*** CRUSH User's Manual Extension for SHARC *** Attila Kovacs Last updated: 2 June 2014 Table of Contents ================= 1. Introduction 1.1 Notes of curiosity 2. Locating Scans 3. Pointing and Extinction Corrections 4. Glossary of SHARC-specific options ##################################################################### 1. Introduction ##################################################################### This document contains information specific to using CRUSH-2 to reduce SHARC data. It is assumed that you are already familiar with the contents of the main CRUSH-2 README (inside the distribution directory), especially its Section 1 (Getting Started). If you run into difficulty understanding the concepts in this document, you will probably find it useful to study the main README document a little longer. 1.1 Notes of curiousity ======================= SHARC represented a particular challenge for integrating into CRUSH (which is why it was interesting), since CRUSH was designed for total-power data, whereas SHARC produces only a dual-beam differential stream via a chop, which requires the extra step of deconvolution. The deconvolution, and map accumulation, happens in Fourier domain, and the dual-beam image that is synchronized to the time-stream is dynamically generated in each iteration, for each scan. The deconvolution method is essentially an Emerson II (Emerson 1995) type algorithm with some extra use of well-placed noise weighting. The new SHARC module mainly serves as a demo for what CRUSH can do for dual-beam (and multibeam) reductions in general. CRUSH can be further tweaked to work with arbitrary wobble patterns too, if there is desire to adapt it... ##################################################################### 2. Locating Scans ##################################################################### SHARC scans were stored in user-specified data files (stored in a poorly documented VAX-binary format), with each file containing data from multiple scans. So, before accessing any given scan, you must specify the data file via the 'file' option, or via the combination of 'datapath' and 'file'. E.g., say your data file is '/home/sharc/mydata.dat'. Then you can specify this file either as: crush sharc [...] -file=/home/sharc/mydata.dat [...] or as: crush sharc [...] -datapath=/home/sharc -file=mydata.dat [..] (Clearly, the second approach is advantageous when you want to reduce scans from multiple files stored in the same location.) Next you can obtain a listing of the scans in any given file using the 'list' directive. E.g. crush sharc [...] -file=myfile.dat list will list the scan entries stored in 'myfile.dat'. Every row of an OTF scan constitutes a separate scan entry in the data file. While the scan headers state how many OTF rows each complete scan should have, the number of rows recorded in the data may often be fewer (aborted scans or crashes) or more (often by +1). Therefore, there is no fool-proof automatic detection of which scan rows 'belong' together. Instead you can specify which entries to treat as belonging to the same 'master' scan. Suppose you judge that entries 3-12 are part of the same OTF, and 15-19 another, and 22 is a single pass over the same data. Then you may reduce these together as: crush sharc [...] -file=myfile.dat 3-12 15-19 22 I.e., unlike for other instruments, ranges for SHARC specify entries that are to be treated as one scan (not a range of scans as for other instruments). ##################################################################### 3. Pointing and Extinction Corrections ##################################################################### This works the same way as for SHARC-2, so you should read the corresponding section of sharc2/README. But, in short you can use 'fazo' and 'fzao' to correct the pointing, and 'tau.225GHz' to set a 225GHz tau value. E.g.: crush sharc [...] -fazo=-49.5 -fzao=10.2 -tau.225GHz=0.043 3-12 [...] There is one small difference, however, is that the 'sharc' tau relation includes the bolometer nonlinearity (which for SHARC-2 is treated separately). ##################################################################### 4. Glossary of SHARC-specific options ##################################################################### chopper.throw=X @Advanced Deconvolve with this chopper throw (arcsec) instead of the value stored in the file. The chopper throws that appear in SHARC data are often approximate only. You can "determine" the effective throw by minimizing deconvolution lobes while reducing bright pointing sources. You can then apply the same values to the science data, assuming that the chopper controller was left in the same state (which is a reasonable assumption :-). deconvolve @Advanced Procude deconvolved maps. Otherwise, CRUSH will produce dual-beam maps. deconvolve.above=X @Expert Set the deconvolution threshold to X. Usually 0 Specify the data file either as a fully specified path, or relative to the path set by the 'datapath' option. pixelsize=X @Expert Set the SHARC pixel spacing to X arcseconds. By default, the nominal 5" spacing is assumed. ------------------------------------------------------------------------------ Copyright (C)2014 -- Attila Kovacs