2. Types of scans

2.1 Scan types and antenna motion

Scanning, if completely generalised, is defined by specified rates of motion in two orthogonal axes which may have no particular relation to the natural axes of the telescope, i.e. Equatorial Hour Angle and Declination.

However it may be useful to categorise scans in order of increasing complexity in relation to the natural axes of the telescope, as subtleties of algorithm design may be involved.

1. passive (drift) scan, telescope stationary
   1. park telescope at an earlier RA than the target, Earth rotation produces the scan
2. active scan at fixed rates in the natural telescope coordinates (HA, Dec)
   1. scan in HA at siderial rate (track RA), park in Dec
   2. scan in HA at non-siderial rate, park in Dec
   3. scan in HA at siderial rate (track RA), scan in Dec
   4. park in HA, scan in Dec
   5. scan in HA at non-siderial rate, scan in Dec
3. active scan at fixed rates in coordinate systems rotated from the natural telescope coordinates, producing variable speeds in the latter
   1. Galactic
   2. Ecliptic
   3. Altazimuth
   4. other Longitude + Latitude

The outputs of the instruments attached to the receiver are recorded during the scan, and the data are internally calibrated at some point in the scan or set of scans by firing a noise diode and recording the change in signal level.

All continuum radiometry comprises the recording of instrument outputs during one or more of the above scans.

It is of interest to see what sort of scans have been predefined for the GBT:

Advanced GBT procedure writing in Glish, at http://www.gb.nrao.edu/ rfisher/Glish/gbt_procedures.html#built_ins

2.2 SCANTYPES implying multiple scans

Two standard SCANTYPE definitions produce multiple scans that are treated as an uninterruptible observation, normally with a single calibration:

• STEP is defined to consist of a series of (track RA, park in Dec) scans, at the step positions defined in the next section.
• SCANPNT is defined to be two orthogonal scans centred on the specified coordinates, first being (scan in HA, park in Dec), the second (track RA, scan in DEC).

2.3 Step positions

Stepping consists of tracking in RA at positions stepped around a beam pattern at some subset of the following positions:

• second null in RA (SNE, SNW) or Dec (SNN, SNS)
• first null in RA (FNE, FNW) or Dec (FNN, FNS)
• half power points in RA (HPE, HPW) or Dec (HPN, HPS)
• beam centre (ON)
• zero-crossing point of dual beam receivers (ZC)

Suffixes A or B denote the beam in use for dual beam systems, e.g. SNEA. Suffix CAL denotes that the calibration noise diode is fired during the track, e.g. SNEACAL, producing a double length track with the noise diode on during the first half.

Stepping therefore comprises a set of a particular type of active scans carried out in a specific sequence as one, uninterruptible unit.

2.4 Mapping options

Maps are made from repeated scans. Large maps are made from scans treated as single entities that are calibrated and recorded individually.

However there remains a need for small maps comprising a set of scans treated as a single entity, calibrated once, and observed without interruption. two specialized examples are Sun and Moon maps. See "NCCS Requirements and Specifications", section 6.1.5. SCANTYPE= STEP and SCANPNT are two undersampled examples already described. Fully sampled examples could be defined to include the SCANTYPEs:

• SMALLMAP, a map of a small area such as the inner sidelobes of the beam, centred on a strong, compact radio continuum source.
• SUNMAP, a map of a small area centred on the Sun
• MOONMAP, a map of a small area centred on the Moon

There are two ways that uninterruptible small maps can be handled:

1. By generating in advance an input file comprising all the scans required, as single entities, and commanding the scheduler to treat the set as uninterruptible;
2. By defining in an input file the minimum set of parameters needed to execute the set of scans and passing these to the observing program to generate internally the required scans.

The first option is identical to making a large map, each individual scan being treated separately, except that all the scans are done without interruption.

The second option is identical to the method used to do STEP and SCANPNT observations, and is how it was done in all pre-NCCS mapping.

Clearly, the relative advantages of the two methods need careful assessment.