APF Target Specification

To observe with the APF requires that you construct a target list which is fed into the observing software. The information is contained in a Google Sheet. Some of the data are straightforward, such as the J2000 coordinates, the proper motion, the magnitude and color, and the requested decker for the observation. Others are details for the observing software that control either total exposure time, the minimum exposure time, and the final signal to noise.

In each section, we discuss the various components of this listing and the requirements. Some components are optional.

Please see the description of coordinates for important details regarding specifying how the telescope software points.

Object ID

The name of the object is required. The name should be an identification that can be used to reference the object in the Simbad database. We use that database for confirming object coordinates.

This name CANNOT have a space, so names like HD 185144 or BD+03 8 must have the spaces removed or replaced with some other character like an underscore.

  • Star Name : HD185144

Owner

The owner of the target. Data will be listed in the archive under this name.

public is a special name, all calibrations are public as are any observations of a B star at the start of the night, so anyone can reduce their own data using the standard calibration observations

  • Owner : B.Holden

Object coordinates

The coordinates should be in sexagesimal format with each component separated by spaces. A plus sign is optional for positive declinations. Following this must be either 2000 or the epoch for the date of observation, so 2000 is preferred. Finally, the proper motion is specified as a pair of keywords with the value in milli-arcseconds per year, just as Simbad returns it.

  • RA hr : 19
  • RA min : 32
  • RA sec : 21.59
  • Dec deg : 69
  • Dec min : 39
  • Dec sec : 40.24
  • pmRA : 598.07
  • pmDec : -1738.40

Photometry

The target must have a V band magnitude and B-V color value. Both are used to estimate the observed photon arrival rate. For the APF, the exposure meter has no filter, so a combination of both magnitude and color are needed to predict the rate of photons arriving in clear conditions. The V band magnitude is also used to predict how long -- given the current transparency -- it will take to achieve the required signal to noise for the science requirements.

  • Vmag : 4.68
  • B-V : 0.78

Scheduling

The scheduler has a number of attributes. The most important is the priority, and that must be balanced across programs. Relative ranking within a program will be preserved, but the over all values may move up or down depending on the TAC ranking of the program.

For time dependent observations, the scheduler has an option to set a UT time and a duration over which that the priority of the target will jump significantly.

  • APFpri : 6 - Priority of the target. Larger numbers are higher priority, negative values are skipped. Usually this is between 4 and 6. The maximum available is 10 and this must be an integer
  • APFcad : 10 - Minimum number of days between observations. This can be less than 1, we have used 0.01. If the (current JD - the JD of the last observation) is smaller than that number, the object is skipped.

Exposure time and number of exposures

These parameters determine the optimal exposure time for target in the current conditions through the precision. The rest allow the user to set minimum and maximum values to ensure that either the target does not take too long or that the target gets enough data.

The Levy exposure time calculator can be used to guide these values. It will provide estimates for the resulting exposure meter value for a given exposure and the precision that was used in the earlier version of the scheduler.

  • texp: 1200 - This is the total possible exposure time. The exposure will go this length unless terminated sooner by the exposure meter.
  • APFnshots: 1 - The number of exposures per observation, if blank defaults to 1.
  • expmeter: 1e9 - This is the total number of counts in the guider aperture required to terminate the exposure. A value of around 2 billion usually yields 60,000 counts per pixel in the spectrum, though can be more at the peak for stars that are very red or very blue. Values larger than 2 billion are not allowed. One billion is the standard for most precision radial velocity work on the brightest stars.

Observation Details

There are two items, one that controls if the iodine cell is in the beam, and the second controls whether or a pointing check should be made to ensure the acquisition of the target because of a close companion.

  • I2 : Y - By default the iodine gas cell is in the beam, replace this with a n or N to acquire data without that cell.
  • Close Companion : N - The acquisition algorithm for the APF acquires the brightest star in 25 arc-seconds of the field center. If there is a star brighter than the target inside that radius, select Y or y for this option. The APF will then perform a local pointing correction and acquire the brightest star within 2.5 arc-seconds of the field center.
  • Template : Y - Has a template observation occurred. This defaults to Yes. If N, a template could be scheduled. These are required for precision radial velocity work.

Observation History

These are values that describe the history of the

  • Nobs : 11 - The number of observations of the star that have been taken since the sheet has been use. This is not the number of exposures, as an observation can consist of multiple exposures.
  • Total Obs : - The maximum number of observations taken of a target. If the value is blank, then there is no limit. Otherwise if the number of observations equals or exceeds this value, then the target is not considered for observing.
  • lastobs : 2458574.02 - The date of last observation, required for cadence (APFcad), starts at 0 for no exposures.

Decker

The aperture used for the opening of the spectrometer is specified by a single letter. Below we list the decker codes with the sizes in arc-seconds

APF decker : W

  • 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

Alternate Observing Modes

These modes are not implemented yet.

APF decker : W

  • Mode
  • obsblock
  • raoff
  • decoff