A New Secondary Mount for the NSRT

The Problem

Since the recommissioning of the NSRT direct imager you might have noticed some odd streaking that occurs in long exposures. For example look at this image of the Eagle Nebula:

The star images are oblong along an axis from the upper left to lower right of the frame. We had also noticed some rather large errors in the telescope pointing that could not be explained a simple instrumental flexure. The telescope is big and heavy but it is built like a battleship thanks to the Warner & Swasey Company so we did not expect the pointing to deflect as far as it appeared to from the telescope sagging as it points closer to the horizon.

A competing problem with the telescope tracking kept us from figuring out what was going on until we had the clock drive and telescope balance fixed. At that point we discovered this image smearing and large apparent flexure errors were due to a wobble in the secondary mirror mount on the telescope.

This problem was not an issue before we installed the electronic camera because the movement of the image plane caused by the floppy mirror would have minimal impact on the cassegrain spectrograph or naked eye observations.

The Old Mount

The old secondary mirror mount (pictured at right) supported the mirror at a single point on a linear ram that moved in and out to change the focus of the telescope. This piece of equipment is about as sturdy as a mount of this design can be but the design is flawed. The ram must be loose in the tube to allow it to move back and forth. However the ram must also have some tension on it to keep it from rattling around inside the housing. On the old mount this was acomplished with a single pin, so when the telescope orientation changed, the ram rolled over the pin and slipped. A very small deflection of the secondary mirror is multiplied by the distance to the image plane and so a very little mirror wobble can cause the effects we saw.

The New Mount

The new secondary mirror mount (show with a standard 12oz Pepsi can for scale) solves the problem of the old mount by supporting the mirror at three points instead of one. The linear ram to move the mirror is a fourth independent arm on the mechanism. The tension/support problems for the moving rams are solved in the new mount by linear berrings that support each ram at four points instead of one, allowing for more stability in each individual ram.

The plan is to have this new mount constructed and installed by May 1, 2002.

Pictures from the installation are online at personal/martin/photos/NSRTApril2002.

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