2011 February GBT KFPA Call for Proposals

The K-band Focal Plan Array Receiver (KFPA) was successfully re-commissioned in August and September 2010. Results of these tests are encouraging and we invite GBT KFPA proposals.

All beams and polarizations are performing well. During the commissioning tests, system performance looked good, with receiver temperatures for the different beams between 13.6 and 27.4 K at 23.5 GHz. A significant improvement in System Temperature, Tsys, was achieved by replacing a lossy type of Gortex radome with a low attenuation type. A reasonable system temperature estimate for high elevation and good weather is 40 to 45 Kelvin.

Observing modes

The KFPA observing modes were previously tested in April 2010. All of these observing modes remain valid and are described in 2010 proposal call.

The KFPA operates in the same modes as current 2 beam K-band, except the widest separation in frequency of spectral windows (spectral bands) is 1800 MHz. The spectral line and mapping modes include:
  • 7 Beams, one spectral window, 2 polarizations, each with 12.5 or 50 MHz bandwidth ( 7+1 Mode )
    • Plus one additional (different) 12.5 or 50 MHz spectral window for one beam, 2 polarizations, within 1.8 GHz of the first spectral window.
  • 4 beams, two spectral windows, 2 polarizations, each with 12.5 or 50 MHz bandwidth ( 4x2 Mode )
  • For 7+1 or 4x2 modes, below are the number of channels and spectral resolutions
    • 9-level sampling: 4096 channels in 12.5 MHz bandwidth, ie. 3.05 kHz channel spacing
    • 3-level sampling: 16384 channels in 12.5 MHz bandwidth, ie 0.76 kHz channel spacing
    • 9-level sampling: 4096 channels in 50.0 MHz bandwidth, ie 12.2 kHz channel spacing ( Usual NH3 Mapping Mode)
    • 3-level sampling: 16384 channels in 50.0 MHz bandwidth, ie 3.05 kHz channel spacing
    • See the GBT proposer's guide for a comparison of sensitivity for 3 and 9 level sampling
  • 4 beams, 1 spectral window, 2 polarizations, 200 or 800 MHz bandwidth
  • 4 beams, 2 spectral windows, 1 polarization, 200 or 800 MHz bandwidth
  • 7 Beams, 1 spectral window, 1 polarization, 200 or 800 MHz bandwidth
    • Previous three modes, with 200 MHz bandwidth: 8192 channels, ie 24.41 kHz channel spacing.
    • Previous three modes, with 800 MHz bandwidth: 2048 channels, ie 390.62 kHz channel spacing.
  • 2 Beams, 4 spectral windows, 2 polarizations, 12.5 or 50 MHz
    • Same spectral resolution options as for 4x2 Mode
  • 2 beams, 2 spectral windows, 2 polarizations, 200 or 800 MHz bandwidth
  • 1 beam, 4 spectral windows, 2 polarizations, 12.5, 50, 200 or 800 Mhz.

w51-NH3-1-1.png w51-NH3-2-2.png
Spectral line image of W51 in the NH3 1-1 Line at 23694 MHz, from the 4 beams, 200 MHz bandwidth, one spectral bands mode observation, with 20 minute total mapping duration.

This map was made scanning along lines of constant right ascension.

Spectral line image of W51 in the NH3 2-2 Line, made simultaneously with the 1-1 observation.

Images in two modes: 7 Beams (+1) and 4 Beams, Two Spectral bands

  • 7 Beam, dual polarization, 50 MHz bandwidth, 4096 channel spectral line mode. Additionally 1 beam was configured for observations at an additional 50 MHz dual polarization, 4096 channel spectral line band. Ie NH3 1-1 and 2-2 lines for all 7 beams, plus NH34-4 lines using beam 1.
  • 4 Beam, dual polarization, dual 50 MHz bandwidth, 4096 channel spectral line mode. Ie HC7N J=16-15 (18048 MHz) and HC5N J=7-6 (18639 MHz) images produced simultaneously.

At the rest frequency of the NH3 1-1 line (23694 MHz), the channel resolution, 12200 Hz, corresponds to a velocity resolution of 0.15 km/sec. In both of these modes, we have tested a spectral integration time of 1 second per set of all beams, polarizations and spectral bands. We anticipate that for frequency switched observations, which generate twice as much data as position switched observations, the minimum integration time is 2 seconds.

orion1-122.png Spectrum of Orion A in NH3 1-1 and 2-2 lines,
produced from a 10 minute Daisy pattern map of the region (Scan 20 of Project TKFPA_16).
The left image is 2-2 line, the center image is the continuum image map
and right image is 1-1 emission.
The continuum image shows the image region, 20' in diameter, was not finely
enough sampled in this 10 minute observation. A 20 minute daisy scan could have fully sampled the central region of the image.

