3. Object Tables (OTs) – Format

Object Tables (OTs) are FITS tables containing all necessary information for the mask design. This section describes the OT format, and the next section how to create an OT.

3.1. Mandatory columns

OTs have the following mandatory columns, ideally in this order:

ID:

Unique object id (16-bit integer)

RA:

Right ascension in hours (float)

DEC:

Declination in degrees (float)

x_ccd:

X coordinate of object position in pixels (float)

y_ccd:

Y coordinate of object position in pixels (float)

MAG:

Relative magnitude (float)

The column names are case sensitive.

The numeric format is mandatory, in particular the ID must be an integer.

Note

The RA and DEC columns are used to plot the object positions in the image. Cartesian x and y coordinates are used to define (and plot) the positions of the slits.

3.2. Optional columns

The following columns (if absent) will be added automatically by GMMPS.

slitsize_x:

Slit extent in X-direction in arcsec (float)

slitsize_y:

Slit extent in Y-direction in arcsec (float)

slitpos_x:

Offset in X-direction relative to the object in arcsec (float)

slitpos_y:

Offset in Y-direction relative to the object in arcsec (float)

slittilt:

Slit position angle in degrees (float)

slittype:

Slit type. Currently, only rectangular slits are allowed (slittype = R; char*1)

priority:

Priority (char*1)

redshift:

Redshift of the source (if any, otherwise zero) (float)

3.2.1. Priority

An object’s priority is represented by a single character with 5 possible values.

  • priority = 0: Acquisition star (at least two required, better three)

  • priority = 1: Highest priority

  • priority = 2: Medium priority

  • priority = 3: Lowest priority

  • priority = X: Ignore object

More information on how GMMPS uses priorities.

3.2.2. slitsize and slitpos

GMOS has a horizontal dispersion (in image coordinates), and therefore slitsize_x/y refer to the slit width and slit length, respectively. To offset the object along the slit you would introduce a non-zero value for slitpos_y. A non-zero value for slitpos_x would quickly lead to flux losses for point-like objects as they are driven off the slit.

F2 has a vertical dispersion. Therefore, slitsize_x/y refer to the slit length and slit width, respectively. To offset the object along the slit you would introduce a non-zero value for slitpos_x. A non-zero value for slitpos_y would quickly lead to flux losses for point-like objects as they are driven off the slit.

Defaults: The following values are used if optional columns are added:

  • slit width = 1.0 (slitsize_x for GMOS, slitsize_y for F2)

  • slit length = 5.0 (slitsize_y for GMOS, slitsize_x for F2)

  • slitpos_x = 0.0

  • slitpos_y = 0.0

  • slittilt = 0.0

  • priority = 2

3.2.3. Tilted and curved slits

  • Positive angles rotate the slit counter-clockwise.

  • Angles larger than 45 degrees are not permitted.

  • A tilted slit is represented by a parallelogram. Its width in dispersion direction remains unchanged (i.e. slitsize_x for GMOS, and slitsize_y for F2). The spectral resolution is conserved among slits with different tilt angles and identical widths.

  • Tilted slits are permitted in Nod & Shuffle masks, provided a q=0 offset is used in the GMOS Nod & Shuffle component of the Observing Tool (see here).

  • Curved slits are not supported. They can be mimicked by concatenating neighboring slits with different tilt angles.