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Resolving limits and Seeing

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#1 astrokeith

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Posted 29 July 2021 - 01:16 PM

A while back we used a resolution test target (USAF1950 pattern) at TSP to measure telescope resolution. The test target was put up on a nearby hill primarily to assist attendees check their scope collimation, in particular check for astigmatism. Thanks to all those who participated and the results were published in Astronomy Technology Today back in 2014,

 

I thought I would reproduce in the next three posts the key findings - for discussion!

 

First the resolution achieved by various apertures and telescope types.

 

The cluster of refractors that have 'beaten' the Dawes limit were all Taks and APs!

The big aperture dobsonians clearly struggled to get near the limit, hampered by seeing across the ranch to the target. They were all looking at very low elevation and pushing their mirror edge supports to the limit!

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#2 AstroVPK

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Posted 29 July 2021 - 01:20 PM

Interesting... Can you tell us a bit more about the Dobs? Whom were the mirrors by and what sort of edge support did they use?

#3 astrokeith

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Posted 29 July 2021 - 01:20 PM

Next, a plot of maximum magnification the observers used to achieve their best resolution.

 

The refractors with their smaller apertures were able to achieve x120 per inch (red line)

whilst the big reflectors only did x20 per inch.

 

The SCTs sit in the middle much closer to the classic x50 per inch rule.

 

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#4 astrokeith

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Posted 29 July 2021 - 01:23 PM

and finally, yours truly volunteered to measure the limiting resolution of my airline portable 10" dobsonian through the night to see how seeing varied.

The short night, is when I fell asleep actually looking through the eyepiece and decided I had better go to bed!

 

The variation through the night and from night to night is enormous.

 

 

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#5 deepwoods1

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Posted 29 July 2021 - 01:47 PM

Do you have a list of the brands/scopes used available? The Mak/Cass data point seems like it was pushed way over its capabilities? Not so surprised about the light buckets as their size isn't "seeing" tolerant. I'd like to know about the SCT's? Thanks for posting these data, always interesting to see tests!



#6 Jon Isaacs

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Posted 29 July 2021 - 01:53 PM

The cluster of refractors that have 'beaten' the Dawes limit were all Taks and APs!

 

To play to devils advocate:

 

The test target (USAF1950 pattern) does not measure resolution in the same terms as the Dawes limit does. It basically measures lines per inch which is quite different from the two overlapping Airy disks of Dawes limit.

 

The Cassini division is a good example of the difference between line pairs and the Dawes limit.  The Cassini division is 0.75" in width and yet can be resolved by a 60 mm whose Dawes limit is about 2" 

 

Jon


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#7 astrokeith

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Posted 29 July 2021 - 01:56 PM

Do you have a list of the brands/scopes used available? The Mak/Cass data point seems like it was pushed way over its capabilities? Not so surprised about the light buckets as their size isn't "seeing" tolerant. I'd like to know about the SCT's? Thanks for posting these data, always interesting to see tests!

I do have that data, but we agreed with the scope owners that we wouldnt declare it. The observers were trying to work in at least pairs to improve accuracy, but there is still too many variables to start showing a 'league table'.

Better scopes tend to have better observers and so they come out especially well, which isnt fair on a budget scope with an observer with limited eyepieces to choose from and less developed observing skills.

 

For the SCTs, you could possible infer brand from the apertures!


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#8 astrokeith

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Posted 29 July 2021 - 02:05 PM

To play to devils advocate:

 

The test target (USAF1950 pattern) at TSP to measure telescope resolution does not measure resolution in the same terms as the Dawes limit does. 

 

It basically measures lines per inch which is quite different from the two overlapping Airy disks of Dawes limit.

 

Jon

Absolutely right. This was clearer in the original article.

It explains why the Cassini division is so visible, when perhaps it shouldn't according to Dawes.

 

We also had a set of 'double stars' of varying separation. (Fibre optics are wonderful), but observers found it very difficult to make observations and they all wanted to vary the brightness.

 

The advantage (and purpose) of the USAF target is that it shows astigmatism so clearly. The target was put up purely as a 'telescope check' aid for all attendees. Quite a few people came to me who had strong astigmatism which we had fun tracking down. (Beware of stock diagonals on some SCTs!)

 

So it could be said these results are only good for scope to scope comparisons. However a planetary observer would say that the test targets are very appropriate for their work.



#9 Supernova74

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Posted 29 July 2021 - 02:08 PM

Personally to a certain degree i feel a high graded optical lens will always out perform a mirror based scope in more mediocre transparent seeing conditions!however tho admittedly the results are very suprising.then on the other side of the coin the larger aperture scopes should out perform hands down regarding the smaller aperture refractors in good seeing conditions.is it one of these scenarios again there are so many variants if this was tested in s different environment would the results come to a completely different conclusion.were the reflectors collimated properly also sufficient cool down times etc.



