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Premium dobs Vs mass produced dobs

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#26 Arctic eye

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Posted 09 September 2020 - 01:38 PM

It is a common misconception that seeing somehow hides optical imperfections and you can't get benefit from good optics if the seeing is average. In reality, first the seeing distorts the wavefront at some amount and then the telescopes optical aberrations add on to that and smudge the already smudged image even further. So better optics give always better image, no matter of seeing, if other factors (aperture etc.) are kept the same. And when we go to high-res and especially lucky imaging, the quality of the optics is even more important in less than ideal seeing: Seeing is a somewhat statistical phenomenon and even in poor seeing, there is always some probability for good frames and if you capture 50000 frames of Jupiter, there is maybe still 300 good frames and you get an image. But if your optics are bad, even those 300 good frames become bad and you don't get an image. 

 

The above post is an exception to the rule (IMO).  I find even a mass produced 12" is more pleasing than a 10" Premium for DSO's.

For Planetary/Lunar It's trickier but the premium mirror needs excellent seeing to overcome even the 2" Aperture difference.

Maybe in general, but the OP was specifically asking about optical quality and resolution on 20" Stargate Dob, so it is not an exeption unfortunately. There are at least two threads in this forum on 20" stargate and maybe one on 18" stargate, and my case is not the worst at all. For example see this post from one of those threads: https://www.cloudyni...vice/?p=9484986

 

If one wants to do high resolution work, Stargate 20" is not the preferred choice. Much better odds with the 16":er.

 

Ironically, after 2 lousy SW20" mirrors, I donated the optics away and modified my Stargate for 24" custom made mirror that turned out to be bad as well... The manufacturer refigured it once, but we still couldn't get it to work, so took it back. Now, after 4 years since I originally bought the Stargate, I have this huge 24" Stargate based scope laying around without optics 4.gif

 

In two months or so, I will receive 24" Mirrosphere (Franck Griere) optics for it. I will then repeat the side by side comparison for my SW16", with and without masking the aperture to the same and even smaller, maybe on several nights to have different seeing, to get some actual images on the "premium effect". axe.gif

 

 


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#27 Keith Rivich

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Posted 09 September 2020 - 01:39 PM

I fear we are getting into comparisons that do not actually reflect real life observing. Each scope has to be pushed to its fullest under varying conditions to fully realize their strengths and weaknesses. Only then can some comparisons be made.

 

IMHO:

Using the planets to compare similar scopes at the same time and same conditions works just fine. 

 

Using DSO's to compare similar scopes at the same time and same conditions also works just fine.

 

Trying to compare two vastly different scopes under different conditions is just wasting keyboard time. To much going on. To really appreciate the similarities and differences one has to own both scopes and put in a lot of hours at the eyepiece. 


Edited by Keith Rivich, 09 September 2020 - 03:00 PM.

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#28 Arctic eye

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Posted 09 September 2020 - 04:00 PM

 

 

IMHO:

Using the planets to compare similar scopes at the same time and same conditions works just fine. 

 

 

Well, that's just what I did with the Moon in the images I provided earlier; Scopes standing side by side, imaging train switched from one scope to the other and again back several times. Taking short videos and stacking. Little seeing fluctuations average out during several rounds. Did not even disconnect the camera from the computer (firecapture) in between. Didn't take many seconds to do the switch. Only way to make it better would be imaging with two cameras simultaneously, but can't really do that alone. Planning to do the same test when I get my new optics, scopes standing side by side. If I mask the aperture to be same, only difference comes from the different focal length, but can compensate for that somewhat in image processing and stacking. Also want to test (by aperture masks) how aperture affects in certain seeing if optical quality stays the same.

 

I have seen this premium vs. mass-produced - discussion coming up again and again without anybody even trying to do anything to actually test it in real life and show some actual images on it. That's what I'd call waste of keyboard time. Same goes with the everlasting discussions whether smaller or larger aperture is better in certain seeing... Well, I'll do the tests anyway. Because I want to know myself, if nobody else does...


