543 replies to this topic

[SandyHouTex]

Don was a mentor to many, however, planetary imaging says little to nothing about the optical quality of telescopes. That's nearly all just conditions for good data, a good camera combined with great processing.

Oh contraire.  No matter how much post processing you do on an image, you can never get any more information than was already in the original image.  You can enhance it, but it must be there to begin with.  So SCTs are extremely good at planetary imaging.  Damien Peach gets images with a C14 that rival Hubble's pictures.

[GJJim]

 

Don was a mentor to many, however, planetary imaging says little to nothing about the optical quality of telescopes. That's nearly all just conditions for good data, a good camera combined with great processing.

Oh contraire.  No matter how much post processing you do on an image, you can never get any more information than was already in the original image.  You can enhance it, but it must be there to begin with.  So SCTs are extremely good at planetary imaging.  Damien Peach gets images with a C14 that rival Hubble's pictures.

 

It's not one image. The processing techniques combine data from multiple images to improve the S/N ratio. They actually do present more detail and contrast in the final image than was visible in the subs.

[PowellAstro]

You can never pull more data than the optics can resolve. The multiple images and stacking are only to overcome the effects of seeing and thermal issues. In no way can you process out details that the optics did not provide. You can enhance them but not create them. The only reason the sub's have less detail are again because of seeing and thermal currents.

Edited by PowellAstro, 03 November 2015 - 11:29 AM.

[Phil Barker]

 

 

But does that explain the Mak/SCT difference? They were both collimated with the artificial star before my shootout that the Mak won so easily.

 

Astro Foren has a test on a Celestron C9.25 with amazing results, 1/11th wave and Sterhl of 0.98;
http://astro-foren.d...r/?postID=49917
So why is Mak, visually, sharper than any SCT, the answer is very simple, smoothness of figure, If you look at some of the images on the above test, particularly Lyot Test, you will see a very rough surface, the hallmark of all SCT's.
The most important optical quality for high power planetary observation is smoothness of figure, the numbers alone do not give a true picture, and remember, we are talking visual only.

 

Vahe

 

You are right on the money here

 

My 10 inch intes micro MC murdered the what  thought was excellent c11 on Jupiter.  The detail in the bands ovals etc were sharper clearer it handled higher power contrast was better.

 

The 11 startested to around 1/6th wave and produced a fine image but the IM1008 was better.

 

The startest did show some minor roughness in the 11.  It still did the business and basically matched a reasonable 10 inch newt for deepsky.

The 10 inch Mak costs about 4 times more clearly optics are on another shelf. 

 

I know from making Newtonian mirrors rough surface can occur with a good figure.  I always used a softer pitch for figuring and rouge not cerium oxide definitely made a difference to surface roughness.

 

My foucault tester has a green led lightsource which really shows up roughness.  Haven't done this for a few years wife hated seeing the blanks/testing gear left lying round but I will retire one day and get back into it.

[maadscientist]

A 1/4 wave instrument is only good if there is no turbulence and zero tube currents. Any outside influences with a 1/4 wave instrument throws additional light outside the airy disk. With a 1/16 wave instrument, it is much more stable under the same conditions. So for example, say we have 1/4 wave tube currents and 1/8 wave seeing:

The 1/4 wave instrument is now acting as if the mirrors has almost a full wave error.

The 1/16 wave instrument is still diffraction limited. All the energy is still within the airy disk.

1/4 wave is fine for the Hubble as it is free from the wavefront damaging seeing as well as a lot of the thermal conditions we deal with.

Hey Powell Astro.....did you ever sell that 10 inch Schmidt Newt I sold you? Dang thing was strangely heavy......

 

There is a lot of information about what actually happens to the wavefront during it's journey through Earth's atmosphere. This is not because we don't know what is going on, but rather it is a complex interaction of phenomenon that makes it ....well complicated. The main factor is temperature differentials between pockets of turbulence. And turbulence models, like the weather, are a bit beyond our capacity to calculate the impacts completely. We have made great strides in adaptive optics, but the modeling is still dependent on super fast calculations and bright objects next to what large observatories are imaging. 

 

We do know that the Fried seeing parameter for average seeing is around 8 inches or slightly larger. Anything larger in aperture is affected by seeing in a different way, I.E. speckling occurs. This cannot be compensated for by "great optics", as the seeing shifts the centroid of the airy disk point spread function. So it becomes irrelevant how tightly the rays are concentrated in the airy disk.

 

Simply put, you cannot beat seeing with better than diffraction limited optics if the aperture resolving limit is beyond seeing.. So in relation to Daniels observation, I personally would say a high resolution planetary image from say, a 10 inch or up aperture basically means you got a diffraction limited image....no way to tell if the optical throughput quality is .95 strehl, or .85.....or .88....or.91...

