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RE: L-Band at Ceduna

From: <David.Jauncey_at_email.protected>
Date: Wed, 20 Jun 2007 23:03:15 +1000

Dear Brett and Aidan,

Bloody well done; this is an excellent start!

However, a couple of comments: firstly, I am concerned about the high
level of "side-lobes" apparent in both scans. In the top scan the start
and end of the scan shows up to ~40% displacement from the baseline
level on both sides. In the second scan, this drops to almost 20% and
appears where you might expect to see the first side-lobes.

These displacements don't appear to be due to bad baselines, i.e. 1/f
noise, as the edges of the second scan repeat very closely at the
beginning and end. So it would appear that the displacements are most
likely real.

I suggest that, as part of your tests that you try mapping the L-Band
beam in Az and El in single dish mode. Perhaps making the scans longer
to begin with and then similar long scans half of the HPBW off from the
centre on either side of the main peak, and so on so as to get long
scans, of at least 12 or so HPBWs, so as to cover the main beam out to
half a dozen HPBWs either side in both co-ordinates.

If these are side-lobes, then two second side-lobes of ~50% would
account for your high apparent Tsys in Jy. So if you get the geometry
right, i.e. focus and alignment, and a bigger tertiary dish then you
should be in excellent shape. Jim's suggestion would then give you every
reason to quit the Naysmith focus and go for the tertiary focus for most
frequencies. However, I do suggest that, if this proves to be the case,
then please make sure that the receivers and noise diodes are properly
thermally stabilised so as to avoid the current "difficulties".

Cheers and again, great stuff,

Dave

-----Original Message-----
From: owner-vlbiobs_at_atnf.<!--nospam-->csiro.au [mailto:owner-vlbiobs_at_atnf.<!--nospam-->csiro.au]
On Behalf Of Brett Reid
Sent: Monday, 18 June 2007 3:33 AM
To: vlbiobs_at_atnf.<!--nospam-->csiro.au
Subject: L-Band at Ceduna

Hi,
Aidan and I have installed a test reflector (1.5m offset satellite dish)

at the vertex of the 30m dish and an L band receiver from Hobart. We
aligned the dish optically so that the ray from the center of the feed
goes to the center of the offset dish and on to the center of the
subreflector. We checked the alignment at 12 GHz using the Optus D1
satellite and the small K band feed used for satellite TV. We were
pleased with a beam width and alignment demonstrated with this test.We
then changed over to the L-band feed. The first scans were pleasing with

a system temperature obtained of around 1000 Jy. No further alignment
has been done. We know that a larger dish will collect more of the
signal. The vertex tube is 2m internal diameter and our experimental
1.5m reflector does not fill this.

The 2 channel plot that Aidan performed is attached. The 2 channels are
linears. The frequency is 1400 MHz, The bandwidth is 64MHz. We intend to

try linear to circular conversion through the quadrature hybrid in the
morning once we have wound a test helix.

Both the L-band feed and extra reflector should be easily removed for
other frequency use.

Pictures are to come.

Regards,
Brett and Aidan
-- 
Brett Reid
Observatory Manager, Radio Telescopes
University of Tasmania
School of Mathematics and Physics
phone + 61 3 6248 5285
email brett.reid_at_utas.<!--nospam-->edu.au
web http://www-ra.phys.utas.edu.au/~breid/
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Received on 2007-06-20 23:03:38