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#29 Keith Rivich

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Posted 09 September 2020 - 04:12 PM

Well, that's just what I did with the Moon in the images I provided earlier; Scopes standing side by side, imaging train switched from one scope to the other and again back several times. Taking short videos and stacking. Little seeing fluctuations average out during several rounds. Did not even disconnect the camera from the computer (firecapture) in between. Didn't take many seconds to do the switch. Only way to make it better would be imaging with two cameras simultaneously, but can't really do that alone. Planning to do the same test when I get my new optics, scopes standing side by side. If I mask the aperture to be same, only difference comes from the different focal length, but can compensate for that somewhat in image processing and stacking. Also want to test (by aperture masks) how aperture affects in certain seeing if optical quality stays the same.

 

I have seen this premium vs. mass-produced - discussion coming up again and again without anybody even trying to do anything to actually test it in real life and show some actual images on it. That's what I'd call waste of keyboard time. Same goes with the everlasting discussions whether smaller or larger aperture is better in certain seeing... Well, I'll do the tests anyway. Because I want to know myself, if nobody else does...

Perhaps a "waste of keyboard time" was a poor choice of words. I'm just saying there are to many variables to just make a blanket statement about custom (I do not like the word premium) built vs mass produced. We can fill these pages with comments but until one observes with the scopes in question its just chatter.

 

What you did is probably a fair test. Hard to say. Was collimation the same? Temperature? Focus? Secondary? Focuser sag? Would be interesting to have each mirror (including the secondaries) tested and see if there is a measurable difference that aligns with your images. 

 

At the observatory that I volunteer at there are a few spots on the observing deck that look out over the A/C condensers. When they cycle on the view is terrible, while just 5 feet left or right one can get perfect seeing. Likewise there are spots on the deck (2nd floor concrete) that vibrate when people walk by. Lots of variables...


Edited by Keith Rivich, 09 September 2020 - 05:34 PM.


#30 junomike

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Posted 09 September 2020 - 04:19 PM

One must also consider these “laws of physics”…

 

Seeing will limit a telescope’s resolving potential and in most US locations the seeing will be the determining factor regarding the potential resolving capability of a telescope.

 

Consider…

 

“How often do nights of excellent seeing occur? At the William Herschel Telescope site in the Canary Islands, even this superb viewing location  (second best in the northern Hemisphere) has many nights of relatively poor seeing: the distribution is positively skewed, and at this excellent site, a 10 inch telescope will be seeing limited on 9 out of 10 nights.”

 

I presume most don’t live in a location with anything like the seeing at the above.

 

Here is a quote “from the link you posted”…

 

“The ability of a telescope to resolve to Dawes' limit is usually much more affected by seeing conditions,...”

 

One must also consider the optical quality of the primary. For example, a 1-wave optic will show soft images no matter how large it is. Images might be bright but resolution will be impacted.

 

Bob

Very true for Planets/Luna, very un-true for DSO's.  Even in poor seeing, the larger aperture always wins.



#31 Glory Eye

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Posted 09 September 2020 - 04:56 PM

Looks like we've arrived at the usual place in this ongoing topic. That each scope is different.

 

Here's a quick, easy, and free experiment that you big scope guys can try tonight. Craft a ~spinap~ off-axis stop for your light bucket, whatever its certified wavefront quality. I made mine out of foam board, 6.5-inch on my 17.5-inch Coulter. Aim at e.g. Jupiter, Saturn, opens, globs, nebulae, galaxies. Be aware that is the combination of your scope, atmosphere, eye... that combine to formulate your perception. Here are my observations, on an averagely decent night:

 

> Saturn - spinning the ap while viewing finds the sweet spot on the high-side of the tube. Image luxuriously-resolved, better than any 6.5-inch refractor, because chromatic aberration is identically zero, wavefront is superb, and atmosphere is supporting good resolution. At F/12 200x 1mm pupil, my eye is enjoying 20/10 acuity. Saturn is razor-sharp and holding that breathtaking presentation. Contrast is wonderful and the ball and rings are bright enough, not too bright. Yank off the mask and the planet degrades to OK but not at all impressive. Seeing is meh and the ball and rings look ~too bright~ ... but... but... a bunch of cute little moons pop into view, that I hadn't noticed before!