 

As to Hubble, since there is no atmosphere to mess with the wavefront, this would be a clear example of why you would want to have a .95 or above strehl, right? The limiting factor to your resolution would be how good you made the optic....but no, as Perkins Elmer and other optical fabricators realize, you can't resolve past the Raleigh criterion, so as long as all wavelengths are at .80 strehl or above, you are good to go.

 

Dan

[Thomas A Davis]

 

A 1/4 wave instrument is only good if there is no turbulence and zero tube currents. Any outside influences with a 1/4 wave instrument throws additional light outside the airy disk. With a 1/16 wave instrument, it is much more stable under the same conditions. So for example, say we have 1/4 wave tube currents and 1/8 wave seeing:

The 1/4 wave instrument is now acting as if the mirrors has almost a full wave error.

The 1/16 wave instrument is still diffraction limited. All the energy is still within the airy disk.

1/4 wave is fine for the Hubble as it is free from the wavefront damaging seeing as well as a lot of the thermal conditions we deal with.

Hey Powell Astro.....did you ever sell that 10 inch Schmidt Newt I sold you? Dang thing was strangely heavy......

 

There is a lot of information about what actually happens to the wavefront during it's journey through Earth's atmosphere. This is not because we don't know what is going on, but rather it is a complex interaction of phenomenon that makes it ....well complicated. The main factor is temperature differentials between pockets of turbulence. And turbulence models, like the weather, are a bit beyond our capacity to calculate the impacts completely. We have made great strides in adaptive optics, but the modeling is still dependent on super fast calculations and bright objects next to what large observatories are imaging. 

 

We do know that the Fried seeing parameter for average seeing is around 8 inches or slightly larger. Anything larger in aperture is affected by seeing in a different way, I.E. speckling occurs. This cannot be compensated for by "great optics", as the seeing shifts the centroid of the airy disk point spread function. So it becomes irrelevant how tightly the rays are concentrated in the airy disk.

 

Simply put, you cannot beat seeing with better than diffraction limited optics if the aperture resolving limit is beyond seeing.. So in relation to Daniels observation, I personally would say a high resolution planetary image from say, a 10 inch or up aperture basically means you got a diffraction limited image....no way to tell if the optical throughput quality is .95 strehl, or .85.....or .88....or.91...

 

As to Hubble, since there is no atmosphere to mess with the wavefront, this would be a clear example of why you would want to have a .95 or above strehl, right? The limiting factor to your resolution would be how good you made the optic....but no, as Perkins Elmer and other optical fabricators realize, you can't resolve past the Raleigh criterion, so as long as all wavelengths are at .80 strehl or above, you are good to go.

 

Dan

 

 

All errors on an optic are cumulative.  Seeing does enter into this.  Thermals as well.  If seeing is 1/4 wave, it will add to the error of a 1/4 wave optic.  Same with thermals.  The atmosphere becomes part of the optical train, whether it is outside of the telescope or within it.  A 1/4 wave system with 1/4 wave seeing will not equal a perfect optic with 1/4 wave seeing.  The error will be cumulative.  This is why a 1/4 wave optic will not generally equal the performance of a 1/10th wave optic on a given night on an in-focus image.  The addition of seeing and thermals to the final image are more evident on the 1/4 wave system.

 

Where this applies to planetary imaging with stacked images is obvious.  You will get frames that have little to no atmospheric distortion, and these make for a great image when stacked with other similar frames.  If you take an optic with better wavefront correction, and examine the individual frames, you will find more good frames, due to the lower cumulative error in the image.  This is how the eye works.  The averaged view will be better and show details to the eye that only can be seen in the better frames on the video image on the 1/4 wave system, that do not show to the eye on the 1/4 wave system.  It really is an apples to oranges comparison.  What stacking programs do is to remove the bad frames and add the good ones.  The eye tends to average the view, so the better the optic, the better it deals with seeing.  Cumulative error does matter.  At least in my experience this has proved true, and I've done visual and imaging of planets with scopes from 1/4 wave or worse to scopes with measured strehls higher than .99.

The better the scope, the more good frames you get.  The final processed image may not differ by much, but visually, you can see the difference.

 

Tom

[Wildetelescope]

 

 

Don was a mentor to many, however, planetary imaging says little to nothing about the optical quality of telescopes. That's nearly all just conditions for good data, a good camera combined with great processing.

Oh contraire.  No matter how much post processing you do on an image, you can never get any more information than was already in the original image.  You can enhance it, but it must be there to begin with.  So SCTs are extremely good at planetary imaging.  Damien Peach gets images with a C14 that rival Hubble's pictures.

 

It's not one image. The processing techniques combine data from multiple images to improve the S/N ratio. They actually do present more detail and contrast in the final image than was visible in the subs.

