Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.naic.edu/alfa/ealfa/meeting1/minutes/optics.html
Дата изменения: Mon May 8 23:01:36 2006
Дата индексирования: Sun Dec 23 01:56:30 2007
Кодировка:
Поисковые слова: п п п п п п п п п п п п п п п п п п п п п п п

ALFA Front End Optics

Next: Front End Construction Up: Minutes of the 1st Previous: Minutes of the 1st

Minutes of the talk on the:
ALFA Front End Optics

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

A more detailed version of this talk by the speaker is available in PDF format. See also the documents available at the NAIC ALFA specs web site.

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 $\pm 100^\circ$

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. ( $T_{scatt} \sim 4$ K, $T_{rec}\sim 8$ K).
Calculated G/T $_{sys}$ - 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 $\phi$ .
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

The ideal is to sample evenly the direction perpendicular to the direction of the scan. It is possible to rotate the feed by some angle Phi so that the beams will move. One could then figure out out how they appear to move along the scan direction and what the spacings will be between the projected beams. The footprint is noticeably elliptical, so that there is a real need to optimize spacing. For this non-uniform beam, the ideal angle Phi is 22 degrees, compared to 19 degrees for uniform beams.

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 T_sys 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 T_sys 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 T_sys 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 T_sys?
German	See Fig 1.9: T $_{\rm scatt}=4$ K T $_{\rm rec}=8$ K T $_{\rm sky}=5$ K. 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.

Next: Front End Construction Up: Minutes of the 1st Previous: Minutes of the 1st

This page created and maintained by Karen Masters, Kristine Spekkens and Martha Haynes.

Last modified: Wed Oct 15 10:59:31 AST 2003

Minutes of the talk on the: ALFA Front End Optics

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