Introduction

A question arose, from Matt Carter: "Could anyone give me the latest views if any on the sun avoidance. i.e the amount of attenuation required. If I put in 90%, would that be a reasonable figure."

There are several items needed for a complete response to this.

  1. Is the current spec on the Solar Filter acceptable?
  2. Is the power from the Sun during a scientific observation, e.g. a flare, enough to damage the receiver?
  3. How will we observe the Sun anyway?
  4. Having observed the Sun, can the scientists get what they want from the data?

Summary:
  • The Specs on the FE filter are satisfactory from a scientific point of view.
  • Even a large solar flare is unlikely to damage the receivers.
  • For very large flares, receiver compression may complicate accurate calibration.
  • Digitization accuracy may present a problem for imaging of the strongest flares with ALMA. Solar active regions present a complex imaging problem for ALMA, in which case poor digitization accuracy is an issue. This issue can be pursued with the ALMA preproduction receivers at the ATF.

Discussion of Item A

A) The current specifications for the solar filter are listed in the (unapproved) Front End Sub-System Technical Specification, which is document 99 in the Document Approval Request queue in almaedm with a URL of: http://almaedm.tuc.nrao.edu/forums/alma/dispatch.cgi/docapproval/docProfile/100636/d20040408155506/No/t100636.htm

4.2.5. Solar Filter A solar filter shall be provided to allow solar observations. It shall be possible to insert the filter into the RF beam of any of the ALMA bands under remote control. [FEND-40.00.00.00-00200-00 / T] 4.2.5.1. Infrared attenuation [FEND-40.00.00.00-00200-00 / T] The solar filter shall attenuate the incident infrared radiation by at least 20 dB. 4.2.5.2. RF attenuation [FEND-40.00.00.00-00205-00 / T] The solar filter shall attenuate the RF signal by 13 ±3 dB.

N.B. Gordon Hurford sends a warning not to trust the filter should the antenna be wet: "Has ALMA evaluated the effect of moisture on the thermal response of the dishes when pointed at the Sun?

"One can get into trouble on those (admittedly rare) occasions when solar-pointed antennas are wet (from dew or melting snow) since this changes the dishes from diffuse to specular reflectors at optical and IR wavelengths. Before we realized what was going on and took preventative action, in the early 80's we lost several feeds on the 27m antennas at OVRO due to such effects. Although not as serious, as I recall even the 2m antennas at OVRO showed evidence of accumulated charring at the center of their feeds. A wet 11 m reflector with 2-3% efficiency can focus a few kilowatts of heat onto a very small ~cm area. While these experiences may not be relevant to your design (or site), we certainly learned that moisture can pose a rare but serious hazard in the context of solar pointing."

Measurements of Prototype Solar Filters

On 2009 Jun 11 Ferdinand Patt wrote: "...the solar filter prototype test report prepared by QMC and IAP (has been received) for review. The report is also placed on EDM, see link.

There are some issues that need to be discussed about the solar filter specification.
  • could the in band attenuation be relaxed by lets say 2 dB, from range 13 to 16 dB to range 12 to 17 dB?

We are planning to send out a Call for Tender for the solar filter production in preparation for the ESO Finance Counsel by early July 2009. So, we need your feedback very soon.

Comments Regarding Solar Filter Spec from Richard Hills 2009-06-18

From an email exchange involving several ALMA/ESO staff and Richard Hills.

Dear Gie Han,

As you know I believe that several changes to this specification are
required.  I sent some detailed comments to Ferdinand on 7th July last
year, but I don't think I heard back from him about those and the points
I made then do not seem to have made it into the version of the spec
that is now in the CRE queue.

Here are the main things that I think we need to sort out:

1) I do not think that the IR rejection is necessary or helpful.  As I
understand it, the IR rejection is what is provided by the multi-layer
mesh and that is where most of the issues about polarization and (I
suspect) most of the costs arise.  I suggest you consider filters that
consist of the neutral density section only.  Presumably these should be
much cheaper.