 

I might be mistaken, but I don't think Dan is saying that the quality of ones optics is not important.  I took his point to be that it is only ONE of many variables that lead to a final processed image. As a result, it is hard to back out from the image WHICH of those variables was dominant.  For example, I have an 8 inch Edge on an Atlas and a not inexpensive planet cam.  A friend has an 8 inch Orion Dob(standard) and a 99 dollar Orion planet cam. His moon and planet images routinely put mine to shame, and a couple have received some recognition.  This is likely because, a. He is really, REALLY patient and good at using his gear, and b. He lives in Florida, while I live under the jet stream at the top of the Chesapeake! ( Option A is probably the more important factor;-)  I doubt is says anything about the relative quality of the optics in either scope(both of which are good examples of their class I think).   Images coming from folks like Mr. Peach or Chris Go have things firing on ALL cylinders, including their C14 optics which I am sure are excellent examples of that class of instrument. 

 

Just thinking out loud...

 

Cheers!

 

JMD 

[TG]

 

 

Don was a mentor to many, however, planetary imaging says little to nothing about the optical quality of telescopes. That's nearly all just conditions for good data, a good camera combined with great processing.

 

This is demonstrably false. Good optics is a necessary but not sufficient condition for good planetary images. So if the images are good, the optics have to be good as well but not vice versa. If this weren't true, seeing would be an irrelevant factor in imaging and that happens to be a necessary condition as well for good images.

 

My personal experience has been that a superb 11 inch aperture produced superb planetary images. A mediocre 11 in aperture produced mediocre ones.

 

There's a lot of nonsense on these forums about how post-processing can compensate for mediocre optics. IME, this simply isn't true.

 

 

 

Tanveer.

 

 

Tanveer,

 

There are many other shootouts I've not bothered to publish. Here's a few reviews for the OP though. What I'm more interested in is your claim that good optics are needed for planetary imaging.

 

http://www.cloudynig...ry_scopes_1.pdf

 

http://www.cloudynig...ts/shootout.pdf

 

http://doctordreview...0-teleport.html

 

Do you think those images you posted are indicative of great optics? Do you see that detail visually? I'd like an answer to my question because I'm this close to dropping a bombshell on all these planetary imagers once and for all, and I'll do it from my own website.

 

 

Daniel, to answer your questions directly:

 

1. The image on the left is not indicative of great optics. Though the one on the right may not seem like it either, it is indicative of great optics because it's not the original and has been scaled down for comparison. Here is the original:

 

2. Much of the detail was seen visually including ovals, festoons, barges, the turbulence around the GRS and Io transiting. In comparison with an A-P 7", this scope performed at least as well if not more so.

 

Awaiting your "bombshell" (should I duck?)

 

Tanveer.

[PowellAstro]

Hey Powell Astro.....did you ever sell that 10 inch Schmidt Newt I sold you? Dang thing was strangely heavy......


Dan

Yes, I sold it to a good friend. It really is heavy but after a good deep cleaning and optical alignment, it really is a great scope! It is now on an alt/az mount!

Edited by PowellAstro, 03 November 2015 - 05:02 PM.

[azure1961p]

 

Don was a mentor to many, however, planetary imaging says little to nothing about the optical quality of telescopes. That's nearly all just conditions for good data, a good camera combined with great processing.

 

This is demonstrably false. Good optics is a necessary but not sufficient condition for good planetary images. So if the images are good, the optics have to be good as well but not vice versa. If this weren't true, seeing would be an irrelevant factor in imaging and that happens to be a necessary condition as well for good images.

 

My personal experience has been that a superb 11 inch aperture produced superb planetary images. A mediocre 11 in aperture produced mediocre ones.

 

There's a lot of nonsense on these forums about how post-processing can compensate for mediocre optics. IME, this simply isn't true.

 

 

 

Tanveer.

 

 

 

That's a point I ponder quite a bit... 

 

In the face of super sensitive ccd imaging cams you'd think surely,  the best scope wins.   In the face of that is post image processing with super aggressive tools and technique. 

 

I'd like to think my 8" newt would turn out a better image than a C8, but so very much of the talent in processing would seem to leave that advantage of my 8" newt as a negotiable technicality. 

 

I'd like to think it'd ace out,  but set up a guy next to me with a mediocre C8 and some serious imaging know-how and it would appear to shrink k or dissolve what edge I thought should show plainly. 

 

That's ccd.  Visual I'll take my newt everytime over a C8.

 

It's an interesting argument for both sides of the table. 

 

 

Pete

[TG]

Tanveer wrote:

My personal experience has been that a superb 11 inch aperture produced superb planetary images. A mediocre 11 in aperture produced mediocre ones.

True, however that is a tautology.  I know its a personal experience, which I dont doubt, and is my experience also.  Evaluation of relative merits is difficult as atmospheric seeing and thermal effects must be accounted for.

