Mopra Calibration @ 115GHz 2005-2009

The MAGMA Perspective

Annie Hughes & Tony Wong

Why this talk?Flux monitoring undertaken as part of the MAGMA survey may be valuable for:•users who want to combine data from MPCOR and MOPS (2006 to present)•managing observer expectations of system performance at 115GHz•quantitative examples for why users should follow good millimetre observing habits at 115GHz•practical feedback from Large Programme to user community (not just their science results)

MAGMA OverviewObservations: OTF mapping of LMC at 12CO, 13CO + …LMC vrad~ 200 to 300 km/s (LSR)12CO Sky Freq ~ 115169 MHz

For MAGMA, consistent calibration over duration of survey is more important than absolute flux scale.

Flux Monitoring at 115GHz:Regular Orion KL position switched spectraOri KL vrad~ 9 km/s, 12CO Sky Freq ~ 115269 MHzBootstrap to SEST Tmb (FWHM = 45” @ 115GHz)

Results

• Assuming Txb (12CO) = 100K, an average bootstrap flux scaling factor for different observing epochs was derived using measurements of Orion KL’s peak CO brightness.• “flux scaling factor” includes efficiency + other system effects• Flux monitoring suggests efficiency consistent with Ladd ea (2004), but with “glitches” in 2006 (?) and early 2008 (focus?).

Peak

Tem

pera

ture

[K]

Inte

grat

ed In

tens

ity [K

km

/s]

Elevation

After excluding OriKL data in poor weather & “glitches”, we find elevation dependence ~ 0.8 to 1.2 K TA

* per 5 degrees for el>35 degrees (steeper at lower elevation).This is within 20-30% absolute flux calibration uncertainty, but is reproducible (in good weather).Origin: beam distortion & pick-up (?). Atmospheric model more uncertain at low elevationsMoral #1: stay above 35 degrees if absolute/consistent flux mattersMoral #2: monitor calibrator and source at similar elevations

Blue/green dots – different MOPS centre frequency settingBlue = 111460 MHzGreen = 111550 MHzPe

ak T

A* [K

]

The choice we faced:MOPS configuration for LMC:12CO sky freq ~ 115169 MHzVcen = 111460 MHz12CO Zoom Window = [54]

Configuration for Ori KL:12CO sky freq ~ 115269 MHzNeed to choose between:Vcen = 111550 MHz12CO Zoom Window = [54] Vcen = 111460 MHz12CO Zoom Window = [55]

Does it matter which of these configurations we choose? In either case, it is nearly inevitable that sky frequency of 12CO line will be different for extragalactic source and Galactic standard.MAGMA tried both configurations …

Peak

TA*

[K]

Vcen scans

• empirical approach: repeat Ori KL spectra with different MOPS vcen

• green/red (2007); grey (2006)• vcen dependence of 0.8 to 1 K TA

* per 100 MHz between 111.5 and 112.0 GHz• net effect may lie within 30% guarantee, but reproducible –should consider during obs. setup?

Origin: consequence of Tsys rising across the 2.2 GHz block at highest MOPS operating frequencies? Moral #3: Keep vcen the same for source and standard

Moral #4: Stay as far from the danger zone as practical for all measurements (dependence probably flattens at lower frequencies)

Summary I• MAGMA flux monitoring of Orion KL suggests system has been reasonably stable since 2005, with small glitches in 2006 and early 2008• Useful to quantify vcen effect across the 3mm band • MAGMA is happy to provide feedback about our flux monitoring to Mopra user community. Could other Large Programmes do the same? How useful would it be?• Useful for Mopra website to explain the rationale for millimetre observing guidelines to interested – but potentially renegade! – observers… (pages like this are currently under construction?)

Summary IIObserving at 115GHz with Mopra is pushing the envelope of the site and system. If a consistent flux scale matters:1. stay above 35 degrees2. be religious about taking standard spectra3. take standards with an observing set-up that mimics target observations (e.g. elevation, vcen)

4.use lowest frequency setting for MOPS that works

Most of 1-4 is not new, but MAGMA flux monitoring analysis could be used to show why the advice is sensible.