2) We need to understand the mechanism of the attenuation much better -
 is it absorption or reflection?  If it is absorption then we will need
to know the temperature of the absorber.  If it is reflection then we
need to control where the reflection goes.  It would be best if we could
return this signal to the sky.  If it is a mixture we need to do both.

3) The RF return to the receiver needs to be better controlled.  It
should certainly be lower than the 20 dB specified.

4) Perhaps most importantly (and this was a topic of separate set of
e-mail exchanges in early August last year) I am not at all convinced
that we can manage without a solar filter in Bands 1 and 2.  I believe
that the decision to exclude these bands was taken by FE without
involving the Calibration Group and may have been based on false
assumptions.  In fact the requirement for attenuation is greatest in the
low frequency bands and I myself would have been asking QMC to try to
develop a filter with attenuation sloping between at least 20 dB at the
low frequency end to roughly 10dB at the highest frequencies.

5) The "channeling" displayed in the test results - oscillations in the
attenuation of 2 or 3 dB with a period of order 100 GHz is not desirable
although not fatal.  It is not clear whether that arises in the neutral
density section or the IR blockage.  If it is still there at this level
when the IR section is removed, then it would be worth asking whether
there are ways of reducing it.

In addition to the above points I have floated the idea of using a
different approach to this problem, which is to use a small and possibly
variable-sized stop in the beam to do most of the attenuation.  This
opens up the possibility of using some auxiliary optics to make a
reduced effective aperture for the telescope to give a larger primary
beam.  The astronomers that I have talked to are positive about this
approach and I have done some simple calculations which look promising.
 This is however new scope and it will take some time to do the
necessary studies of both the detailed requirements and the best
solutions.  At present this is being discussed as an item for
development line funding and I think it is probably best to leave it there.

There are however some applications - e.g. mapping the structures on the
quiet sun for which we would want to use the full aperture.  I think
that we should therefore still look for a cheap and simple form of
filter taking into account the points above.

Needless to say I do not think that you proceed with the call for
tenders with the current design.

Best Richard

Discussion of Item B

B) How bright is a flare?

Stephen White notes:

"A 12m ALMA dish has a beam of 70" at 85 GHz. A large flare at 3 mm is 100 sfu (10^6 Jy): the corresponding increase in brightness temperature over the ALMA primary beam is

"1.22e10 * 100. / (85 GHz)^2 (70")^2 = 34000 K.

"The denominator is frequency-invariant since beam size goes down as frequency goes up.

"The quiet Sun is 7000 K, so this is a factor of 5 brighter. A VERY large flare at 10000 sfu (of order tens per cycle but chance during ALMA observation is zero) is of course 100 times as much again but at this flux the bright region is likely to be bigger than the ALMA beam, particularly at shorter wavelengths."

B+) Can it damage the receiver?

Recall the spec: "The solar filter shall attenuate the RF signal by 13 ±3 dB." Presumably this is written with the quiet sun in mind; after attenuation the receiver would be confronted with a 500K source.

So the power into the 4GHz IF B3 receiver without the attenuator is kTB= 1.38E-23 * 34000 * 4E9=1.9E-9 W = -87.2 dBW = -57.2 dBm. From considerations discussed in ALMA Memo No. 504, this would not endanger the receiver, nor would the rare event of a flare one hundred times this large.

Discussion of Item C

C) How will we observe the Sun anyway?

Tests are scheduled as part of the AIVC plan to test the solar loading on the antenna. These tests may have been done on the prototypes but results are unknown. We assume the antenna will survive. Whenever the telescope is pointed to the Sun, the solar filter should be in place. From the above argument, we believe the receiver will survive even a bright flare.

