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MKV
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Reged: 01/20/11

Re: Optimized Newts vs APOs new [Re: Darren Drake]
      #5590942 - 12/27/12 10:29 AM

Quote:

According to my calculations this is reasonable. It's possible a Q could resolve a license plate as far away as 2.75 miles



Dawes limit is 4.6/D in arcseconds, where D = aperture diameter in inches. In theory, a perfect 3.5 inch aperture can clearly resolve 1.3 arseconds.

A 0.25 inch wide letter over 2.7 miles is below the ability of a 3.5 inch aperture to resolve.

A quarter inch (6.35 mm) over 2.7 miles (4.32 million mm) forms an angle of 0.0000842 degrees, which is 0.303 arcseconds.

This is what is meant by the Dawes limit:

http://upload.wikimedia.org/wikipedia/commons/thumb/4/46/Airydisks_dawes_sqrt...

Mladen


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MKV
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Re: Optimized Newts vs APOs new [Re: magic612]
      #5590949 - 12/27/12 10:36 AM

Quote:

It's that old saying in any telescope discussion:

"All things being equal."

As we know, they rarely are equal. An APO has no central obstruction. Then we get into optical windows, and CO, and how big is that CO, and what is thermal cool-down time, and... well, every other argument already discussed in this thread and many others.



Well there is a saying "to each his own".

Some differences are without distinction. Just because we can measure something doesn't mean it's (always) significant.

Theory (MTFs) clealry suggest that small COs are not significant. Some people claim they can see even the smallest CO. Yet no one has dared to take Ed Turco on his challenge!

Mladen


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Dave O
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5590997 - 12/27/12 11:11 AM

Quote:

Quote:

According to my calculations this is reasonable. It's possible a Q could resolve a license plate as far away as 2.75 miles



Dawes limit is 4/6/D in arcseconds, where D = aperture diameter in inches. In theory, a perfect 3.5 inch aperture can clearly resolve 1.3 arseconds.

A 0.25 inch wide letter over 2.7 miles is below the ability of a 3.5 inch aperture to resolve.

A quarter inch (6.35 mm) over 2.7 miles (4.32 million mm) forms an angle of 0.0000842 degrees, which is 0.303 arcseconds.

This is what is meant by the Dawes limit:

http://upload.wikimedia.org/wikipedia/commons/thumb/4/46/Airydisks_dawes_sqrt...

Mladen




Dawes’ limit applies to observations of two point sources (double stars) only.

Extended objects follow different "rules"; and in the case of dark markings, if they are linear (such as a license plate), the Dawes' limit can be exceeded. Experiments by W. H. Pickering indicate that if such a marking is as thick as one-fifth Dawes' limit it can be seen.


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kfrederick
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Re: Optimized Newts vs APOs new [Re: Dave O]
      #5591023 - 12/27/12 11:30 AM

How big was the plate numbers? some are large .The color of number and plate. The numbers are raised 3d might help. Lighting could help.Looking over water might be more stable air .not sure. .The eye of the observer ,could be a big factor some eagle eyed guy . lots of factors I could count the points on a deer horns over 2 miles away 20 inch f5 newt .

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Darren Drake
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Re: Optimized Newts vs APOs new [Re: kfrederick]
      #5591036 - 12/27/12 11:36 AM

Looking at the brochure I see that the pic was taken with a 180 or Q7 not a 3.5. This explains why it is so clear but still a Q3.5 could do it.

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Asbytec
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5591038 - 12/27/12 11:37 AM

Mladen,

That's all true, depending on what one means by resolution. I've "seen" double stars well below the Sparrow limit in a 6" scope, though not technically resolved with a black space separating them.

The letter might be 0.25" wide, but they are much taller. And they (might) obey the edge spread function, not the point spread function defining Dawes' limit. This is why Cassini can be resolved in modest apertures. You may not be able to "read" the license plate or resolve the letter "E", as you say.

But it wouldn't surprise me if you could "see" something was printed on the plate especially with an obstruction (struggling hard to stay on topic.) So, yea, maybe a bit of marketing in that claim and who is to say it's not accurate? Raleigh?

