ALMA engineering and Solar Filters

Presented by P. YagoubovTechnical department

ALMA European Support CenterEuropean Southern Observatory

High thermal load on Front End receivers High radio flux High dynamic range required Wide field imaging

2

Solar observations with ALMA –Challenges:

Problem Thermal load on Front End receivers, irradiance of sunlight –

~130kW per 12m diameter collecting areaSolution Antenna panels scalloped to diffuse visible and near infrared

radiation Implemented and proven to work, ALMA can be pointed at

the Sun safely, also without solar filters Drawbacks None

3

Solar observations with ALMA –Challenges: high thermal load

Problem ALMA receivers are designed for the maximum RF signal

~800K (effective brightness temperature) at the receiver input (=cryostat vacuum window)

4

Solar observations with ALMA –Challenges: high radio flux

ALMA: signal path

5

Front End dynamic range

• Mostly determined by the SIS mixers gain compression, in Bands 3-10 (Bands 1-2 will use HEMT amplifiers with higher saturation level)

• Tsat of SIS mixers is ~5000K, so the receivers are in “soft” (quadratic) saturation already with ambient load at the input, few % gain compression

• FE IF components designed to stay 20dB below 1dB compression point

• FE attenuator can be adjusted to deliver nominal power level to the Back End, however its dynamic range of 15 dB is used to cope with sub-assemblies gain variations and cover range of 40K<Tsys<800K

6

Back End dynamic range

• BE analog components designed to stay 20dB below 1dB compression point for the nominal input power level

• IF gain and thus power level of signal fed to the digitizer can be adjusted by the BE basebands attenuators

• However, there is no automatic gain control!• BE dynamic range mostly limited by the digitizers, +/-2dB. This

sets the fundamental limit for the IF total power dynamic range, within one observation scan

7

Problem ALMA receivers are designed for the maximum RF signal

~800K (effective brightness temperature) at the receiver input (=cryostat vacuum window)

Solution Optical attenuator in the receiver optical path, between

antenna and Front End: Solar Filter

8

Solar observations with ALMA –Challenges: high radio flux

Solar filter as part of the Amplitude Calibration Device

9

Amplitude calibration device

10

WVR pick up mirror

Calibration wheelSolar filter

Front End cryostat

Calibration load

Solar Filter Requirements:

Nominal RF attenuation (dB) = 4 + 2λ, where λ is the wavelength in millimetres. Deviation from this nominal attenuation level no more than +4/-2dB;

Return Loss <–25dB below 400GHz and <–20dB above 400GHz

Cross polarization induced by the Solar Filter<–15 dB relative to the co-polar component, goal of <–20db

11

Solar filter core

Absorber

Optical path

Solar filter core design

Three layers:- Inner layer is a Frequency Selective Surface, acting as a low pass filter;- Outer layers are dielectric films

12

Solar Filter attenuation

13

0

5

10

15

20

25

30

0 5 10 15 20 25 30 35 40 45 50

Atte

nuat

ion,

dB

Frequency, cm-1

Atten new spec (dB)low limit (-2db)upper limit (+4db)ALMA Solar Blocker SFC-061 inclined at 15 degrees ALMA Solar Blocker SFC-062 inclined at 15 degrees ALMA Solar Blocker SFC-063 inclined at 15 degrees ALMA Solar Blocker SFC-064 inclined at 15 degrees ALMA Solar Blocker SFC-065 inclined at 15 degrees ALMA Solar Blocker SFC-066 inclined at 15 degrees ALMA Solar Blocker SFC-067 inclined at 15 degrees ALMA Solar Blocker SFC-068 inclined at 15 degrees ALMA Solar Blocker SFC-069 inclined at 15 degrees ALMA Solar Blocker SFC-070 inclined at 15 degrees ALMA Solar Blocker SFC-071 inclined at 15 degrees ALMA Solar Blocker SFC-072 inclined at 15 degrees ALMA Solar Blocker SFC-073 inclined at 15 degrees ALMA Solar Blocker SFC-074 inclined at 15 degrees ALMA Solar Blocker SFC-075 inclined at 15 degrees ALMA Solar Blocker SFC-076 inclined at 15 degrees ALMA Solar Blocker SFC-077 inclined at 15 degrees ALMA Solar Blocker SFC-078 inclined at 15 degrees ALMA Solar Blocker SFC-079 inclined at 15 degrees ALMA Solar Blocker SFC-080 inclined at 15 degrees

Problem ALMA receivers are designed for the maximum RF input signal with

800K effective brightness temperature Solution Optical attenuator in the receiver optical path: Solar FilterDrawbacks (also potential) Reduced SNR – all calibrations are more difficult Complex gain introduced, frequency and maybe time dependent.

