Diff for "Iram30mEfficiencies"

Telescope efficiencies and beam widths

Update on telescope efficiencies and errorbeam parameters

The following table is taken from the report on "Improvement of the IRAM 30m telescope pattern" of 26-August 2013 which is available here. It gives the strengths (in percent) of the main beam, the three errorbeams, and the forward efficiency, as a function of observing frequency.

• Freq	Beff	P1'	P2'	P3'	SumP	Feff	eta_fss
  86	81	0	7	6	12	95	2
  115	78	1	8	6	14	94	2
  145	74	2	9	6	17	93	2
  210	63	4	11	9	24	94	6
  230	59	4	11	11	25	92	8
  280	49	3	11	15	29	87	9
  340	35	2	11	14	28	81	19
  345	34	2	11	14	28	80	18

See the report for explanations on the errorbeam parameters P1', P2', P3', SumP. The half power beamwidths (HPBW) of the main beam are described to a good accuracy by HPBW(arcsec) = 2460/Freq(GHz). For the aperture efficiencies and point source sensitivities see previous observations listed below.

Parameters describing the beam of the 30m telescope: HPBW, Feff, Beff, Aeff, point source sensitivity, Errorbeams

1. Half power beam width HPBW. The observed HPBWs can be well fitted by HPBW/arcsec=2460/Freq/GHz or HPBW/rad=1.166 Lambda/D, with the wavelength Lambda and the telescope diameter D.

2. Forward efficiency Feff. The values for Feff were updated after the 12th of December 2000 when a new reflecting ring was put around the secondary mirror. Forward efficiencies are derived from skydips.

3. Main beam efficiency Beff. Beff is the ratio of main beam solid angle over the entire antenna pattern solid angle. It is best derived from a source which has a diameter comparable to the size of the main beam. It can be calculated from the peak antenna temperature TA*, the HPBW, the source diameter, and source brightness temperature Tb (see Eq. 18 of cali_rep.pdf). For a source which fills the main beam, Beff=TA* Feff/Jnu(Tb), where Jnu(Tb) is the Rayleigh Jeans brightness temperature at frequency nu. When using p.59, Eq.4.13, Eq.5.33 of Baars (2007), we find

that the observed relation, HPBW=2460/nu with HPBW in arcsec and nu in GHz, is consistent with an edge taper of -16.2dB, HPBW=1.19 lambda/D in radian, and Beff=1.27Aeff (cf. te.greg, CK, 8-May-2015). Previously, we had sometimes assumed, HPBW=1.17Lambda/D, corresponding to an edge taper of 13dB and Beff=1.21 Aeff.

1. Aperture Efficiency Aeff. Aeff can be obtained via pointings on point-like celestial calibrators with a well known flux, like Uranus or Mars, when it is small. Aeff can be computed from 3.906 K TA* Feff / Ssou, where K is the correction factor that considers the coupling of the disk size of the planet to the HPBW, TA* is the peak antenna temperature, and Ssou is the intrinsic flux density of the planet. (see Eq.16 in cali_rep.pdf or spatial_response_framework_v1.8.pdf)

2. Point source sensitivity S/TA*. S/TA* is expressed as 3.906 Feff/Aeff in Jy/K (see Eq.17 in cali_rep.pdf)

3. Error beams. A part of the power pattern is distributed in three error beams (see the analysis of greve_1998.pdf). The size of the described Gaussians is unchanged, however the main beam efficiencies have been improved since 1998, lowering the strengths of the error beams. A new report by Kramer, Penalver, Greve of August 2013 is available online here. Astronomers should take the contribution of the error beam into account when converting antenna temperatures to brightness temperatures, especially when mapping extended sources.

Gain elevation curves

The point source sensitivity or aperture efficiency drops for large and for low elevations above/below the optimum of about ~50deg. This effect becomes more pronounced with increasing frequency. At 210GHz, the aperture efficiency drops to 80% of its optimum value at an elevation of 20deg or 80deg (cf. Report of April-2012 by J.Penalver where we also offer a CLASS script to observers to correct for the gain elevation curve).

Gain variation with wobbler throw

The point source sensitivity or aperture efficiency also drops with increasing wobbler throw. This effect becomes more pronounced with increasing frequency. At about 230GHz, the point source sensitivity drops to 80% of its optimum value for a wobbler throw of +-2' (cf. Figure 6 in Greve et al. 1996).

Past efficiencies

Efficiencies measured with EMIR in 2009

• Freq	HPBW	Feff	Beff	Aeff	S/TA*	Date	Comments	Beff/Aeff
  GHz	arcsec	%	%	%	Jy/K
  86	29	95	81	63	5.9	4.4.2009		1.29
  115			78			predicted	(1)
  145	16	93	74	57	6.4	4.4.2009
  210	11	94	63	49	7.5	29.3.2009		1.29
  230	10.7	92	58	46		14.11.2012	(2)	1.26
  260	9	88	53	41	8.4	29.3.2009		1.29
  330
  340	7.5	81	35	29	10.9	24.11.2009		1.21

Comments: (1) The beam efficiency at 115GHz is predicted from the measured beam efficiency at adjacent frequencies, using the Ruze formula. <CK, 15-Jan-2013> See also the EMIR Users Manual and the EMIR Commissioning Report. (2) See report by JP.

Efficiencies measured with ABCD receivers in 8/07 (and 6/08)

• Freq	HPBW	Feff	Beff	Aeff	S/TA*	Comments
  GHz	arcsec	%	%	%	Jy/K
  72	33.4	98	79	65	5.9	estimated
  86	28.5	98	78	64	5.9
  145	16.9	95	64	53	6.9
  210	11.3	94	62	51	7.2
  260	9.0	90	53	44	8.0
  345	7.0	87	39	32	10.6	estimated

• Measurements were conducted during night time when effects of anomalous refraction and any panel buckling are strongly reduced. In addition, the panel backstructure is heated in a random fashion since 8/05, improving on its thermal balance. (JP, CT, CK 2/09). ABCD receivers were used to observe Uranus and Mars, while small. Receivers were tuned to single sideband. Planetary brightness temperatures Tb from ASTRO/GILDAS:
  • Mars: 215K constant with frequency

  • Uranus: 139K at 86GHz, 116K at 145GHz, 102K at 210GHz, 94.5K at 260GHz, 85.6K at 345GHz following Griffin & Orton 1993

Efficiencies before 2007

• Efficiencies measured on August, 31, 2007, Annual Report 2007,page 15ff

• Efficiencies of 3/2005, see also the IRAM Newsletter 8/05.

• Plot of efficiencies against frequency, measured in 2000,

• Compilation of efficiencies obtained in the past till 2001.

Back to top

Back to the IRAM 30m System Summary