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Possible Modifications to Improve Observing Efficiency

The overhead might be reduced if (1) the settle time of the telescope is shortened, and (2) guiding is turned off when one is observing sky. The main part of the overhead is in reacquiring the guide star each time the telescope is offset. At present, there is a built in 2 sec delay to allow for settling before the telescope tells ICE to resume integrating: this could be a variable parameter on the telescope operator's GUI. The settling time seems to be shorter for smaller offsets, and may be unnecessary for most offsets 2 arcmins.

We tried offsets as large as 13 arcmin (with FORMAPS off) and the telescope behaved reasonable well. The settling time was longer (6 secs), but we never lost the star through 20 to-and-fro offsets. The settling time will likely be longer with FORMAPS on, since the telescope tries to readjust the mirror support after every offset, depending on the size and direction of the offset and the current position of the telescope. Throws larger than about 3 arcmin generally make FORMAPS reconfigure the mirror support.

A new guider is critical, especially if one intends to guide in the off position during small offsets. On 00mar16ut we worked through light cirrus and with a 10-day old moon, but were able to use  15.5mag guide stars successfully as long as the seeing was stable. Many fields are likely to need fainter guide stars, and a high-throughput CCD guider capable of performing small offsets that keep the guide star within the camera field and guiding at more than one position within the field, would reduce the overhead. A more sensitive guider would also reduce the overhead in searching for a second guide star if a large throw is required by the nature of the observation.


next up previous
Next: Modifications Needed to Make Up: Report on the Nod-and-Shuffle Previous: Overhead for the Nod-and-Shuffle
Jim DeVeny 2002-05-20