On The Fly Mosaicking (OTFM)

On the fly mapping or on the fly mosaicking (OTF, OTFM) scans are different from other scan modes as the data is actually taken with the telescope moving in a scanning motion between two points on the sky. This move is done in a linear fashion with a constant speed with respect to the sky: the starting and ending coordinates (RA and Dec) are given in the OPT scan, and the telescope moves in the given stripe direction (see below) at a constant speed linearly in RA and linearly in Dec. Due to the sky's natural sidereal motion with respect to the telescope, OTF scans from east to west and west to east in principle require different absolute telescope slewing rates seen by the operator, but the slewing rate on the sky is not affected by the direction of the slew. The scan duration, the number of phase center steps, and the number of visibility dumps per integration determine the interpolation mapping grid and map sensitivity as discussed on the mosaicking page of the Observing Guide. Note that these OTF slews are using the Mercator projection of the coordinates in RA and Dec and therefore might have map deformation implications for OTF close to the (north) celestial pole.

For the remainder of this section it is assumed that the observer has consulted the guidelines given in the mosaicking page, and that the phase center grid and integration time per pointing are known. As OTF modes typically are used to cover a large area of sky in a short time, the default resource integration times are likely to be too long for OTF observations. How to set up an OTF resource is described further below.

To observe in OTF mode, the OPT needs to know both the beginning and end points of the OTF scan in RA and Dec (J2000). The scan from the beginning to the end point is referred to as a "strip" or "stripe". Per strip these two boundary target positions must be defined in the source list (SCT) as two distinct sources. The intermediate phase centers for the mapping between the beginning and end points are determined by the on-line system as described below. These intermediate points are not displayed in the Reports tab of the SB, only the boundary positions. Of course these boundary target points can be entered in the SCT manually, but for any larger (rectangular, hexagonal, etc) grid mosaic, by far the easiest way to define sources in the SCT is to generate the grid of boundary points (e.g. pairs of RA values with each pair having the same Dec) using a text editor and to import this grid with the text import tools given under the Source Lists section of Text Files and Catalogs in the OPT. Some examples are given below; boundary points provided in coordinates other than equatorial will be converted to J2000 RA and Dec internally by the SCT.

Setting Up an OTFM Scan

To utilize the OTFM scan mode, select On The Fly Mosaicking from the drop-down menu under the scan mode section of a scan (Figure 4.23). Selecting this scan mode reveals the required fields to schedule an OTF strip. These fields are not only the beginning and ending position of the strip (as defined above), but also some specific settings for defining the strip. The number of steps and the integrations per step both require user input. The value for integration time is provided from the selected "hardware setup" resource, whereas the stripe duration is equal to the number-of-steps-plus-one multiplied by the number-of-integrations-per-step, then multiplied by the integration time. The plus-one is added to the number of steps to account for the time required for the telescope to reach the appropriate constant speed. (The telescope actually begins its motion from just beyond the boundary point, in order to be in motion at the appropriate scanning speed from the starting point for the OTF scan. The time for this speed build-up results in an extra "step" that will be flagged in the visibility data). Note that the stripe duration is not the same as the scan timing duration. The latter should include at least 10-15 seconds of additional overhead to position the telescopes for the beginning of the strip (e.g., to include turn around and settle time) and thus somewhat depends on the angular distance to slew between the observed strips.

Figure 4.23: Example of an OTFM scan from a VLASS scheduling block.

The on-the-sky slew rate is determined by the strip angle (in arcmin or arcsec) and the stripe duration (in minutes or seconds), where the maximum tested slew rate in S-band currently is of the order of 10 arcmin per second, which is the same as 10 degrees per minute:

slew rate = (Change in angle)/(N*M*Tvis)

As for standard scan modes, the data rate is determined by the resource details. That is, as determined by the amount of correlator capacity used and by the dump time (Tvis). The resource summary in the RCT will show this number when creating the resource, and the reports summary page top table will also summarize it. Note that for the typically short integration times used in OTF, the data rate quickly will approach or exceed the limits set for General Observing Capability modes.

Create an OTF Resource

The main difference between a normal observing scan resource and an OTF resource is that typically the visibility dumps should occur at a faster rate than the potential smearing effects on the data. It is thus advisable to dump at least a few times while the telescopes move a primary beam on the sky, say Tvis(sec) <~ 0.1 x freq(GHz) / slew-rate(arcmin/sec), but this is dependent on the required sensitivity and science goal. Detailed guidance is provided in the mosaicking page of the Observing Guide

The resource can be created as described in the OPT RCT section, with the visibility integration time entered in the first tab. However, care should be taken not to exceed the current data rates (shown in the box in the RCT), which happens easily with demanding correlator setups as well as with shorter integration times.

