ALFA FITS format for ALFALFA

The ALFA data are delivered to the user as binary FITS tables. The format of these tables is still under development, so be aware that this document will certainly need regular updating over the next months. In particular, we anticipate that some columns now blank or hardwired will be fixed and more columns will have to be added to record additional parameters such as Doppler corrections, etc.


Look at Jeff Hagen's header definition for the latest and also his documentation page

Look at Mike Nolan's notes on suggested changes

Look at Phil's page describing how the FITS positions and offsets are calculated

Here is a listing of the header contents of a file written on 28Sep2004.


On 28 Sep 2004, the FITS tables produced by ALFA contained 118(+1) columns, though not all of them were correctly set. Here's what a single row contains:   (note: this page is a work in progress!!!!)
Col#TitleDescriptionFormat# bytes
1 DATA Spectral values; an array of dimension = # channels = # lags. See Jeff's documentation for further info. 4096*4 16384
2 OBJECT Name of source, as input by user. A16 16
3 CRVAL1 Center Frequency, in Hz, of spectrum. D13.5 8
4 CDELT1 Channel separation, in Hz (delta f), hardwired to be = (bandwidth/# lags); this is currently a positive number, but if freq decreases with increasing channel number, it should be negative. D13.5 8
5 CRPIX1 Pixel of center frequency, hardwired to be = (# lags/2); currently, we do not know if this is before or after flipping which would make a difference. D13.5 8
6 CRVAL2 Requested source coordinate 1, either RA or AZ "depending on equinox". Currently for FIXEDAZ, this is the requested azimuth, in degrees. See note under EQUINOX. D13.5 8
7 CRVAL3 Requested source coordinate 2, either Dec or ZA "depending on epoch". Currently for FIXEDAZ, this is the ZA that corresponds to the requested DecJ, in degrees. See note under EQUINOX. D13.5 8
8 EQUINOX Epoch of requested CRVAL2,3. If "0", then CRVAL2 and CRVAL3 are azimuth and zenith angle. As currently implemented, this does not distinguish between current and epoch coordinates in systems other than RA, Decl. and will cause problems for either Az,ZA or l,b users. D13.5 8
9 CRVAL4 A code which designates the type of polarization data, where applicable, (neg -> Pol, pos -> Stokes). See Jeff's documentation. D13.5 8
10CRVAL5 Time at start of data block, in hours since midnight. D13.5 8
11DATE-OBSDate of this observation, in yyyy-mm-dd. D13.5 8
12TSYS Tsys, as last computed; available only if "standard cal" is invoked. D13.5 8
13BANDWID Overall bandwidth of spectrum, in Hz. D13.5 8
14RESTFRQV Rest frequency, in Hz; currently, this is defined as the "rest frequency at band center" but is hardwired to the center frequency input to the IFLO configuration widget for topocentric V = 0 km/s. Thus, it will be be the same as CRVAL1 for such modes, and not 1420.4058 MHz. D13.5 8
15CRVAL1V Requested velocity, in m/s or z (?) D13.5 8
16CDELT1V Velocity, or z, per channel, in m/s or z; supposedly hardwired to be = (CRVAL1V/# lags), so that usage is entirely unclear(?). Furthermore, delta v will not be constant across the band. It is not clear how or when the frequency and velocity implementations of spectral coordinates are applied. Not applicable for 100 MHz BW. D13.5 8
17CRPIX1VCenter channel; supposedly hardwired to be = (#lags/2), but actually is "0". Should be different from CRPIX1 if (# chans) is even, so usage unclear. Currently it is set to a mysterious "4000f" for FIXEDAZ ALFA data. D13.5 8
18CUNIT1Vunits of CRVAL1V; m/s or z; currently: "m/sd" (?) 8A 8
19CTYPE1Vsupposedly "units" of velocity, but appears to refer to velocity definition, e.g. "VOPT-F2V" refers to optical convention for doppler correction. 8A 8
20SPECSYSspecifies frame for velocity, as chosen by user, e.g. "topocent" 8A 8