Amplitude and phase need to be calibrated Short time scale phase correction with WVR not possible in high

ALMA bands, 7-10 Beam properties, focus, and pointing Polarization purity and polarization calibration

14

Solar observations with ALMA –Challenges: high radio flux

SNR is decreased by two factors:- the signal is decreased due by the attenuation in the filter- noise is added due to termination at ambient

Trx(sf) = Trx/t + Tsf(1-t), where

Trx is the receiver noise temperature without solar filterTsf is the solar filter termination load physical temperature (~290K)t is the solar filter transmission

ExamplesBand 3, t=0.1, Trx increases from 40K to ~660KBand 9, t=0.2, Trx increases from 100K to ~730K

15

Solar Filter drawbacks: reduced SNR

Solar filter drawbacks: no short time scale phase correction using WVR

ALMA 12m antennas of the main array are equipped with Water VapourRadiometers (WVR)

WVR correct for the excess optical path due to water vapour and thus the phase of each antenna signal

WVR are used to correct phase fluctuation for the short time scale: 1sec < t < fast switching period

Problem Solar filter is installed on the Amplitude Calibration Device. With solar

filter inserted in Bands 7-10 (other tbc) receiver beam, calibration wheel blocks WVR optical path.

Solution None

16

Amplitude Calibration Device

17

Complex gain calibration- Phase: covered by Shimojo-san presentation- Amplitude: low resolution FTS laboratory data available. High resolution

measured in coherent mode in Band 9, spectral response is flat

Trx with and without solar filter Attenuation level derived from Trx

18

Solar Filter drawbacks: gain calibration

600 620 640 660 680 700 7200

200

400

600

800

1000

1200

Tnoise pol. 1

FLO (GHz)

T nois

e (K

)

600 620 640 660 680 700 7205

5.2

5.4

5.6

5.8

6Attenuation pol. 1

FLO (GHz)

dB

Primary beam properties- Focus and pointing can be done with solar filters- Astrophotography measured with solar filter- Band 9 receiver beam measured with solar filter, sidelobe detected however

terminated at the sky (sub-reflector angular dimensions are shown by dashed lines)

Band 9 receiver far field beam patternWithout solar filter With solar filter

19

Solar Filter drawbacks: primary beam

(-1.192, -1.454)

Xangle (deg)

Y angl

e (deg

)

FLO = 670 GHz; dFocus = 120 mmCopolarisation: pol. 1

-8 -6 -4 -2 0 2 4 6 8

-8

-6

-4

-2

0

2

4

6

8

Am

plitu

de (d

B)

-81-78-75-72-69-66-63-60-57-54-51-48-45-42-39-36-33-30-27-24-21-18-15-12-9-6-30

(0.060, -1.185)

Xangle (deg)

Y angl

e (deg

)

FLO = 670 GHz; dFocus = 120 mmCopolarisation: pol. 1

-8 -6 -4 -2 0 2 4 6 8

-8

-6

-4

-2

0

2

4

6

8

Am

plitu

de (d

B)

-81-78-75-72-69-66-63-60-57-54-51-48-45-42-39-36-33-30-27-24-21-18-15-12-9-6-30

Polarization purity- Tested by the manufacturer at one unit. Confirmed at Band 9- Polarization calibrations are more difficult due to low SNR

20

Solar Filter drawbacks: polarization purity

0

10

20

30

40

50

60

0 5 10 15 20 25 30 35

Atte

nuat

ion,

dB

.

Frequency, cm-1

ALMA Solar Blocker SFC042 Longside at 15 deg incidence cross pol

ALMA Solar Blocker SFC042 Longside at 15 deg incidence

Solar Filters installation status

• All solar filters delivered to JAO in 2012• 36 antennas equipped with production SF (32 at the AOS)• 1 antenna with pre-production SF (at the AOS) • 17 antennas w/o SF

21

Conclusions High dynamic range needed for solar observations is difficult to achieve with

ALMA:o Quite Sun and solar flares up to10 SFU can be observed with solar filter.

However, ALMA receivers in soft saturation, so amplitude scale not accurate, tbd if needs and can be calibrated

o +/-2dB dynamic range of the digitizers limits total IF power variation within one observation scan

No WVR corrections possible when solar filter is used for Bands 7-10, other Bands tbc

Solar filter reduces SNR, all calibrations are more difficult Solar filters introduce complex gain - need to be calibrated Solar filters have been produced and delivered, most of antennas are

equipped already Bands 1-2 will use HEMT amplifiers instead of SIS mixers, higher radio flux

can be measured22

Schematics of IF processor

23

IF switch attenuator BB attenuators

TP detectors BB detectors