The IRS instrument offers a powerful form of spectral imaging known as "Spectral Mapping Mode", in which the telescope moves by small steps through a grid of positions, stopping at each step to collect one or more spectral exposures. Large regions can be mapped to relatively faint surface brightnesses, providing 3D spectral cubes covering extended objects. This document offers some suggestions for designing IRS spectral maps from which accurate flux intensities and spectral images of extended sources can be obtained. Sparse spectral maps, which can be considered equivalent to a collection of non-contiguous spectra, are not covered here. For more general information on the IRS and planning IRS observations, see the Observing Planning Cookbook. An example Short-Low nuclear and Long-Low strip map on the nearby galaxy M51 from the SINGS Legacy program is shown at right.
Note that spectral maps are always performed in a coordinate frame parallel and perpendicular to the slit. It is not possible to map at, e.g., 45 degrees to the slit.
A very general rule of thumb, if there are no other constraints, is to construct maps with approximately 1/2 slit length and 1/2 slit width steps.
The recommended step sizes are:
| Step Size | ||
|---|---|---|
| Module | Parallel | Perpendicular |
| Short-Low (SL) | 26" | 1.85" |
| Short-High (SH) | 5" | 2.3" |
| Long-Low (LL) | 79" | 5.1" |
| Long-High (LH) | 10" | 4.5" |
Note that the recommended steps are slightly less than 1/2 the slit dimensions in each direction, to provide redundancy and safely avoid the edges of the slits where flat fielding errors are largest. If your mapping observation requires the use larger steps (at the cost of redundancy), it is important to maintain the factor of two redundancy in the perpendicular direction.
When designing maps, prioritize redundancy where possible. At least a factor of two perpendicular stepping redundancy is essential for the highest quality maps, which is why the maximum recommended step size is 1/2 of the slit width. Although very useful maps can be made without any parallel (along the slit) redundancy (as is done, for instance, in the SINGS Short-Low maps), this type of redundancy can improve the quality of faint object maps considerably.
For slit-parallel steps, the redundancy at each position will not be uniform over the entire map. Some useful formulae for calculating map sizes and areas of redundant coverage are given here. These apply to a single map, specifically for either Short-Low or Long-Low, with "n" parallel steps of size "d" arcseconds, a sub-slit length of "s" arcseconds, and a gap between sub-slits of "g" arcsecs. The dimension of the map in the slit-parallel direction has the following properties:
(n-1) d + 2s + g
(n-1) d - g
(n-3) d - g
The second approach, which takes advantage of the orientation of the slits in the focal plane to construct two or more perpendicular, yet overlapping maps, is usually preferred. An example of this for the Long-Low and Short-Low slits is illustrated above in the case of M51. Although the Short-Low map is small in this case, the general idea is the same. Here, the "Low Long Both" option was chosen with enough perpendicular steps to cover the length of the Short-Low slit. This map produces full Long-Low and Short-Low coverage around the nucleus. A larger Short-Low map (more perpendicular steps) would increase the area covered by both slits. Obviously, since the Short-Low and Long-Low slits are not precisely perpendicular, the maps will not have full overlap at large radii.
For an 1xn Short-Low map, 11 Long-Low (perpendicular) steps of five arsec each will suffice to cover one of the sub-slits. The "Both" mapping method treats low-resolution IRS slits as a single full slit. The center of the full slit is placed at each mapping grid position. Since the central ~25" of each slit is obscured, this method requires enough slit-parallel steps to achieve uniform coverage in both sub-slits, and at least one slit-parallel step to obtain any coverage on the targeted position. This type of map leads to along-the-slit extensions on either side of the main mapped area with coverage in a single sub-slit only. Rather than being wasted, these areas are often very useful for background estimation (see below).
The high-resolution slits are small enough and close enough to square that it is not difficult to create grouped maps in Short-Hight and Long-High which cover roughly the same area. Typically 4 times as many Short-High steps are needed to cover the length of the Long-High slit.
In terms of mapping efficiency, overhead can be a concern when mapping bright sources. Since it takes roughly 6-8secs for Spitzer to offset position, finish the conditioning frames, and settle before another map exposure can begin, using the 6 sec ramp time, unless necessary to avoid saturation, can be very wasteful. For a Short-Low map, with 20% or less additional total AOR time, you can increase the BCD exposure time per position by a factor of 2.3 simply by choosing the 14sec ramp time, instead of the 6sec ramp time. Use Spot to try various combinations and pay close attention to the exposure vs. total observing time estimates.
In order to calculate the required exposure per pointing for sources of
a given surface brightness, observers should use SPECPET and
assume 3x3 detector pixels correspond to a point source, in order to
compute expected S/N for a given flux intensity in MJy/sr.
However, scheduling a tiled group of AORs is complicated by the fact
that all mapping AORs rotate about their geometric center.
Unfortunately, a group of spectral mapping AORs cannot be made to rotate
about a common center. Therefore, even if all AORs are grouped within a
time period which limits their relative slit position angles to a degree
or less, large gaps can open up within the tiled AORs, depending on the
absolute roll angle of the slit at the date of observation. There is no
simple solution to this problem, but timing constraints may help.
One option is to place absolute constraints on the date(s) of
observation, consulting the BIC
to pick a date within an IRS campaign. For coordinate positions for
which the slit rotates quickly (near the ecliptic poles) a one day
absolute constraint is required. For lower ecliptic latitude
observations, this can be relaxed somewhat. Visualize the group of AORs
in Spot, and vary them about the chosen day by 1-3 days to see how tight
of a constraint is required to avoid gaps in the map. Remember that the
slit footprint visualized in Spot is slightly larger than the usable
slit area.
