16 replies to this topic

[JimP]

In terms of telescope resolution and planetary detail, I may have this wrong but it seems somewhere I read that the very best seeing conditions would be something around 0.4 arc seconds. Is this about right? If a 12” inch telescope has the ability to resolve approximately 0.4 arc seconds does this mean anything larger than 12” inches will show a brighter image but one is not likely to have seen conditions that will allow any finer detail to be seen. I know there are those of you out there who understand this far better than me so I await your thoughts.

Jim

Edited by JimP, 27 February 2020 - 06:46 PM.

[MitchAlsup]

What you state in amateur level astronomy is basically correct. Down hear in Texas away from the jet stream, I used to get about 3-5 nights per year where my C11 could not detect the atmosphere. I don't think in the 20 years I have owned my 20" F/4 that I have ever had a night where one could not detect the presence of the atmosphere.

 

At certain places, where they put large professional telescopes (Cerro Pranal, Cerro Tololo), the seeing can get down to about 0.2" for 10-20 minutes at a time.

 

A 12" telescope has the ability to resolve 0.5 arc seconds (Rayleigh limit).

 

As a telescope gets larger that the air currents refracting star light, the Airy disk breaks down into speckles, these speckles smear up the image, as the image gets brighter. HOWEVER, the images actually DO get better. The image in the scope properly sized for the atmosphere of the moment will look nice and crisp, but if you look through a telescope that is (a bit) disturbed by the atmosphere it will show features the smaller scope cannot, you may have to wait a while until the atmosphere has a large air pocket pass in front of the scope; but in those moments of "better" seeing, the larger telescope will let you see stuff the smaller one will not. {Caveat: all things being equal--which they seldom are in telescopy.}

[ngc7319_20]

Yes, that is pretty much correct... larger than about 12" will not add detail.  Larger may reduce noise, and it might let you employ techniques better like "lucky imaging" or computed deconvolution.

Edited by ngc7319_20, 27 February 2020 - 07:21 PM.

[Tom Polakis]

Unless the seeing is abysmal, a larger telescope will always show finer detail than a smaller one with equivalent optical correction.  My driveway typically has seeing that's on the order of 3 arcseconds, and my 15-inch shows better planetary detail set up right next to the 10-inch.  And the 10-inch similarly does better than my 4-inch refractor.

 

The quoted seeing is typically FWHM (full width half maximum) of a profile of a star's brightness based on images with long exposures, so it's not good at telling an observer what the instantaneous seeing is.  Veteran lunar/planetary/double star observers know to wait for moments of good seeing.  The FWHM value may be 3 arcseconds, but during those brief intervals, it's sub-arcsecond.

 

Tom

[Jon Isaacs]

This the way I see it.

 

The Dawes limit of a 12 inch scope is 0.38". 

 

But the Dawes limit not a good measure of the resolving power. In a Dawes limit split, the Airy disks are overlapping to the point where there is only a slight minima between the two centers.

 

Even the Rayleigh criteria involves overlapping Airy disks. In a Rayleigh split, the first minimum of one star passes through the center of than other star. A Rayleigh split in a 12 inch is "0.45". 

 

If one thinks that the image is being drawn in pixels the diameter of the Airy disk to the first minimum, the those pixels are 0.91" in diameter. This also is a measure of what a clean wide split is.

 

Once you are in the regime where the Airy disks are overlapping, contrast is reduced. 

 

One has to think in terms of a mapping function, points in the field plane are being mapped to the focal plane. Once the points are overlapping, contrast is lost.

 

Example:

 

The Dawes limit of a 4.56 inch scope is 1.0". That's a very difficult low contrast split, low contrast that requires excellent seeing and high magnifications. In a 10 inch scope, in 1", that will be a clean split because you're not fighting the limited resolution of the 4.56 inch scope.

 

This example is based on personal experience. The Dawes limit only applies to the high contrast situation of two stars ..

 

Jon

[TOMDEY]

Hi, guys! I used an excellent 12.5-inch Cave Astrola Newt for about a decade and stared at stars directly and with knife-edge to scrutinize wavefront and Airy Disc. Thousands of hours of imagery and hundreds of hours staring at guide stars or KE patterns. I'd enjoy reasonable Airy Disc (tight core and one pretty stable ring) maybe 10% of the time. On great nights that would hold in, sometimes for up to a half-hour straight  And that is indeed in the neighborhood of the half-arc-sec you mention. Keep in mind, that is a very squishy benchmark, depending on the behavior of both your telescope and the atmosphere. The BIG scope 0.2 arc-sec transitions into the realm of ~wishful thinking~ even for the professionals. They are just as prone to exaggeration as we amateurs are... possibly more so.

