The recommended raster step/line size settings (see below) have been optimised only for zero degree map orientation in instrument coordinates. In case of applying sky reference frame then optimal spatial sampling cannot be guaranteed because the PACS footprint rotation with respect to the raster line orientation depends on the position angle of the detectors footprint (determined by the day of the observation). The sky reference frame can be selected only in unchopped grating scan mode. Please note, in unchopped grating scan mode the HSpot default option is 'sky' reference, but we highly advise to switch to 'instrument' mode except for cases described in Section 6.1.5.2.

Note

Time constraints as well as the chopper avoidance angle may be used to put restrictions on the map orientation angle if the map is defined in 'instrument' reference frame. The time constraint can indirectly limit the array position angle in the optimal range, while the chopper avoidance can be used to avoid a certain chopping direction with respect to the target position (see details in the HSpot Users Manual). Map orientation direction restrictions are possible only for targets at high ecliptic latitude, here the spacecraft orbit may result different position angles at different observing epochs.

In unchopped grating scan mode, if an AOR raster covers an elongated area (e.g. a nearby edge-on galaxy) then the observer might have no other option than using sky reference frame and turn the raster to the right direction. If the target is at higher ecliptic latitude then you may select instrument reference frame and put a time constraint on the AOR. The time window can be identified in HSpot "Overlays/AORs on images..." option by changing the tentative epoch of observation. This way the array can be rotated to the desired angle by the time dependent array position angle. Please consult the HSpot Users Manual for a detailed description how to set time constraints and what consequences apply.

If the map is defined in sky coordinates, the map "orientation angle" in the "Observing Mode Settings" HSpot panel can be used to rotate the raster map, this angle is counted from the celestial north to the raster line direction counterclockwise.

If the map is defined in sky coordinates and the observer wants to cover a contiguous area in the sky, under any position angle he or she shall not define a step size larger than 34 arcsec, i.e. the size of the array (47") divided by √2.

Chopped rasters cannot be rotated with a specific orientation angle, the chop direction is hard-coded in instrument reference frame with zero angle orientation (i.e. the chop direction is perpendicular to a raster line). If the target is at higher ecliptic latitude then put a time constraint on the AOR. This way the array and the whole raster can be rotated to the desired angle by a time dependent array position angle.

For raster maps with stepsize greater than 30" (i.e tiling the sky rather than oversampled rasters) there are no particular recommendations for step sizes. Typical step sizes are 47" (no overlap between the different raster positions) and 38" (approximately one row or column of spatial pixels overlap between the different raster positions).

For extended objects, mapping with oversampling, i.e. with step size smaller than one spaxel, may be very time consuming. Therefore this mapping strategy is suggested with step sizes larger than one spaxel, but such that the beam is Nyquist sampled. Taking into account that what is defined as raster point step in HSpot corresponds to the spacecraft Z axis, and the line step to the Y axis, the recommended raster step sizes (see ???) for Nyquist sampled maps of larger areas translate to the following HSpot settings:

Figure 6.2.  Spatial sampling by all PACS spaxels when using a 5x5 raster with step size 14.5"/16" for the blue (left) and a 3x3 raster with step size 22"/24” for the red (right)

In order to map the sky at full PACS spatial resolution, step sizes smaller than a spaxel have to be used. Since this increases the observing time, this mode is strongly suggested to be used only for mapping point-like or almost point-like objects. In order to recover the best PACS resolution we recommend the following minimum number of raster positions and maximum step sizes:

In this default version of the unchopped grating scan mode, the line is scanned with the same grating step as in chopped line spectroscopy, i.e., every spectral pixel samples at least every 1/3 of a resolution element. The number of grating steps is fixed to 75 irrespective the spectral band in which the line is observed. Other grating scan parameters and block duration are shown in Table 6.2.

A key element in the unchopped mode is to make this technique robust against instantaneous responsivity changes caused by cosmic ray hits in the Ge:Ga detector pixels. For this purpose, the up/down grating scan have been made much faster than in the chopped mode, and every line repetition requested by the observer is doubled internally by PACS. On every grating position, four integration ramps are taken. Each integration lasts 1/8 s, resulting in a grating scan which is four times faster than the one in chop mode.