21MJD-OBSModified Julian date (MJD) at start of data block (= JD-240000.5). D13.5 8
22LST Local mean sidereal time, in hours, at start of data block. D13.5 8
23EXPOSUREIntegration time, in sec, of fundamental data block. D13.5 8
24ENC_AZMITHEncoder azimuth on sky (not dome/feed), in degrees; note that it can be > 360, to distinguish wraps; is it on the sky? Get Az=539 for obj at 180. in north. This is uninterpreted engineering info. D13.5 8
25ENC_ELEVATIOEncoder elevation, in degrees; uninterpreted engineering info. D13.5 8
26ENC_ALTEL Encoder elevation, of the other carriage house in deg; uninterpreted engineering info. D13.5 8
27CROFF2 RA offset applied at req_time, in hours (?) of J2000(?); note says that "offsets include encoder errors and reflect actual telescope position of this beam as well as can be known at the starting time of this scan". Possibly related to map construction as "Offset to commanded map center" noted in header. Unclear if user can control this; if so, should allow for coordinate offsets in other than RA,Dec. D13.5 8
28CROFF3 Dec offset applied at req_time, in deg. See CROFF2. D13.5 8
29OFFC1 Engineering offset 1, in radians. D13.5 8
30OFFC2 Engineering offset 2, in radians. D13.5 8
31OFF_TIMETime of offsets OFFC1 and OFFC2, in seconds from midnight AST D13.5 8
32RATE_C1 Rate of change of offset C1; in header: rad/sec; in FITS file: deg/sec. D13.5 8
33RATE_C2Rate of change of offset C2; in header: rad/sec; in FITS file: deg/sec. D13.5 8
34OFF_CS Coordinate system of offsets C1,C2; currently set to "0". 8A 8
35RATE_CS Coordinate system of rates; currently set to "0". 8A 8
36RATE_DURHow long has rate been applied?, in sec. Note that these rates are valid at the time specified by OFF_TIME and not necessarily at the start of the scan. D13.5 8
37CUR_TOL Computed great circle tolerance, in sec. Currently, this is not interpolated; needs to be fixed. D13.5 8
38REQ_TOL Requested tolerance, in sec, as set by user. D13.5 8
39MODEL_OFFAZPointing model offset Az, in deg, for this beam; read from /share/wappsrc/alfa_pointing.tcl (cannot find this file?) D13.5 8
40MODEL_OFFZAPointing model offset ZA, in deg, for this beam; see above. D13.5 8
41BEAM_OFFAZ Unrotated ALFA beam offset in Az, in deg, of this beam from Beam 0. Is this read from /share/wappsrc/alfa_offsets.tcl? D13.5 8
42BEAM_OFFZA Unrotated ALFA beam offset in ZA, in deg, of this beam from Beam0. D13.5 8
43 USER_OFFAZ User commanded pointing offset in Az, in deg D13.5 8
44 USER_OFFZA User commanded pointing offset in ZA, in deg D13.5 8
45 RFEED_OFFAZ Rotated offset of this beam in Az, in deg; refers to offset along ellipse, given the ALFA rotation angle. Changes with each beam. D13.5 8
46 RFEED_OFFZA Rotated offset of this beam in ZA, in deg; see above. D13.5 8
47 PRFEED_OFFAZOffset from nominal to the beam selected as the "centered" one, in Az, in deg, corresponding to the beam selected in the CIMA pointing widget to which all offsets are referenced. Normally this will be 0 and for Beam 0, these are the negative of RFEED_OFFXX above; "same for all beams of a scan". Not 100% sure of this. D13.5 8
48 PRFEED_OFFZAOffset to "centered" beam, in ZA, in deg; see above. D13.5 8
49 BEAM_OFFRAJ Total RA offset to this beam from "center beam", in deg of J2000 D13.5 8
50 BEAM_OFFDECJ Total Dec offset to this beam from "center beam", in deg of J2000 D13.5 8
51 CRVAL2A True RA pointing of this beam on the sky, in hours. D13.5 8
52 CRVAL3A True Dec. pointing of this beam on the sky, in deg. D13.5 8
53 CRVAL2B True Az pointing of this beam on the sky, in deg. D13.5 8
54 CRVAL3B True ZA pointing of this beam on the sky, in deg. D13.5 8
55 CRVAL2C RA J2000 antenna pointing without beam offset, in deg. Same for all beams. D13.5 8
56 CRVAL3C Dec J2000 antenna pointing without beam offset, in deg. Same for all beams. D13.5 8