 

We were testing/scrutinizing a 12-inch imager for professional use, out in the lot aimed at a distant cell tower. Crummy atmospherics, sunny summer day, terrible thermals occasionally calmed by passing benign breezes. I put a beamsplitter in there and watched real-time video, so I could snap the shutter, whenever the image looked sharper than average. (Technique I had learned for improved stats on lunar and solar imagery). We collected hundreds of "select" images, and then went inside, to sift thru for the best ones. And, something rather amazing happened --- anticipated, but still surprising. A few of the select images manifested half-arc-sec "perfect" resolution. I already knew that the lens wavefront was crummy, but the explanation goes something like this: If the system's Zernike wavefront is badly-aberrated but smoothly-so (aka continuously differentiable) then at times (rare times) the atmosphere will improve (rather than degrade) the wavefront presented to the image. So, with sufficient (large) # of short-exposure snapshots, a few will be "diffraction-limited". 

 

Most of my assigned metrologies were spaced-based imagers. So, it is indeed true that an e.g. "Hubble-Class" big scope will and must resolve ~1/20 arc-sec to be considered ready to launch. Anything less is deficient. And that's because the atmosphere is no longer the limiter.    Tom

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[Keith Rivich]

In terms of telescope resolution and planetary detail, I may have this wrong but it seems somewhere I read that the very best seeing conditions would be something around 0.4 arc seconds. Is this about right? If a 12” inch telescope has the ability to resolve approximately 0.4 arc seconds does this mean anything larger than 12” inches will show a brighter image but one is not likely to have seen conditions that will allow any finer detail to be seen. I know there are those of you out there who understand this far better than me so I await your thoughts.

Jim

Theoretical limits aside my 25" blows away my 12 1/2" scope any time any conditions. 

[Starman1]

While larger scopes can resolve smaller perturbations in the atmosphere, which means they virtually never see a "still" atmosphere,

nonetheless, the seeing varies on a scale of days, hours, minutes, and even seconds.

When the seeing is appreciably "sub arc-second", the larger scope will reveal the details visible with that kind of resolution, while the smaller scope

will see a more stable, yet lower-resolution, image.

So in a given hour of the night, seeing will vary and the larger scope will occasionally reveal an image resolution not obtainable by the smaller scope.

And, in the event of truly spectacularly-good seeing conditions (which happens occasionally where I observe), the larger scope will simply make the smaller scopes

seem like regular resolution broadcast TV versus a 4K Blu-Ray disc image.

 

You may not like the image in a big scope under mediocre seeing conditions, relative to the small refractor, but you will always see more if you look for longer than a few seconds.

 

That assumes equal optical quality, though, a condition I think may be rare.

[JimP]

I always observe longer than a few seconds at a time. Lol!

We get nights of pretty darn good seeing here as well, maybe not as good as where you observe but good enough to show me the kind of detail on Jupiter that I have spent hundreds of hours trying to see and draw in the past with a large number of different scopes of various apertures. I guess rather than worry about it especially if you have to sit observing for hours to get a more detailed view for several seconds at a time with a larger scope I will just be happy with what I have. And on those nights of super duper seeing when the big DOB will show finer details  than my 10”, I can assure you, based on personal experience the view with my 10”, under those kinds of seeing conditions, will blow you away. My 10” apo on a night like that is quite impressive and certainly the comparison with a larger DOB will never be like a regular tv vs a Blu ray disc. Lol

 

My experience over 55 years of observing goes something like this. The larger the aperture the less number of nights per year of really good seeing although on those nights the view is better than anything smaller. But, is it worth it if you get one or maybe two nights a year where the larger aperture really works at its theoretical  limit?

 

best,

 

Jim

Edited by JimP, 28 February 2020 - 05:12 PM.

[Starman1]

But the larger scope doesn't have to operate at its theoretical limit to exceed the resolution of a smaller scope.

It does have to have good optics, collimated and cooled, something I don't see all that often.

[JimP]

I can agree with you on the collimators issue for sure. I have just spent the last hour working on collimating my New Moon 14.7”. Laser tublug then autocollimator. Up down and my back aches. But it’s clearing off and in a few hours I will be able to do some Deep Sky observing. Seeing is poor but the sky is clear. 

