0.9m Field Corrector

The Richey-Chretien design of the 0.9-m telescope introduces a curved focal plane. With a flat detector, the best focus becomes position dependent across the field of view.

In order to improve the f/7.5 wide-field performance of the telescope, a doublet corrector was installed in 1993. The effect of the corrector is to produce a nearly uniform point-spread-function across the full 23 arcmin field of a Tektronix 2048x2048 CCD.

The theoretical image quality of the telescope plus corrector is better than 0.3" from 3300 A to 1 micron. The technical details of the optical system are available.

The corrector is designed to work only at f/7.5. It is desirable to remove the corrector under the following conditions:

to use the f/13.5 secondary
to reduce the already very low potential for scattered light with the additional optics
to improve the U-band throughput, which is reduced by 35% when the corrector is in place
for marginal improvement at other bands - the corrector transmission is 87% at B, 94% at V, 98% at R, and 94% at I.

The corrector was designed to be easily removed from the light path. There is a plunger on the side of the Gold Guider opposite the ILS camera. When pulled out, the corrector is removed from the beam.

Installing the Corrector

The corrector is placed in the beam according to the instructions noted on the telescope.

Push the plunger with the T-handle with some effort toward the telescope.
Open the cap covering the alignment hole.
Check that the 2 holes are exactly aligned, that the plunger has been pushed as far as possible.
Close the cap