by Dr. Arthur Babcock |
|||
| As a consequence of the recent upgrading
of the MIRA research telescope and its control system, MIRA astronomers have been delving
into the dark art of collimation, that is, aligning the primary and secondary mirrors so
that the best images are produced. |
|||
|
|
||
| After installing the realuminized secondary mirror (Ivan Eberle and Bruce Weaver, shown at left, perform the operation), the telescope’s optical system was completely out of adjustment. A rough collimation was achieved just by looking through an eyepiece as the two mirrors were adjusted, but the result (right) is far from perfect: the "fuzz" on the upper edge of these stellar images is coma, meaning that the camera is not aligned with the optical axis of the telescope. (All photos by Dr. Arthur Babcock.) | |||
|
Much better results were obtained when we placed a Hartmann screen (Ivan Eberle is shown at left holding the Hartmann screen manufactured by DFM Engineering) over the aperture of the telescope. The way this device is used is to take two CCD exposures, one on each side of focus. Then the images are inspected visually and analyzed with special computer software. We repeated this process after every adjustment to the mirror. | ||
The Hartmann image on the right was taken with the telescope in the condition that produced the comatic image shown above. Note how the spacing of the spots is irregular (see circled areas) and how the spots form curved lines where they should be straight (arrow). |
|
||
| After 18 iterations of the inspect-adjust-inspect cycle, note in the left-hand image how the spacing of the spots is more regular in the left-hand image (compare the circled pairs of spots) and the spots aligned in straighter lines (arrow). |
|
||
| The result at right: some further refinement of the collimation is no doubt possible, but this image of the open cluster NGC 7790 is entirely suitable for science. |
|
||
Last updated 1/10/01 et |
|||
