Telescope Pointing

  In order to achieve the best performance of the Telescope Pointing Model, the following zeropoint determinations have to be carried out daily, as part of the start-up procedure for the INT:

• A hardware zeropoint, determined by the initialization of the Ferranti Encoders with the ZEROSET procedure (section ), which involves carefully moving the telescope across hardware markers on both axes.
• A software zeropoint, which has to be determined by pointing the telescope to a star with known position, and use a SNAFU procedure to find a correction to the pointing-model's zeropoint. SNAFU comes in two versions: a long one, the 2-Star SNAFU (section ), which has to be performed after an instrument change-over, and a short one, the 1-Star SNAFU (section ), which can be used on subsequent nights. The recommended procedure is to do a 1-Star SNAFU at the beginning of the night. With the improvement in performance of the telescope the longer version is rarely used. However, failure to do a SNAFU at the beginning of the night, will seriously impair the pointing of the telescope. A SNAFU at Prime Focus (section ) is different from one at Cassegrain because at prime there is no TV camera for field viewing.

Further, the parameters of the Pointing Model itself have to be determined at regular intervals of several months (see section ).

In order to achieve the maximum accuracy and consistency in the determination of zeropoints it is essential that these ZEROSET and SNAFU procedures are adhered to and that variations are not introduced for purposes of expediency or personal preference.

• Encoder Zeropoints: ZEROSET
• Software Zeropoint: 1-star SNAFU
• Software Zeropoint and Collimation Error: 2-star SNAFU
• SNAFU at Prime Focus
• Reasons behind the ZEROSET and SNAFU procedures
• Testing the Encoders
• Pointing Tests using POINT and AUTOPNT