IME, reducing my CO by about 10% (.37D to .28D optimistically) showed an immediate improvement in stellar and lunar diffraction effects. Basically, bright stars lost the very dim third outer ring and the second was noticeably dimmer (now barely visible at all.) Planetary detail was much less obvious, less conclusive. In fact, I can only surmise there must be some improvement, even if it is so difficult to really tell.

But, can one notice improvement in stellar diffraction from 15% CO to zero? Dunno, maybe not. The first maximum is probably already relatively faint and the central maximum pretty close to EE ~ Strehl. So, if the 6" optimized Newt had a comparable Strehl to the refractor (~0.98 for both), a 15% CO would knock EE down to about 0.95. Then it becomes a test to see the difference between 0.98 (real) and 0.95 (nominal) Strehl. That might be impossible, even if all other things are held constant.

That's the math, anyway.


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ed_turco
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Re: Optimized Newts vs APOs new [Re: Asbytec]
      #5591071 - 12/27/12 11:56 AM

Dawe's Limit does not apply to lines, folks. You can see stuff on the moon or Saturn that you have no right to see using that limit.

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MKV
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Reged: 01/20/11

Re: Optimized Newts vs APOs new [Re: Darren Drake]
      #5591122 - 12/27/12 12:35 PM

Quote:

Looking at the brochure I see that the pic was taken with a 180 or Q7 not a 3.5. This explains why it is so clear but still a Q3.5 could do it.



That ad has been out for more than 30 years now. I remember it from the Questar catalogue from the late '70s and it left the impression the image was taken with the Q3.5 (don't forget this was film photography, not digital!)

Anyway I'd like to see some sort of equation that calculates extended object resolution.

Mladen


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MKV
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Re: Optimized Newts vs APOs new [Re: Darren Drake]
      #5591153 - 12/27/12 12:49 PM

Darren, maybe you should have used the USAF Test Pattern (rather than ordinary text) in conjunction with the Aberratior.

USAF 1951 Test Pattern


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gatorengineer
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5591254 - 12/27/12 02:06 PM

Ed,

You have raised a good point optical windows for long focus newts. There are very few long focus newts out there, there are very few of these with optical windows. As far as I know they are not made commercially. It would be neat to see someone start producing them for the ATM market.


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dpwoos
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Re: Optimized Newts vs APOs new [Re: gatorengineer]
      #5591260 - 12/27/12 02:15 PM

Is there any good evidence, or even good reasons to believe, that an optical window is a (significant, big, huge) improvement over an open tube and a spider?

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Jeff Morgan
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Re: Optimized Newts vs APOs new [Re: Dave O]
      #5591353 - 12/27/12 03:23 PM

Quote:



Dawes’ limit applies to observations of two point sources (double stars) only.

Extended objects follow different "rules"; and in the case of dark markings, if they are linear (such as a license plate), the Dawes' limit can be exceeded. Experiments by W. H. Pickering indicate that if such a marking is as thick as one-fifth Dawes' limit it can be seen.




And especially when it is a known and expected shape, such as letter of the Roman alphabet. The brain can infer the correct response with rather minimal data.


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ed_turco
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Re: Optimized Newts vs APOs new [Re: dpwoos]
      #5591548 - 12/27/12 06:00 PM

The difference is significant enough to make that Newt, given sufficient optical accuracy to challenge any APO of equal aperture. Why settle for a halftone image caused by a spider? That is what my challenge was all about.

And by the way, windows are not that tough to make.

Ed


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azure1961p
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Re: Optimized Newts vs APOs new [Re: magic612]
      #5592305 - 12/28/12 09:00 AM

"BTWm, Questar still makes fantastic claims that defy everything we know about optics. In this Questar brochure, which I remember back from the late 70's, the company claims a 3.5 inch clearly resolves license plate number (1/4 inch wide) at a distance of 3.5 km (2.2 miles) over water (!). That's a resolution of less than half an arc second, and theory (Dawes limit = 4.6/D in inches) says 3.5 inch aperture (assuming wavelength of 0.55 um) can resolve no more than 1.3 arc seconds under ideal conditions. "

Yes I had that beautiful red jacketed plush Questar catalog when I was a kid. It was Puget Sound on the west coast and I recall you COULD read the license plate numbers on the cars. Resolving that however isn't a function of Dawes...