> Jupiter - ditto.

> Open Clusters - ditto, but generally prefer full aperture.

> Globulars - interesting, prefer full aperture.

> Nebulae - interesting, generally prefer full aperture.

> Galaxies - full aperture blows the socks off any smaller scope.

 

I have the best of both worlds in this one cheap scope, on a night of decent, but not superb seeing, just for building a half-pound foam-board OAS.

 

You can craft other experimental stop boards. Be sure to make them so you can readily pop on and off, and spin them, while looking through the eyepiece. On the very best of nights, you will want to leave the stops off... if your scope is superb, if the sky is superb, if your eyes are superb... noting that each of those three is very rare. For most of us, that last one is most commonly deficient and most commonly ignored.    Tom

When seeing is bad, I put a 12" concentric mask on my UTA and I get an immediate improvement in the image, but you seem to be saying an off axis mask as you have described will really do the trick.


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#32 Asbytec

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Posted 09 September 2020 - 05:15 PM

At F/12 200x 1mm pupil, my eye is enjoying 20/10 acuity. Saturn is razor-sharp and holding that breathtaking presentation. Contrast is wonderful and the ball and rings are bright enough, not too bright. Yank off the mask and the planet degrades to OK but not at all impressive. Seeing is meh and the ball and rings look ~too bright~ ... but... but... a bunch of cute little moons pop into view, that I hadn't noticed before!

 

Tom, what about the 17.5" at some higher magnification approaching 400x or closer 1mm exit pupil? I know you are also magnifying seeing, but might also see more of the scope's resolution given the seeing. Even in seeing, up to a point of bloating, point sources become uglier but they are also smaller. I am not sure how that applies to extended objects, but surely there is a higher resolution image in the 17/5" that remains unseen at 200x. 



#33 TOMDEY

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Posted 09 September 2020 - 05:45 PM

It is a common misconception that seeing somehow hides optical imperfections and you can't get benefit from good optics if the seeing is average. In reality, first the seeing distorts the wavefront at some amount and then the telescopes optical aberrations add on to that and smudge the already smudged image even further. So better optics give always better image, no matter of seeing, if other factors (aperture etc.) are kept the same. And when we go to high-res and especially lucky imaging, the quality of the optics is even more important in less than ideal seeing: Seeing is a somewhat statistical phenomenon and even in poor seeing, there is always some probability for good frames and if you capture 50000 frames of Jupiter, there is maybe still 300 good frames and you get an image. But if your optics are bad, even those 300 good frames become bad and you don't get an image. 

 

Maybe in general, but the OP was specifically asking about optical quality and resolution on 20" Stargate Dob, so it is not an exeption unfortunately. There are at least two threads in this forum on 20" stargate and maybe one on 18" stargate, and my case is not the worst at all. For example see this post from one of those threads: https://www.cloudyni...vice/?p=9484986

 

If one wants to do high resolution work, Stargate 20" is not the preferred choice. Much better odds with the 16":er.

 

Ironically, after 2 lousy SW20" mirrors, I donated the optics away and modified my Stargate for 24" custom made mirror that turned out to be bad as well... The manufacturer refigured it once, but we still couldn't get it to work, so took it back. Now, after 4 years since I originally bought the Stargate, I have this huge 24" Stargate based scope laying around without optics 4.gif

 

In two months or so, I will receive 24" Mirrosphere (Franck Griere) optics for it. I will then repeat the side by side comparison for my SW16", with and without masking the aperture to the same and even smaller, maybe on several nights to have different seeing, to get some actual images on the "premium effect". axe.gif

Hi, Ari! Believe it or not, that statement is not true. Although highly counterintuitive, a fundamentally-worse wavefront (but otherwise identical) telescope can (admittedly only occasionally) give a better image. It is consequent of the random-walk theorems of statistics, applied to the Zernike constitution of the ever-varying atmosphere. A special (common) case is where the atmuspheric column is a continuous and continuously-differentiable function in lateral space and some continuous subsets of time (and therefore piecewise Zernikeizable). In layman' terms, the atmosphere will occasionally/fleetingly ~correct~ the wavefront to better than it would be in a quiescent atmosphere.