I spoke with Stephen White. At BIMA, the problem is not so large as the antennas are small, but they do nothing special with the digitizers. I'd have to go look up our digitizer range but normally the flare would not result in more than a factor of five or so increase. This shouldn't cause us very much problem I think. The very bright mm flares only occur once per year and we are extremely unlikely to have to deal with them. BIMA does use cascaded junctions to deal with saturation but of course there are a different number on each antenna (the joys of a University facility). They do the calibration by putting in a semitransparent vane (we will probably have one handy; this is the basis of our experiment along these lines anyway). Stephen felt comfortable with the ALMA scheme; his main worry at BIMA has been filtering out the IR, which our filter should take care of.

Discussion of Item D

D) Can one do science with the result one obtains?

Two areas of concern were discussed: digitization saturation and receiver saturation. Darrel Emerson noted: "There is no AGC as such in the system, but there are software controlled step attenuators. I believe there's one controllable attenuator inside the receiver, coming out at the 4-12 GHz IF, and there's another step attenuator in the downconverter, essentially just before the digitizers.

"In operation, these attenuators will be set by software to ensure a reasonable level to the digitizers, as well as to the various analog stages after the frontend. I presume the attenuation will be held constant between calibration and observations. There is no AGC that would reduce the gain during a "scan". I suppose the step attenuators in principle could be changed at any time to reduce the gain, but I believe the current philosophy is to hold the gain and attenuators constant during a scan.

"To accommodate large variations in signal, I think we just have to allow enough headroom in the signal path and digitizers, by keeping the signal level going through the IFs sufficiently low, so that the signal (flares or whatever) can be allowed to increase enough without running into saturation. The digitizers probably present the biggest limitation - obviously we don't want to keep the gain so low that digitization steps become much larger than the rms noise."

It seems to me that this philosophy is fine for flares of the size which we expect to encounter. If we were lucky enough to get a big flare, the receivers would not be endangered but the digitization could cause data interpretation problems. There may also be receiver nonlinearities which compromise calibration for these very bright flares.

Darrel forwarded a note from Tony Kerr: "Gain compression depends on the mixer gain and the number of junctions in series in the mixer for Band 6 (a lot of assumptions are built into the figure for which see ALMA Memos 401 & 460.1). With the SSB mixer gain in the 5-10 dB range, you can see that a single-junction mixer would have ~ 4-11 % compression looking at a 300 K load (red lines in the fig.), while a 4-junction mixer would have only 0.2-0.9 % compression. One of the reasons the Band 6 mixers have 4 junctions is to reduce gain compression. Ditto, the Band 3 mixers which were designed by Pan.

If significant gain compression is present, it may be possible to correct it by applying a correction factor, depending on antenna temperature, at some point in the data processing, as described in ALMA Memo 460.1.

Amplitude Calibration

When the filter is in, the amp cal device is obstructed. The WVR cannot be used. The filter can only go in front of one receiver at a time. Therefore the calibration modes will be restricted--for instance fast switching to another band will not be possible. Calibration accuracy and methods of achieving it are research topics. It would be very useful to have a description of the sequence of calibration and observation when targeting the Sun.

Summary

Summary:
  • The Specs on the FE filter are satisfactory from a scientific point of view.
  • Even a large solar flare is unlikely to damage the receivers.
  • For very large flares, receiver compression may complicate accurate calibration.
  • Digitization accuracy may present a problem for imaging of the strongest flares with ALMA. Solar active regions present a complex imaging problem for ALMA, in which case poor digitization accuracy is an issue. This issue can be pursued with the ALMA preproduction receivers at the ATF.

System Requirement Review Item

2.1-R5 (from SSR), Solar flare tracking and pulsar observing No real work has gone into these areas as we consider them lower priority than the bread-andbutter observing modes.

-- AlWootten - 13 Apr 2006 update.

-- AlWootten - 24 Jan 2005
Topic attachments
I Attachment Action Size Date Who Comment
SolarFlareFluxesMay2011.eml.txttxt SolarFlareFluxesMay2011.eml.txt manage 8 K 2011-06-06 - 11:09 AlWootten Solar Flux Spec May 2011 discussion
Topic revision: r8 - 2011-06-06, AlWootten
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