Support and adjustment of the SDSS 2.5-m telescope conical baffle

Sloan Digital Sky Survey Telescope Technical Note 19980421

Contents

• Introduction
• Conical baffle adjustment
• Results

Introduction

The Sloan Digital Sky Survey (SDSS) 2.5-m telescope is based on a wide-field fast two-mirror optical design, i.e., a 3° diameter field of view at f/5. Such a design is difficult to baffle adequately against stray light sources without excessive central obstruction of the entrance pupil. The adopted solution adds a conical baffle in the converging beam between the primary and secondary mirrors ("2.5-m telescope light baffles" SDSS Telescope Technical Note 19980215). Together with the usual primary and secondary baffles, the design rejects stray light efficiently and blocks only 28.6% of the pupil area on-axis.

A unique challenge is posed by the design of the conical baffle. It is suspended between the primary and secondary baffles. However, its suspension system may only minimally obstruct the light entering the telescope and reflecting from the primary mirror. Its location must be controlled to an accuracy of ±3 mm.

The conical baffle has three translational and two rotational degrees of freedom that can be controlled by adjusting the length of 8 wires using turnbuckles. In addition, the natural frequency of the torsional vibration mode of the baffle about its axis is determined by the tension in the wires. The number of degrees of freedom and adjustments imply that a systematic approach to adjustment is needed.

Conical baffle adjustment

Prior to adjusting the baffle, the following steps should be taken.

1. The inside of the wind baffle is poorly illuminated. Install temporary high intensity illumination before proceeding.
2. Adjust the release force of the conical baffle support wire anchors. A spring scale attached to the ball plunger end of the lever should read between 50 and 60 N when the lever releases. Adjust the release force by turning the two ball plungers clockwise or counterclockwise equal amounts. The lever has a ratio of 6:1. The corresponding maximum tension in the wire is 300 to 360 N.
3. Insure that the secondary mirror is centered on the rotator axis and at the nominal imaging mode focus.
4. Mount the collimating telescope adaptor (manhole cover) to the instrument rotator. Install the alignment telescope in the adaptor. Care must be taken since these components are delicate. Rotate the alignment telescope so that the crosshairs are diagonal. Adjust the micrometers so that the crosshairs are centered on the dimple ground into the secondary mirror.
5. Install cross-hairs at the front and back of the conical baffle. Holes that are approximately 1 mm in diameter have been drilled along the two diagonals approximately 12 mm from the front edge and 9 mm from the back edge of the baffle. White or gold-colored Kevlar™ thread (approximately 100 x 200 microns) intended for fly tying is excellent. Route the thread so that only one joint is needed. Use hot melt adhesive to join the two ends while keeping 100 N or so of tension in the thread until the adhesive cools. Wrap one thread around the other to improve the strength of the joint. Make the joint on the lower side of the baffle to prevent adhesive from dripping on the baffle.
6. Hang a steel tape measure from one of the wind baffle panels so that it passes just in front of the front edge of the conical baffle. Attach a ø27 mm metric crossed scales reticle (Edmund Scientific Co. A39450) to the tape measure so that the scales are diagonal and centered on the front crosshairs. This grid is in focus with the front crosshairs but not the rear crosshairs. Mount a similar reticle behind the rear crosshairs. Displacement of the crosshairs or the baffle should be avoided. I found that the rear grid could be used to measure the error at the front crosshairs by refocussing the alignment telescope.

The adjustment process is divided into four steps.

1. Translate the baffle along the telescope optical axis so that its distance from the vertex of the secondary mirror is correct.
2. Rotate the baffle so that it is parallel to the telescope optical axis.
3. Center the baffle on the telescope optical axis.
4. Tension the baffle.

Initially, the tension in the upper wires should be uniform. The translation and rotation adjustments described below should not change the tension in the wires. However, tensions should be checked periodically. Note that when making adjustments, it is easier to turn the turnbuckles if loosening adjustments followed by the tighting adjustments.

