Rensselaer Astrophysical Society

Archives - Mirror Overhaul Project

As with any viewing instrument, the quality of the image is determined by the quality of the optics. In the summer of 1998 the RAS took the primary mirror on the 16" Cassegranian telescope in for servicing. This is how we did it. (Thank you Jeffrey LaCombe for putting together the original page.)

Disassembly

The process of overhauling the primary mirror on the Hirsch Observatory's 16" Classical Cassegranian telescope began with a thorough inspection of the telescope construction. It was first determined that the focusing drawtube needed to be removed, to allow inspection of the back side of the primary mirror cell. Additionally, the counter-weights and the cone were also to be removed to allow access to the bolts which attach the primary mirror cell to the Optical Tube Assembly (OTA). Additionally, any parts of the guide scopes or support equipment and wires were removed or restrained so as to allow easier working conditions. From here, it was anticipated that the mirror and the cell would probably weigh quite a bit, and that it would be necessary to exercise caution when detaching these from the OTA.

The mirror was removed with the assistance of a temporary wooden structure constructed of 2x3 studs. The weight of the mirror and cell is probably in the 75lb range, and hence can be difficult to maneuver once the attachment bolts are removed. Two people at the minimum are necessary to lift the mirror into its final position, with additional people helping lift and inserting or removing the attachment bolts that join the cell assembly to the OTA.

When disassembled, there are many parts to keep track of. Here we see the various major components that were detached during the procedure (left). Starting at the upper left and moving clockwise, we have the primary mirror, the mirror retaining ring, the light baffle, assorted counter-weights, the mirror cell, the focuser drawtube, and the cone.

A degree of confusion was encountered when trying to detach the mirror backing plate from the mirror. This is accomplished first by removing the retaining ring from the front via the socket cap screws. However, at this point, it is likely that the backing is still firmly attached to the mirror. The backing is removed by loosening the four set screws that extend into the backing plate from the rear (along the edges of the center tube) and cause friction pads located in the tube to expand their diameter and thereby allow adjustment (left-right etc.) of the primer mirror. Once these set screws are loosened, the backing plate should detach from the mirror.

Mirror Alignment and Collimation

By the time of its completion, the mechanical aspects of the project would prove to be the easier part. The best resource that could be found on the subject was the book "Star Testing Astronomical Telescopes" by Harold Richard Suiter (available at Orion). Unfortunately however, this resource, as well as most other mentions found online and elsewhere, only discussed adjustment of the secondary mirror in Schmidt-Cassegranians. For the case where primary mirror adjustment is necessary, they say something to the effect that we need to send it to the factory for adjustment. Since we clearly can't do this, difficulties were in store for us!

The procedure followed was basically a best-guess adaptation of the Newtonian optics collimation method. The details won't be discussed here. Instead, only the important tips will be mentioned. The first step was to achieve the objective (described in the text) of centering the image of the primary in the secondary mirror, as viewed from the eyepiece (a Cheshire eyepiece was helpful). This was done by moving and tilting the primary using the set screws that extend along the center tube (N/E/S/W) and four set screws that were at first concealed by cover screws with flathead screwdriver slots in them. Remove these to get access to the set screws that control the primary mirror tilt.

At this point, the quality of the image was moderate. Further adjustment was made via the secondary mirror, which has quite a few little screws to confuse matters. Some of these are actually lock-down screws, so once you figure out which to turn, the iterative process can proceed. The schematics of the secondary apparatus are available (either in the physics department or in the observatory) and were useful when determining the function of each screw.

Jeffery LaCombe, RAS Member involved in the mirror overhaul project, wrote, "At the conclusion of the rather difficult collimation process, it is my opinion that the image quality should be considerably better. However, it seemed that no amount of further adjustments would improve the image. It seems that there is a degree of astigmatism, that I can't work out of the system. I feel that the resolving capability should be better than it is (an 8" Celestron SCT does quite a bit better on bright objects like Jupiter and Saturn). I can't say if the resolution was ever was better in its past, but it should be in an instrument of this purported caliber. With this observation aside, it is the general consensus of the physics department users as well as the RAS club members that the telescope now exhibits considerably higher brightness, and at least as much resolving capability as it did prior to the overhaul project."

Polar Alignment

The telescope (in general) had not been tracking all that well. Of even greater importance, the setting circles did not operate properly. It was suspected that these problems were likely due to inadequate polar alignment of the equatorial mount. This was fixed by using the star-drift method to move the mount into better alignment. Subsequent testing has shown that the setting circles are now exceptionally useful, and the tracking has improved to the point where any difficulties are now attributed to the motor drive, which appears to be in need of some work.

Resolving Ability

The September 1998 issue of Sky and Telescope magazine lists double stars in Cepheus that can be used to test the resolving ability of a telescope.

Resolution Tests

Some of the above double stars were observed to determine the resolving limit and to get an idea of the general quality of the Campus's observing conditions. The sky conditions on this evening (10/19/98) were clear (at the time of these observations) with an extremely light haze.