1. Molecular Imager: Focal Plane Array
Glen Langston
Motivation: Molecular discoveries
Focal Plane Arrays important for all observing bands
First 7 pixel Array for 18 to 26 GHz
NRAO
Glen Langston: KFPA CDR: Jan 2009

2. Molecular Discoveries: 100m
World’s most sensitive Telescope in frequency range 10 to 50 GHz
Discovered >7 new large interstellar molecules, including propenal and propanal
Widely distributed, not exclusively in compact cores
Hollis, Snyder, Remijan and Jewell
Langston, Turner 2007
Glen Langston: KFPA CDR: Jan 2009

3. Molecular Line Surveys
Science goals require wide instantaneous bandwidths
Prime targets have many widely spaced narrow lines.
Widely separated (Delta nu > 1 GHz)
Glen Langston: KFPA CDR: Jan 2009

4. 1st Pixel Tests Completed
September 11, 2008 – Single Pixel Tests on GBT
October 6, 2008 – IDM1 IF Spur studies – Measure versus feed position
October 12, 2008 – Comparison of Current GBT K-band (K0) and Single Pixel (K1)
December 28, 2008 – IDM 2 – Check of System temperature and compare K0 with K1
Glen Langston: KFPA CDR: Jan 2009

5. First Astronomical Tests
Peak and Focus results good.
Obs revealed IF spurs, that are now understood.
Extensive tests of interfaces.
Web based monitor
CLEO /Astrid scripts worked for test obs.
Obs on Sept 11, 2008
Glen Langston: KFPA CDR: Jan 2009

6. 1st Pixel Tests: Frequency Range
Orion A GHz
H and He Recombination Lines
Water Line and molecules
Negative features are IF spurs
Initial tests showed IF spur problems (now fixed)
Glen Langston: KFPA CDR: Jan 2009

7. Glen Langston: KFPA CDR: Jan 2009
Gain and Tsys Tests
Performed intensity measurements as a function of pixel offset from center location.
Gain versus Position Offset 3.8’ in Azimuth and Elevation directions (LCP and RCP)
Frequency Offset-Azimuth Offset-Elevation
(GHz) Average Average
/- 2.5 % /- 2.5 %
/- 4.4 % /- 4.5 %
Performed a sequence of obs with K0 Feed system followed by K1 sequence.
Found higher system temperatures
Cause: Low gain and IF spurs
Obs Oct 6, 2008
Glen Langston: KFPA CDR: Jan 2009

8. Glen Langston: KFPA CDR: Jan 2009
Spectral Line Tests
Configured for TMC
NH3 1-1 and 2-2 Lines, plus HC7N
Baseline Performance Good, similar to K0
Glen Langston: KFPA CDR: Jan 2009

9. Baseline Stability Check
Comparison of twenty five 90 second observations
Obs. Sig-Ref/Ref calibrated with same average reference scan
No significant baseline structure changes.
No baseline fit subtracted, only single average value plus an offset to separate spectra
Glen Langston: KFPA CDR: Jan 2009

10. 1st Pixel Tests: Line Mapping
Taurus Molecular Cloud
HC7N
NH3
Simultaneous Maps of several lines
Note High GBT Sensitivity: Only 2 seconds per pixel!
Glen Langston: KFPA CDR: Jan 2009

11. Glen Langston: KFPA CDR: Jan 2009
RMS noise Versus Time
RMS decreases roughly as expected with time.
Plot shows measured RMS in spectra and sqrt(T) model.
Common Reference may add to systematic noise.
Glitch may be due to weather or
Processing (?)
Glen Langston: KFPA CDR: Jan 2009

12. Glen Langston: KFPA CDR: Jan 2009
K0/K1 Tsys Comparison
K0/K1 Comparisons at 19, 21, 23 and 25 GHz show comparable performance
Weather Variations limit comparison accuracy
All obs show increasing Tsys with time.
23 GHz results typical
Obs on Dec. 28, 08
Glen Langston: KFPA CDR: Jan 2009

13. Efficiency Estimate at 23 GHz
Efficiency estimated by comparison of 3C123 peak intensities (K) for K0 and K1.
Two K0 measurements and one K1 Measurements at 23 GHz.
Weather dominates uncertainty (and cal values)
K1/K0 Efficiency
LCP RCP
==== ===
99% %
95% %
96% %
88% %
Ratio of 3C123 Peak
Heights
Glen Langston: KFPA CDR: Jan 2009

14. Mapping Tests: Multiple Beams
HC7N
HC5N
Software for Mapping Tests using Current GBT Rx: Two Beam mapping works, must write software for 7 beams.
Glen Langston: KFPA CDR: Jan 2009

15. Glen Langston: KFPA CDR: Jan 2009
IDM 2 Tests
2nd IDM showed no IF spurs
Tsys performance similar to K0
Glen Langston: KFPA CDR: Jan 2009

16. Summary: Planned Tests Accomplished
First On Telescope tests showed good basic performance, and identified engineering tasks
Tests of gain offsets versus feed position were consistent with predictions.
Second IDM seems free of IF Spurs
Baseline stability seems good
Comparison of K0/K1 is good (Weather limits accuracy)
RMS decreasing with integration time. Further observations needed for firm conclusions
Wide frequency (18 to 26 GHz) observations are critical for study of important large molecules and Chemistry.
Glen Langston: KFPA CDR: Jan 2009