Current constraints:

• Phase centers (N) cannot be changed faster than at 0.5 second intervals.
• Visibility integration (dump, Tvis) times must be at least 250 ms (which would be the integration time in the RCT).
• Resource data rates may not exceed the limits for (Resident) Shared Risk as stated in the current OSS.
• Limited slew rates in Azimuth (40 deg/min) and Elevation (20 deg/min) will pose a limit on the maximum slew rate in RA and Dec, depending on geometry (and LST) of the observation. It is best to stay on the safe side of these limits.

Other notes:

• Slewing at high elevations should be minimized due to potential tracking and dish deformation issues close to zenith. That is, for OTF targets close to 34 deg Dec, map well before or well after transit, i.e. close to rise or set!
• Times for dumps are in UTC (which is very close to LST for Tvis << 1 min), scan times are in LST.
• Individual antennas may start and stop scanning a strip up to 0.5 seconds late with respect to the rest of the array. This discrepancy will introduce a pointing offset of up to 0.5(sec) x slew-rate for that antenna. To limit imaging problems without flagging such antennas as bad (where one may known which antennas were affected), observers should keep the offset smaller than about 1/4 of the primary beam FWHM, as calculated at the highest frequency in the observing band, when planning for potential slew-rates.
• See mosaicking page for Tvis and other mosaicking hints.

Examples

Suppose we observe in X band and follow the mosaicking web page to get the sampling distances, i.e., we assume an observing frequency of 9.33 GHz and hexagonal packing at sqrt(3) which is almost the same as using the upper part of the band and sqrt(2). The primary beam is thus 4.5 arcmin (270 arcsec) and we use a strip separation of 2.6 arcmin (156 arcsec). Remember that (ΔRA in angle) is (ΔRA in time) multiplied by 15 × cos(Dec), e.g. 4min at 60d = 4min x 15'/min x cos(60d) = 30 arcmin.

The minimalistic SCT input files then may look like this (see text-file input):

1a.  A 20-arcmin length, 4-row (10' wide) rectangular area scanned in RA (constant Dec) centered on (J2000) RA=12:00:00, Dec=00:00:00

row01west ; ; ; ; 11:59:20; +00:03:54; ; ; ; ;
 row01east; ; ; ; 12:00:40; +00:03:54; ; ; ; ;
row02west ; ; ; ; 11:59:20; +00:01:18; ; ; ; ;
 row02east; ; ; ; 12:00:40; +00:01:18; ; ; ; ;
row03west ; ; ; ; 11:59:20; -00:01:18; ; ; ; ;
 row03east; ; ; ; 12:00:40; -00:01:18; ; ; ; ;
row04west ; ; ; ; 11:59:20; -00:03:54; ; ; ; ;
 row04east; ; ; ; 12:00:40; -00:03:54; ; ; ; ;

1b.  A 20-arcmin length, 4-row (10' wide) rectangular area scanned in RA (constant Dec) centered on (J2000) RA=12:00:00, Dec=+60:00:00

row01west ; ; ; ; 11:58:40; +60:03:54; ; ; ; ;
 row01east; ; ; ; 12:01:20; +60:03:54; ; ; ; ;
row02west ; ; ; ; 11:58:40; +60:01:18; ; ; ; ;
 row02east; ; ; ; 12:01:20; +60:01:18; ; ; ; ;
row03west ; ; ; ; 11:58:40; +59:58:42; ; ; ; ;
 row03east; ; ; ; 12:01:20; +59:58:42; ; ; ; ;
row04west ; ; ; ; 11:58:40; +59:56:06; ; ; ; ;
 row04east; ; ; ; 12:01:20; +59:56:06; ; ; ; ;

2.  A 7-point (3-row, ~8' diameter) hexagonal scanned in RA (constant Dec) centered on (J2000) RA=12:00:00, Dec=00:00:00

row01west ; ; ; ; 11:59:49.6; +00:02:36; ; ; ; ;
 row01east; ; ; ; 12:00:10.4; +00:02:36; ; ; ; ;
row02west ; ; ; ; 11:59:39.2; +00:00:00; ; ; ; ;
 row02east; ; ; ; 12:00:20.8; +00:00:00; ; ; ; ;
row03west ; ; ; ; 11:59:49.6; -00:02:36; ; ; ; ;
 row03east; ; ; ; 12:00:10.4; -00:02:36; ; ; ; ;