 

I'm not sure what you mean. It doesn't seem like a tautology to me unless I slept through my entire mathematical logic course. According to the assertion I'm arguing against: mediocre optics can also produce good planetary images. Yet I found that this is patently false. It's should be easy to test in an experimental setup: introduce an aberration such as coma (miscollimation) and then try to post-process it away. Bad seeing's effect is also to degrade optical quality on average. Show me a picture taken with bad seeing that in matches one with the same optic during good seeing.

 

Tanveer.

[Asbytec]

I've owned several newts and dobs from 6 to 18", more than a few SCTs in the 10 and 11" aperture range, and a couple of small achros. But, I am having the time of my observing life with a 6" Mak. I've seen things never thought possible in a modest aperture. 

 

I believe Vahe said it best earlier, it's not that Maks are easy to produce. But, I believe, they can be made very smooth. Was it Roland who said there are machines that can lay down 1/20th wave smoothness? Mine is reasonably well corrected and very smooth. I modded the obstruction (removed the secondary baffle) to get it down to ~30%. So, I think the peak intensity and distribution is right at "diffraction limited." Give or take...

 

I can tell it's smooth because the seeing here is often very good and ambient temperatures are modest and easily dealt with. Defocused and focused star images are not dancing blobs of light, they are steady with diffraction rings very visible allowing for "perfect" collimation as humanly possible. All the way down to 1 wave defocus. Way out of focus there are no obvious zones and in focus and collimated the first ring is pretty darn uniform, best I can tell. 

 

I could use a little more aperture, but the images are excellent. Smooth, cooled, collimated, and in good seeing...the Mak (in fact any descent scope) rocks when conditions are right. But, maybe if the Mak has any advantage, it's in it's smooth figure and reasonably good correction, even for mass produced versions. 

 

The one advantage the modest aperture Mak has, in my view, is I love the images it's capable of putting up. Not that other scopes cannot do it, just that the Mak is proven to do so. 

[saemark30]

My experience is that thermals kill Newtonian and Cassegrain reflectors and SCT's for planetary use. That includes some 12" and 20" Cassegrains and at 28" Dall Kirkham telescope.

They should murder a 4" refractor but the small scope shows more when I look 99% of the time.

I wonder if the glass element in front of the MAK helps or hinders it further?

[Asbytec]

This is why a 1/4 wave optic will not generally equal the performance of a 1/10th wave optic on a given night on an in-focus image.

 

 

This is an interesting concept. In about 8/10 Pickering or better, the seeing error seems to be confined to within the Airy disc. The Airy disc is pretty darn stable, as is the first ring. All rings, when visible, are stable...and are visible because they are stable. But, are we saying seeing that good adds light to the rings as an optical aberration would? I am sure it has to do with phase, but the diffraction (creating the rings within which to put that aberrant light) of the seeing induced wave front does not occur until it reaches the aperture. Afterward, the aperture itself and the less than perfect correction does its damage to the intensity distribution - of the diffracted image - as the aberrant optic tries to bring the (now) diffracted wavefront to focus.

Edited by Asbytec, 03 November 2015 - 08:49 PM.

[Asbytec]

I wonder if the glass element in front of the MAK helps or hinders it further?

 

 

In my experience and opinion, it's not the thick meniscus causing thermal equilibrium problems. Probably no different than the thick glass in a refractor. It's the embedded mirror and supporting structure that has difficulty radiating heat into the environment. I actively cool the primary mirror end (primarily) with an ice pack. The whole OTA is cool to the touch and have not seen a heat plume in years. 

[Bomber Bob]

My 6" f/20 Cassegrain shows more fine detail than my 4" f/15 refractor with its fine Carton lens.  Not just on planets.  As I've posted before, the Tinsley showed me the bright thin rinds of The Ring nebula for my first time ever, and with direct viewing, not averted.  But, the Cass requires planning & preparation to maximize its capabilities, while the Edmund is ready to go whenever I get a break in the clouds.

[DesertRat]

Tanveer wrote:

I'm not sure what you mean. It doesn't seem like a tautology to me unless I slept through my entire mathematical logic course.

I already agreed with you.  Good scopes are good and mediocre scopes are mediocre.  That is a tautology, in the sense it says that good scopes will do good and mediocre scopes will do  - well not so good.  This is a case of grammatical tautology not mathematical.  Sorry if I offended you in some way, I dont claim to be a stylistic expert either.

 

I dont generally agree with Daniel on everything, but I think it is true that a good planetary image by itself, says virtually nothing about the quality of the optic.  It says more about the quality of technique, the aperture and even more importantly on how good the seeing was.  As I mentioned earlier a scope of sufficient aperture even compromised by aberrations giving a Strehl of 0.5 can produce stunning images.

 

Many dont appreciate that under normal backyard seeing a larger scope is normally running under effective Strehls  in the low 20's, sometimes single digits!