Because of the weight of the baffle, the tension in the lower wires will be less that that in the upper wires. During the first three steps, it is easier to turn the turnbuckles if the tension is the wires is low. The natural frequency of the fundamental string vibration mode in the lower wires should be kept below about 60 Hz. (The frequency can estimated by plucking the wires and listening to the tone. Alternatively, a light stroboscope can be used to measure the vibration frequency.)

Each set of four wires along one diagonal of the conical baffle mounting frame acts largely independently of the other set. The exception is if the front set along one diagonal is tightened and the front set of the opposite diagonal is loosened. This causes uneven tension in the wires and distortion of the baffle. Consequently, it should be avoided.

The turnbuckle threads have a pitch of 635 microns (5-40 machine screws). It is convenient to adjust the turnbuckles in quanta of one-sixth turn since the turnbuckles have hex wrench flats machined on their surfaces. Since the turnbuckle is threaded on both ends, the adjustment quanta is 211 microns, more than fine enough for our purposes. The opening half-angle of the wires is 15°.

An axial translation adjustment consists of loosening the four rear turnbuckles one quantum and tightening the four front turnbuckles one quantum. This adjustment translates the baffle 815 microns toward the secondary mirror. Set the spacing between the secondary vertex and front crosshairs to 994.8 mm (39.17 inches).

A rotation adjustment consists of loosening the rear turnbuckle one quantum and tightening the front turnbuckle one quantum in one corner of the frame. In the diagonally opposite corner, these adjustments are reversed. This adjustment rotates the baffle about the opposite diagonal of the frame in the direction of the tightened turnbuckle of each corner pair. The front and rear crosshairs move in opposite directions 449 microns and 596 microns respectively per quantum.

A centering adjustment consists of loosening the turnbuckle pair in one corner of the frame one quantum and tightening the pair in the opposite corner one quantum. This adjustment translates the baffle 204 microns along the diagonal of the frame.

To adjust tension, first verify that the tension in the upper wires is uniform by plucking them and listening to the tone. Then, attach a spring scale to each of the lower anchor levers in turn. Adjust the tension in each of the lower wires so that a spring scale attached to the ball plunger end of the lever reads 35 N with the ball plungers just out of contact with the ball buttons. This step should not translate or rotate the baffle a significant amount as long as the initial tension was approximately correct.

Figure 1: One of eight conical baffle support wire anchors.

Results

Table 1 gives the spacings of the baffles with respect to the optical surfaces for the imaging mode ("2.5-m telescope imaging optical design", SDSST Technical Note 19960410). These numbers are from the drawing "baffle layout97".

Table 1: Spacings of baffle features and optical surfaces.

Feature	x (mm)
Focal surface	0
Common corrector front surface	784
M1 baffle tip	2065
Conical baffle rear tip	2697
Conical baffle front tip	3423
M2 baffle tip	3945
M2 vertex	4406

Table 2 gives the error in position of the critical features of the light baffles on May 23, 1998. They were measured with the telescope pointed at the horizon. Standing behind the focal surface facing the secondary with the telescope pointed at the horizon, +x is right, +y is up and +z is toward the focal surface.

All measurements are within the tolerances that were used to design the baffle system ("2.5-m telescope light baffles", SDSST Technical Note 19960616) except the error in z of the conical baffle location. It was adjusted to the wrong value. The location in the z direction is less critical than in x and y and gravity loading at smaller zenith angles will reduce the error. Consequently, it should have no effect. It will be set to the correct value the next time the baffle is adjusted. Visual inspection did not indicate any discrepancies, e.g., direct views of the sky from the focal surface.

Table 2: One of eight conical baffle support wire anchors.

Feature	x (mm)	y (mm)	z (mm)	tolerance (mm)
M1 baffle tip	1.9	-0.8	*	2
Conical back	-0.3	0.5	*	3
Conical front	-0.8	0.8	-4.7	3
M2 baffle tip	1.8	-0.6	-0.6	2

* Not directly measured but not likely to be out of tolerance. 

Date created: 4/21/98
Last modified: 2/12/99
Copyright © 1998, 1999, Walter A. Siegmund

Walter A. Siegmund