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