Millimetre and Radio Astronomy  Techniques for Star‐Forma:on 

Studies‐II John Conway 

Onsala Space Observatory, Sweden 

&Nordic ALMA ARC node 

(john.conway@chalmers.se) 

Today prac:cal details.... 

For details see ‘Introduc:on to Millimeter/Sub‐millimeter  astronomy’   by T. Wilson, March 2009, astro.ph 0903.0562, and textbook ‘Tools of Radio Astronomy’ Wilson,Rohlfs and HuYemeister. Also a few viewgraphs adapted from, 2007 IRAM summer school especially  talks of Thum and Pandre. 

The APEX (sub)millimetre wavelength telescope 

Antenna ‘op:cs’ 

Onsala 20m IRAM 30m/APEX 12

Single pixel vs Camera obs 

•  Simplest case (e.g. Onsala 20m) radio‐telescope as ‘light‐bucket’ dominated by radia:on coming from patch on sky of angular diameter  θ= λ/d, the ‘main lobe’ 

•  Hence in simplest form a ‘single pixel’ device, ‐ ‐ though we can make images by poin:ng at different posi:ons and integra:ng – ‐ ‐‐‐ but takes a long :me. 

•  Can build ‘radio cameras’ with mul:ple receivers at primary or secondary focus. BUT  spectral line sensi:ve systems expensive and difficult to build, max  can be 3x3  or 7x7 pixels(!). Con:nuum only ‘bolometers’ can have more pixels,but  at cm wavelength number of pixels that can be crammed into focus limited, each pixel is many wavelengths in size.  

Radio telescope as a Thermometer 

•  Last :me, strength of signal in Wm‐2Hz‐1 if source more extended than antenna beam can be expressed conveniently as Temperature in K (the measured ‘Antenna temperature’ ) using Rayleigh‐Jeans approxima:on. If source op:cally thick and in LTE this antenna temperature will equal kine:c  temperature, Tkin otherwise is (1‐e‐τ)Tex Direct link to physical processes. 

•  Convenient to get output in Temperature units directly  without calibra:ng  Wm‐2Hz‐1 coming into telescope aperture and conver:ng to Kelvin. We just compare powers coming out of receiver/amplifier electronics in milli‐WaYs when on our target source and when looking at loads of different temperature.  

Complica:ons 

•  The receiver adds power (TRX, receiver temperature). 

•  At mm atmosphere adds power and also absorbs target sources.  

•  In fact telescope not just sensi:ve to small patch on sky   

      θ= λ/d, but has ‘side‐lobes’, which pick up power from range of direc:ons including ground.  

Beam‐switch to subtract atmosphere 

RX

Amp Feed Spectro

meter Vout(v) in milli-watts

Incoming for 20m telescope with 1MHz BW – 10-17 W

Beam-switch (needed only at mm wavelengths)

Beam‐switch to subtract atmosphere 

RX

Amp Feed Spectro

meter Vout(v) in milli-watts

Incoming for 20m telescope with 1MHz BW – 10-17 W

Beam-switch (needed only at mm wavelengths)

Single beam switching 

RX

Vright (ν)

Adds receiver noise TRX and then multiples by gain G

Single beam switching 

RX

Vleft(v)

Adds receiver noise TRX and then multiples by gain G

Single beam switching 

RX

Vleft(ν)

Vsig(v) = Vright – Vleft

So get spectrum where receiver power removed and additive power effect of atmosphere also removed

Gain G

Improved method ‐Double beam switching 

Von = Vright - Vleft with source in right beam path, form using fast mirror switching at 2Hz. Integrate for say 10sec – 30sec.

Then slew telescope so now source is in left beam path. Again form Voff = Vright – Vleft with fast mirror switching at 2Hz, integrate again for say 10sec -30sec.

Form Vsig = (Von – Voff)/2, which is proportional to received source power

Advantage; compared to single beam switching, eliminates difference in gain between left and right paths. Slight disadvantage; lost slew time, but best choice for weak lines

•  Single and double beam‐switching efficiently remove addi*ve power effects of receiver and atmosphere, and ground (coming in from far side‐lobes). 

•  They do no remove mul*plica*ve (aYenua:on) effects of atmosphere, nor do they do the conversion from powers to temperature. 

•  To solve above problems, need to make calibra:on observa:ons, switching between blank sky and load temperature.  

Calibra:on Spectra (‘flats’) 

RX

Vright(v) = Vamb(v)

Gain G

Ambient temperature load

Calibra:on Spectra 

RX

Vleft(v) = Vsky(v)

Gain G

Ambient temperature load

Calibra:on Spectra 

RX

Vright(v) = Vamb(v)

Gain G

Ambient temperature load

Vcal(v) = Vright(v) – Vleft(v)

or

Vcal(v) = Vamb(v) – Vsky(v)

Can be used together with Vsig(v) to get opacity/gain corrected spectra in temperature units.

All quantities below are functions of velocity/frequency. All V quantities are output powers in mWatts, G is the electronics gain. With an ambient temperature absorber in signal path.

When pointing on blank sky

Where Feff is the forward gain of the telescope (approx 0.9), first term above is power contributed from atmosphere, second term pickup from the ground. Using that

We obtain that the difference in power between blank sky and ambient load is

‘Chopper mirror’ calibra:on 

This atmosphere opacity corrected TA’

versus frequency is what is delivered from the telescope, all ON-OFF, and ‘flat field’ done automatically, no need to manually ‘reduce’ this data (!!)

Where TA’ is the antenna temperature that

would be measured if telescope was at top of atmosphere (i.e. before atmospheric attenuation), i.e the astronomical quantity of interest.

From previous slide, substitute into above equation and rearrange....

So to get opacity corrected calibrated data in K units, take the ratios of Δvsig spectra to Δvcal spectra multiply by TambFeff

Final reduc:on steps 

•  Look for bad integra:ons 

•  Remove ‘spectral baselines’ by fiong polynomial to line‐free parts. 

•  Francesco will explain more.....  

Final Result Tant(K)

Taken with Onsala 20m telescope 17 days ago. Spectra of water maser emission at 1.3cm from disk surrounding super-massive black hole in NGC4258