The Dawes limit is for point sources and it's a standard the good reverend created to find uniformity in resolution perception over differing apertures per observation of that "notch" first appearing. The license plate res is something altogether different. Here we are seeing the results of LINEAR resolution. Typically we can resolve/observe fine lines 10 to 11 times finer than Dawes and the great Harvard refractor at one point resolved a distant human hair at 1/15 the Dawes limit for that 10" scope. Clearly the airy disc patterns are NOT the finest features an aperture will resolve. This is a hot topic among some however as the term "resolved" then comes into question as linear resolution is not the same as true angular resolution and that's a given. But as in the case of those license plates it's nothing to seize at either. If infact the photo of those parked cars was resolved strictly to Dawes and linear resolution was erased the plates would be smears and many things lumpy and ambiguous.

Darren did a nice article supported with revealing images that's revealing.

Pete


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magic612
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Re: Optimized Newts vs APOs new [Re: ed_turco]
      #5592347 - 12/28/12 09:35 AM

Quote:

The difference is significant enough to make that Newt, given sufficient optical accuracy to challenge any APO of equal aperture. Why settle for a halftone image caused by a spider?




Ed, shouldn't that - more accurately (perhaps?) - refer to a straight vane spider? I'm not trying to take anything away from the window. But my 3 vane curved spider makes for unnoticeable diffraction, and having looked through many Newts and refractors, provides a refractor-like view. No "spider spikes" whatsoever.

And it wasn't that tough to make either - a few pieces of brass, some solder and a propane torch.


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Asbytec
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Re: Optimized Newts vs APOs new [Re: magic612]
      #5592367 - 12/28/12 09:56 AM

Pete, I just worked the math, and the Dawes limit at 2.2 miles for a 3" Q is two parked cars separated by their side view mirrors. At that distance over water, you should just be able to tell they are cars. Two SUVs is another story.

Now, thats provided the numbers on the licence plate emit nor reflect no light. Once they do, they become subject to the MTF and low frequency resolution.


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gatorengineer
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Re: Optimized Newts vs APOs new [Re: Asbytec]
      #5592393 - 12/28/12 10:20 AM

There is calculational evidence. If you look at a 1.125 secondary in an 8 F8, its roughly 1 square inch of obstruction. When I look at my destiny 3 vane curved the thickness of the vanes approaches the same obstruction area. So I am in a very crude approximation "doubling" the total difraction, but the curved vane simply spreads it around, but it doesnt go away....

In the Northeast, I would need a truly spectacular night of seeing to be able to actually realize the benefit, but I think the benefit is there.


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Jon Isaacs
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Re: Optimized Newts vs APOs new [Re: MKV]
      #5592432 - 12/28/12 10:47 AM

Quote:

Yet no one has dared to take Ed Turco on his challenge!




A challenge accepted or ignored does not make a victory. I could venture a challenge to arm wrestle all takers. If no one were to accept my challenge, the wise course of action would be to not claim victory.

Jon


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Pinbout
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Re: Optimized Newts vs APOs new [Re: Jon Isaacs]
      #5592522 - 12/28/12 11:33 AM

Quote:



Quote:
--------------------------------------------------------------------------------

Yet no one has dared to take Ed Turco on his challenge!


--------------------------------------------------------------------------------



A challenge accepted or ignored does not make a victory. I could venture a challenge to arm wrestle all takers. If no one were to accept my challenge, the wise course of action would be to not claim victory.






without the scope created first, there is no challenge at all cause there is nothing to compete against except for dreams and promises.



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MKV
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Reged: 01/20/11

Re: Optimized Newts vs APOs [Re: Jon Isaacs]
      #5592544 - 12/28/12 11:48 AM

Quote:

Quote:

Yet no one has dared to take Ed Turco on his challenge!




A challenge accepted or ignored does not make a victory. I could venture a challenge to arm wrestle all takers. If no one were to accept my challenge, the wise course of action would be to not claim victory.

Jon



True, but then claims to the superiority of APOs should be mum.

Mladen


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