If you are planning a very large
map requiring two or more grouped AORs, you should consult Observer
Support through the Help Desk for more information on scheduling your
observations.
An example large map created with 4 grouped SL mapping AOR, comprised of
over 1200 total individual pointings is shown below. Sample AORs which produce
a square area of uniform coverage if
executed on 5 Jan 2005 can be found here.
The total time to spend on the background depends on the signal quality
requirements of your program. Typically, spending 10% or less of the
total map time on the background is sufficient: this is true for
high-resolution as well as low-resolution maps. Even a single
background frame can radically improve the data quality, but five or
more frames are recommended. In all cases, use the same ramp time for
the background exposures as for the main map. If there is a large
gradient in the background expected at your position, consider using
cluster mode to group backgrounds in an AOR on either side of the
target. Be sure to keep the full slit clear of any source emission at
all permitted rotations.
When using the "Both" method of mapping, in which both
subslits are treated as a single larger slit, to obtain matched coverage
in both sub-slits over a given area of interest, one sub-slit will
extend much further in one direction along the slit, and the other will
extend a similar amount in the opposite direction. Often 1, 2, or even
3 "columns" of a map designed this way can be useful for background
frames. Note that the background for the two spectral orders will come
from opposite regions centered on the mapping target. An example AOR
with marginally usable Short-Low outrigger background frames (purple and
yellow) is shown here:
The outrigger method does not work for high-resolution modules (which
have only a single sub-slit). If in doubt, include a grouped, dedicated
background. This can never hurt, and is typically quite inexpensive for
large maps. If outrigger background data is, in the end, usable, it can
be combined with the dedicated exposures to improve the background
signal even further, or look for nonuniformity in the background across
the map.
In all cases, it is very important to visualize your AOR within SPOT to
make sure you are getting well off your science target with all IRS
slits of interest.
The basic approach is to search for staring mode data (typically) which
were obtained within a few days of the observation, and within a small
range of ecliptic latitude of the mapped target, using the same
instrument module and (ideally) the same ramp exposure time. Because of
the time-varying nature of the warm pixels in the Long-Low and Long-High
modules, background observations taken very nearby in time to the
science data are best. This method should be considered as a last
resort for dealing with poorly designed maps, and we strongly recommend
obtaining your own background data as part of your IRS mapping AOR via
one or both of the methods outlined above.
Very Large Maps
The maximum IRS AOR duration is six hours. Mapping large regions (e.g.,
larger than 30 square arcmin) typically requires more mapping steps than
this AOR limit will allow. To achieve large maps in this case, two or
more mapping AORs can be tiled to cover a large region. The IRS slits
can rotate up to 1 degree per day, and since Spitzer cannot roll, you
are forced to live with this rotation. For a 6hr map, the differential
roll is small enough that no gaps open up in a properly designed map.
Backgrounds
The best scientific results are obtained from IRS spectra when a proper
background is subtracted at the BCD-level from each position in the map.
The background exposures should be taken as close in time and position
to the primary target as is possible. Not only does this remove the
astrophysical foreground emission (zodiacal and cirrus), it eliminates
many systematic artifacts which exist at or even above the level of the
background in unsubtracted frames, and mitigates the effects of
time-varying warm pixels (an especially significant factor in the LL and
LH modules). While staring mode observations of point sources with the
Short-Low and Long-Low slits have a
built in background observation, since each science target is placed at
two nod positions for every AOR, you must explicitly obtain your own
background exposures for all mapping mode observations. High-res staring
observations also use two nods per slit, but the nods are very close together
and there is significant PSF overlap.
Care must be
taken to avoid the outer regions of extended science targets when
selecting regions for the background. The best method for deciding
which type of background to use or where to place background
observations is overlaying AORs directly on an available Spitzer IRAC or
MIPS image within Spot.
Dedicated Backgrounds
The recommended way of obtaining
background frames is to create an additional dedicated background AOR
which maps adjacent dark sky. Either mapping or staring mode can
be used for this. A typical format might be a 1x2 map with 2 cycles at
each position. Remember that, for the low-resolution modules,
background is obtained in both subslits (e.g. SL1 and SL2) at once.
Outrigger Backgrounds
For sources which are small enough, and which will be mapped
sequentially with both orders of a low-resolution module, you can use
the non-primary, or "outrigger" order to obtain an
accurate background spectrum without spending any time on dedicated
observations. Since, while, e.g. data is being collected in SL order 1,
SL order 2 is also integrating 70" away, this is often enough separation
to provide clean background data. Assuming both orders of the module
are being used to map the primary target, often no additional
observations will be required.
Archive Backgrounds
When all else fails, e.g. for working with older spectral maps from the
archive which offer no in situ backgrounds, you may be able to
obtain useful background data directly from the Spitzer archive. Often
this means waiting up to a year after the data of interest were first
placed in the archive, so that data from the same instrument campaign
are freely available.
Data Reduction and Flux Calibration
The CUBISM
tool, which is a SINGS Legacy team deliverable and will be made available and supported through the SSC in Spring 2006, is
designed to reconstruct and extract full spectral cubes from
well-designed spectral mapping data sets. It relies on several
extended-source specific calibrations and corrections provided by the SSC.