Heterodyne Calibration

Observations with RxA and HARP are calibrated on a TA* scale (in Kelvin), in which the spectra are corrected for atmospheric attenuation and scattering and spillover seeing ambient temperature.
This is done by combining measurements of hot and cold loads and knowledge of the sky and ambient temperatures. Note there are special considerations when observing targets with a strong continuum such as the moon, the Sun, or planetary atmospheres.

The temperature scale is checked by observing a number of standard sources at certain standard frequencies (depending on the observing program for the night). Also the bandwidth chosen depends on the observing program. Some of these source/frequency observations have been observed very frequently (e.g. CO in IRC+10216, CRL2688 and CRL618) and can be used to monitor the calibration of the telescope. Other combinations have been observed only once.

Source RA(2000) Dec(2000) Switchmode Vlsr(km/s)
W3(OH) 02 27 03.83 +61 52 24.8 PS -600″,0″ RJ -45
L1551-IRS5 04 31 34.14 +18 08 05.13 PS 1200″,0″ RJ +6
CRL618 04 42 53.672 +36 06 53.17 BM 180″,0″ AZ -22
OMC1 05 35 14.373 -05 22 32.35 PS 0″,2100″ RJ +10
N2071IR 05 47 04.851 +00 21 47.10 PS 2400″,0″ RJ +10
OH231.8 07 42 16.83 -14 42 52.1 PS 300″,0″ AZ +30
IRC+10216 09 47 57.382 +13 16 43.66 PS 300″,0″ AZ -26
16293-2422 16 32 22.909 -24 28 35.60 PS -800″,0″ RJ +4
NGC6334I 17 20 53.445 -35 47 01.67 PS 2400″,0″ RJ -7
G34.3 18 53 18.569 +01 14 58.26 PS -3120″,1800″ RJ +58
W75N 20 38 36.433 +42 37 34.49 PS -1800″,0″ RJ +13
CRL2688 21 02 18.75 +36 41 37.80 BM 180″,0″ AZ -35
NGC7027 21 07 01.598 +42 14 10.02 BM 180″,0″ AZ +26
N7538IRS1 23 13 45.346 +61 28 10.32 PS 1200″,0″ RJ -58
Frequency (GHz) Transition
218.222186 H2CO 303-202
219.5603568 C18O 2-1
220.3986765 13CO 2-1
230.538000 CO 2-1
241.791431 CH3OH 50-40A
244.9356435 CS 5-4
260.255478 H13CO+ 3-2
265.88618 HCN 3-2
267.557619 HCO+ 3-2
271.981142 HNC 3-2
329.3305453 C18O 3-2
330.5879601 13CO 3-2
329.3305453 + 330.5879601 C18O + 13CO 3-2 Dual channel 250 MHz
338.408681 CH3OH 70-60A
342.883 CS 7-6
345.7959899 CO 3-2
354.5054759 HCN 4-3
356.734288 HCO+ 4-3
362.736048 H2CO 505-404
372.672509 N2H+ 4-3

Results are given separately for `Ū`ū, RxA3 (retired 230GHz receiver) and HARP

In addition regular observations are made of Mars, Uranus, and Jupiter in order to monitor the main beam efficiency ηmb (and the aperture efficiency ηa) of the JCMT. Also some observations of the full Moon have been made. Most observations are made for RxA3 at 230.538 GHz (CO 2-1) and for HARP at 345.796 GHz (CO 3-2), both for a bandwidth of 1000 MHz. (On Mars, the spectral regions from 345.7-345.93 GHz and 230.4-230.7 GHz are excluded from efficiency calculations to avoid the  CO 3-2 and 2-1 absorption lines.)

Results are given separately for RxA3 and HARP.

For point sources the conversion from TA* in Kelvin to flux density in Jansky for the JCMT is
S(Jy) = 15.6 TA*(K) / ηa.

In general, our rule of thumb is that night time spectral standard observations should be within 10% of the ‘canonical’ value (see below for a more thorough discussion of the true uncertainties). For convenience, we have analysed observations (considered as ‘good’, and not including very anomalous results or poor baselines) towards some of our standard sources at CO 3-2 (HARP) and CO 2-1 (RxA3). The mean peak and integrated value, along with the standard deviation and percentage error are shown in the table below for these sources. Please note that in general the distribution may not be Gaussian (due to pointing errors, which will tend to skew the distribution below the ‘true’ value).  This does not include any RxA3m observations, and excludes day time observations and observations when RxA3 is believed to have been misaligned.

Mean and standard deviations for good, night time RxA3 and HARP spectral standard observations towards a subset of standard sources.
Instrument Source Type mean std % error
HARP CRL2688 PEAK 9.4 1.1 12.1
HARP CRL2688 INTEGINT 237.2 28.3 11.9
HARP CRL618 PEAK 4.4 0.5 11.9
HARP CRL618 INTEGINT 139.8 19.3 13.8
HARP IRC+10216 PEAK 31.2 3.2 10.2
HARP IRC+10216 INTEGINT 672.0 70.4 10.5
RxA3 CRL2688 PEAK 6.4 0.6 8.9
RxA3 CRL2688 INTEGINT 155.4 9.3 6.0
RxA3 CRL618 PEAK 3.5 0.2 6.3
RxA3 CRL618 INTEGINT 99.9 7.4 7.4
RxA3 IRC+10216 PEAK 21.7 1.6 7.5
RxA3 IRC+10216 INTEGINT 463.7 26.5 5.7

Considerations for Calibration Uncertainty

Categorising and understanding each component of the inherent scatter in the Standard observations is a difficult task that is not well-defined. For small sources compared to the beam, the typical uncertainties in peak flux measurements tend to be in the range of 10-20%, with brighter sources (such as our calibrators) nearer to 10% and faint peaked sources/marginal detections closer to 20%. For observations of extended (especially diffuse) structures, the observing mode, data reduction, and method of analysis can all affect the uncertainties with no simple characterisation of the uncertainties. For extended, diffuse structures, varying uncertainty can be introduced depending on the size of the region and the chop-angle used for background subtraction (as extended emission components can simply be “chopped out”), so this is difficult to analyse systematically. Indeed it is difficult to constraint faint, extended flux measurements obtained by any millimetre or submillimetre telescope to better than 30%.

A few issues to consider when using the HARP instrument are:

1. The SSB noise factor when observing far from the center of the band (5 GHz IF). While most observations are centred optimally on this IF, data can be noticeably affected across the 2 GHz band.

2. The nominal value assumed for the separation between HARP receptors is 30″, but that is only correct to the 2” level.

3. Calibration differences between receptors will also play a role when comparing observing modes because rasters will move the array more across the source than jiggles, where each receptor stays its own small area of the map. Standard source observations are performed by “staring” at the source with just the pointing receptor (H05) so there is no way to collect enough information on a night-to-night basis to compare receptors without the calibration time becoming prohibitive. For more information, see the work by Curtis et al. (2009) in assessing individual factors for different receptors to mitigate striping artefacts: (Section 3.1, “The HARP flat-field”)

https://academic.oup.com/mnras/article/401/1/455/1007610#92356426

This is also described by Jenness et al (2009) for the ORAC-DR pipeline here: (Section 4.8, “Flat-fielding”)

https://academic.oup.com/mnras/article/453/1/73/1747692#25829784

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