System Design Notes

SDN 0021.09 – Light Source (Telescope Simulator) for Flexure Test Rig

1. Introduction

This document describes the light source to be used in testing GNIRS on the Flexure Test Rig.

2. Overview

The flexure test rig under construction is to evaluate the performance of GNIRS through the full range of gravity vector orientations that it is likely to experience in actual operation.  While this testing will cover many facets (mechanism operation, electronics cooling, thermal stability, etc.), it will also be necessary to evaluate the optical performance of the instrument at different orientations to demonstrate compliance with the flexure requirements.

Three types of flexure can be evaluated:

• Flexure or nonrepeatability within the spectrograph due to motion or bending of one or more of the optical elements or the bench itself or hysteresis in the mechanisms.  These have different specifications that must be met, but all are manifested as a motion of the optical image of the input slit on the detector.  For this reason, an external light source is not required, but will facilitate the analysis and will be required for the other flexure measurements in any case.
• Flexure of the entire spectrograph with respect to the ISS (and thus the telescope).  This will require a light source firmly referenced to the flexure rig, projecting a point image onto the slit.  One may also evaluate the motion of the pupil on the Offner cold stop utilizing the pupil viewer in the instrument.
• Flexure within the spectrograph between the input slit and OIWFS detector.  Since the OIWFS will be used for guiding, motion of the entire spectrograph with respect to the ISS is not serious as long as there is no differential motion between the slit and OIWFS.  This will require projecting two point images.

3.  Requirements

1. The light source shall project a variety of target images (single or multiple points, lines, crosses) to the input focus of GNIRS mounted on the flex rig; that is, 300 mm behind the ISS simulator plate.  The input focal ratio shall be that of Gemini (f/16.2) with an apparent pupil imaged at the same distance as the exit pupil on Gemini.
2. The light source must install within the hole in the flex rig beam and fit within the rotational envelope of the rig.  It will be desirable to mount the projector to the backside of the ISS simulator plate using some of the supplied boltholes.
3. The light source shall employ a continuum source to permit evaluation of the full spectral coverage through the order separating filters as well as the cross-dispersed modes.
4. It shall be possible to substitute an emission line source (e.g., a Pen-Ray lamp) for the continuum source.
5. There is no requirement a priori which dictates a reflective or refractive system.  However, a reflective re-imaging system would likely require a refractive element to generate the appropriate pupil image, so it would not be entirely achromatic.  A fully refractive solution must be sufficiently achromatic over the 1 – 2.5 micron range to permit evaluation of the cross-dispersed operation.
6. The projector shall be functional over at least part of the 3 – 5 micron range, but need not be achromatic with respect to 1 – 2.5 micron operation (i.e., refocusing of the source is permitted).
7. The source should project a field at least 20 mm in size.
8. The projector shall be installed on a mechanical stage permitting motion in all three axes (two spatial and focus), with total travel of at least 10 mm in each direction.  Remote operation of these stages is not required.  The optical system must accommodate the field motion without vignetting.
9. The geometric blur circle (50% encircled energy) over the 1 – 2.5 micron range shall be less than 40 microns.
10. The projected image shall move no more than 100 microns as the flex rig is moved from the vertical to horizontal position.

4. Possible Solutions

4.1 Light Projector

We have in hand a light projector used for the alignment of SQIID.  It is a small integrating sphere into which a small incandescent bulb is installed.  This gives a spectral continuum whose intensity can be easily controlled.  The resultant change in color with intensity is not a problem.  The integrating sphere has an output aperture of 20 mm and provision for installing shim stock slides (33 x 50 mm x .010”) in which the desired target have been cut.  These may be either fabricated in-house or by the vendor used by the Observatory for cutting slit masks.

4.2 Translation Stage

The projector used for the SQIID alignment is mounted on a stage capable of X-Y motion using a rack/pinion microscope stage and focus over 25 mm with a micrometer stage.  This arrangement is compact and lightweight, and seems to be sufficiently robust to permit full motion with respect to the gravity vector without exceeding the flexure requirements.

4.3      Optics

A refractive design has advantages in simplicity, since it would permit a tube structure with a mounting flange and perhaps gussets for structural stiffness.  Depending on the tolerances from the optical design, the lens mount could be fairly simple, since it will not be required to withstand cryogenic operation.  The lenses would not require anti-reflection coatings, although it may be desirable for high-index materials.

