Effelsberg: going broadband New receivers: UBB (0.6 3.0 GHz) C+ - - PowerPoint PPT Presentation

▶
effelsberg going broadband
SMART_READER_LITE
LIVE PREVIEW

Effelsberg: going broadband New receivers: UBB (0.6 3.0 GHz) C+ - - PowerPoint PPT Presentation

Oct 06, 2023 225 likes •597 views

Broadband calibration for Single-dish radio telescopes Benjamin Winkel Alex Kraus Uwe Bach Effelsberg: going broadband New receivers: UBB (0.6 3.0 GHz) C+ (4 9.3 GHz) Ku (12 18 GHz) K (18 26 GHz) Q (33

slide-1
SLIDE 1

Broadband calibration for

Single-dish radio telescopes

Benjamin Winkel Alex Kraus Uwe Bach

slide-2
SLIDE 2

Effelsberg: going broadband

  • New receivers:

– UBB (0.6 – 3.0 GHz) – C+ (4 – 9.3 GHz) – Ku (12 – 18 GHz) – K (18 – 26 GHz) – Q (33 – 50 GHz)

  • New Backends:

– 64k FFTS – Stacking: 1+M

slide-3
SLIDE 3

Effelsberg: going broadband

Calibration is frequency-dependent! Calibration is frequency-dependent!

slide-4
SLIDE 4

Overview

  • Motivation
  • Introduction (continuum calibration)
  • Spectroscopy calibration

– Classic – Unbiased

  • Conclusion / Outlook
slide-5
SLIDE 5

Intro: fundamental equation

slide-6
SLIDE 6

Intro: fundamental equation

Calibrating a system means to determine G Calibrating a system means to determine G

slide-7
SLIDE 7

Intro: continuum calibration

  • Use known source

(aka “calibrator”) to infer G

slide-8
SLIDE 8
  • Use known source

(aka “calibrator”) to infer G

  • Problem: G is not

perfectly stable

Intro: continuum calibration

slide-9
SLIDE 9

Intro: continuum calibration

Gain variations Gain variations

slide-10
SLIDE 10
  • Use known source

(aka “calibrator”) to infer G

  • Problem: G is not

perfectly stable

  • Solution: use a

stable reference → Noise diode (Tcal)

Intro: continuum calibration

slide-11
SLIDE 11

Solution: use a noise diode

We now have It follows

slide-12
SLIDE 12

We now have It follows This is really noisy, because

Intro: using a noise diode

slide-13
SLIDE 13

Solution: use a noise diode

We now have It follows

slide-14
SLIDE 14

We now have It follows

No averaging With averaging

Intro: noise diode + gain model

slide-15
SLIDE 15

We now have It follows We still need to use a calibration source to infer Tcal! We still need to use a calibration source to infer Tcal!

No averaging With averaging

Intro: noise diode + gain model

slide-16
SLIDE 16

Spectroscopy: basics

Again we have But now everything is a function of frequency

slide-17
SLIDE 17

Spectroscopy: basics

Again we have But now everything is a function of frequency Idea: Calibrate each spectral channel independently

(using the same method as before)

slide-18
SLIDE 18

Spectroscopy: basics

Again we have As before, but vectorized

slide-19
SLIDE 19

Spectroscopy: basics

Again we have As before, but vectorized

slide-20
SLIDE 20

Spectroscopy: basics

Again we have As before, but vectorized Denominator is too small → numerically unstable

slide-21
SLIDE 21

Spectroscopy: basics

Average in time? → only possible if G is very stable

(long integration periods needed, because of small bandwidth per channel)

Average in frequency? → only possible if G is very flat

(usually not the case, especially not for ultra-wideband systems)

Average in time? → only possible if G is very stable

(long integration periods needed, because of small bandwidth per channel)

Average in frequency? → only possible if G is very flat

(usually not the case, especially not for ultra-wideband systems)

Again we have As before, but vectorized

slide-22
SLIDE 22

Spectroscopy: position switching

Observe ON and OFF-source

slide-23
SLIDE 23

Observe ON and OFF-source It follows

Spectroscopy: position switching

slide-24
SLIDE 24

Observe ON and OFF-source It follows But: Tsys depends on time and frequency → need to relate this to Tcal again But: Tsys depends on time and frequency → need to relate this to Tcal again

Spectroscopy: position switching

slide-25
SLIDE 25

Spectroscopy: inferring Tsys

Observe ON and OFF-source Compute

slide-26
SLIDE 26

Observe ON and OFF-source Compute This can be approximated by a constant (in frequency)! This can be approximated by a constant (in frequency)!

Spectroscopy: inferring Tsys

slide-27
SLIDE 27

Observe ON and OFF-source Compute However, denominator is small → numerically unstable However, denominator is small → numerically unstable

Spectroscopy: inferring Tsys

slide-28
SLIDE 28

“Classic” solution with

Spectroscopy: classic solution

slide-29
SLIDE 29

Spectroscopy: unbiased method

Now switch to larger bandwidth... Now switch to larger bandwidth...

slide-30
SLIDE 30

Model this quantity and invert afterwards

(avoids numerical instability)

Gauss-filtered

Spectroscopy: unbiased method

slide-31
SLIDE 31

From Gauss-filtered From mean

Spectroscopy: unbiased method

Model this quantity and invert afterwards

(avoids numerical instability)

slide-32
SLIDE 32

Spectroscopy: unbiased results

slide-33
SLIDE 33

Correct continuum signal!

Spectroscopy: unbiased results

slide-34
SLIDE 34

RRL: H109α

Spectroscopy: unbiased results

slide-35
SLIDE 35

RRL: H112α Classic method: line ratio systematically wrong!

Spectroscopy: unbiased results

slide-36
SLIDE 36

Conclusion

Need to incorporate frequency dependence But

  • Modeling not always robust, may need supervision

(e.g., in case of standing waves)

  • Tsys may not be stable between ON and OFF

Weather can hurt a lot! → Solution: cross-scanning

  • Frequency dependence also for opacity,

Elevation-gain curve, taper function Need to incorporate frequency dependence But

  • Modeling not always robust, may need supervision

(e.g., in case of standing waves)

  • Tsys may not be stable between ON and OFF

Weather can hurt a lot! → Solution: cross-scanning

  • Frequency dependence also for opacity,

Elevation-gain curve, taper function