Imaging

An application using the new ETC was made that provides specific signal-to-noise calculations as a function of exposure time for all the stars in a given standard star field for ALFOSC. See

http://www.not.iac.es/observing/forms/signal/v2.0/standard.php

A fully automatic focusing script was implemented for ALFOSC imaging. For this script, a new catalogue of focus fields spread over the whole sky was generated from the Guide Star catalogue. The script selects an optimal focus field from this catalogue based on the current telescope position (a field that is at an intermediate distance from zenith and which involves the least movement of the telescope and building), it sets-up the instrument and CCD, takes and analyzes the focus images and applies the resulting correction in an iterative process.

The instrument and detector setup present when the script is started is restored when the script terminates so observations can continue immediate. No user interaction is required, and the whole procedure typically takes around 1 minute. This is specifically intended for users that (need to) use ALFOSC (for Target-Of-Opportunity observations) but are not familiar with the instrument, but the speed of the process makes this a very useful tool for anybody.

A common issue with ALFOSC is that different optical elements (like filters, but also the calcites) change the required telescope focus. In principle it would be possible to build-in the relative focus offsets in commands that put an element in the light path, but there are different ways in changing the setup (e.g., through the user interface, but also through sequencer commands), while also more elements might be put in the beam at the same time. The plan is to implemented a script that will check the current instrument set-up, look up the relative focus offsets in a data base, and apply the cumulative offset to the telescope focus.

The automatic script that is provided to take twilight sky flat fields with ALFOSC have on occasions been used with a wrong telescope or instrument setup which effectively resulted in useless data. To limit improper use of this script error and warning checks and messages were added that will either abort the script in case an improper setup is used (e.g., the light path is being vignetted), or give a warning when there might be a problem (e.g., the CCD is windowed and/or binned).

An upgrade was made of the list of photometric standard fields for ALFOSC based on the latest version of the Landolt catalogue (AJ 137, 4186). The new catalogue has 27 Landolt fields with three or more `well' observed standard star in the ALFOSC field-of-few compared with 14 in the old catalogue. New finding charts are provided indicating the colors of the starts and listing their magnitudes.