A dysh glossary

See also the GBO Glossary

In this glossary we also note overloaded terms.

Argus

A 16-pixel W-band (74-116 GHz) focal plane array in use at the GBT. Named after a mythical figure with 100 eyes. See also https://www.gb.nrao.edu/argus/

band

A contiguous section of the radio spectrum. In radio astronomy bands are often referred to by an alphabetical designation, e.g. L-band covers 1-2 GHz. A summary of the bands commonly used at GBO can be found on https://gbtdocs.readthedocs.io/en/latest/references/receivers.html

bank

A VEGAS bank is a single ROACH-2 board with 2 inputs, typically for two polarizations of one IF. A VEGAS bank can produce between one and eight spectral windows, which are identified by an ifnum.

baseline

Baseline is a generic term usually taken to mean the instrumental plus continuum bandpass shape in an observed spectrum, or changes in the background level in a continuum observation.

beam

The footprint of one receiver horn on the sky.

See also fdnum, multi-beam, and horn.

beam switching

This is a variation on position switching using a receiver with multiple beams. The signal and reference positions are chosen so that one of the feeds is always pointing at the source. Using this method requires that the source be smaller than the on-sky separation between feeds. Nodding and subreflector beam nodding are forms of beam switching.

See also position switching.

BINTABLE

Binary table. In dysh, bintable is an index running from 0 to N-1, where N is the number of binary tables in the SDFITS file.

See also FITS.

blanking

Blanking is the process of replacing data values with a blank. For SDFITS files, the blanking value is Not-a-Number (NaN). For example VEGAS will blank data while it is switching states (Kepley et al. 2015). Not to be confused with the concepts flagging and masking in dysh.

CAL

Column in SDFITS files indicating if the noise diode was being fired at a particular time.

caloff

Signal with no calibration diode in the signal path (CAL="F").

calon

Signal with a calibration diode in the signal path (CAL="T").

Chebyshev

A type of orthogonal polynomial that is commonly used in numerical methods due to its optimal convergence properties and connection to the Fourier transform. One of the options in baseline fitting in dysh.

CoG

Curve of Growth: integrating the flux from the center of a line outwards.

See also Curve of Growth for the dysh implementation.

DYSH_DATA

(optional) environment variable pointing to a directory with local copies of SDFITS data for developers.

See also SDFITS_DATA.

ECSV

Enhanced Character Separated Values: a self-describing ASCII table format popularized by astropy. See also astropy/astropy-APEs

fdnum

Feed number. An integer, starting at 0, used to identify different beams. Also used as the fdnum keyword in the getXX() routines. For example, to select the first beam one would use fdnum=0.

See also beam.

FITS

Flexible Image Transport System: the export format for data-cubes and images.

flag files

SDFITS files created by GBTIDL can have a separate ASCII flag file. By default, GBTFITSLoad reads this file and applies the flags therein.

flagging

Flagging is a non-destructive operation, where data in the time-frequency domain is flagged to be skipped, masked or blanked.

Flagging specific to the VEGAS backend, which has bad channels also known as ‘spurs’ at regular channel intervals. VEGAS flagging is done automatically by GBTFITSLoad.

The data are flagged by GBTFITSLoad (or the user). Masking is the application of flags using apply_flags.

See also masking and blanking.

frequency switching

This is an observing technique in which a local oscillator is alternated to switch the IF into signal and reference states.

FWHM

Full Width at Half Max. A measure of the width of a curve. It reports the width of the curve at its half power point. It is commonly used to describe the angular resolution of a telescope (also referred to as half power beam width, HPBW, in this case), or the width of a spectral line.

The FITS keywords BMAJ, BMIN, and BPA are used for the major axis, minor axis, and position angle respectively when referring to a spatial beam.

GBTIDL

Green Bank Telescope Interactive Data Language. The GBT data reduction package written in IDL for analyzing GBT spectral line data.

getXX()

Generic name for any of the dysh calibration routines, e.g. getps, getfs, getnod etc.

horn

Another term used for beam or pixel.

IDL

The Interactive Data Language program, currently of ITT Visual Information Solutions but with a long history of owners.

IF

Intermediate Frequency, is a frequency to which a carrier wave is shifted as an intermediate step in transmission or reception. The term window is often used as well, where it means the contiguous range of frequencies being recorded by the spectrometer (e.g., VEGAS).

See also ifnum.

ifnum

IF number (0,1,…). These are used to identify spectral windows. Also used as the ifnum keyword in getXX(). For example, to select the first spectral window one would use ifnum=0.

See also band and window.

intnum

Integration number, starting at 0. Also used as the intnum keyword in the getXX() routines. For example, to select the first integration one would use intnum=0.

KFPA

K-band Focal Plane Array, a hexagonal set of beams, with a central beam. Covers 18-26 GHz. See the KFPA receiver page for more details.

masking

Masking hides the value in the spectrum. As in numpy, as mask value of True means the underlying value is not used. In dysh masks are set on individual integrations during calibration [getXX()]; resultant spectra will have the final mask set in Spectrum.mask.