 

I've actually taken advantage of this! One of the departments had commissioned a telescope to an outside vendor, and brought into our test labs for certification. I set it up in a superb autocollimation test tower and found it grossly out of alignment. I had the techs tune it up to best achievable... and it was still pretty stinky deficient. The guys who brought it in were at wit's end, probably figuring they would be blamed for foolish outsourcing a custom build. I told them to return the next day. We took it outside and imaged a distant cell tower through crummy atmosphere. I used a familiar trick that relies on that Zernike stats theorem. Snapped a pile of images when the live-view image looked unusually good, hauled that data into the lab, culled through the bests to further down-select, and then derived, coded and ran a Zernike max variance sorter to further cull. These images did indeed satisfy the isotropic Rayleigh, Dawes, and Sparrow extensions in Zernike-Space. I delivered those images to the engineers, when they slithered back in... and handed them the true statement... "These images, captured with your system, are consistent with diffraction-limited performance". I refused to call the system good. They were satisfied, and slithered back out... no doubt to create charts extolling the virtues of that crummy scope. Any of you optical test guys who have worked in industry know the feeling. Often under pressure to call something good, even when you know it's not. That little referenced scope was only twelve inches. Rumor has it that such legerdemain worked its way all the way up to a 2.4 meter scope! Is that possible?!    Tom


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#34 TOMDEY

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Posted 09 September 2020 - 06:06 PM

Tom, what about the 17.5" at some higher magnification approaching 400x or closer 1mm exit pupil? I know you are also magnifying seeing, but might also see more of the scope's resolution given the seeing. Even in seeing, up to a point of bloating, point sources become uglier but they are also smaller. I am not sure how that applies to extended objects, but surely there is a higher resolution image in the 17/5" that remains unseen at 200x. 

That was a Coulter Dob with good but not magnificent wavefront. Now I'm using a 36-inch Fullum tnat has great wavefront.    Tom


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#35 CHASLX200

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Posted 09 September 2020 - 06:19 PM

”I was wondering would a 12.5 teeter match a 20" skywatcher Stargate dob ?”

 

Yes it would, at least based on my experience on two of those 20”:s (actually original optics and then a replacement mirror Synta sent me).

 

My SW16” dob was far better than the 20” stargate, hands down. I think an 8” can beat the 20”. Below a comparison between my 16” and the better of those 20” mirrors. Imaged using same imaging train in turns in both scopes, side by side, several videos in turns...

 

The second 20” mirror was so bad that stars looked like gearwheels. 

 

400p_500p_comparison.gif?img=full

 

Platoncomparison.gif?img=full

I had a 18" F/5 Tectron with a Nova mirror. It was ok at low power but at high power planets looked like they needed half a turn of the focus knob to snap into focus but never would. Looks just like your first pic of the moon.


Edited by CHASLX200, 09 September 2020 - 06:19 PM.


#36 havasman

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Posted 09 September 2020 - 06:34 PM

I have scanned, but not really carefully read this string so I apologize if this has been covered but as aperture increases significantly in a Dob the difficulty of controlling critical structural characteristics of the scope also increase and not necessarily in a linear manner. These characteristics definitely influence the performance of the scope both optically and ergonomically. Experienced custom scope makers are often challenged when customers want larger and larger scopes. Even they find a new learning curve must be negotiated. They almost always do it well but still it must be done.

 

I think it less likely a mass market maker will often do more than scale up their smaller designs.

 

The problems big mirrors present are different from these matters.

 

There are advantages beyond just aperture, as several posters with experience with larger aperture Dobs have said.


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#37 havasman

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Posted 09 September 2020 - 06:39 PM

I have a friend who owns an Orion 12i dob. The views are quite nice. I know he bought it used on the cheap. Can’t think a premium dob would give better views, IMHO.

