An alternative is to simply add signal from synthetic astronomical sources to real time-series. This feature was relatively easy to add and can be quite versatile. The user simply specifies an external 2-dimensional map using the new configuration parameter fakemap to makemap, containing the simulated sources. There are several things to be aware of when using this facility:
- the map of fake sources must have the same pixel dimensions as the output map
- the units are the same as the output map from makemap (normally pW if working from raw data)
- a scale factor can be applied to the map using the fakescale configuration parameter
Response to a point source
In this example we create a beam map using a scan of Uranus (s4a20091214_00015), and add it to the time-series for a scan of a blank field (s4a20100313_00029) to establish the effect of map-making and matched-filtering on point sources. Both of these data sets are public (See SC/19 and this web page).
First, maps of the blank field and Uranus are both reduced:
makemap s4a20100313_00029\*.sdf blank450 method=iterate config=^$STARLINK_DIR/share/smurf/dimmconfig_blank_field.lis
makemap s4a20091214_00015\*.sdf uranus_450 method=iterate config=^$STARLINK_DIR/share/smurf/dimmconfig_bright_compact.lis
Then we cut the central region out of the Uranus map and copy it into a map (using KAPPA:NDFCOPY) with the same pixel dimensions as the blank450.sdf so that we can use it to provide fake signal:
ndfcopy 'uranus_450(3~31,1~31)' fakeblank450 like=blank450
We create a modified dimmconfig that instructs makemap to add in this additional source of signal, but applying a scale factor such that the amplitude of Uranus is 0.01 pW:
^/stardev/share/smurf/dimmconfig_blank_field.lis
fakemap = fakeblank450
fakescale = 0.0471865
We run makemap using this new config:
makemap s4a20100313_00029\*.sdf blank450_fake method=iterate config=^dimmconfig.lis
Finally, we produce match-filtered maps for both the original map, and the map with the fake source added in:
picard SCUBA2_MATCHED_FILTER blank450
picard SCUBA2_MATCHED_FILTER blank450_fake
The following image shows all four maps. The left column contains the raw output of makemap for the original map, and the map with the source added in. The right column shows the corresponding match-filtered images.
The input source had a known amplitude of 0.01 pW. In the makemap output, however, we can see that the filtering during map-making has reduced the amplitude by about 12% to 0.0088. The peak in the match-filtered image, however, is not attenuated quite as severely, at 0.0096. In addition to the peak response, this test also clearly shows the effective PSF for the map: there are faint negative streaks along the scan directions caused by the high-pass filtering. This example suggests that upward corrections to the FCF of order 5 to 10% should be applied to maps of points sources using dimmconfig_blank_field.lis (although this factor should be established on a case-by-case basis).
Response to extended structures
It is known that maps produced with SMURF do not have useful information on scales larger than roughly the array footprint due to the common-mode rejection, and high-pass filtering. Using the fakemap facility, we can now add sources with complicated structures to SCUBA-2 data to see this effect quantitatively.
In this example we use an 850um scan of Orion (s8d20100216_00022), and to it we add some diffuse structure from a publicly available map of Vela at 350um from the Balloon-borne Large-Aperture Submillimeter Telescope:
Similar to the previous example, we first reduce the map of Orion (using dimmconfig_bright_extended.lis),
and then we reduce it again using the BLAST image as a fakemap (positioning it and padding it to the correct size using KAPPA:NDFCOPY) and scaled to a similar signal range as the actual structure in Orion:
Comparing the upper-left region of this map where the Vela data were added with the original BLAST data, it is clear that all of the compact structures are faithfully identified, but the extended structure is not. Since we know what those underlying structures are, we can simply difference the input and output maps to see what we are missing in the SMURF reduction:
For reference, the upper-centre green blob is about 3 arcmin across, of order the array footprint. The smaller-scale dark features at the right are simply an artifact of the real Orion 850um emission that extends into the patch of the map where we inserted the Vela data!
2 comments:
That's great work. I've been doing the exact same thing with HiGAL data and Bolocam (some of that work is in an accepted but not quite published paper). If you come up with any interesting ways to quantify the sensitivity to larger scale structure or the effective filter function, I'm definitely intrigued.
It's worth emphasizing that the fake map must have exactly the same pixel dimensions as the map produced by makemap.
The consequence of this is that a new fake map must be created for every data set given separately to makemap. In the case of the pipeline, that's every observation,
which is a bit tedious for big projects. However, it wouldn't be too difficult to write some code for the pipeline (and/or PICARD) which can automatically create appropriate fake maps.
A couple of other things that might be worth noting:
* WCSSLIDE can be used to shift the fake map so that it doesn't lie on top of the source (if using, say, existing SCUBA data). Do this before the NDFCOPY step.
* While the fake map doesn't have to have the same pixel scale as the SCUBA-2 map, it helps if you're trying to assess how structure on a given angular scale is affected by the map-making process. WCSALIGN with a pixel-spreading scheme other than nearest-neighbour does the trick. Again do this before NDFCOPY.
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