[Moon] [Moon-Net] Partial eclipse tomorrow

Sat Mar 21 09:14:46 CET 2015

Roger and all, I was looking into this subject when working with Doug on his EMECalc software and the issue of beam filling factor when using narrow beamwidths and non-point sources came up. I came accross an interesting  book chapter titled  4.1.1.6 Radio Emission of the Quiet Sun by Arnold.O.Benz . I have it as a pdf and Im not sure now where I found it, it was a free download. Figure 5 which you can see here http://www.authormapper.com/search.aspx?val=publication%3ASolar+System&val=name%3ABenz%2C+Arnold+O.  shows the effect of radiation from the halo or corona quite clearly. Many of the measurements were made during eclipse. At 3cm it looks like the solar radius is about 5% larger and at 23cm 15% larger. This is why I thought it would be interesting to try and get some multi frquency data when the sun was not quiet. Obviously our data quality will be mixed but it will be interesting to see what can be extracted from the results.
73 Peter G3LTF 

From: Roger Rehr W3SZ 
Sent: Saturday, March 21, 2015 2:21 AM
To: JJ Mx ; 73w3sz at gmail.com ; moon-net at mailman.pe1itr.com 
Subject: Re: [Moon-Net] Partial eclipse tomorrow

Hi JJ and Ed et al,

I need to correct at least part of my statement about the effects of the antenna beamwidth on the depth and width of the notch in sun noise during an eclipse.

My thinking was faulty.  Detector beamwidth [antenna or eye] will affect the apparent diameter of an object that is captured in transit mode, either by allowing the object to transit across the beamwidth of the detector, or by sweeping the detector's beamwidth across the object; just consider the two extremes:  A tiny object will be detected by a sufficiently sensitive detector when it enters the beamwidth at one edge of the detector's field of view and continue to be detected until it leaves the beamwidth at the other edge.  So its apparent angular diameter will be on the order of the beamwidth of the detector, as long as it is well centered in the detector's field of view.  On the other hand, the angular diameter of an object much larger than the detector beamwidth can be accurately measured as long as it is well centered in the detector's field of view.

But for the eclipse experiments, the detector would be locked onto the sun, with the sun centered well within its beamwidth.

So the larger beamwidth of the RF array vs the optical array does not directly cause a widening of the notch nor a decrease in the depth of the notch for the RF experiment vs the optical experiment by way of the mechanism described above.

What the larger beamwidth does do is to contribute to a reduction in the signal to noise ratio of the RF system as compared to a system where the entire beamwidth is filled by the sun [as it would be in an optical system with appropriately chosen region of interest].

For this case we have:

Diameter of sun and moon 30 minutes, or 0.5 degree.

Optical resolution of the human eye 1 minute, or 1/60 degree.  So each pixel will be much smaller than the sun.

Optical resolution of a telescope is even better, ~ wavelength/Objective diameter.  So again each pixel will be much smaller than the sun.

Beamwidth of the antenna is 4.4 degrees. So each pixel is much larger than the sun [and yes, there is only one pixel].  The sun fills only (0.5/4.4)**2 or 1.2% of the pixel.

So with the human eye or telescope the sun can be very accurately represented and a region of interest accurately drawn.  Relative detector sensitivity will be 100%.

With a 4.4 degree beamwidth dish the relative detector sensitivity based solely on this geometric argument will be 1.2%.  

For this reason and others the RF system will have poorer signal to noise ratio than the optical system.

I believe any effect of the larger beamwidth of the antenna vs the optical systems will be due to the poorer statistics of the RF system, due to the reduced signal to noise ratio of the RF system vs the optical system.  This would be expected to produce a shallower null, but instead of broadening the null as I first stated, this statistical effect would narrow the null. This latter effect would because with poorer signal to noise ratio, the small difference in signal level caused by a very slight occultation of the sun at the very beginning and end of the eclipse would be hidden due to the poorer signal to noise ratio of the RF system.

JJ had mentioned the possibility of an RF halo [my words] affecting the results.  I had also done some thinking along those lines, but then I remembered that when I use moon noise to keep my 0.79 degree beamwidth dish peaked on the moon on 10 GHz, the moon seems like a tiny object and it does not seem to subtend a greater angle than my beamwidth.  That observation suggests that there is not a significant RF halo, at least at 10 GHz.  I checked on the net, and found an NRAO page that says that at 4.6 GHz the edge of the RF limb of the sun is only about 20,000 km above the optical edge of the sun; a tiny difference given that the diameter of the sun is 1,392,000 km.  Given these results on 10 GHz and 1.2 GHz, I suspect that on 1-1.5 GHz the result is similar.  No halo sufficiently large or intense to affect the results directly.

On an image, the RF limb of the sun looks like the thickness of an eggshell above the optical limb.  There is a nice [and brief] discussion of this below a picture showing the RF limb superimposed on the optical limb at 

http://www.cv.nrao.edu/course/astr534/Tour.html

Scroll down to the second image, the one showing a blue green disk that is the sun and read the caption for more info on this.  The rest of the page is very interesting too, as you might guess since it is titled "A Tour of the Radio Universe".

Hope the above is of interest to some folks out there!  

Maybe our radioastronomers in the group will tell us what is really going on :)

73,

Roger Rehr
W3SZ

On 3/20/2015 6:55 PM, JJ Mx wrote:

  Hi Roger,

  The beamwidth of my dish is 4.4° (3.3m @ 1.4 GHz)

  Find below the geometry of the sun/moon eclipse for my antenna location.

  I did the comparison with the optical eclipse time for the beginning and end of contact between moon and sun.

  Maximum was around 9:22 UTC.

  De : moon-net-bounces at mailman.pe1itr.com [mailto:moon-net-bounces at mailman.pe1itr.com] De la part de Roger Rehr W3SZ
  Envoyé : vendredi 20 mars 2015 22:00
  À : moon-net at mailman.pe1itr.com
  Objet : Re: [Moon-Net] Partial eclipse tomorrow

  Hi JJ,

  Re:

  The differences you observed RF vs optical [both depth and width of null]:

  What was the beamwidth of your dish?  

  I would think that the wider RF beamwidth could explain both observations, depending upon just how wide it was compared to the optical beamwidth, which would of course be quite narrow.  One would need to know the beamwidth and the geometric details of the occultation to do the calculation.

  73,

  Roger Rehr
  W3SZ  

  On 3/20/2015 2:13 PM, JJ Mx wrote:

    Moreover the radio eclipse started before the optical eclipse and finished later than the forecast. The sun seemed to be bigger at this frequency (probably coming from its hot chromosphere – I am not an expert and I have to find more info).

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