I too have observed with another club member's XX12i and think it quite good. But you should avail yourself of the opportunity to observe with a premium Dob if it ever presents itself as I think it might offer some valuable insight into their relative performance.


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#38 KBHornblower

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Posted 09 September 2020 - 06:44 PM

My 17.5" Coulter mirror, made in 1985, shows a distinctly triangular pattern just outside and inside of focus.  I rotated the mirror 60o and the pattern rotated with it, indicating that it is frozen into the mirror rather than being caused by my 9-point cell.  Nevertheless in good seeing it makes 1 arcsecond spots with faint stars and resolves the planets about as well as my 6" Newtonian and my Celestron 8.  For the price I paid, at most 1/3 of what I would have needed for a better mirror, I would say it is satisfactory for my primary use, which is going deep.  From what I have been seeing in this forum, I guess I did well in a roll of the dice.  For this aperture Dawes' limit would be about 1/4 arcsecond, seeing permitting which is seldom.

 

My educated guess is that with these thin mirrors, the fine figuring needs to be done TOT with subdiameter laps and the mirror supported by the same sort of flotation system as in the finished telescope.  Any comments on this remark would be most welcome.



#39 Keith Rivich

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Posted 09 September 2020 - 10:10 PM

When seeing is bad, I put a 12" concentric mask on my UTA and I get an immediate improvement in the image, but you seem to be saying an off axis mask as you have described will really do the trick.

When the seeing gets that bad I break out the ice cold Grey Goose and my 8x56 binocs!


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#40 Jon Isaacs

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Posted 10 September 2020 - 04:50 AM

When the seeing gets that bad I break out the ice cold Grey Goose and my 8x56 binocs!

 

When the seeing is on the poor side, I don't bother with the planets, I take what the skies gives me.

 

When the seeing is on the good side and the skies are dark and clear, I don't bother with the planets, that's the time for the good views of DSOs.

 

For me, the planets and double stars are backyard urban objects.  My backyard often has quite good seeing and sometimes it's under 1/2". I think top notch 12.5 inch is a good planetary/double star scope, big enough to take advantage of very good seeing, not so big that it's a hassle, not so big it's got issues when the seeing is only so-so.

 

I didn't buy my 22 inch nor the 25 inch before it to view the planets. They have provided me with some wonderful planetary views but it's about deep sky.  That's why I bought it, that's what I use it for.  It lives out in the high desert.. where it belongs.

 

Jon 


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#41 bobhen

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Posted 10 September 2020 - 08:08 AM

One must also consider these “laws of physics”…

Seeing will limit a telescope’s resolving potential and in most US locations the seeing will be the determining factor regarding the potential resolving capability of a telescope.

Consider…

“How often do nights of excellent seeing occur? At the William Herschel Telescope site in the Canary Islands, even this superb viewing location  (second best in the northern Hemisphere) has many nights of relatively poor seeing: the distribution is positively skewed, and at this excellent site, a 10 inch telescope will be seeing limited on 9 out of 10 nights.”

I presume most don’t live in a location with anything like the seeing at the above.

Here is a quote “from the link you posted”…

“The ability of a telescope to resolve to Dawes' limit is usually much more affected by seeing conditions,...”

One must also consider the optical quality of the primary. For example, a 1-wave optic will show soft images no matter how large it is. Images might be bright but resolution will be impacted.

Bob

 

Very true for Planets/Luna, very un-true for DSO's.  Even in poor seeing, the larger aperture always wins.

You must have missed my post # 23...

 

"For deep sky observing (not double star splitting) like hunting down galaxies etc., light gathering is important. Resolving fine lunar and planetary details is more dependent on seeing and optical quality whereas observing faint fuzzies is more dependent on contrast (hence the need for dark skies) and light gathering (hence the need for size).

"A large “decent quality optic” in a dark sky will satisfy many. A “same size or maybe “slightly” smaller but higher quality optic will deliver better contrast and therefor better performance for those that don’t want to leave anything on the table."


