----------------------- LINEPOINTING.DOC -----------------------------

Updated Line Pointing at the SMT		      D. Muders, June 1998

Line pointing provides a possibility to check the telescope pointing during
heterodyne observations on the left flange where no bolometer is available.
It will be mainly used with the CO 2-1, 3-2 and 4-3 lines observed towards
evolved stars. We use the list of sources compiled by C. Loup et al., A&AS,
99, 291, (1993). It contains 444 sources and it lists several names per
source so that you should be able to use your favorite name.

The Loup catalog is stored on the VAX computers in KDATA:[SMT.CAT]. There
are two versions: LOUP1950.CAT (B1950 coordinates) and LOUP2000.CAT
(J2000 coordinates). It is recommended to check also the papers about
CO observations of evolved stars and Mira variables by Stanek et al. (ApJS,
100, 169, (1995)) and by Young (ApJ, 445, 872, (1995)). Copies of all these
papers are provided in the "Submillimeter Calibration" folder in the control
room.

Although the Loup catalog contains 444 sources, not all of them are suited
for line pointing. Some are too weak, others are extended in CO (e.g.
IRC+10216). Be careful about choosing sources. Stanek et al. provide some
sample CO maps to estimate the source sizes. The SMTO provides a special
catalog with sources that have been checked to be OK for line pointing.
This catalog is KDATA:[SMT.CAT]LPOINT.CAT. It has also been converted to
Xephem's EBD format and the sources can be displayed in the XEPHEM window
by selecting KDATA:[SMT.CAT]LPOINT.EDB as the objects database.

Line pointing is implemented via the LPOINT (On-Off) and LWPOINT (Wobble-
switched) commands in OBST. It sets up an observation of a cross of N On/Off
type measurements per axis (azimuth / elevation) relative to a given reference
position. The syntax is similar to the POINT and PSWITCH/WSWITCH commands, e.g.

LPOINT -120 0/NUM 3/TIME 15 15/LEN 50/ON 3/BE AOSB

or

LWPOINT -120 0/NUM 3/TIME 15 15/LEN 50/ON 3/BE AOSB

would observe 3 points on each arm spaced by 25" with a reference point at
(-120",0"). The integration time per point is 15 seconds. The /ON 3 qualifier
tells LPOINT to observe 3 ON's per OFF (recommended under good weather
conditions, /ON 2 or /ON 1 otherwise). See HELP LPOINT or HELP LWPOINT in
OBST for more details.

The /BE adverb selects the backend to be used for the fit. It must be one
of the backends currently enabled by the BACKEND command. Preferably, one
of the broadband AOS (AOSA or AOSB) should be used to avoid virtual
memory problems in VAX CHEF. The default at startup is "*" which means
all enabled backends would be used. This is not recommended since it would
lead to resampling to the smallest bandwidth (e.g. 8 MHz for the 62.5 kHz
part of the filterbanks), thus making fits impossible for the typical line
widths seen in the CO stars. One can, however, select individual parts of
the filterbank to be used in the line pointing fits. The names are FBSA
for the 62.5 kHz part, FBSB for the 250 kHz part and FBSC for the 1 MHz part.

The LPOINT observation is reduced and displayed in the online display (CHEF).
It subtracts a baseline of order 1 off all spectra and calculates the line
area. It needs source specific velocity windows to do that. Those windows are
specified in the catalog using the four new OBSINP commands VLOW, VHIG, WLOW
and VHIG. VLOW and VHIG define a window for the baseline calculation (like
SET MODE X in CLASS). This is necessary because of edge effects (e.g. isolator
in the SIS230). The second pair (WLOW, WHIG) defines a line window for the
area calculation (like SET WINDOW in CLASS). The Loup catalogs already contain
those velocities. If you want to use your own catalog you must provide those
parameters. A typical line in the catalog would be:

SNAM=o Cet        ; SLAM=02 16 49.11S; SBET=-03 12 22.4; -
VLSR=+47; VLOW=-14; VHIG=108; WLOW=36; WHIG=58;

Note the dash at the end of the first line. This is the continuation character.
The head of that catalog section must contain the extra parameters:

COMMON: SBAS=1
INDIVIDUAL: SNAM SLAM SBET VLSR VLOW VHIG WLOW WHIG

CHEF writes those velocity windows are written in the CLASS spectrum
header so that they can later be used for the fit. The online CHEF mails
the fit results to OBST where they can be used with CORRECTION F F, etc.
like with a continuum pointing. The new LPOINT command in CHEF does the
fitting and mailing. It can also be used offline for later analysis (e.g.
using baselines of different order or different velocity intervals).
The syntax is:

LPOINT <Start scan number> [[[[[[[[<End scan number>|*] <Base Order|*>]
       <VLOW|*>] <VHIG|*>] <WLOW|*>] <WHIG|*> <Backend Name|*>]

The starting scan number must be given, the other parameters are optional.
One can specify a range of scan numbers to sum several scans for better
signal-to-noise ratio. Currently, it is not possible to sum scans with
different COL*/NULE values. That means that one can not take the corrections
between scans.

A "*" tells LPOINT to use wildcards (scan number range) or to fetch the
velocties from the header, take a baseline of order 1 and to use all
available backends. LPOINT expects the line pointing spectra in the current
input file opened by FILE IN in CHEF. The backend name can be either the
names used in LPOINT and LWPOINT (AOSA, AOSB, AOSC, FBSA, FBSB and FBSC)
or the "TELESCOPE" names used in CLASS (SMT-10M-B30, SMT-10M-B40, SMT-10M-B50,
SMT-10M-B71, SMT-10M-B72 and SMT-10M-B73).