 

Coma is a really nasty aberration.  It is difficult to correct for it in IP as it is not constant for the field, but it is possible.  You can have fairly significant spherical errors and correct for them quite easily.  The original HST had an absurd level of spherical, yet with IP it could produce really good images of Saturn and R Aquarii.   You can duplicate some of the early HST deconvolution today quite easily.

 

 

Dan (maadscientist) wrote:

We do know that the Fried seeing parameter for average seeing is around 8 inches or slightly larger.

The Fried coherence length r0 is rarely as good as 8" or 20cm.  Normal backyard seeing is more likely given by a 2 arcsec seeing disk.  The seeing disk is the time averaged psf of a point source (so called long exposure).

 

 

The full width half max of the seeing disk in radians as a function of r0 is given approximately by:

 

FWHM = 0.976*(lambda/r0)

 

where lambda is the operating wavelength.  Hence for lambda 550nm and doing a little algebra (and recalling that 1 radian is ~ 206264.8 arcsec) we find that for 2" seeing r0 is ~ 5.5cm and for 1" seeing it is ~ 11cm.  Now 1" seeing is pretty good and does happen often enough to keep us productive.  In lucky imaging 1" seeing will achieve diffraction limited imaging with larger apertures.

 

For brief moments the effective r0 may improve to yield nearly diffraction limited frames in lucky imaging.  However as the aperture goes up the frequency of those frames drops exponentially.  This exponential drop is not appreciated by many, unless they have a collection of scopes small and large and pursue high resolution, visual or imaging.

 

And finally, back to the OP.  I have a very good IM715 mak.  But honestly I can tell you I have seen plenty of SCT8's (both C & M !!!) which will easily match if not eclipse it.  Mechanically this Mak is superior, it includes a fan, and has a decent focusing mechanism.  I like the scope very much,  but there is nothing magic about it.

 

Glenn

Edited by DesertRat, 03 November 2015 - 09:22 PM.

[azure1961p]

My experience is that thermals kill Newtonian and Cassegrain reflectors and SCT's for planetary use. That includes some 12" and 20" Cassegrains and at 28" Dall Kirkham telescope.

They should murder a 4" refractor but the small scope shows more when I look 99% of the time.

I wonder if the glass element in front of the MAK helps or hinders it further?

 

The seeing has to be pretty bad for any 4" of any design to match or beat my 8".   I use  a fan behind the mirror so thermals are not an issue.   Moreover,  my 8" in mediocre seeing will define Titan as a disc.   A four inch couldn't do that atop pic do midi. 

That you claim a 4 can best a 28" DK and the other instruments just makes no real world sense.  It might appear more tidy and clean as the seeing is less obvious in the four but unless there's some awful optics and seeing combined those instruments would relegate any four inch as an inspired finder scope. 

 

Pete

[Ptkacik]

Thanks DesertRat:

 

"Nothing magic about it". I was thinking there would be a Mak versus SCT optics chart showing the advantage in green light or something. Something about Coma at 635nm (or similar jazzy math). Or are the jazzy plots optics or physics?

 

BTW, I mounted two fans on my 7" Mak (Orion 180) but I'm not sold on their benefit.

 

Regarding imaging, I don't care for it but twenty five years ago I did my PhD work on image processing and can tell you that with enough stacked images, YES, you can get far superior images. It is just digital mathematics. Yes, better optics does improve the images.

 

In fact, I recall a method for focusing a single image through unfocused lenses with an inverse Fourier transform on a single spot of light. I never did it but remember thinking how cool that was.

 

Clear skies,

Peter

[Ptkacik]

BTW, did we answer the question of Mak/SCT versus the Dall Kirkham or even the Celestron Edge optics, (heaven forbid the Meade ACF)? Big refractors or Newtonians are very cumbersome to carry so I may restrict the question to Cassegrains or folded optics systems, (and restrict to visual rather than imaging).

 

Clear skies,

Peter

[Jon Isaacs]

 

 

A 1/4 wave instrument is only good if there is no turbulence and zero tube currents. Any outside influences with a 1/4 wave instrument throws additional light outside the airy disk. With a 1/16 wave instrument, it is much more stable under the same conditions. So for example, say we have 1/4 wave tube currents and 1/8 wave seeing:

The 1/4 wave instrument is now acting as if the mirrors has almost a full wave error.

The 1/16 wave instrument is still diffraction limited. All the energy is still within the airy disk.

1/4 wave is fine for the Hubble as it is free from the wavefront damaging seeing as well as a lot of the thermal conditions we deal with.

Hey Powell Astro.....did you ever sell that 10 inch Schmidt Newt I sold you? Dang thing was strangely heavy......