See also flagging.

metadata

Describes data. Examples for a spectrum are the right ascencion (RA) and declination (DEC) associated with the spectrum. Typically GBT spectra have 70 items in the metadata, implemented as columns in the BINTABLE and accessed via keyword in GBTFITSLoad, e.g., sdf[“object”].

dysh spectra have metadata in Spectrum.meta and Scans in Scan.meta.

multi-beam

If an instrument has multiple beams that typically point to different sky locations (e.g. Argus in a 4x4 configuration, and KFPA in a 7 beam hexagonal shape).

nod

An observing mode where two beams alternatingly look at source and (different) sky.

See also position switching and beam switching.

noise diode

A device with known effective temperature (see tcal) that is coupled to the receiver to give a measure of system temperature (TSYS). When the telescope is pointed on blank sky, the noise diode can be set toalternate between on and off states to determine the system temperature. This device is also refered to as the “Cal”.

See also calon and caloff.

ON/OFF

In the context of position switching, the on-source (target or signal) and off-source (reference) positions. In the context of the noise diode, it being on (firing) or off (not firing). The ON/OFF references are an overloaded term for when we refer to the SIG and REF respectively.

OTF mapping

On-the-fly mapping: in this procedure the telescope is scanned across the sky to sample the emission. The samples are “gridded” on to a map using the tool gbtgridder. The gridding is not implemented in dysh.

pixel

An overloaded term. Sometimes referred to as the beam, but usually interpreted in image processing as the size of a single (usually square) element in a gridded map (e.g. from an OTF), which is commonly referred to as a picture element.

plnum

Polarization number (0,1,…). An integer used to identify the polarization. Usually 0 and 1, but of course up to 4 values could be present for full Stokes observations. Averaging two orthogonal polarizations should reduce the noise by \(\sqrt{2}\)

Also used as the plnum keyword in getXX(). For example, one would use plnum=0 to select the first polarization.

position switching

This is a standard way to obtain spectra by switching between a SIG and REF position on the sky, usually using a single beam.

See also beam switching and Single Dish Math

project code

A code designating the year and proposal number, e.g. GBT21B-024. Data associated with a project are found in /home/sdfits (or $SDFITS_DATA), with a slight twist of the name. In the example this becomes AGBT21B_024.

See also Data : Project Code / Session ID

REF

Reference point, meant to have no signal.

See also Single Dish Math.

region

Region or regions of spectrum, used for flagging/masking, or baseline subtraction.

scan

A unit of observing, usually in some common mode, with one or more integrations. Scans are referred to as 1-based integers. The observing procedures (e.g., OnOff or Track) are commonly referred to as scans (e.g., an OnOff scan or a Track scan). A scan can contain multiple integrations.

ScanBlock

A container for a series of scans.

See also ScanBlock for more details.

SDFITS

Single Dish FITS format, normally used to store raw or even calibrated spectra in a FITS binary table (BINTABLE) format. Each row in a BINTABLE has an attached RA,DEC (and other meta-data), plus the whole spectrum. This standard was drafted in 1995 (Liszt), and has been implemented by many telescopes (Arecibo, FAST, GBT, Parkes, ….), albeit with slightly different conventions. An SDFITS file can have more than one BINTABLE extension.

See also GBT SDFITS Files.

SDFITS_DATA

(optional) environment variable pointing to a directory where SDFITS project directories and files are stored.

session

Or Session ID. This is the number (starting at index 01) denoting the observing sessions within a given project. It is concatenated with the project code to define a unique observing session, for example AGBT21B_024_01.

See also Data : Project Code / Session ID.

SIG

Signal. In the context of position switching the ON or target position in an ON/OFF observation. In the context of frequency switching, the signal state has SIG=”T” and the reference state SIG=”F”.

See also Single Dish Math.

spectral window

Subdivision of a band or bank to a set of linearly spaced channels in frequency space. For GBT data a spectral window is also refered to as an IF.

See also ifnum.

spectrum

A coherent section in frequency space, with its own unique meta-data (such as polarization, right ascencion, declination, and time). Normally the smallest portion of data we can assign. A spectrum is defined by its own seting of (crval, crpix, cdelt) in a FITS WCS sense.

subbeamnod

Subreflector beam nodding. In this observing method the subreflector is used to alternate the signal and reference positions between one or more beams.

See also beam switching.

TCAL

Equivalent temperature of the noise diode. Usually given in K. This is a function of frequency and time. It is stored as a single value in SDFITS as the TCAL column. This single value is the average of the noise diode frequency-resolved temperature measurements over the corresponding IF.

VEGAS

Versatile GBT Astronomical Spectrometer - https://www.gb.nrao.edu/vegas/

VEGAS spurs

Artifacts introduced by VEGAS to the data. These artifacts manifest as bad channels at known locations. See https://gbtdocs.readthedocs.io/en/latest/references/backends/vegas.html#the-spurs for more details.

waterfall plot

A plot (or two-dimensional image) that shows time vs. frequency.

window

See spectral window.

Data : Project Code / Session ID

Generally projects are assigned a project code, e.g. AGBT21B-024, which is then observed in a number of sessions, numbered starting with 01. The SDFITS data associated with these are stored under $SDFITS_DATA, e.g. for session 5 in this example, this would be in $SDFITS_DATA/AGBT21B_024_05/.

Possible confusion: a project code “GBT21B-024”, is labeled “AGBT21B_024” as the filename prefix for file storage, which is the name that users need for dysh.