Noise reduction versus Integration Time.

We have checked the RMS noise level continues to decrease as the expected function of integration time. Maser observations were accumulated and the resulting RMS noise level in a single channel continued to decrease as expected over the allotted time, as is shown in the figure below. This suggests that the KFPA may be used instead of the current GBT K-band receiver for observations requiring long integrations.

kfpaNoiseVsTime.png tRmsVsTimeKfpa.png
Spectral Noise as a function of averaging individual spectral 5-second integrations for a Maser search observation. The measured values are diamonds, and the theoretical curve is a dashed line. Spectral Noise as a function of averaging pairs of 150-second Nod scans, in the same Maser search observation. The channel width is 24.4 kHz. The diamonds are measurements and the dashed line is the theoretical curve.

Performance of individual Beams of 7 Beam Receiver

The KFPA beams are configured in a hexagon shaped pattern, with number 1 the central beam. The receiver refrigerator placed to the side of the hexigon, and beams 2, 3 and 4 are closest to the refrigerator, and are coldest. Beams 1,2,3 and 4 are used in the four beam, dual frequency band mapping mode, as these have the lowest receiver temperature. The beam spacing is 94.88" and the maximum beam separation is twice this, 189.66".

Note that the KFPA can be placed in one of four orientations (see detailed description) , and if one of the other orientations is critical to the proposed science case, this could be requested during the call for proposals.
KFPA Orientation C, used for Commissioning tests.

Beam Polar- Trs Trx Tsys
Num. ization Specification Measured Measured
  (Left/Right) (K) (K) (K)
1 L 25.0 13.6 33.5
1 R 25.0 16.1 30.9
2 L 25.0 13.9 39.0
2 R 25.0 16.4 37.2
3 L 25.0 20.9 36.0
3 R 25.0 18.4 36.2
4 L 25.0 13.6 36.0
4 R 25.0 16.7 36.4
5 L 25.0 17.8 36.9
5 R 25.0 16.8 34.7
6 L 25.0 26.6 38.7
6 R 25.0 26.9 39.2
7 L 25.0 27.4 37.5
7 R 25.0 26.1 35.2

Table of specified and measured receiver temperatures for each of the 7 beams and polarizations. A list of measured system temperatures is given for an observation at 65 degrees elevation. The values are taken at 23.5 GHz, near the NH3, 1-1 and 2-2 lines. The differences in receiver temperatures are due to differences in the thermal connections between the amplifiers and the refrigerator. Beam 4 is close to the refrigerator, and has the coldest amplifiers.

KFPA Receiver temperatures (Trx) versus frequency for L polarization for all beams (TOP) and for R polarization for all beams (BOTTOM). kfpaTRx10Aug14.png

More information about the KFPA is available on the Project WIKI page.

Please contact Glen Langston (glangsto@nrao.edu, 304-456-2224) if you have questions about the KFPA configurations.

-- GlenLangston - 2010-09-13
Topic attachments
I Attachment Action Size Date Who Comment
kfpaNoiseVsTime.pngpng kfpaNoiseVsTime.png manage 16 K 2010-09-08 - 10:59 GlenLangston Test of Noise reduction as a function of integration time from Maser search observation
kfpaSfo-38.pngpng kfpaSfo-38.png manage 43 K 2010-09-10 - 10:06 GlenLangston Daisy pattern image
kfpaTRx10Aug14.pngpng kfpaTRx10Aug14.png manage 85 K 2010-09-10 - 08:27 GlenLangston Recommissioned Rx Temperatures
orion1-1-2-2.pngpng orion1-1-2-2.png manage 85 K 2010-09-13 - 07:47 GlenLangston Orion A
tRmsVsTimeKfpa.pngpng tRmsVsTimeKfpa.png manage 17 K 2010-09-20 - 16:39 GlenLangston Longer Time average
w51-NH3-1-1.pngpng w51-NH3-1-1.png manage 47 K 2010-09-12 - 23:31 GlenLangston W51 NH3 1-1 line
w51-NH3-2-2.pngpng w51-NH3-2-2.png manage 47 K 2010-09-12 - 23:31 GlenLangston W51 NH3 2-2 line
Topic revision: r10 - 2016-06-08, PatrickMurphy
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