Scan maps taken with the ``Emerson2'' technique

Scan maps taken with the ``Emerson2'' technique, i.e. basket weaving in Fourier space, have to be run through rebin without restoring them from dual to single beam maps and in the same coordinate system that was used for the chop throw (e.g. RJ or PL). You have to make one map for each chop throw and each map has to have identical dimensions and pixel size. We recommend that you make the maps larger than the area mapped and then cut them in size after running them through remdbm, the final reduction stage for ``Emerson2'' scan maps. Since remdbm makes use of fast Fourier transforms, it is advisable to choose map sizes, which are a power of two. Make sure that you do not choose a size equal to the default size for any of the sub-maps, because in that case rebin will choose its own map center, which is not equal to the center pixel of the map. The end result will be a garbled map. To make it easy to identify the map sets we need for remdbm one can give the calibrated, noise weighted and co-added maps names like : m30ra.sdf, m44ra.sdf, m30dec.sdf, m44dec.sdf etc., not because the tasks need it, but it easier for book keeping purposes. These maps also have to be corrected for pointing drifts but they should not be restored. The maps are still dual beam maps and each source in the map should show up as a positive and a negative feature in the image. Fig. shows the first sub-map of RNO1b, i.e. scan rn14_lon_sky, which we used to test calcsky in Section . It has now run through rebin and given a size 384 $\times$ 384 with a pixel size of 1''. Since this map was taken with a 20'' chop in RA, we called it ra20.sdf. We should have made the map slightly bigger, because the maps with 30'' and 65'' chops will cover a larger area. Note that this map was taken at a time when the recommended chop throws were 20'', 30'', and 65'', now the recommended minimum set is 30'', 44'' and 68'', which gives a somewhat better recovery of spatial frequencies.

Figure: An example of the first map in a series of 6 maps taken with the ``Emerson2'' mapping technique. This is a calibrated map with a 20" chop in RA, which is converted to RJ with rebin but still unrestored. We can see the positive and the negative features in the map, and a few noise spikes at the edges of the map.

Once we have all maps pointing corrected and co-added to the same pixel size and dimension, we can run remdbm which converts the maps into our final image. In the example below I take the six sub-maps of RNO1b and convert them into a map called rno1b_lon_reb using remdbm. We specify the name of the final map with the parameter out, and a provide the task with a listing of the files, see below:

% remdbm ra20 ra30 ra65 dec20 dec30 dec65 -out=rno1b_lon_reb
Perl/ADAM messaging is present. Good
Starting monoliths...Done
Loop number 1
Chop: PA=90 THROW=20

Doing forward transformation
....................................................

Loop number 6
Chop: PA=0 THROW=65

Doing forward transformation

There was 1 element changed in the Data array.
Running inverse FFT...
Maximum difference between estimates of the same Fourier component is
0.02118739.

Doing inverse transformation

Result stored in rno1b_lon_reb
ADAM exited

remdbm also accepts wild-cards and we could therefore have listed the files as ra* dec* and it would have picked up the whole set of six maps. Neither do we have to give it chop throw or chop direction. It will extract that information from the FITS-header. The task can restore a single map file, but even two maps in orthogonal directions looks rather ugly, and it is strongly recommended to use a set of 4 or 6 maps.

Due to the way the Fourier transformations are done, remdbm forces the sum of all pixels to be zero. This will introduce a small negative background level in the map. We can remove this baseline by analyzing the map with KAPPA's stats or with GAIA and add back the level we deduce with cadd. For our map we find the negative baseline to be $\sim$ 0.3 Jy/beam, which we add to the map. The final, baseline subtracted map is shown in Fig. .

Figure: Our final ``Emerson2'' scan maps of RNO 1b. The map has a little bit of negative residuals in the scans that went over the strongest part of the map (edge of the map at p.a. $\sim$ 30 and 210 degrees), indicating that the baseline removal was not perfect, but it does not show up with the contrast used for this figure. For a published version of the map, see Sandell and Weintraub [16].