Measuring efficiencies at the HHT			D. Muders, December 1998

Efficiencies for heterodyne receivers are necessary to establish standard
intensity scales for line observations. Planets are usually used to derive
the efficiencies. Depending on the planet size one gets e.g. beam or forward
efficiencies. Essentially, one needs a continuum flux measurement on the
planet, a sky measurement at the same elevation and a scan on a hot and
a cold load. In addition, one should get a skydip measurement and also note
the value of the tipper tau (those values can then also be used for the
calibration of the tipper).

We now have an OBST commandi (EFFICIENCY) to make all the measurements
necessary to calculate efficiencies. It has one adverb (/NUM_REPEATS) to
specify the number of repeats of the sequence of pointings and ctrack's
(default is 3, recommendation is at least 2).

To make the measurement, simply type:
 
     SOURCE <Planet>
     EFFICIENCY/NUM <number of repeats>

This will do <number of repeats> times the sequence

POINT
CTRACK on the planet
CTRACK on sky
CTRACK on the hot load
CTRACK on the cold load

and a final skydip (please start another tipper measurement when the
skydip begins).

To get the drumbeat counts you can use Robert Zylka's SKYDIP program.
Give it the scan and channel numbers and specify "1 9" for airmass. This
will cause SKYDIP to try to read 9 subscans which is the maximum number
we usually get (from the final skydip w/ hot and cold) plus one to prevent
SKYDIP from actually making a fit (because it will always search for more
data). SKYDIP will print the average drumbeat counts and RMS. You will
also get the skydip numbers, but PLOTS_KY is much more comfortable to get
the plot (see the document about Bill's skydip kluge).

Alternatively (not recommended), you can use Toolbox to get the counts.
In one of the OBSHOME windows on FRANKENHEIM type CONT2 to run the Toolbox
continuum reduction package. Type the following sequence:

     CO=FDATA:[OBS]SKY.C44
     READ
     ON=<Scan number of one CTRACK>
     END

"END" starts the reduction. You will be asked "to PRINT <Scan number>". Just
type return. You will then get a line saying "PRINT SCAN ..." and the next
line will contain the counts for this CTRACK. There will be another question
("to PLOT <Scan number>") which you should also answer with just a <Return>.

Back at the "Cont2?" prompt you can then enter "ON=<Scan number of the other
CTRACK>" and "END" to reduce the second scan. Finally exit from CONT2 by
typing "FINIS".

To calculate the efficiencies we usually use Tom Phillips' report (in the cali-
bration folder on the mountain) or the JCMT notes (Bob Hayward printed them
from their Web page).

The necessary brightness temperatures for the planets can be estimated with
the programs "astro" or "mopsi" on jever / hannen. In "astro" type:

     TIME [HH:MM:SS.SSS [DD-MMM-YYYY]]		! After starting "astro" the
						! current time is already set.
     LET FREQUENCY <Observing frequency in GHz>
     PLANET

This will print a list of all planets and their brightness temperatures TB
at the observing frequency.

In "mopsi" just type "PLANET" and answer all questions. Be careful to enter
the right distances (-> Almanac) since we do not have the 1998 ephemeris for
"mopsi" in the computer.

For the Moon there is a paper from Krotikov and Pelyushenko (in the calibration
folder in the control room) which gives an estimate of TB(Moon) in dependence
of the Moon's phase for centimeter to millimeter wavelengths. I have added a
plot of TB versus the Moon's phase for lambda=1mm (the smallest wavelength
covered by this paper). However, it would always be best to catch the Moon
when it is full to avoid inhomogeneous illuminations.