 

There is a lot of information about what actually happens to the wavefront during it's journey through Earth's atmosphere. This is not because we don't know what is going on, but rather it is a complex interaction of phenomenon that makes it ....well complicated. The main factor is temperature differentials between pockets of turbulence. And turbulence models, like the weather, are a bit beyond our capacity to calculate the impacts completely. We have made great strides in adaptive optics, but the modeling is still dependent on super fast calculations and bright objects next to what large observatories are imaging. 

 

We do know that the Fried seeing parameter for average seeing is around 8 inches or slightly larger. Anything larger in aperture is affected by seeing in a different way, I.E. speckling occurs. This cannot be compensated for by "great optics", as the seeing shifts the centroid of the airy disk point spread function. So it becomes irrelevant how tightly the rays are concentrated in the airy disk.

 

Simply put, you cannot beat seeing with better than diffraction limited optics if the aperture resolving limit is beyond seeing.. So in relation to Daniels observation, I personally would say a high resolution planetary image from say, a 10 inch or up aperture basically means you got a diffraction limited image....no way to tell if the optical throughput quality is .95 strehl, or .85.....or .88....or.91...

 

As to Hubble, since there is no atmosphere to mess with the wavefront, this would be a clear example of why you would want to have a .95 or above strehl, right? The limiting factor to your resolution would be how good you made the optic....but no, as Perkins Elmer and other optical fabricators realize, you can't resolve past the Raleigh criterion, so as long as all wavelengths are at .80 strehl or above, you are good to go.

 

Dan

 

 

All errors on an optic are cumulative.  Seeing does enter into this.  Thermals as well.  If seeing is 1/4 wave, it will add to the error of a 1/4 wave optic.  Same with thermals.  The atmosphere becomes part of the optical train, whether it is outside of the telescope or within it.  A 1/4 wave system with 1/4 wave seeing will not equal a perfect optic with 1/4 wave seeing.  The error will be cumulative.  This is why a 1/4 wave optic will not generally equal the performance of a 1/10th wave optic on a given night on an in-focus image.  The addition of seeing and thermals to the final image are more evident on the 1/4 wave system.

 

Where this applies to planetary imaging with stacked images is obvious.  You will get frames that have little to no atmospheric distortion, and these make for a great image when stacked with other similar frames.  If you take an optic with better wavefront correction, and examine the individual frames, you will find more good frames, due to the lower cumulative error in the image.  This is how the eye works.  The averaged view will be better and show details to the eye that only can be seen in the better frames on the video image on the 1/4 wave system, that do not show to the eye on the 1/4 wave system.  It really is an apples to oranges comparison.  What stacking programs do is to remove the bad frames and add the good ones.  The eye tends to average the view, so the better the optic, the better it deals with seeing.  Cumulative error does matter.  At least in my experience this has proved true, and I've done visual and imaging of planets with scopes from 1/4 wave or worse to scopes with measured strehls higher than .99.

The better the scope, the more good frames you get.  The final processed image may not differ by much, but visually, you can see the difference.

 

Tom

 

 

Tom:

 

The difference between your analysis and Dan's is that he is looking at the absolute seeing and you are looking at the relative seeing.  Consider this simplified example:

 

A 1/4 wave seeing in a 6 inch maybe something like 1 arc-second and in that case the seeing and the optical error would be additive.   On the other hand, 1/4 wave seeing in a 24 inch might be something like 0.25 arc-seconds and if the seeing were that good, again, the seeing and the optical errors would be additive.

 

But if the seeing is 1 arc-second, that would be 4 waves of seeing error in the 24 inch and whether the optics are 1/10wave, 1/4 wave or 1/2 wave really doesn't matter because the error in the optics is small relative to the seeing error. 

 

One more tidbit.. 

 

A discussion like this can be on two levels, the practical, how do existing SCTs and Maks compare, and the abstract, on a fundamental level, how do the two designs compare.  

 

When considering the various existing SCTs and what is possible, rik ter horst's 8 inch F/25 seems to be at the summit of perfection...  After reading about his 30mm (that's 1.25 inch) solid SCT), I wondered who is this guy?

 

I still don't know.. But I know he is an optical designer and found this reference to his work on the MeerLICHT telescope.  It doesn't say much but under optical design, there are two names listed:

 

Optical Design: Harrie Rutten (Castor Optical Design) & Rik ter Horst (NOVA)

 

We all know Harrie Rutten.. from his books.. 

 

I am impressed.