3.  A 14-point (5-row, ~13' diameter) hexagonal scanned in RA (constant Dec) centered on (J2000) RA=12:00:00, Dec=+60:00:00

row01west ; ; ; ; 11:59:39.2; +60:05:12; ; ; ; ;
 row01east; ; ; ; 12:00:20.8; +60:05:12; ; ; ; ;
row02west ; ; ; ; 11:59:18.4; +60:02:36; ; ; ; ;
 row02east; ; ; ; 12:00:41.6; +60:02:36; ; ; ; ;
row03west ; ; ; ; 11:58:57.6; +60:00:00; ; ; ; ;
 row03east; ; ; ; 12:01:02.4; +60:00:00; ; ; ; ;
row04west ; ; ; ; 11:59:18.4; +59:57:24; ; ; ; ;
 row04east; ; ; ; 12:00:41.6; +59:57:24; ; ; ; ;
row05west ; ; ; ; 11:59:39.2; +59:54:48; ; ; ; ;
 row05east; ; ; ; 12:00:20.8; +59:54:48; ; ; ; ;

Example SB

An example of an SB that can be created with a text editor and imported in the OPT using the third example, may look like this (see explanation of the fields):

# example SB for OTF on W51
#
VERSION; 3;
#
# rectangular OTF in S band, 8 strips, 0.5s 
#
# define catalogs in which Sources and Resources can be found by name
 SRC-CAT; OTF, VLA;
HDWR-CAT; OTF, NRAO Defaults;
# Scheduling Block details
SCHED-BLOCK; OTF_W51_S; Dynamic; 1; 2014-06-07; 16:00:00-21:00:00, ; 25.0; D; 225.0; 35.0; N; N; w=100.0,p=60.0; ; 
#
# startup scans on calibrators
 STD; Dummy; 3C286_S; S16f_otf_0.5; DUR; 1m; cw; N; N; N; N; SetAtnGain,; ;
 STD;3C286;3C286_S; S16f_otf_0.5; DUR; 9m; cw; N; N; N; N; CalFlux, CalGain, CalBP; ;
#
# Slew to calibrator close to mapping area
# slew from calibrator to beginning point of mapping row 1
# first 4 rows of map
 STD;J1925_S; J1925_S; S16f_otf_0.5; DUR; 6m; ; N; N; N; N; CalGain,; ;
 STD;Setup01; row01left; S16f_otf_0.5; DUR;35s; ; N; N; N; N; ObsTgt,; ;
 OTFM; row01; row01left; row01rght; S16f_otf_0.5; DUR; 144s; 32; 8; 0; ; N; N; N; N; ;
 OTFM; row02; row02rght; row02left; S16f_otf_0.5; DUR; 144s; 32; 8; 0; ; N; N; N; N; ;
 OTFM; row03; row03left; row03rght; S16f_otf_0.5; DUR; 144s; 32; 8; 0; ; N; N; N; N; ;
 OTFM; row04; row04rght; row04left; S16f_otf_0.5; DUR; 144s; 32; 8; 0; ; N; N; N; N; ;
#
# Slew to calibrator
# slew from calibrator to beginning point of mapping row 5
# next 4 rows of map
# final calibrator scan
 STD;J1925_S; J1925_S; S16f_otf_0.5; DUR;95s; ; N; N; N; N; CalGain,; ;
 STD;Setup05; row05left; S16f_otf_0.5; DUR;35s; ; N; N; N; N; ObsTgt,; ;
 OTFM; row05; row05left; row05rght; S16f_otf_0.5; DUR; 144s; 32; 8; 0; ; N; N; N; N; ;
 OTFM; row06; row06rght; row06left; S16f_otf_0.5; DUR; 144s; 32; 8; 0; ; N; N; N; N; ;
 OTFM; row07; row07left; row07rght; S16f_otf_0.5; DUR; 144s; 32; 8; 0; ; N; N; N; N; ;
 OTFM; row08; row08rght; row08left; S16f_otf_0.5; DUR; 144s; 32; 8; 0; ; N; N; N; N; ;
 STD;J1925_S; J1925_S; S16f_otf_0.5; DUR;95s; ; N; N; N; N; CalGain,; ;
#---END---