- At 30 and 90 degrees elevation we have 7 rows with a separation
.
In order to obtain a map sampled
in elevation we choose an increment
in elevation of
.
Consider a single scan-line (subscan).
For simplicity let the first row of the array be at elevation offset 0,
we can write down the following table of elevation offsets
for three subscans:

sub- row row row ... row scan n n-1 n-2 ... n-7 1 0 - *D*-2 *D*... -7 *D*2 - ... - 3 ... -

and so on. We see that after 6 subscans we have sampled the elevation offset and thus the final map will be fully sampled in elevation. Thus the minimum map size in elevation is 6 subscans and the array should have more than 6 rows. - At 0 and 60 degrees elevation we have 13 rows with a separation . An increment of in elevation results in a map sampled better than where the gaps are filled in with every other row of pixels. Here we also need 6 subscans or more and more than 11 rows of pixels.

In Figure 2 we show a normal and a fast map of Saturn (Fig. 2a and b). The maps were interpolated onto a Azimuth/Elevation grid. The diagonal elongated structures are caused by the quadripod sub-reflector support legs, these merge into a patchy ring of emission with diameter about . These features agree well with those seen in holography maps, in spite of large disk of Saturn ( by ). The main beam is also well reproduced in the fast maps, comparing Gaussian fits to the main beam in our four Saturn maps showed a total variation of about 2%. This could easily be caused by changes in the opacity and we can only say that we did not detect any deterioration of the main beam due to fast mapping.

Finally in Fig. 2c we show a fast map of OMC1 in Orion. In Azimuth/Elevation we mapped a region of by . At a scanning speed in azimuth of sec each subscan took about 133 seconds. With a spacing in elevation of the whole map consisted only of 29 subscans and took a little over 64 minutes to complete.

One can improve the double beam restoration by combining the data of several fast maps done with different chopper throws by using a improved restoration method as described by Emerson and Payne 1995 [2].

Obviously this method can be used also with a fast scanning total power mode developed by the MPIfR in Bonn. In this case, a four-fold increased scanning speed of per second would allow to cover the region of the OMC1 map in Fig. 2 in 16 Minutes. You could map the full moon in about 1.5 hours!

- [1]
- David Teyssier, Albrecht Sievers 1999, IRAM Newsletter Aug 1999 (http://www.iram.es/Telescope/manuals/Report/Fast_bolo.ps)
- [2]
- Emerson, D.T., Payne,J.M., 1995, Multi-Feed
Systems for Radio-Telescopes, PASP, Vol.
**75**, p. 332