Units for data analysis pipelines

Definitions of terms:

One "chop":  One full period of chopper motion. The Mac
computes the demodulated signal once per chop.

One "frame":  The mac combines the computed demodulated
signals from several "chops" and combines these into one
"frame".  Each "frame" is plotted to the stripcharts and also
saved to disk.  Typically there are 2 chops per frame.

"Frame-rate data":  The data saved by the Mac to disk is
referred to as the "frame-rate data".

One "int":  The integration time at a given postion (either
left or right) is called an "int".  One "int" is set to be an
integer number of "frames".  Typically six "frames" constitute
an "int".

One "nod-pair":  One left-beam "int", followed by two
right-beam "ints", and then by another left-beam "int"

A "photometry file":  Consists of a specified number of
"nod-pairs" at a given postion.

A "map" is created by taking a "photometry file" at each of a
series of sky positions as specified in a tcl mapping script.

A "polarimetry file" consists of exactly one "nod-pair" at
each of six half-wave plate angles.

Definitions of units:

For all the discussion below, we will assume that the chopping
frequency is 3.1 Hz, the global attenuation is 1000, the gain
on the preamp is 10,000 and the "chops-per-frame" is 2.  A
discussion of how changing these values might affect the data
appears at the end of this document.

Also, for much of the discussion below we are dealing with raw
demodulated signals, so keep in mind that the "total signal
change on phase flip" is twice the value of the signal.  Also
the "total signal change on switching from left-beam to
right-beam" is twice the value of the signal.

Mac-units:  Units of the data stored to disk in binary form
from the Macintosh/Yerk data system.  I.e. these are the
units of the "frame-rate data".

Stripchart-units:   A source with a signal of 1
"stripchart-unit" will produce a signal of 1 inch amplitude on
the Macintosh screen when the strip-chart attenuation is set
to "1".  So this same source would produce a signal of (1/8)"
when the strip-chart attenuation is set to "8".

1 stripchart-unit ~ 85,000 Mac-units

Integ-units: Units of the data after the "integ" routine has
combined all of the "frame-rate data" from a single "nod-pair"
into a single integrated "L-R signal" that is the result of
that nod-pair. This "L-R signal" or "nod-pair result"
represents the signal from the source with synchronous offset
removed, and is stored in the ascii files that are the output
of "integ".  

1 Integ-unit = (1/4) Mac-unit.

Photometry Units:  These are the units used by the software
that makes the maps of sources.  They are basically
Integ-units reduced by an attenuation factor which is settable
by the user of "premat".  The default setting is 10,000.

Details - following the signal through from the bolometers:

SPARO Data System:  The units output from sparo depend on
preamp gain. Chops per frame and chopper frequency are not
supposed to have any effect on the average signal level. If
preamp gain is held constant(as is the case for normal
operation) the SPARO mac will write to disk a consistent
number which is proportional to power measured by the
bolometers. Normally, we operate at a chopping frequency of
3.1 Hz, with a preamp gain of 10,000, and 2 chops per frame.
Global attenuation is a parameter which only affects the strip
chart.  It is normally 1000.

Integ:  Integ's output is independent of the number of frames
in each integration. It takes a straight average of the frame
rate data and then adds the four nodpair positions together
using the proper sign for each left and right beam
integration.  Thus, this introduces a gain of 4.

Phot(sp_phot): This code takes a weighted sum for each pixel
from the data available in each nodpair and thus there is no
gain due to changing the number of nodpairs in a file.  

Premat adds an attenuation factor set by the call line tag -a

The default(and most often used) value for atten is 10,000. 
This is a convenience so as to not carry around such large

In the noise histograms, the horizontal axis is in units of
10^5. Thus, a peak of around 1 corresponds to an signal of
100,000 integ-units.

references:  logbook 5, p. 66 & 73 for stripchart units to
Mac-units.  Also p. 66 vs. analysis of maps from that day for
integ-units to Mac-units (verifying factor of four).