 

Jon

 

 

 

Jon

[A6Q6]

  I was out this morning with my 1978 Quantum 6 Mak,  # 72 from 288 optical sets. I don't know how many made it into telescopes. The Mak has enhanced silver coatings.  I also had my new to me, mint,1985 Super C8 Plus SCT with StarBright coatings. The coatings on both look new. Jupiter was great in both,  with the nod going to the C8, its just plane brighter, along with great contrast, the extra 2" is a big deal to me, after 35+yrs with the 6" Mak.  But I was repeatedly pulled away by the Moon.   Small craterlets around Rima Birt was just one of the views I really enjoyed in the C8.  I was having so much fun with the C8, by the time I looked at Rima Birt in the Quanutm 6, the sky was getting too bright.  Christmas is coming and  I think I will ask for a couple of popular optical books, (Thanks Eddgie) so I can test the C8 and have a more in depth understanding of a SCT that is not too optically or mechanically ( no image shift) common.   I love my Mak, but as I said in the past,  I will say again, If Meade and Celestron were consistent in what they are capable of making, people would be buying less Maks.  I think its relevant to add that every telescope is an individual and every used SCT should be tested on that bases, since you never know what you may come across and that's good to know for folks who think they could never afford a high quality Mak.

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Edited by A6Q6, 04 November 2015 - 01:12 PM.

[BLACKDRAGON]

My experience is that thermals kill Newtonian and Cassegrain reflectors and SCT's for planetary use. That includes some 12" and 20" Cassegrains and at 28" Dall Kirkham telescope.

They should murder a 4" refractor but the small scope shows more when I look 99% of the time.

I wonder if the glass element in front of the MAK helps or hinders it further?

 

I have the 9.25 SCT and the Mak 180 and I've never had any problems with thermals........possibly due to the fact that when using either they're outside and on the NEQ6 at least a couple of hours before use.   I also cover them in a thick dust sheet if the Sun is out for any length of time.

Edited by BLACKDRAGON, 04 November 2015 - 05:14 PM.

[Thomas A Davis]

 

 

 

A 1/4 wave instrument is only good if there is no turbulence and zero tube currents. Any outside influences with a 1/4 wave instrument throws additional light outside the airy disk. With a 1/16 wave instrument, it is much more stable under the same conditions. So for example, say we have 1/4 wave tube currents and 1/8 wave seeing:

The 1/4 wave instrument is now acting as if the mirrors has almost a full wave error.

The 1/16 wave instrument is still diffraction limited. All the energy is still within the airy disk.

1/4 wave is fine for the Hubble as it is free from the wavefront damaging seeing as well as a lot of the thermal conditions we deal with.

Hey Powell Astro.....did you ever sell that 10 inch Schmidt Newt I sold you? Dang thing was strangely heavy......

 

There is a lot of information about what actually happens to the wavefront during it's journey through Earth's atmosphere. This is not because we don't know what is going on, but rather it is a complex interaction of phenomenon that makes it ....well complicated. The main factor is temperature differentials between pockets of turbulence. And turbulence models, like the weather, are a bit beyond our capacity to calculate the impacts completely. We have made great strides in adaptive optics, but the modeling is still dependent on super fast calculations and bright objects next to what large observatories are imaging. 

 

We do know that the Fried seeing parameter for average seeing is around 8 inches or slightly larger. Anything larger in aperture is affected by seeing in a different way, I.E. speckling occurs. This cannot be compensated for by "great optics", as the seeing shifts the centroid of the airy disk point spread function. So it becomes irrelevant how tightly the rays are concentrated in the airy disk.

 

Simply put, you cannot beat seeing with better than diffraction limited optics if the aperture resolving limit is beyond seeing.. So in relation to Daniels observation, I personally would say a high resolution planetary image from say, a 10 inch or up aperture basically means you got a diffraction limited image....no way to tell if the optical throughput quality is .95 strehl, or .85.....or .88....or.91...

 

As to Hubble, since there is no atmosphere to mess with the wavefront, this would be a clear example of why you would want to have a .95 or above strehl, right? The limiting factor to your resolution would be how good you made the optic....but no, as Perkins Elmer and other optical fabricators realize, you can't resolve past the Raleigh criterion, so as long as all wavelengths are at .80 strehl or above, you are good to go.

 

Dan

 

 

All errors on an optic are cumulative.  Seeing does enter into this.  Thermals as well.  If seeing is 1/4 wave, it will add to the error of a 1/4 wave optic.  Same with thermals.  The atmosphere becomes part of the optical train, whether it is outside of the telescope or within it.  A 1/4 wave system with 1/4 wave seeing will not equal a perfect optic with 1/4 wave seeing.  The error will be cumulative.  This is why a 1/4 wave optic will not generally equal the performance of a 1/10th wave optic on a given night on an in-focus image.  The addition of seeing and thermals to the final image are more evident on the 1/4 wave system.

 

Where this applies to planetary imaging with stacked images is obvious.  You will get frames that have little to no atmospheric distortion, and these make for a great image when stacked with other similar frames.  If you take an optic with better wavefront correction, and examine the individual frames, you will find more good frames, due to the lower cumulative error in the image.  This is how the eye works.  The averaged view will be better and show details to the eye that only can be seen in the better frames on the video image on the 1/4 wave system, that do not show to the eye on the 1/4 wave system.  It really is an apples to oranges comparison.  What stacking programs do is to remove the bad frames and add the good ones.  The eye tends to average the view, so the better the optic, the better it deals with seeing.  Cumulative error does matter.  At least in my experience this has proved true, and I've done visual and imaging of planets with scopes from 1/4 wave or worse to scopes with measured strehls higher than .99.

