(based on Philip Massey's "whydra")

"Hydraassign" is an useful program for assigning fibers that comes as part of the Hydra-CTIO software distribution. It takes an input file containing celestial coordinates of objects and determines a "pretty good" assignment of fibers for Hydra. Phil Massey has written an instruction guide.


Basic Use:

The objects in the input file should be listed in priority order, with the most important objects at the top of the list. The coordinates of clean sky positions and field-orientation-probe stars ("fops stars") should also be included in the same file. The file must also contain the coordinates of the field center, along with a number of things that are needed to properly account for the effects of differential refraction. The output file will contain all of the input objects, assigned or not, with fiber assignments indicated with a numerical value in the "STATUS=" field. The stars with assigned fibers are rotated to the bottom of the list, but the input order is otherwise preserved; thus, one can use each output file as a new input file, with previously assigned stars dropped in priority.

The input file must begin by specifying nine keywords, one per line. Each keyword ends with a colon followed by a value. The order in which they appear is not relevant.

FIELD NAME - <= 64 character ID

INPUT EPOCH - Equinox of the input coordinates; must be 1855-2010.

CURRENT EPOCH - Epoch of the proposed observations, e.g., 1992.251, must be 1991-2010

SIDEREAL TIME - LST at mid-exposure, must be 0.00-23.999

EXPOSURE LENGTH - Decimal hours expected for exposure, 0.0-10.0

WAVELENGTH - Wavelength of spectrograph in A


WEIGHTING - Possible answers are STRONG (default), WEAK, or NONE

GUIDEWAVE - wavelength of TV camera, defaults to 6000 A

After the keywords is the list of coordinates. Each coordinate entry consists of a single line, and includes the following. The format is fixed.

Integer ID (4 digits or less). This should be a unique number that allows you to tell the fiber positioner what star you are talking about if you wish to drive the gripper over that locale. It will also be kept as a secondary identifier when you reduce the data with IRAF. COLS1-4

Name (20 characters or less). This name is the principle object identifier; it will be kept with the individual spectrum when you reduce the data with IRAF. COLS 6-25

RA. This should be specified the usual way, e.g., "02 12 14.123". The leading zeros are not required. Since we are reading this numbers in a fixed format, you may include colons (or anything else) where the spaces go. COLS 27-38

DEC. This should be specified the usual way, e.g., "-01 12 13.21". For positive declinations the plus sign is optimal. The value of the degrees must be between -89 and 89, and the values of the minutes and seconds < 60. The SIGN goes into COL 40; the rest go in COLS 41-51.

CLASS. This is a one letter code that specifies what type of object this is. COL 53. Possible answers are:

C Center. There must be exactly one of these in the file; it is the coordinates of the plate center.

O Object. This is the default if the item is unspecified, and is a normal program object. Currently the maximum number of objects is 2000.

S Sky. Currently the maximum number of sky positions is 2000.

F Field orientation probe star (FOPS). Currently the maximum number of FOPS is 2000.

In order to provide some capability for you to document what you are doing, we interpret any line that begins with a pound sign ("#") to denote a comment field. These will be retained in the output file. Items with double pound signs will not be retained ; these are used in the output files to denote warning messages.

When running hydraassign you will be asked for the input and output file names, as well as the number of fops and skies you require. In the example below user hydra specified the name of the input file as lepusastrom.hydra and the output file name as test2.hydra. The user required 3 FOPS and 6 skies to be assigned :

hydra% hydraassign
## CTIO Concentricities file Version 2.1 12/8/98
## v2.1 FOPS, Large fibers, no small fibers
## proper slit to fiber assignments for large cable
## -----------------------------------------------
## Astroparams Version 3.0 10/05/98
## -----------------------------------------------
## Systemparams Version 3.0 12/23/98
## -----------------------------------------------
Input file name: lepusastrom.hydra
Output root name: test2
There are 3 fops stars found
How many fops need to be assigned to fops stars?: 3
There are 75 objects found
There are 20 skies found
How many skies are required?: 6
Would you like to tweak the center? y or n: n

Field center= 15.4505 22 22.456 -19 06 25.67 1950.00 Ag= 179.988
Assigned: 49/ 75 objs; 6/ 20 skys; 3/ 3 fops; Total wts= 4.01

Last Modified: April 27, 2000

Updated on June 15, 2021, 7:42 am