Mapping - MAP

The MAP program collects data on a regular grid by position switching, frequency switching, or total power drift scans in continuum modes. It is activated by typing
START MAP
at the VT100 observer console.

MAP is rather simple in concept. The observer defines a regular grid of observations on the sky, and the MAP program takes the data on this grid by position switching, frequency switching, beam switching or in total power scans. Unfortunately, in order to maintain versatility in the mapping algorithm, a large number of parameters must be specified to define the characteristics of the grid. Thus, users should plan their maps carefully before arrival at the telescope to be sure that their selection of input parameters will yield the map they desire.

The MAP coordinate system is defined by the KMAP parameter. KMAP allows the user to select the AZ-EL, RA-DEC, or L-B coordinate system and to determine which axis is to be scanned most rapidly as the map is constructed. For example, KMAP = 3 yields a map in the AZ-EL coordinate system. The map X coordinate corresponds to the AZ direction and the map Y coordinate corresponds to EL. The map is obtained by scanning the telescope along the map X axis (AZ) to make the rows of the map and stepping the telescope in the map Y direction (EL) between rows.

The coordinate system used by the MAP program attempts to maintain consistent definitions of its X and Y coordinates regardless of whether the map is scanning along the X or Y axis. Thus, in AZ-EL coordinates, X is generally aligned with AZ and Y is generally aligned with EL. However, with the ROTANG parameter, the observer may rotate the map X-Y coordinate system with respect to the sky coordinate system specified by KMAP and change the orientation of the map and the alignment of the axes. All references to X and Y in the program input parameters refer to the MAP coordinate system and depend, therefore, on the value of ROTANG. In all coordinate systems, ROTANG is defined to be the angle, measured counter clockwise, between the Y map axis and the Y sky coordinate. An illustration for the RA-DEC system is given in Figure 4.3 below.

Figure 4.3: Orientation of map X and Y coordinates with respect to RA-DEC system. Note that the definition of ROTANG is consistent with that of astronomical position angle.

The building blocks of maps made with the SEQUOIA receiver are "footprints" of the array on the sky. Footprints are our way of describing the pattern of SEQUOIA observations that is repeated many times to build up a complete map. The simplest footprint is that of a single SEQUOIA observation, which yields the 16 pixel, unfilled pattern of GRID option 1. This pattern is filled in by making additional pointings of the array as in the Figure 4.4.

Figure 4.4: Telescope pointing sequences for GRID=1,3,4, and 5 options.

With RAST=1 in spectral line mode, when the sequence of observations that fill in the footprint are completed, the individual observations are sorted according to their sky position and stored in 32 pixel blocks. Therefore, for a given footprint, multiple scans are generated according to the sampling interval. For example, for GRID=3, 64 pixels distributed in 2 scans are generated. For GRID=5, 256 pixels within 8 scans are produced. Figure 4.5 illustrates the pattern of the pixel storage for the GRID=3 and GRID=5 options.

Figure 4.5: Scan storage sequence for GRID=3 and GRID=5. Upon completion of the footprint sampling, the observations are sorted and stored into $8{\times }4$ pixel scans according to their sky position.

The location of the first footprint to be observed in the map is given by the XOFF and YOFF parameters in MAP. NX and NY give the X and Y dimensions of the map in footprints. In order to tie the footprint scaling used by XOFF and YOFF to the scale of sky coordinates, we provide the fundamental spacing of the array pixels in arcmin and give the array dimensions in pixels. The BX and BY parameters give the pixel spacing in arcmin along the X and Y MAP dimensions, respectively. The DX and DY parameters define the footprint dimensions in pixels along the X and Y MAP directions. In addition, the sign of DX and DY is used to tell the map program how to increment footprints after the first one is observed, with positive numbers increasing the position offset values as the map proceeds and negative numbers causing the offsets to decrease. It is important to note that the BX, BY, DX, and DY parameters refer to the X and Y MAP dimensions and NOT to the X and Y axes of SEQUOIA itself, and in principle, the SEQUOIA array could be commanded to have an arbitrary orientation with respect to the MAP coordinate system. However, to avoid confusion on this point, the MAP program sets the SEQUOIA angle so that the X array axis is aligned with the X axis of the map. With this definition, the DX parameter should always be $\pm$8 and the DY parameter should always be $\pm$8 for maps with 1 footprint spacing.

With reference sharing (RAST=1 in spectral line mode), no data is stored until the sequence requiring NONS telescope pointing is complete. The reference position can be sampled more frequently than NONS. The number of telescope pointings between references is set by the observer with the NBREF keyword.

Table 4.5

Map Keywords

RAST

Defines the mapping technique. When RAST=0, MAP simply goes to the points of the map grid and takes a DPS or FS scan. However, when RAST=1, then the MAP program uses a more sophisticated data collection procedure. In spectral line mode, RAST=1 permits the observer to specify that many ON source positions will share a single reference integration (via the NBRE keyword). In continuum mode, RAST=1 is used to specify a map made up of total power scans across the source (i.e. as raster scanning map).