#10 Jon Isaacs

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Posted 29 July 2021 - 02:16 PM

So it could be said these results are only good for scope to scope comparisons. However a planetary observer would say that the test targets are very appropriate for their work.

 

 

I think there are many factors involved. The ideal resolution target for a telescope is an object at high elevations in excellent seeing.  Dawes did a good job of establishing the limit for skilled observers for equal magnitude pairs and it agrees quite nicely with the basics of optics. 

 

It seems to me it is somewhat misleading to suggest some of the scopes beat the Dawes limit.. I have to think the seeing was the biggest factor for the larger scopes, I have made near Dawes limit splits with my larger scopes, under 0.5", but the pairs were at high elevations. Viewing objects near the horizon is fraught with difficulties even with relatively small scopes, that's one reason why spotting scopes generally max out at 60x-80x, even over short distances the thermal issues near the ground are overwhelming.

 

In my mind, the comparisons are probably best optimizing an individual scope but not for comparing maximum resolution under optimal seeing conditions...

 

Jon



#11 astrokeith

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Posted 29 July 2021 - 02:26 PM

Personally to a certain degree i feel a high graded optical lens will always out perform a mirror based scope in more mediocre transparent seeing conditions!however tho admittedly the results are very suprising.then on the other side of the coin the larger aperture scopes should out perform hands down regarding the smaller aperture refractors in good seeing conditions.is it one of these scenarios again there are so many variants if this was tested in s different environment would the results come to a completely different conclusion.were the reflectors collimated properly also sufficient cool down times etc.

The results were taken over a series of nights and their was an air of 'competition' among the observers to get the best out of their own scopes. Nearly all observers were what I would describe as 'advanced' and their scopes were well collimated. They also knew to wait for glimpses of good seeing etc.

 

There are interesting 'conclusions' that can be seen. The maximum magnification is one. Given poor seeing (looking to close to the ground) smaller apertures can take magnification many times higher than larger apertures.

 

The cluster of scopes below the Dawes line have apertures in the range 150-300mm. This is probably related to the Fried Parameter which describes the coherence length for any particular atmospheric turbulence.


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#12 Supernova74

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Posted 29 July 2021 - 02:30 PM

The results were taken over a series of nights and their was an air of 'competition' among the observers to get the best out of their own scopes. Nearly all observers were what I would describe as 'advanced' and their scopes were well collimated. They also knew to wait for glimpses of good seeing etc.

 

There are interesting 'conclusions' that can be seen. The maximum magnification is one. Given poor seeing (looking to close to the ground) smaller apertures can take magnification many times higher than larger apertures.

 

The cluster of scopes below the Dawes line have apertures in the range 150-300mm. This is probably related to the Fried Parameter which describes the coherence length for any particular atmospheric turbulence.

Well it’s definitely an interesting thread keith



#13 Supernova74

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Posted 29 July 2021 - 02:36 PM

Maybe irrelevant however what were the age groups of these amateurs?



#14 AstroVPK

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Posted 29 July 2021 - 02:40 PM

How do the Big Dobs compare when stopped down to the same aperture as the Apos? 



#15 astrokeith

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Posted 29 July 2021 - 02:44 PM

Maybe irrelevant however what were the age groups of these amateurs?

Not recorded, but from memory a good spread. I would say 35 - 65. But most measurements were repeated by 'buddies', which would have spread the age distribution even more.



#16 astrokeith

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Posted 29 July 2021 - 02:46 PM

How do the Big Dobs compare when stopped down to the same aperture as the Apos? 

I dont believe anyone did that, or at least they didnt report the results.



#17 astrokeith

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Posted 29 July 2021 - 02:48 PM

Do you have a list of the brands/scopes used available? The Mak/Cass data point seems like it was pushed way over its capabilities? Not so surprised about the light buckets as their size isn't "seeing" tolerant. I'd like to know about the SCT's? Thanks for posting these data, always interesting to see tests!

The MakCass stands out in its results and perhaps because it was f15.5!



#18 Supernova74

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Posted 29 July 2021 - 03:03 PM

.I was thinking does exit pupil come into play a little maybe to the overall results.

it may have a minuscule role as a guy of around 35 can have a exit pupil as much as 7.5mm compared to a 65 year old man low as 5mm this could effect the resolving power to a degree surly.


Edited by Supernova74, 29 July 2021 - 03:05 PM.