[Asbytec]

If a 12” inch telescope has the ability to resolve approximately 0.4 arc seconds does this mean anything larger than 12” inches will show a brighter image but one is not likely to have seen conditions that will allow any finer detail to be seen.

Jim

The way I see it, as Tom Polakis said above, this is a measure of width of the seeing induced in-focus diffraction pattern over time. At times seeing will be as good or better and easily diffraction limited, at other times it will be less and fall below the diffraction limit. Both over shorter intervals. During the better moments, a slightly larger aperture should be diffraction limited for short intervals at least some of the time for a "diffraction limited" scope that is not otherwise compromised by poor collimation or thermal instability. When the scope and the seeing are both diffraction limited, the view can be amazing. The diffraction limit for seeing is about Pickering 7/10 or better, which should easily be the case for a 12" resolving to the Dawes limit. I'd think the 12" would be relatively steady and the larger aperture less so, but the larger aperture still performing to it's limit at least some of the time. 

 

This may likely be the case for a slightly larger aperture, adequately prepped and thermally stable for observing, so long as seeing effects at that larger aperture do not fall below the diffraction limit, i.e., in seeing less than about Pickering 7/10 all of the time. But, even then, a larger aperture is packing those induced seeing effects into a slightly smaller seeing induced diffraction artifact due to it's increased resolution, so a larger aperture continues to retain most of it's resolution advantage until, as Don says "you may not like the image in a big scope under mediocre seeing conditions", which means and as Mitch describes, the aperture is bloating or speckling it's star images. At that point, all bets are off. For stars at 0.4" arc, anyway. 

 

For extended object resolution, I agree with Jon in that the Dawes limit (0.4" arc in this case) is not a good indicator of lunar or planetary resolving power. Dawes applies to two relatively bright high contrast point source diffraction patterns, not extended objects. When seeing is cooperating, we can actually see higher contrast detail to some degree well below Dawes. Even in the best seeing there are even better moments. During the best moments of the best seeing conditions, I've seen craters, in full crater form with a bright rim and a dark pit, on Plato's floor that subtended an angular diameter of ~ 0.70" arc less that Dawes calculated at 0.77" arc for a 150mm aperture. I saw it three times during the time I was observing Plato's floor at high magnification around 0.5mm exit pupil (300x in a 6" aperture). That crater was less than a mile in diameter (near apogee IIRC). It has nothing to do with Dawes, only of (high) object contrast "transferred" to the image on very small scales. 

 

Bottom line, as I understand it and somewhat by anecdotal evidence, in seeing that good there are better moments, coupled by the higher resolution of the larger aperture packing energy into smaller diffraction patterns, and that Dawes has nothing to do with extended object resolution. So, in my view, a larger aperture will still hold some or much of its resolving power until the tiny image begins to speckle and bloat in lesser seeing being affected by the aperture itself. I recall the general rule of thumb is, in theory and maybe empirically so, bloating will begin at about 3 times the aperture in diffraction limited seeing conditions. 

[Cotts]

Here is a way of looking at the flaw in using Dawes or Rayleigh limits to discuss the amount of detail/contrast transfer in telescopes.

 

Regarding the in focus diffraction pattern of a star.  There's a central disc of light, then a dark 'ring' - the first minimum - and then the first bright ring, a second minimum, a second (much fainter) maximum and so on.....

 

The radius, in arc seconds, to that first minimum is 138/D where D is aperture in millimetres..

To the first ring the radius is 163/D,  (telescopoptics.net)  These numbers are a few percent smaller for increasingly obstructed scopes but for this discussion we will stay with unobstructed....Also let's assume a fairly ordinary star of around 6th magnitude where the second bright ring is too dim to see...

 

Imagine your 150mm scope is showing a beautiful, motionless first ring on a night of good seeing at 300x or so.

 

The first minimum is at a radius of 138/150 = 0.92"  and the first ring is at a radius of  163/150 = 1.09".   These numbers differ by only 0.17", the dark space (depends on the brightness of the star) is, at most about 0.3" across.  Yet you can clearly see it....if seeing allows. 

 

This must mean that the  'balls of confusion' are only about o.3" or a bit less in size,  allowing you to see the diffraction pattern in all its glory in your 150mm scope.  At lower magnifications views will be 'sharper' and the views will be 'tighter' or more 'refractor-like' or whatever non-empirical descriptors you care to use...  Your 6-inch scope will perform to its resolution capability and deliver its very best contrast transfer in this scenario...  