The better the scope, the more good frames you get.  The final processed image may not differ by much, but visually, you can see the difference.

 

Tom

 

 

Tom:

 

The difference between your analysis and Dan's is that he is looking at the absolute seeing and you are looking at the relative seeing.  Consider this simplified example:

 

A 1/4 wave seeing in a 6 inch maybe something like 1 arc-second and in that case the seeing and the optical error would be additive.   On the other hand, 1/4 wave seeing in a 24 inch might be something like 0.25 arc-seconds and if the seeing were that good, again, the seeing and the optical errors would be additive.

 

But if the seeing is 1 arc-second, that would be 4 waves of seeing error in the 24 inch and whether the optics are 1/10wave, 1/4 wave or 1/2 wave really doesn't matter because the error in the optics is small relative to the seeing error. 

 

One more tidbit.. 

 

A discussion like this can be on two levels, the practical, how do existing SCTs and Maks compare, and the abstract, on a fundamental level, how do the two designs compare.  

 

When considering the various existing SCTs and what is possible, rik ter horst's 8 inch F/25 seems to be at the summit of perfection...  After reading about his 30mm (that's 1.25 inch) solid SCT), I wondered who is this guy?

 

I still don't know.. But I know he is an optical designer and found this reference to his work on the MeerLICHT telescope.  It doesn't say much but under optical design, there are two names listed:

 

Optical Design: Harrie Rutten (Castor Optical Design) & Rik ter Horst (NOVA)

 

We all know Harrie Rutten.. from his books.. 

 

I am impressed.

 

Jon

 

 

 

Jon

 

What I was getting at Jon was two equal sized scopes with similar seeing on the same night.  The better optics will find more good frames than the other, due to cumulative error from seeing issues.  The fact is that 1 arc second seeing for a scope one  given night is an average, not a moment to moment thing.  For any given size optic, there will be moments, however fleeting where the seeing steadies enough for my assertion to apply.  From my location in the tropics, I can routinely resolve double stars to Dawes limit in my EdgeHD 8" or 180 Mak-Cass.  Having owned a 20 Starmaster under far less ideal seeing in NC, I get your point.  On about 4 nights a year, it would work to the limit of it's capability.  It had a killer Zambuto mirror, which gave astonishing views when cooled on those rare nights.  Most nights, though, my AP180EDT would step on it, since it was seeing challenged.  Apples to oranges, since seeing would not allow the larger scope to work to near its potential.  Location, location, location.  Still, under decent, but not great seeing, a webcam will reveal detail hidden to the eye.  The eye can't register the fleeting fractions of a second that seeing steadies, while the camera can, and software can separate them from the blurred frames due to seeing.  I still say, the better the scope, the better it deals with seeing issues.  What you refer to are the nights when seeing is just not up to allowing a large scope to work, and I've seen enough of them over the years.  I ended up selling the Starmaster due to the relatively few nights it would work well from my location in NC.  We're actually agreeing with each other, just looking from different perspectives.  When the seeing reaches a certain level, only an aperture mask will help improve the aesthetic quality of the view.  What I was referring to was seeing effects in more modest apertures (the realm of Mak-Cassegrains).  There more frames will be in the 1/4 wavefront or better range, and cumulative error does apply in the equation.  With large apertures, either a location with excellent seeing, adaptive optics or stopping down to where the seeing matches the aperture will be the ticket.  Since this thread is discussing Mak-Cassegrains (where aperture is generally 10" or less), I was speaking from that perspective.

 

Tom

[PowellAstro]

Reasons the Mak may perform better than a standard Cass.


1..The Mak has better glass usually for the corrector. The casses most times use clear float glass while the Maks usually have bk7.

2..The Mak also in most cases, uses a primary with a slower f/ratio(easier to produce) than the Cass primary.

3..Most Maks have a longer total f/ratio usually around f/15, etc. This makes the Mak more steady while viewing. Seeing and thermals have less damaging effects than on a faster Cass.

4..The Mak, if made right will also have much less coma producing a wider corrected field(what's visible in any given eyepiece off axis) which will give a better overall image. The center of field should be the same.

5..The Mak also usually has a smaller central obstruction which also helps produce a little cleaner image with higher contrast.

So the Mak should preform better for these reasons compared to a Meade or Celestron SCT. Now when compared to the ACF or Edge scopes, the Mak looses most of these benefits.

Edited by PowellAstro, 04 November 2015 - 09:11 PM.