Nooelec Mesh Antenna spatial resolution

[kb3puw]

I may have missed previous discussion on this, but does anyone have any data on the highest spatial resolution they have achieved with the Nooelect scope in a box type antenna?

[Marcus D. Leech]

On 17/12/2024 12:41, kb3puw wrote:

I may have missed previous discussion on this, but does anyone have any data on the highest spatial resolution they have achieved with the Nooelect scope in a box type antenna?

Because of the asymmetric beam of that grid antenna, it's likely equivalent to a 0.8m parabolic dish in terms of
  beam-width.  Which means about 18deg beam-width.   You can muck-about with feeds on it to optimize the pattern
  a little bit, but no amount of post-processing gets you past the inherent diffraction limit of the antenna.

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[kb3puw]

Thanks - that is what I wanted to know.  So to summarize, it is not theoretically possible to statistically distinguish two points closer than 18 degrees apart?

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[Marcus D. Leech]

On 17/12/2024 12:55, kb3puw wrote:

Thanks - that is what I wanted to know.  So to summarize, it is not theoretically possible to statistically distinguish two points closer than 18 degrees apart?

Essentially, yes.  The details depend on the beam-shape and how close it is to a Gaussian spatial distribution, the
  shape and magnitude of side-lobes, etc.  But anything you do to the "optics" is only going to have a minor
  impact on spatial resolution for a given aperture.

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[Alex P]

Paul,

18 degrees equates to 1.2  hours of RA drift ..   I can detect level shifts in the data in the 5 minute intervals ..

The plots reveal changes better than that, and these are after all, just large fuzzy ISM clouds with no 'points' to resolve ..

an 8 hour period of time

[b alex pettit jr]

Radio telescopes are somewhat the equivalent of  an optical telescope with a  1 pixel frequency and amplitude  measuring camera ( spectrometer.)

[kb3puw]

Please excuse any naivety on my part - I am still using the excuse that I have only been involved in astronomy and radio astronomy for months 😀

These are some photos I took of Saturn.  The first is a single frame.  The second an average of about 300 frames.  The third was application of wavelet filtering.

I was hoping that with enough data and application of computational manipulation I might be able to significantly exceed the beamwidth of the antenna.  

Below the Saturn photos is the manufacturer's plot of the Nooelec antenna characteristics.  Because it looks kind of "pointy" I thought that with enough data it might be

possible to get better resolution than the beam width.  Is this an unrealistic expectation?

On Tue, Dec 17, 2024 at 12:20 PM 'b alex pettit jr' via Society of Amateur Radio Astronomers <sara...@googlegroups.com> wrote:

Radio telescopes are somewhat the equivalent of  an optical telescope with a  1 pixel frequency and amplitude  measuring camera ( spectrometer.)

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[Marcus D. Leech]

On 17/12/2024 13:33, kb3puw wrote:

Please excuse any naivety on my part - I am still using the excuse that I have only been involved in astronomy and radio astronomy for months 😀

These are some photos I took of Saturn.  The first is a single frame.  The second an average of about 300 frames.  The third was application of wavelet filtering.

I was hoping that with enough data and application of computational manipulation I might be able to significantly exceed the beamwidth of the antenna.  

Below the Saturn photos is the manufacturer's plot of the Nooelec antenna characteristics.  Because it looks kind of "pointy" I thought that with enough data it might be

possible to get better resolution than the beam width.  Is this an unrealistic expectation?

What you're effectively doing with "stacking" is both improving low-light sensitivity in your optical telescope, and
  allowing the *sensor* to achieve close to the diffraction limit of the underlying optics.

Now, there are transforms that effectively "hallucinate" data that isn't really there in order to provide better-looking
  images, but my understanding is that you cannot exceed the diffraction limit of your single-mirror telescope
  using "fancy post-processing".

The only way to improve spatial resolution with a small antenna is to use more than one of them at a time, and
  use interferometry to achieve image synthesis.  Full-on image synthesis requires a real dedication to your
  craft that many amateurs have neither the skill nor the patience nor the budget for.  There are exceptions, of course.
  Jim Abshier has done image synthesis at ~400MHz or so (correct me if I'm wrong).

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[b alex pettit jr]

THIS is what your Radio Telescope 'sees'

Similar to what you would get if you put a piece of tissue paper over the end of the optical telescope.

The  FOV is the 'beamwidth', and the light falloff is the beam pattern ..

= There is nothing to 'sharpen'

Alex

[Anthony]

I remember the article by Jim Abshier, Marcus. 

Amateur Radio Astronomy: 400 MHz Interferometer.

https://websites.umich.edu/~lowbrows/reflections/2007/jabshier.1.html

by Jim Abshier
Printed in Reflections: March, 2007.

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[Anthony]

I like that example, Alex! 

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[kb3puw]

This diagram is how I was thinking about the problem.  Does this simply not work out mathematically?

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[b alex pettit jr]

beamwidth is typically defined as  being the half-power width ( 0.707 of peak amplitude )  .. as shown

Nothing precise, just a gradual roll-off

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.

[kb3puw]

The reason I am delving a bit deeply into this subject is that I have successfully mounted my antenna on my Celestron AVX motorized GoTo equatorial mount.

I have written software to both control the mount position and use ezRA to collect the data.  I will be able to find pretty much any position

precisely by substituting a small optical telescope with a camera and using Sharpcap software GoTo function (which uses plate solving for precise localization).

The camera is very closely aligned with the antenna post so when I switch over the antenna should be on axis.  My goal is to create a mosaic of some

celestial object by sampling in a matrix around the center of the object.

The problem I am working on now is selecting a target which has some reasonable chance of being detectable and large enough to image with my very primitive antenna -

no doubt the Achilles heel of my whole project.

