A single data dump occuring every D seconds consists of R spectra where R = (B beams times P polarizations) + 2 = 16. ALFA records 7 beams and 2 polarizations. The reason for the extra 2 is that the WAPPS have 8 inputs; we might use the last two someday for something else. Currently, the last two are junk.
A single dumped spectrum contains N spectral points, in our case 4096.
Each of the R spectra is dumped to a separate row of the binary table, so that each dump occupies 16 rows.
A single dumped spectrum of N spectral points is written to column 1 of the table as N elements of 4 byte real (7 significant digits).
The remaining columns 2-NNN occupy NNN bytes. See below for current column assignments. For higher precision, many of the columns are written as 8 byte real (15 significant digits).
A single row therefore has a width of (N times 4) bytes plus NNN bytes.
Thus a single data dump contains R rows of (4 N + NNNNNNN) bytes.
| Col# | Title | Description | Format | # 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 |
| 10 | CRVAL5 | Time at start of data block, in hours since midnight. | D13.5 | 8 |
| 11 | DATE-OBS | Date of this observation, in yyyy-mm-dd. | D13.5 | 8 |
| 12 | TSYS | Tsys, as last computed; available only if "standard cal" is invoked. | D13.5 | 8 |
| 13 | BANDWID | Overall bandwidth of spectrum, in Hz. | D13.5 | 8 |
| 14 | RESTFRQV | 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 |
| 15 | CRVAL1V | Requested velocity, in m/s or z (?) | D13.5 | 8 |
| 16 | CDELT1V | 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 |
| 17 | CRPIX1V | Center 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 |
| 18 | CUNIT1V | units of CRVAL1V; m/s or z; currently: "m/sd" (?) | 8A | 8 |
| 19 | CTYPE1V | supposedly "units" of velocity, but appears to refer to velocity definition, e.g. "VOPT-F2V" refers to optical convention for doppler correction. | 8A | 8 |
| 20 | SPECSYS | specifies frame for velocity, as chosen by user, e.g. "topocent" | 8A | 8 |
| 21 | MJD-OBS | Modified Julian date (MJD) at start of data block (= JD-240000.5). | D13.5 | 8 |
| 22 | LST | Local mean sidereal time, in hours, at start of data block. | D13.5 | 8 |
| 23 | EXPOSURE | Integration time, in sec, of fundamental data block. | D13.5 | 8 |
| 24 | ENC_AZMITH | Encoder 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 |
| 25 | ENC_ELEVATIO | Encoder elevation, in degrees; uninterpreted engineering info. | D13.5 | 8 |
| 26 | ENC_ALTEL | Encoder elevation, of the other carriage house in deg; uninterpreted engineering info. | D13.5 | 8 |
| 27 | CROFF2 | 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 |
| 28 | CROFF3 | Dec offset applied at req_time, in deg. See CROFF2. | D13.5 | 8 |
| 29 | OFFC1 | Engineering offset 1, in radians. | D13.5 | 8 |
| 30 | OFFC2 | Engineering offset 2, in radians. | D13.5 | 8 |
| 31 | OFF_TIME | Time of offsets OFFC1 and OFFC2, in seconds from midnight AST | D13.5 | 8 |
| 32 | RATE_C1 | Rate of change of offset C1; in header: rad/sec; in FITS file: deg/sec. | D13.5 | 8 |
| 33 | RATE_C2 | Rate of change of offset C2; in header: rad/sec; in FITS file: deg/sec. | D13.5 | 8 |
| 34 | OFF_CS | Coordinate system of offsets C1,C2; currently set to "0". | 8A | 8 |
| 35 | RATE_CS | Coordinate system of rates; currently set to "0". | 8A | 8 |
| 36 | RATE_DUR | How 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 |
| 37 | CUR_TOL | Computed great circle tolerance, in sec. Currently, this is not interpolated; needs to be fixed. | D13.5 | 8 |
| 38 | REQ_TOL | Requested tolerance, in sec, as set by user. | D13.5 | 8 |
| 39 | MODEL_OFFAZ | Pointing model offset Az, in deg, for this beam; read from /share/wappsrc/alfa_pointing.tcl (cannot find this file?) | D13.5 | 8 |
| 40 | MODEL_OFFZA | Pointing model offset ZA, in deg, for this beam; see above. | D13.5 | 8 |
| 41 | BEAM_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 |
| 42 | BEAM_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_OFFAZ | Offset 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_OFFZA | Offset 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 | ||
| ..... | ..... | ..... | ..... | |