 

 Now crunch these same numbers for a 16-inch scope.  400mm of aperture, so, first minimum will be at 0.35" and the first ring will be at 0.41" which differ by only about 0.06" now and the black space between the disc and the ring will be barely 0.1".

 

On the same night assuming the same approx 'balls of confusion' of about 0.3" the diffraction pattern,disc, dark space and ring will be smeared by the atmosphere into speckles or a fuzzball. No diffraction pattern for you!   The scope will not perform to its resolution capability nor achieve its best contrast transfer...

 

BUT....

 

The radius of this fuzzball in the 16-inch will be approximately 0.7" (radius to first ring of 0.41" plus about 0.3" of 'confusion').  

 

The bigger scope will still out-resolve and out-contrast-transfer more detail than the 6-incher!!  Even when you cannot see the diffraction pattern of stars in the 16-incher...

 

Now all of the above are for a night of decent seeing where a 6-incher can clearly see its diffraction ring.

 

Imagine much bigger 'balls of confusion' such that the 6-incher cannot see its diffraction pattern.  Just a fuzz ball maybe 1.0" or 1.5" in size.  'Balls of confusion of 1.0" or even larger..... In this situation the 16-inch  won't out-resolve or out-contrast the 6-inch.  The bigger scope will just show more detail/speckles in the fuzzball. 

 

The latter scenario plagues most of us on most nights. 

 

The TL;DR of all the above:

 

On nights of mediocre/poor seeing where the diffraction pattern is completely smeared out in a 6-inch, the 16-inch will have little or no advantage in resolution or contrast transfer.  These nights are all too common.

 

On nights of good/excellent seeing where the diffraction pattern is clearly seen in the 6-inch the 16-inch will still out-resolve and have better contrast transfer than the 6-inch even though the diffraction pattern is not visible.  These nights are less common but, at least in my neck of the woods, happen a dozen or so nights a year....

 

On nights when the diffraction pattern is discernable in a 16-inch there will be glorious viewing for its owner and the 16 will truly 'blow away' the 6 in all categories of viewing..  These nights are exceedingly rare even in the florida Keys and are unknown where I usually observe...

 

Dave

[skybsd]

Theoretical limits aside my 25" blows away my 12 1/2" scope any time any conditions. 

Yup..,   

 

Best.., 

 

skybsd 

[TOMDEY]

Theoretical limits aside my 25" blows away my 12 1/2" scope any time any conditions. 

Very nice! I occasionally mention that sort of thing... modestly holding up my 36" as the benchmark, relative to others' cute little scopes. But I never mention it, relative to my own smaller scopes, else suffer the paradoxical Catch-22 of becoming envious of myself!     Tom

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[Spikey131]

So I guess this all plays out like this:

 

On a typical night in the NE USA, Bortle 4 skies, and the seeing is OK but not great.  The moon is in the first quarter.

 

I am looking at beta Monceres in a 12.5" Dob, a C8 and an NP101.

 

In the NP101, I see three pearls in a comma shape.  Aesthetically, very pleasing.

 

In the C8, I see three pearls with a little  fuzz around them.  Almost as nice as the NP101, but not quite as perfect.

 

In the Dob, I see three fuzz balls which occasionally thin out into pearls, but most of the time, they are fuzzy.

 

Now I turn the scope to the Whirlpool galaxy.  The NP 101 shows a faint fuzzy spot, perhaps a little brighter in the middle.  The C8 shows a brighter fuzzy spot, maybe a little bigger than the refractor.  The Dob shows hints of spiral structure.

 

Now I turn to the terminator of the Moon.  It looks great in every scope, but the Dob clearly shows much more detail of every feature and greater contrast even than the refractor.  Looking through binoviewers on the Dob, I feel like I am about to land a lunar module.

 

(Sorry, but no near planets were available last week).

 

For me, I don't feel like any of the  telescopes employed was necessarily "better" than the others, just different in their strengths.  No doubt, if I had a scope with 8" more aperture, or Tom's 36", more galaxy details and a brighter view would be there to enjoy.  All of these tools of our hobby serve to help us appreciate the heavens above and they all have their strengths and weaknesses.

[AstroVPK]

We were very very excited to measure 0.2 arcsec seeing with the Subaru telescope when the first few HSC images started getting processed. 0.2 arcsec seeing is exceptional, even from the summit of Mauna Kea. Apparently there is a location on the south Pole that has 0.07 arcsec seeing, but I don't think my wife will approve new moon trips to observe from the South Pole 😁