Minutes of the talk on the:
      ALFA Front End Optics

German Cortes (NAIC; on telecon from Ithaca) and
Avinash (Desh) Deshphande (NAIC)

ALFA specifications:

• Bandwidth 1.225 to 1.525 GHz
• 7 pixels
• Return Loss > 20 dB
• Linear, dual polarization
• Polarization isolation: > 20 dB
• Edge Illumination (central freq) > 12 dB
• A few dB variation in Gain across band
• Horn aperture diameter is 250 mm, separation 260 mm (practically touching)
• Dewar can rotate by

A layout scheme figure was shown. Also Gregorian focal plane scanning losses. The asymmetries between feeds could be overcome by shifting feeds by a small offset (2.7 cm) in the X-direction.

Antenna Performance:

Now assume the 25cm x 26cm, 2.7cm X-offset configuration

• Central pixel is characterized by a gain of 11 k/Jy; the others should have 8-9 k/Jy.
• Calculated system temperature: 25-26K at 1525 MHz, 29-30K at 1375 MHz and 34-25K at 1225 MHz. ( K, K).
• Calculated G/T - again, center pixel better

Far Field Characteristics:

• Coma lobe quite strong for feeds offset from center.
• Multi Beam Sky Footprint: Separation  2 beam widths between individual beams
• Far Field Characteristics for each feed: 3.5 to 4 arcmins beam width, vary f or different pixels
• Sky Footprint Pixel for 1.375 GHz; shows sky area 50'x50', each beam shows a strong coma lobe that varies in intensity with .
• Total Incoherence Multi Beam Pattern 6-6.5 dB in various beams
• Calculated Beam Comatic Level about 8 dB

Polarization Characteristics: Pol Probes Orientation in OMT. The calculated Cross Polarization is better than 20 dB in most horns across the frequency range.

Drift Scan Sampling

Discussion/Questions

Lister S-S	The system temp seems disappointingly high especially at high frequency; can you explain why it is so high?
German	The answer is spillover. This type of horn has a lot of spillover and it is worse at lower frequencies than at higher ones.
Lister	Will the tertiary skirt help?
German	The tertiary skirt will help more at the lower frequency. Its details are currently being designed; the limitation is its weight right now. There are still some problems with design. The timescale is a few months for definite design.
Bob B.	Can you estimate how much Tsys will go down?
German	Hopefully, 5K across band. Hopefully down to about 30K at 1225 MHz; 25K or better at 1525 MHz.
Riccardo G.	How will the skirt affect the different beams?
German	Slide 1.9 shows calculated Tsys with 2 points for each frequency; one is for the center beam and other is for the outside. About 2K difference at outer freq, 1K difference in center and same at high freq.
Bob B.	Is Tsys measured with OMT? What else adds?
Ganesh	Yes it is measured with OMT. The rest is spillover, and power scattered into main beam from platform. The tertiary skirt will help some.
Steve S.	Back to the overall beam map. It says -8.5dB for outer sidelobes. Is that relative to the center beam or relative to their own beam.
German	See Fig: 1.24, which is the calculated beam comatic level. Again, there are 2 points wrt respective main beams. Clearly the center pixel is much nicer. In Fig 1.23: Everything is relative to the central pixel.
Riccardo	What are individual contributions to the Tsys?
German	See Fig 1.9: TK TK TK. The rest is spillover inside dome.
Bob B.	That suggests that the tertiary skirt is really important.
Lister	When calculation of beam is done, are the beams in isolation or is cross coupling included?
German	In isolation. Coupling -50 to -60 dB down in terms of noise. The amplifiers in general only go in one direction (amplification) but in terms of noise, there is an equivalent temperature looking out of the amplifier. The energy coming out is in general very low, so I didn't include that.