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[b alex pettit jr]

Hi Anthony,

Some interesting math :

Calculate the equivalent  wavelength aperture of an optical refractor telescope at green  500nm

to a  radio telescope  2m dish at H1 21cm  

Alex

[bsn...@gmail.com]

Coming a little late to this disussion ...and a bit off-topic...

A few years ago I used  visible band satellite images to study glaciers. 

For a while the best imagery that was generally available had a spatial resolution of 45m per pixel. Then 15m and 5m imagery became available for the same glaciers. Histororical images are important in order to "see" how a glacier changed over time.

Question: Could you "tease" more details out of a glacier if you had 45m (old) and 10m (new) imges of the same glacier???

Turns out that by merging the two images using various wavelets schemes the resultant image was "better" than the old 45m image.

BUT , as Marcus observes, , "there are transforms that effectively "hallucinate" data that isn't really there in order to provide better-looking

  images, but my understanding is that you cannot exceed the diffraction limit of your single-mirror telescope
  using "fancy post-processing".

Nor can you really "see" details in a 45m image that are not there. But, if you are interested in recording the position of the termius terminus or width or the velocity of a glacier every little of "resolution" helps.

....bill...

[Adrian]

According to the Co Pilot AI :  To achieve the same resolution with an optical refractor telescope at a wavelength of 500 nm as a 2-meter dish radio telescope at 21 cm, you would need an aperture of approximately 4.76 micrometers.

As a comparison, the diameter for a pinhole optical camera with a focal length of 100mm would be an aperture hole of  0.015 mm or 15 micrometers.

   This really brings to perspective the huge difference in wavelengths involved in observations between optical astronomy and observations of the HI hydrogen 21cm line. We really need really huge radio telescopes for our backyard hobby.

Adrian

[Marcus D. Leech]

On 19/12/2024 21:30, Adrian wrote:

According to the Co Pilot AI :  To achieve the same resolution with an optical refractor telescope at a wavelength of 500 nm as a 2-meter dish radio telescope at 21 cm, you would need an aperture of approximately 4.76 micrometers.

As a comparison, the diameter for a pinhole optical camera with a focal length of 100mm would be an aperture hole of  0.015 mm or 15 micrometers.

   This really brings to perspective the huge difference in wavelengths involved in observations between optical astronomy and observations of the HI hydrogen 21cm line. We really need really huge radio telescopes for our backyard hobby.

Adrian

Or two or three modest dishes and a lot of land (and cables!).

On Wednesday, December 18, 2024 at 2:03:45 PM UTC-8 b alex pettit jr wrote:

Hi Anthony,

Some interesting math :

Calculate the equivalent  wavelength aperture of an optical refractor telescope at green  500nm

to a  radio telescope  2m dish at H1 21cm  

Alex

On Tuesday, December 17, 2024 at 01:51:07 PM EST, Anthony <itpart...@gmail.com> wrote:

I like that example, Alex! 

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[Adrian]

LOL

[Marcus D. Leech]

On 19/12/2024 21:56, Adrian wrote:

LOL

Well may you LOL,  but interferometry is an approach to the resolution issue that doesn't require huge antennas, if
  the goal is largely slanted towards resolution.   Obviously, *sensitivity* is proportional to the size of your dish.

But in interferometry, resolution is proportional to dish spacing.

So, let's say your observing goal was to get decent-resolution measurements of the top 20 brightest discrete
  sources.    You could do that with a few modest dishes, and some patience.

So, someone who lived on a chunk of land out in the countryside could easily build-out an instrument capable
  of impressive (for an amateur) resolution.   I myself used to live on 37 acres.  I had plans to build an interferometer
  of 3 or 4  3.7m dishes, with the widest spacing being over 300m.   But my wife at the time had other plans....

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[b alex pettit jr]

Yes  : its worth the simple math to 'see'  the Huge Diff betw ( back yard )  Optical and Radio Telescopes !

==========================================================================

            Alex

[A. C.]

Marcus, thank you for reminding me of the incredible power of interferometry with the ability of dispersed receiving antennas for proportionally increasing the resolution of a synthesized aperture. I was totally flabbergasted when initially the Event Horizon Telescope Collaboration group managed to resolve the SMBH in M 87 at a resolution of 25 milliarcseconds by use of up to 10 observation radio telescopes around the globe for the super VLBI resolution required for such a feat. Then when they were able to repeat that and observe our own  Milky Way, Sagittarius A* SMBH with at a resolution of only 19 milliarcseconds. So I agree, the ability to utilize interferometry to better resolve astronomical objects has been a great boon for radio astronomy. Alas, I LOL at the still required necessity of being able to space even just two antennas in a measly urban backyard of maybe 15' diagonally. I've seen old Winegard C-band dishes bigger than that not able to fit in my allotted space. I am sorry that your better half prevented you from establishing what truly would have been an incredible amateur interferometer. In your 37 acres at minimum you would have been able to rival the resolving power of the 100 meter GBT. I also can't imagine as you also stated the incredible expenditure of cable runs for those far flung dishes that they would require. I guess something like LMR1700 would be needed and even at an attenuation of only ~1.25 db/100' @ 1420MHz it might require some specialized amplification along the way in order to keep the proper phasing of the dishes and all.  If we all had the purchasing power of one of the ultra billionaire types we could even all record our observations around the world with atomic clock accuracies and send in our observations to a centralized supercomputer SARA equipped correlator facility and duplicate the same black hole observations, truly what an accomplishment for amateur radio astronomy that would be. Well we can always dream of that imaginary perfect dish or set of dishes for the ultimate amateur radio telescope. So meanwhile I just have to plod along with the wife not giving too much grief for that 180 cm ugly dish in the backyard.

Adrian

 

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 Dave Bowman