The sensitivity of an observation performed in the unchopped line-scan mode is quite similar to that obtained in chop-nod when the time spent on source is taken into account. This conclusion is based on actual observations of several galaxies in which bright lines were observed both in chop-nod and in unchopped mode. Both the line shape and the strength of the line (when differences in the calibration procedures are taken into account) are very similar. Figure 6.8 shows the observation of a line in the red spectrometer observed in both chop-nod and unchopped mode. Except for differences in the continuum level, the line is well reproduced in both cases. Note that in this example both unchopped observations over-estimate the continuum by 30% compared with the chopped case, and are also different by 10% between two separate spectra taken in separate observations. This continuum uncertainty for faint targets has been discussed previously, observers should not use this mode to measure the continuum.

Figure 6.8.  A comparison of same faint red emission line source observed with unchopped mode and a chop/nod observation. The turquoise and blue spectra are the two unchopped spectrum taken in different observations and show a variation between each other in the continuum of the order of 10%. The Green spectrum shows the same source observed in chop-nod mode where the continuum is much more accurately measured, and is lower than the unchopped mode by approximately 30%. Note that the line itself is well reproduced in both flux and line shape. The systematically larger amplitude of the line-signal seen in the unchopped mode compared with Chop/Nod (roughly 10%) is a known artifact of the way the line is calibrated in this example, using methods designed for chop-nod calibration. Future calibration tables designed for the unchopped Mode will remove this scaling difference.

The example of an observing block sequence of the unchopped grating scan mode is sketched in Figure 6.9. In the on-source block, the observer defined line repetitions are internally multiplied by two, i.e. a single repetition is made of two fast scans. The on-source integration time in this mode is half the time PACS spends on-source in the chopped mode (including both nod positions).

In pointed mode, following an on-source block the spacecraft slews to an off-position within two degrees radius with respect to the target coordinates. The off-position has to be carefully selected for an efficient subtraction of the telescope background. Ideally, the background sky emission towards the target and towards the off-position is at the same level respectively, and the background field is free of confusing structures (i.e. the 25 spaxels see a similar level of sky emission).

The off-position is always executed with a single repetition, irrespective what the observer has defined for line repetition factor in the PACS Line Editor. This means, the duration and signal-to-noise in the off-position blocks will be lower than the on-target block, if line repetition factor has been set larger than one.

Note

The repetition cycle has to be used to increase the exposure, the entire on-off block is repeated as is in the chopped mode. For instance, in two cycles the spacecraft follows an [ON-OFF-OFF-ON] pointing sequence between the two positions. For deep observations, if necessary, the off-position duration could be kept identical to the on-source duration by specifying single line repetitions and higher number of observing cycles. However, this parameterization is inefficient in terms of slewing overheads.

	Note
	While the scheme in the block diagram Figure 6.9 can be executed, it may turn more advisable to rather do this line by line, particularly for rasters with not too frequent off positions.

	Note
	A shorter integration time in the OFF-position results in a RMS noise of the reference continuum higher than that of the source continuum. To avoid to increase the RMS noise when subtracting the OFF-position, the OFF-position data can be binned using larger wavelength intervals. In case of flat continuum, a single value can be computed for the reference continuum.

Figure 6.9.  Instrument observing blocks are shown for a typical unchopped grating scan measurement combining two lines with single repetition in B3A and a third line with two repetitions in band R1 (3rd + 1st orders setting). Line repetitions adjust the relative depth required for spectral lines, while the number of observing cycles is used to repeat the entire on-off cycle. In this example, the off-position is only 6' away from the target what results only short on-off slewing overhead.

In raster mapping mode the observing block sequence is similar to the chopped mode but here the observer could define the number of raster positions to visit before moving to the off-position.

	Note
	In mapping mode, the observing cycle is repeating the entire raster. At least one OFF-position has to be specified for a single cycle, i.e. the number of raster positions to visit before moving to the OFF-position has to be equal or lower than the number of raster positions ([num. raster lines] x [num. of raster points])

In raster mode, PACS executes an obligatory OFF-position before the first science block starts at the first raster position. For instance, in case of a 2x2 raster, with a single cycle and with an off position request after 2 raster positions, the following spatial pattern will be executed: [OFF-ON-ON-OFF-ON-ON-OFF]. The same raster with two cycles: [OFF-ON-ON-OFF-ON-ON-OFF-ON-ON-OFF-ON-ON-OFF].

This bright-line option has been made available in HSpot version 5.3.1 and later for science cases where very high signal-to-noise (S/N > 100) line detections are expected applying a single line repetition factor. For such bright targets, the integration time can be kept shorter than as is in the default faint-line mode, even on the expense of compromised observing performance.