Bob



#42 Mitrovarr

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Posted 10 September 2020 - 02:46 PM

When seeing is bad, I put a 12" concentric mask on my UTA and I get an immediate improvement in the image, but you seem to be saying an off axis mask as you have described will really do the trick.


I like that you consider seeing that allows you to usefully use a 12" aperture "bad".

When the seeing is bad here, I can't get 3" refractors into focus.

#43 Jon Isaacs

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Posted 10 September 2020 - 05:20 PM

"A large “decent quality optic” in a dark sky will satisfy many. A “same size or maybe “slightly” smaller but higher quality optic will deliver better contrast and therefor better performance for those that don’t want to leave anything on the table."

 

 

There is always a bigger scope, there is always a better scope. Something is always left on the table.

 

Something someone once wrote:

 

"There are two kinds of contrast. One needs to define which contrast you mean. If it's planetary contrast, then baffling plays no part. Planetary contrast is strictly a function of how well the optic is figured.

 

If it's deep sky contrast, then optical perfection plays little or no part, and the main contributer will be how well the tube has been made to exclude stray light (i.e. baffled).

 

Rolando"

 

(Roland Christen)

 

Jon



#44 junomike

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Posted 10 September 2020 - 05:39 PM

One must also consider these “laws of physics”…

Seeing will limit a telescope’s resolving potential and in most US locations the seeing will be the determining factor regarding the potential resolving capability of a telescope.

Consider…

“How often do nights of excellent seeing occur? At the William Herschel Telescope site in the Canary Islands, even this superb viewing location  (second best in the northern Hemisphere) has many nights of relatively poor seeing: the distribution is positively skewed, and at this excellent site, a 10 inch telescope will be seeing limited on 9 out of 10 nights.”

I presume most don’t live in a location with anything like the seeing at the above.

Here is a quote “from the link you posted”…

“The ability of a telescope to resolve to Dawes' limit is usually much more affected by seeing conditions,...”

One must also consider the optical quality of the primary. For example, a 1-wave optic will show soft images no matter how large it is. Images might be bright but resolution will be impacted.

Bob

 

You must have missed my post # 23...

 

"For deep sky observing (not double star splitting) like hunting down galaxies etc., light gathering is important. Resolving fine lunar and planetary details is more dependent on seeing and optical quality whereas observing faint fuzzies is more dependent on contrast (hence the need for dark skies) and light gathering (hence the need for size).

"A large “decent quality optic” in a dark sky will satisfy many. A “same size or maybe “slightly” smaller but higher quality optic will deliver better contrast and therefor better performance for those that don’t want to leave anything on the table."


Bob

I did.  Glad to see we're in agreement.



#45 stargazer193857

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Posted 10 September 2020 - 08:13 PM

Hi, Ari! Believe it or not, that statement is not true. Although highly counterintuitive, a fundamentally-worse wavefront (but otherwise identical) telescope can (admittedly only occasionally) give a better image. It is consequent of the random-walk theorems of statistics, applied to the Zernike constitution of the ever-varying atmosphere. A special (common) case is where the atmuspheric column is a continuous and continuously-differentiable function in lateral space and some continuous subsets of time (and therefore piecewise Zernikeizable). In layman' terms, the atmosphere will occasionally/fleetingly ~correct~ the wavefront to better than it would be in a quiescent atmosphere.