#19 Keith Rivich

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Posted 29 July 2021 - 03:52 PM

I did this experiment. Not sure I turned in the data sheet but my results jibe with the data.

 

25" scope.

Galaxy mirror.

Astro-systems support band. 

 

FWIW the seeing was much better just a handful of degrees above the target. I had to sit on the ground to see the graph on the mountain top. 


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#20 Keith Rivich

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Posted 29 July 2021 - 06:00 PM

How do the Big Dobs compare when stopped down to the same aperture as the Apos? 

As interesting as this test is to be fair the target would need to be near zenith to produce actionable data. 


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#21 Asbytec

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Posted 29 July 2021 - 06:42 PM

 

It explains why the Cassini division is so visible, when perhaps it shouldn't according to Dawes.

 

I am sure you know Raleigh and Dawes do not apply to extended objects. Extended object resolution depends very much on the contrast of the object and seeing conditions. It's not uncommon to best the Dawes limit. So, it's not surprising high contrast square wave line pairs can be resolved close to the maximum spatial frequency and even higher. As noted, Cassini division is a textbook example. I've seen fairly high contrast craters in full crater form on Plato's floor about 1 mile in diameter or subtending around 0.72" arc in a 150mm aperture (Mak Cas).



#22 astrokeith

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Posted 30 July 2021 - 02:38 AM

I did this experiment. Not sure I turned in the data sheet but my results jibe with the data.

 

25" scope.

Galaxy mirror.

Astro-systems support band. 

 

FWIW the seeing was much better just a handful of degrees above the target. I had to sit on the ground to see the graph on the mountain top. 

Do you still have the results?

 

I guess you where on the the upper field. The difference in elevation between the fields is bigger than most think (unless you are coming back from the snack bar, tired, and in the soft gravel.)

 

Most of the big apertures were probably hit by the Fried Coherence Parameter.

 

As interesting as this test is to be fair the target would need to be near zenith to produce actionable data. 

Haha! Perhaps one day drones will be stable enough to carry a target up?

 

We've now put the target up for about 6 years, and the consensus I think is that it is a useful 'quick check' tool. There were times when we had around 6 scopes all looking at it at the same time from the same part of the field. That was fun.



#23 Keith Rivich

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Posted 30 July 2021 - 07:56 AM

Do you still have the results?

 

I guess you where on the the upper field. The difference in elevation between the fields is bigger than most think (unless you are coming back from the snack bar, tired, and in the soft gravel.)

 

Most of the big apertures were probably hit by the Fried Coherence Parameter.

 

Haha! Perhaps one day drones will be stable enough to carry a target up?

 

We've now put the target up for about 6 years, and the consensus I think is that it is a useful 'quick check' tool. There were times when we had around 6 scopes all looking at it at the same time from the same part of the field. That was fun.

The data sheet is probably mixed in with old observing notes and charts. I'll take a look this weekend and see if I can't find them. And, yes, I set up on the upper field near the vendor building. 

 

Can additional data be gained by adding in a series of double stars of varying separations? Maybe have a time window for each pair so the altitude is similar for each observation. 


Edited by Keith Rivich, 30 July 2021 - 08:02 AM.


#24 astrokeith

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Posted 30 July 2021 - 08:34 AM

The data sheet is probably mixed in with old observing notes and charts. I'll take a look this weekend and see if I can't find them. And, yes, I set up on the upper field near the vendor building. 

 

Can additional data be gained by adding in a series of double stars of varying separations? Maybe have a time window for each pair so the altitude is similar for each observation. 

The double stars is probably the only way of achieving high altitude measurements, at the moment. I guess there are already reference lists of such doubles.

 

This would be a major exercise for each observer, probably taking some hours?



#25 Jon Isaacs

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Posted 30 July 2021 - 08:55 AM

The double stars is probably the only way of achieving high altitude measurements, at the moment. I guess there are already reference lists of such doubles.

 

This would be a major exercise for each observer, probably taking some hours?

 

Several hours?  I think somewhat longer.. how about a lifetime? 

 

Making difficult splits with larger aperture scopes are long term projects, waiting for that perfect night when the seeing is super, making sure the scope is fully thermally equilibrated, it's not something that just happens when you want it too.

 

A Rayleigh split in Kevin's 25 inch is 0.22" and that's only a 25% drop in brightness from the maximum. Twice the Rayleigh Criterion is a full contrast split, the first minima overlap, that's 0.44", such seeing is very rare..

 

My efforts in cooling my 13.1 inch are driven by my interests in splitting close doubles. I've managed a few under 0.5", most recently a 0.44" but they are projects..

 

Jon


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