination be uniform. The wings of the point spread function (PSF) should fall smoothly with field angle (I. King, PASP 83, 199, 1971). Need better wording and more quantitative criteria.

Conditions

  • W < 5 m/s; Z don't care
  • a = 30°, >80°; A don't care
  • lunar phase within 60 degrees of full at suitable elevation angles
  • Jupiter, Venus, Sirius, ..., at suitable elevation angles
  • enclosure open

Special tools

  • integrating CCD cameras and mounting
  • script to move telescope and take baffle images

Technique

  • Make finding chart. Take images of baffle system viewed from focal plane both on-axis and at field edge. Use diffuse sky illumination. Label all baffles.
  • Visual inspection. From focal plane, especially field edge, look at baffles. Start with diffuse illumination, then solar (field angles > 30° for safety), then lunar. Look for gaps in baffles and glints.
  • Use script to take integrating CCD images of baffles from edge of focal surface ("Detection of scattered light in telescopes", Frank Grundahl and Anton Norup Soerensen, Astronomy and Astrophysics Supp., 116, 367-371). Begin with the moon at 1.8°, 2.7°, 4.5°, 7.4°, 12.1°, 20°, 25°, 30°, 35°, 40° and 50° off-axis and left, right, up and down with respect to the boresight. Look for asymmetries, glints and deviations from Breault Research Organization report dated Aug 5, 1996.
  • Take images along four radii centered on the moon (left, right, up and down) at 0.25° intervals from 1.8° to 20° off-axis (264 images at 10 seconds per image is 44 minutes). These images provide a permanent archive that can be consulted if a scattered light problem is suspected.
  • Using photometric camera, take images with Jupiter or Venus just outside the field of view. Rotate camera 180° between images and compare images to get the large field angle PSF.
  • Obtain PSFs over the photometric field of view, at least one per photometric CCD.

Record

  • Images from items 3 and 4.
  • PSFs from items 5 and 6.

Comment

  • It would be helpful to have software tools to reduce the baffle images to determine quantitatively the amount of scattering from each baffle surface.

Test collimation

Purpose of test

Align optics with the instrument rotator axis. This test insures that image quality is not compromised by optics misalignment. Also, it insures that the point spread function depends only on field radius.

Criteria

On the surface of best focus, image size and aberrations should be only a function of field radius. The image shape should be consistent with the optical design convolved with the seeing.

Conditions

  • W < 5 m/s; Z don't care
  • a = 30°, >80°; A don't care
  • seeing < 1 arc second
  • enclosure open
  • spectrographic corrector installed

Special tools

  • Collimation computer
  • Integrating CCD camera and mounting

Technique

  • Coma: At field center, defocus until central obscuration is visible in the image. Insure that central obsuration is centered in image (“Proposed Process for 2.5m Collimation” E. Mannery, 18 July, 1996 and "Collimation of Fast Wide-Field Telescopes", Brian A. McLeod, PASP 108:217-219, 1996 February).
  • Astigmatism: At field edge, defocus until astimatism can be measured. Insure that astigmatism is independent of rotator angle.
  • With the telescope in focus, verify that the PSF core is consistent with the optical design convolved with the seeing over the field of view.

Record

  • Magnitude of residual coma
  • Uniformity of astigmatism
  • Uniformity of PSF

Test mirror ventilation

Purpose of test

  • Mirror temperature nonuniformities cause figure errors.
  • Thermal inertia contributes to temperature differences between the mirror surface and the ambient air and thereby contributes to image degradation.
  • Axial temperature gradients change the power of optics, require refocussing and change the scale of the focal surface thereby degrading astrometric precision.

Criteria

  • The temperature uniformity of the primary mirror must be 0.1 °C peak - valley after the axial gradient and mean face plate and back plate temperature distribution is removed.
  • The maximum mirror surface - air temperature difference is 0.25°C.
  • These criteria are for an ambient temperature gradient of 0.25°C/hour.
  • In terms of time constants, the thermal time constants of the face and back plates must be uniform to 0.4 hours peak - valley after the axial gradient and mean face plate and back plate thermal time constant distribution is removed.
  • The mean thermal time constant of the face plate may not be larger than 1 hour.
  • For the secondary, the mean thermal time constant of the face plate may not be larger than 1 hour.
  • The thermal time constant distribution need be uniform to 0.8 hours peak - valley.

Conditions

  • W don't care; Z don't care
  • a = 30°, >80°; A don't care
  • enclosure don't care

Special tools

  • temperature measurement system
  • temperature analysis software

Technique

Record

  • Thermal time constant distributions and temperature v. time for the primary and secondary.

Comments

  • These tests should indicate if active ventilation of the secondary is needed to achieve the project astrometric goal.
  • Several days of data acquisition will be necessary, more if problems are found. Data acquisition can occur in parallel with other tests.

Date created: 10/9/96
        Last modified: 10/9/96
        Walter A. Siegmund
siegmund@astro.washington.edu