I've actually taken advantage of this! One of the departments had commissioned a telescope to an outside vendor, and brought into our test labs for certification. I set it up in a superb autocollimation test tower and found it grossly out of alignment. I had the techs tune it up to best achievable... and it was still pretty stinky deficient. The guys who brought it in were at wit's end, probably figuring they would be blamed for foolish outsourcing a custom build. I told them to return the next day. We took it outside and imaged a distant cell tower through crummy atmosphere. I used a familiar trick that relies on that Zernike stats theorem. Snapped a pile of images when the live-view image looked unusually good, hauled that data into the lab, culled through the bests to further down-select, and then derived, coded and ran a Zernike max variance sorter to further cull. These images did indeed satisfy the isotropic Rayleigh, Dawes, and Sparrow extensions in Zernike-Space. I delivered those images to the engineers, when they slithered back in... and handed them the true statement... "These images, captured with your system, are consistent with diffraction-limited performance". I refused to call the system good. They were satisfied, and slithered back out... no doubt to create charts extolling the virtues of that crummy scope. Any of you optical test guys who have worked in industry know the feeling. Often under pressure to call something good, even when you know it's not. That little referenced scope was only twelve inches. Rumor has it that such legerdemain worked its way all the way up to a 2.4 meter scope! Is that possible?! Tom


So if I see a great view of saturn just once in an 8", it might have been corrected by bad air, and not a great scope held back by bad air the other times.

#46 TOMDEY

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Posted 10 September 2020 - 08:52 PM

So if I see a great view of saturn just once in an 8", it might have been corrected by bad air, and not a great scope held back by bad air the other times.

Yes, that's indeed possible... less likely, but at least possible... We will all admit that better scopes most certainly have the statistical edge for performance, and better scopes will almost always out-perform lesser ones... but not always. It's like going to the casino and playing most any of the games. e.g. slot machines. The odds mildly favor the house, but occasionally a customer will walk away with the big jackpot. Indeed, that's what's so enticing ... the NY "Dollar and a Dream" Lottery Commercials. 

 

Here's the bigger scope advantage, even bigger imperfect scopes. >>> When the atmosphere rarely corrects the wavefront, for that fleeting moment, the image of a point is the Airy Disc. That disc subtends a smaller object-space angle for the bigger vs smaller scope... indeed, in inverse proportion to aperture. So, the smaller scope's fleeting perfection is more common, but less magnificent than the bigger scope's.

 

And the overall thrust remains the same --- one would ideally want a scope that is both big as possible and perfect as possible.    Tom

 

PS: There are even more bizarre implications from information theory, that border squarely on the turf where quantum mechanics, (un)certainty, object-truth, and metaphysics (seem to) verge.    Tom



#47 stargazer193857

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Posted 10 September 2020 - 08:59 PM

Do makers of 15-18" mirrors worry that customers will point them side by side with a 10" at a planet through mediocre seeing and conclude the big mirror is low quality and give a bad review?

#48 rowdy388

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Posted 10 September 2020 - 09:34 PM

Do makers of 15-18" mirrors worry that customers will point them side by side with a 10" at a planet through mediocre seeing and conclude the big mirror is low quality and give a bad review?

From what I've seen, some customers have unrealistic expectations and consideration of the reviewer's experience

is important. I'm glad I'm not a vendor and don't have to navigate those shoals.



#49 Jon Isaacs

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Posted 10 September 2020 - 09:44 PM

“How often do nights of excellent seeing occur? At the William Herschel Telescope site in the Canary Islands, even this superb viewing location  (second best in the northern Hemisphere) has many nights of relatively poor seeing: the distribution is positively skewed, and at this excellent site, a 10 inch telescope will be seeing limited on 9 out of 10 nights.”

 

 

That might be true if you uses the Dawes limit as a measure of resolution. 0.46". Not a good measure. 

 

But the real question is, does the larger aperture show more.. 

 

In 1 arc-second seeing, a 5 inch struggles with a 1 arc-second double. In 1 arc-second seeing a 10 inch has an easy time of a 1 arc-second double.

 

Jon


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#50 TOMDEY

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Posted 16 September 2020 - 08:12 PM

That might be true if you uses the Dawes limit as a measure of resolution. 0.46". Not a good measure. 

But the real question is, does the larger aperture show more.. 

In 1 arc-second seeing, a 5 inch struggles with a 1 arc-second double. In 1 arc-second seeing a 10 inch has an easy time of a 1 arc-second double.

 

Jon

So true! In actual practice, if you want to comfortably realize the ~diffraction limit~ of a smaller scope --- just use a decent bigger scope! That gives you comfortable margin on everything ... and more light!    Tom


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