Instructions for the Analog Computer

The G-star analog computer is a simple device enabling rapid selection of a bright G-star at a specific airmass. G-stars are good spectral standards for atmospheric transmission calibration of infrared spectral data. One wants to match the airmass of the calibrator to the airmass of the object observation, which this computer facilitates.

The G-star analog computer consists of a star plot and an airmass overlay, valid for 33 degrees north latitude. Also avaliable is a split version of the G-star plot, which is helpful when the LST is between 18h and 6h. The system works as follows: print the airmass overlay on a transparent sheet, and print the star plot on paper. Align the meridian (central) axis of the overlay onto the Right Ascension of the star plot corresponding to the current LST (Local Siderial Time). The airmass for each star is seen at a glance. The number next to the star is the HR designation, taken from the list described below.

The list of F8 through G9 stars on which this computer is based can be viewed as text, or downloaded as PostScript. The list has the HR number for each star (which appears on the star plot), as well as an SAO number. For stars without an HR designation, capital letters have been substituted.

If desired, the target objects can be plotted onto the star plot, thus enabling one to easily locate G-stars in the same region of the sky as the target object.

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