KMAP

Map coordinate system definition

KMAP	X	Y	Handedness	Map Coords.	Sky Coords
1	AZ	EL	RIGHT	Y	EL
2	RA	DEC	LEFT	Y	DEC
3	AZ	EL	RIGHT	X	AZ
4	RA	DEC	LEFT	X	RA
5	L	B	LEFT	X	L
6	L	B	LEFT	Y	B

ROTANG

Angle between map rows and columns and the coordinate system that is defined by KMAP parameter. ROTANG is given in units of degrees and it is defined to be the angle, measured counterclockwise, between the Y axis of the map and the Y axis of the coordinate system defined by KMAP. The SEQUOIA dewar angle is commanded so that the X axis of the array is aligned with the X axis of the map coordinate system.

GRID

Specifies SEQUOIA footprint type. The basic SEQUOIA footprint is defined to be a 8 X 8 HPBW (5.9'$\times$5.9') region on the sky. The GRID parameter determines how the MAP program will sample this region.

GRID	Footprint Description
1	One pointing of SEQUOIA; Center pixel on 0,0 position
2	RESERVED
3	4 pointings of SEQUOIA to give 1 HPBW sampling
4	8 pointings of SEQUOIA to give hexagonal spacing
5	16 pointings of SEQUOIA to give 1/2 HPBW sampling
6	36 pointings of SEQUOIA to give 1/3 HPBW sampling
7	64 pointings of SEQUOIA to give 1/4 HPBW sampling

BX

Pixel spacing along the X map dimension in arcmin. For SEQUOIA observations BX should be set to 1/2 of the spacing of the beams in the array (i.e. 0.734 arcmin).
Don't change this keyword unless you REALLY know what you are doing

BY

Pixel spacing along the Y map dimension in arcmin. For SEQUOIA observations BY should be set to 1/2 of the spacing of the beams in the array (i.e. 0.734 arcmin).
These values will be slightly modified from season to season according to the statistical errors of the pointing model calculation. If data is taken over several observing seasons, it is prudent to keep BX and BY fixed. If you modify these values, inform the next observer or change back to the original values

XOFF

Offset of the first footprint in the map from the 0,0 position. The units of XOFF are "footprints", whose X dimension in pixels is given by the DX parameter. Since BX gives the pixel dimension in arcmin, the observer may determine the X offset in arcmin by multiplying XOFF*ABS(DX)*BX. Fractional values are legal, though in most cases observers will wish to use integer values in order to maintain proper alignment of map grids.

YOFF

Offset of the first footprint in the map from the 0,0 position. The units of YOFF are "footprints", whose Y dimension in pixels is given by the DY parameter. Since BY gives the pixel dimension in arcmin, the observer may determine the Y offset in arcmin by multiplying YOFF*ABS(DY)*BY. Fractional values are legal, though in most cases observers will wish to use integer values in order to maintain proper alignment of map grids.

DX

Spacing between footprints in the X map dimension in units of the pixel spacing along the X axis (BX). The sign of DX determines whether footprints in the map will increment in the positive (for positive values of DX) or negative (for negative values of DX) X direction. Fractional values are permitted, though for typical SEQUOIA observations, $\vert DX\vert$=8.

DY

Spacing between footprints in the Y map dimension in units of the pixel spacing along the Y axis (BY). The sign of DY determines whether footprints in the map will increment in the positive (for positive values of DY) or negative (for negative values of DY) Y direction. Fractional values are permitted, though for typical SEQUOIA observations, $\vert DY\vert$ = 8.

NX

Number of footprints to be mapped along the X axis of the map. The spacing and direction of the map is determined by the DX parameter.

NY

Number of footprints to be mapped along the Y axis of the map. The spacing and direction of the map is determined by the DY parameter.

NONS

Number of on-source observations to be carried out before storing data in a RAST=1 spectral line map. NONS must be an integer multiple of the number of pointings in a single footprint. NONS is ignored in RAST=0 maps.

NBREF

Number of on-source observations to be carried out between observations of the reference position in a RAST=1 spectral line map. NONS must be an integer multiple of the number of pointings in a single footprint. NONS is ignored in RAST=0

NCALS

Number of telescope pointings between system temperature measurements. For RAST=1 spectral line maps, NCALS must be a multiple of the number of map pointings carried out between reference observations (NONS).

NMAIN

Number of telescope pointings between observations of the map's MAIN position (i.e. no offsets) for calibration. For RAST=1 spectral line maps, NMAIN must be a multiple of the number of map pointings carried out between reference observations (NONS).

SKIP

Number of telescope pointings to skip at the beginning of the map. SKIP is useful for restarting maps which were terminated before completion. For RAST=1 spectral line maps, SKIP must be a multiple of the number of pointings in a single footprint.

TMAIN

Integration time on each map position per on-off cycle (seconds).

TREF

Integration time on REF position per on-off cycle (seconds).

RPT

Number of on-off cycles per scan.

TBLANK

Integration time of detector zero level measurement (seconds).

TSAMPLE

CONTINUUM MODE ONLY. Integration time of a continuum sample during MAIN or REF integration (seconds). TMAIN and TREF must be integer multiples of TSAMPLE.

TSCAN

Integration time per point in a frequency switched map.

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