57 CRVAL2G True galactic l pointing this beam on the sky, in deg. Computed from CRVAL(2,3)A. D13.5 8
58 CRVAL3G True galactic b pointing this beam on the sky, in deg. See above. D13.5 8
59 CROFF2B True Az offset to commanded map center, in deg. Since this is not "0", we are not sure what it means for FIXEDAZ modes. D13.5 8
60 CROFF3B True ZA offset to commanded map center, in deg. See above. D13.5 8
61 ALFA_ANG ALFA rotation angle D13.5 8
62 PARA_ANG Parallactic angle, in deg D13.5 8
63 OBSMODE Name of pattern: e.g. FIXEDAZ. This is a character string intended to identify the overall type of observation collecting groups of scans into one "observation". A8 8
64 OBS_NAME Name of lowest obs: e.g. FIXEDAZ; other options include ON, OFF, CALON, CALOFF, DRIFT. This is a string intended to identify a particular group of scans within an "observation". A8 8
65 BACKENDMODE Backend mode (e.g., 2-chan, 3-level auto); specifies backend setup. A24 24
66 CALTYPE Type of cal selected (eg., HCORCAL) A8 8
67 FRONTEND Name of receiver (e.g., ALFA) A8 8
68 PLAT_POWER Last power measurement from the platform meter; for engineering purposes. D13.5 8
69 CNTRL_POWER Last power measurement from the control room power meter; ditto D13.5 8
70 TOT_POWER Scaled power in the 0th lag D13.5 8
71 TCAL Tcal MHz/K for (dimension 64?) 0:freq0 1:tcal0 2:freq1 3:tcal1 ... 64D13.5 512
72 SYN1 Platform synthesizer setting, in Hz. D13.5 8
73 SYNFRQ Control room sythesizer settings, in Hz; an array of 4. 4D13.5 32
74 PATTERN_SCAN Scan number of **first** scan at which pattern selected (e.g., if part of a LOOP). Written as YDDDnnnnn; see below. I9 8
75 SCAN_NUMBER Scan number of data block (e.g. within scan within loop), written as YDDDNNNNNN = Year (starting at 2000) + Daynum + number of seconds since midnight at start. I9 8
76 PATTERN_NUMBER Number of current dump within scan - 1 (i.e. the first one is "0"). I8 8
77 TOTAL_PATTERN Total dumps for this scan, as requested I8 8
78 ENC_TIME Time when enc_Az and enc_ZA recorded, in msec (?) I8 8
79 LAGS_IN Number of correlator lags; hardwired to be = # bytes(data)/4.0; declared integer, but then written as D? D13.5 8
80 WAPPMASK Code to specify what other WAPPS or ALFAs enabled. Currently set to "252645135"; note says to check WAPP web page(?) I8 8
81 NTCAL Number of valid Tcal pairs(?); currently "31"? I8 8
82 PRFEED Master ALFA feed for this scan 1B 1
83 NIFS # of IFs in this observation; typically 2 for 2 pols. 1B 1
84 IFVAL Which polarization? 0-1 or 0-3 for Stokes 1B 1
85 ATTN_CORR Correlator attenuator 1-15 1B 1
86 UPPERSB Flag to indicate if spectrum is flipped. Currently "1". 1B 1
87 INPUT_ID WAPP number 0-3 or 0-7 for ALFA 1B  
88 MASTER Which is the master? 0: Gregorian dome; 1: carriage house. OK if "0". 1B 1
89 ONSOURCE Flag to indicate if on-source at ENC_TIME. OK if "1" 1B 1
90 BLANKING Flag to indicate if blanking turned on. 1B 1
91 LBWHYB Flag to indicate if LBandWide Hybrid is in. OK if "0". 1B 1
92 SHCL Flag to indicate if receiver shutter is closed. OK if "0". 1B 1
93 SBSHCL Flag to indicate if S-band receiver shutter closed. OK if "0". 1B 1
94 RFNUM Platform position of the receiver selector 1B 1
95 CALRCVMUX   1B 1
96 ZMNORMAL   1B 1
97 RFATTN Platform attenuator position, array of 2 2B 2
98 IFNUM   1B 1
99 IFATTN   2B 2
100 FIBER   1B 1
101 AC2SW   1B 1
102 PHBSIG   1B 1
103 HYBRID   1B 1
104 PHBLO   1B 1
  XFNORMAL   1B 1
  NOISE   1B 1
  GAIN   2B 2
  INPFRQ   1B 1
  MIXER   4B 4
  VLBAINP   1B 1
  AMPINP   4B 4
  EXTINP   4B 4
  SYNDEST   4B 4
  CALSRC   1B 1
  CAL   1B 1
  VIS30MHz   1B 1
  PWRMET   1B 1
  BLANK430   1B 1
  FILL   4B 4
  ..... ..... ..... .....
         

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This page created and maintained by Martha Haynes and the members of the Cornell ExtraGalactic Group

Last modified:Tue Dec 14 10:07:48 EST 2004