Observation Planning for the APF

There are a couple of important pieces of information to know about the APF. First is the rough exposure time needed for to get the signal to noise required for the observation. Second is the resolution needed. Third is how to properly acquire the target.

Exposure Time

The best way to get a handle on the data quality is with actual data. This catalog contains plots of observations of stars with known magnitudes and exposure times across a number of orders.

There also is the APF exposure time calculator. The output agrees with actual measurements, as seen in the APF Prospectus.

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Slit Length and PA

There are a number of slit lengths available, from 3 to 12 arc-seconds. At redder wavelengths, the orders will overlap for slits with widths of greater than 8 arc-seconds.

  • K - 1 by 12 arcsec
  • L - 2 by 12 arcsec
  • M - 1 by 8 arcsec
  • B - 2 by 8 arcsec
  • W - 1 by 3 arcsec
  • T - 2 by 3 arcsec
  • S - 0.75 by 8 arcsec
  • N - 0.5 by 8 arcsec
  • O - 8 by 8 arcsec

The spectrometer is not on an instrument rotator. So, despite having slits with lengths of 8 arcseconds, long slit spectroscopy of extended targets is not option.

Spectrometer Resolution

The spectrometer resolution is, in effect, fixed. The choice the width of the slit, the particular decker required, will result in different resolutions for the data. The resolution is not a simple function of the slit width. The 2 arcsecond width delivers around 80000 at 550nm while the 0.5 arcsecond width delivers around 100000 to 120000 at the same wavelength. The resolution is wavelength dependent but the system is optimized for the best quality in the Iodine region of the spectrum.

Guider Acquisition

That guider has a 55 arcsecond field of view and can detect objects down to 15 magnitude in V.

Acquisition is done by placing the brightest object in the search on the center of the slit. By default, the search radius is 22 arcseconds.

For objects that have brighter neighbors or are in crowded fields, we have a second acquisition mode where a bright star within a few degrees is acquired. After that acquisition, the telescope is offset and the brightest object within 3 arcseconds is placed on the slit.

We are looking into an offset mode where the observer can define a blind offset from a point star for acquisition of fainter targets.