We have designed a triplet refractive solution that appears to meet the above requirements, utilizing ZnSe, BaF₂, and CaF₂ lenses.  This provides 1:1 imaging of the source onto the input focal plane of the instrument and has a total length of ~1200 mm, so it should project no more than ~300 mm above the beam of the flexure rig.  A 20 mm diameter stop 300 mm from the source will be re-imaged by the lenses to the apparent distance of the Gemini secondary (16 m) as seen from the focal plane and provide f/15 input to the instrument.  The geometric spot diagram yields essentially complete containment within a 40-micron diameter aperture over the 1.0 – 2.4 micron wavelength range and the 20 mm diameter field of the source.  Over the 3.0 – 4.2 microns range, the spot diameter is less than 60 microns, smaller than the Airy diameter.

Maximum clear aperture for the lenses is 63 mm.  Allowing the normal 10% overage for mounting, etc., and specifying that all three lenses have the same diameter for ease of mounting will give a physical diameter 75 mm or smaller, so the cost of the lenses should not be excessive.  Since one of the lenses (ZnSe) has a high index, we will probably specify a quarter-wave AR coating centered at 1.6 microns.

5.  Test Plan

5.1 Instrument Focus

While there are other ways to carry out this measurement, one can determine the position of the instrument focus with respect to the ISS mounting plate by adjusting the focus of the light source to obtain the best focus on the GNIRS slit, removing the instrument, and measuring the position of the projected image.  One would presumably carry out this experiment at the very end of a testing sequence, when the instrument would be coming off the flex rig in any case.  If the light source utilizes refractive optics, it will be necessary to apply a small correction to the observed optical focus location.

5.2 Instrument Motion

This measurement refers to the bulk motion of the instrument focal plane with respect to the ISS as a function of gravitational loading.  Since some motion (on the order of 1 mm) is anticipated and unavoidable, this experiment would simply provide a measure of this effect, although an unusually large or discontinuous deflection would be cause for concern.  The experiment would entail centering the projected spot in a small aperture with the instrument at zenith, then measuring the motion of the light source necessary to recenter the spot with the instrument at different orientations.  This could be done with the acquisition mirror in place.  Any motion of the image of the aperture on the detector would be a result of flexure in the collimator, acquisition mirror, or camera (see next section).  It would be necessary to correct for the flexure of the projector by measurements of the projected image with the instrument off the flex rig, which can be carried out as part of the focus check (5.1).

5.2.1           Pupil Motion

With the instrument pointing at the zenith, one may adjust the support truss to center the pupil image of the projector onto the internal cold stop, while in pupil viewing mode.  Any motion of the cold stop with respect to the pupil image can then be measured as a function of gravity load, after any effects of projector flexure have been removed.

5.3 Internal Flexure

This important component deals with any motion in the optical axis between the slit and the detector, that is, one that would cause a shift of the re-imaged slit on the array greater than the requirement of 0.1 pixel for a 15-degree motion of the instrument tilt.  One does not require a focused point image on the slit for these measurements, and the use of a large projected spot that overfills the slit will permit evaluation of this effect without having to deal with the bulk motion of the instrument.  Use of an emission line source for these tests will yield the flexure in both the spectral and spatial directions.  All scientifically viable combinations of grating and cross-dispersers should be evaluated.  The thermal continuum from the emission lamps overwhelms the emission line spectrum beyond 3 microns, so investigation of flexure using the red cameras may require working at shorter wavelengths and readjusting the detector focus.  This should not be a problem, since all of these measurements are differential in nature.  Alternatively, it may be possible to utilize strong telluric absorption lines in the 1 m atmospheric path between the light source and the GNIRS window. 

5.4      Differential Flexure

Differential flexure between the GNIRS slit and the OIWFS detector will result in a false command to recenter the telescope pointing.  This flexure may be measured by using a target plate with two spots in the projector.  One of these, representing the object, would be centered on the slit; the other would be centered in the OIWFS target box using the gimbal mirror.  Any differential motion can be measured directly as a function of the instrument orientation, after readjustment of the projector to recenter the target on the slit after a change in orientation.

6.  Other Tests

This projector will be useful for other characterization tests unrelated to flexure, such as setting up the user grating positions, evaluation of spectral resolution and scattered light, tests of the polarimetry modes (using a linear polarizer in front of the projector), etc.  We may wish to carry out all instrument testing in the flexure rig bay, since all of the infrastructure (cryocooler compressors, workstations, etc.) will be in place in that location.  In such a case, it may be desirable to design a fixture for attaching the light projector to the instrument truss so that optical tests in a fixed (presumably horizontal) orientation can be carried out without the need to install the instrument on the flexure rig.

R. Joyce

J. Elias

18 January 2002

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