In this mode, grating scan parameters and observing logic remain unchanged, except the number of grating steps visited by a single scan. Instead of applying the default 75 steps, a bright line scan is done through 50 steps, resulting a factor 1/3 reduction in integration time. The shorter duration scan is therefore achieved via a shorter wavelength coverage, the scan speed (number of integration ramps per grating position) has not been changed. Due to instrument and spacecraft overheads the 33% gain in integration time does not apply to the total duration of a bright line AOR. Typically, a bright line AOR is shorter by 25-30% than its faint-line homologue. We show in Figure 6.10 the spectral coverage of the bright-line and standard unchopped mode.

Figure 6.10. The unchopped bright line mode (left, 50 grating steps) covers a narrower wavelength range than the standard faint-line implementation (right, 75 grating steps). Colours represent the coverage of individual 16 spectral pixels in a module while the grating scans through a spectral line.

	Note
	The shorter wavelength coverage on both sides of the line profile may result a higher uncertainty in the continuum level reconstruction. HSpot sensitivities therefore report 30% increase in S/N if the bright-line mode is used. You can see a comparison of bright- and faint-line mode observations on Figure 6.11.

Because the bright line mode is faster than the standard unchopped mode (and because we recommend only using the mode with 1 repetition), then automatically the “off position” will be observed quite soon after the target is observed. This may be a small advantage compared with the standard unchopped mode since, in principal, detector transients may be better handled. At the time releasing this document do not have sufficient data to determine if this is a significant gain.

It should be noted that the unchoppped bright-line mode does not sample the spectrum as uniformly, nor as comprehensively as the standard unchopped mode (50 instead of 75 grating steps). Thus there is much less redundancy in the measured sampling of the edges of the band in the bright-line case. This has two effects. Firstly, the number of points that can be binned in a given spectral resolution element is less at the spectrum edges—leading to a larger rms away from the center of the observed spectrum. Secondly, systematic uncertainties in the RSRF, which are partly averaged-out when many spectral pixels contribute to a given resolution element, are subject to systematic effects at the edge of the band. This can lead—in some cases, to slopes in the baseline of the spectrum near the spectrum edges. Such slopes are much less noticeable in the standard unchopped mode. However, if the line is strong enough these effects will be relatively insignificant when compared to the strength of the line—even in the bright line mode. However if the line is too faint, this could become a serious problem and should be avoided.

Figure 6.11.  A comparison between standard unchopped (faint) and bright line mode for on and off observations. The upper figures show the on and off observations (blue and green) and the lower figures show the difference spectra. The green line shows the spectral coverage frequency, which is a measure of the number of samples averaged in each wavelength bin. Notice how the broader coverage of the faint line mode produces a flatter baseline than the bright-line mode.

The choice of “off” frequency is a user-defined quantity and will depend on the science goals. If you are primarily interested in line properties then obtaining an “off” at a convenient point in your observational sequence roughly every 30-60 minutes would be ideal.

For unchopped line scan the mode is set up so that the observer gets an “off” at the end of the observing sequence. To ensure that the “off” is observed every 30-60 minutes, it is desirable to arrange that the main observing block does not last more than 30-60 minutes. So for example, for a single “pointed” observation, since a single line takes rough 10 minutes to execute, three to six separate lines could be observed within the AOR. At the end of this period an “off” observation will be executed. If a small map is executed, the user should balance the efficiency of the map making with the need for a frequent “off”. It may be more efficient to allow the map to execute to the end of a small map before going to the “off”. This would be achieved by setting the “Repeat off position after nth rater position” variable an appropriate value. To give an example. If a single line is observed on a 2 x 2 raster, setting the “repeat off position after nth raster” to 4 will ensure that the “off” is taken at the end of the sequence resulting in an AOR of duration 23 minutes—which would be ideal. If two lines were observed for the same map, the same parameters would lead to an off being taken at the end of the sequence, which in this case would be after approximately 40 minutes—which is still acceptable. Observations with the “off” taken up to 2 hrs from the “on” still seem to produce results that show only a small degradation in S/N ratio, although we recommend keeping the interval as short as possible. In this mode “off” positions of up to 2 degrees from the target may be specified.

	Note
	The user can specify an “off” position, either by offset or by RA and Dec specification. Note that by default these offsets are set to zero and it is important that the user specify a non-zero value away from the target. The specification of an “off position” is provided in a special part of the HSpot window for both the “Pointed” and “Mapping” component of the “Set Observing Mode” option.