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what is it that makes you believe you really do have good optics?

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#226 bobhen

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Posted Today, 07:42 AM

"If an optic produces better results in excellent seeing than another optic, why would it not produce better results in average or bad seeing? The seeing does not change the optical quality of the lens."

That's the $10k question. The short answer is something close to seeing affecting both apertures equally while the optics differ. A premium optic focuses a distorted wavefront into a 1/10 pv mess, and a lesser optic focuses the same wavefront into a 1/4 pv mess.

It's not the optical quality, which is different in both scopes, that composes the final seeing induced image, it's the messy wavefront coming to focus the same way in both. This is the effect of seeing where the optic is dealing with a disrupted wavefront (seeing) instead of creating an abberant one from a perfect wavefront (quality). At least in my intuition, anyway, if it makes any sense.

It's why we sometimes hear or understand a premium optic cannot fix an already distorted one into something better. It only forms a 1/10 pv messy star image from an already distorted wavefront long before the optic tries to form a good or bad image from it. The poor image is due to the seeing, not the optic.

"If seeing varies from moment to moment that means the seeing is “not excellent” seeing. With excellent seeing there are no jumps, scintillation and no movement."

This is not my experience. There is always movement however slight. Even in the best seeing, which is hardly ever completely stable, there are even better moments. Its during those brief best moments where finest Jovian detail rolls in and out of view.

One night I was checking out Plato's craterlets in great seeing through a 6" aperture. I held at least 6 of them mostly steady in crater form during above average seeing good enough to do so. Then out of nowhere, a 7th smaller crater form rolled into clear view briefly, then faded from view. It reappeared 3 times during the observation of Plato. Even during great seeing, patience pays off.

I've not compared with a certified premium optic, but I have used some good ones. At least as good is defined by whomever.

I think Suiter’s “wobbly stack” is the correct analogy. Everything along that stack can impact the final image: Optical quality, diagonal, focus, eyepieces, etc. etc.

 

In your example you need too add the aberrations in the optics to those in the seeing. That is what makes the difference. Because you can’t do anything about the seeing, the goal for an observer is an “aberration free” “wobbly stack”.

 

Think of it this way… two telescopes with the same optical quality but one uses a diagonal that is really dirty and smeared the other diagonal is spotless.

 

Which telescope will produce the better image? Why? The seeing is the same. The optic is the same.

Because the light path before the eye has been compromised in the scope with the filthy diagonal (a diagonal with aberrations) that scope will deliver a poorer image. Because those aberrations in the diagonal have been "added" to those in the seeing.

 

Now just change the aberrations in the diagonal to those in the primary optic and it is the same process.

 

Why do people want good eyepieces? Why do people want their scopes to be thermally stable? Why do people want focusers that allow for exact focus? Etc.

 

Anything that adds aberrations along Suiter’s “wobbly stack” will worsen the image regardless of seeing.

 

That is why along with an excellent optic you need an excellent diagonal, perfect focus, excellent eyepieces, thermal stability, etc. etc. etc.

 

You can’t do anything about the seeing but you can do some of these other things that will improve performance in ALL seeing conditions.

 

Bob


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#227 Asbytec

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Posted Today, 07:59 AM

Bob, I am pondering your comments and really cannot see any conflicting or incorrect information. Yes, it is about the wobbly stack, probably the diagonal, the optic, etc., but I believe there is something to the idea only the optic forms an image. The rest simply disturb the wavefront and the image forming optic has to deal with it as is. The atmosphere is not a very good interferometer delivering a perfect wavefront leaving only the aberration of the scope. It might be like testing a scope with a poorly calibrated IF(sic), it cannot say anything meaningful about the image of even a perfect optic because the input (garbage) is corrupted. It'd be interesting to see a DPAC that includes miles of atmospheric seeing. lol.gif

 

Still, it's hard to argue anecdotal evidence to the contrary. It's also interesting some folks also say it's the good seeing where the differences can be best seen. My experience in good seeing suggests the latter is probably true, and nothing much about the former either way. So, I have been pondering this as an amateur on and off for a while. I guess because we still do not have a common view of Jupiter in perfect seeing with the best scope in the world to use as a standard comparison. Except Damien's images above, maybe. We only have our own experiences. So, how do we know we are seeing a great image others rave about or just one we appreciate fully because it's nice to us and we rave about it, too (which is what matters, anyway, I guess). How do we even know we're on the same sheet of music? Maybe the answer is we talk about it. 


Edited by Asbytec, Today, 08:10 AM.


#228 bobhen

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Posted Today, 08:09 AM

But it's not the quality of these points that matter, it's their apparent roughness and failure to come to focus at a single point that matters. That's seeing acting on an optic and all same apertures equally.

 

Exactly.  If I go outside on a clear dark night, and the stars are twinkling like maniac strobe lights, I'm not gonna attempt planetary.  I'm also not gonna assess a new scope's performance.  I'll wait.

 

Yes, that's obvious to us old refractor fans, but we have new members who may not realize that, and may think their brand new scope is a dud when it's not.

True, the seeing acts on an optic equally but that does not mean that optics act on the seeing equally.

 

That’s what makes the difference. It’s not the seeing it’s the optic. Or it maybe something else along the “wobbly stack”.

 

Perfect seeing on a perfect optic equals a perfect image.
Perfect seeing on an optic with ¼ wave aberrations equals an image with ¼ wave aberrations.

 

Bob



#229 Asbytec

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Posted Today, 08:11 AM

 

You can’t do anything about the seeing but you can do some of these other things that will improve performance in ALL seeing conditions.

 

Bob

That's true. hmm.gif

 

True, the seeing acts on an optic equally but that does not mean that optics act on the seeing equally.

 

That's the key question that requires some out of the box thinking to answer. I've been of the school of thought that it's not the fine image(s) formed by the optic that matter, what matters is the seeing affect on the image formed by the otherwise fine optic. That affect should be pretty much the same in either with similar aperture. This is consistent with folks who say it's the better seeing where the fine optic makes a difference. Just debating because it's interesting topic and it is cloudy tonight. 


Edited by Asbytec, Today, 08:20 AM.


#230 t.r.

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Posted Today, 08:19 AM

“True, the seeing acts on an optic equally but that does not mean that optics act on the seeing equally.” ...That’s it!!! Let that absorb in! Take 1/4 wave seeing...look through it with a 1/4 wave optic...the image produced will be 1/2 wave!! Below the diffraction limit of 1/4 wave and most would pack it in as a bad night! Now, look through that same 1/4 wave seeing with an 1/8 wave optic...the resulting image will be 1/4 wave diffraction limited and one would shout hurray, a great night of seeing! I’m working on providing you the formula but this chit is deep and I have to cross reference back and forth. A lot of this is implied by reading and rereading the material over and over and picking out little phrases like “and optical quality adds it’s own aberrations to the resulting image”!
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#231 Asbytec

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Posted Today, 08:27 AM

...this chit is deep and I have to cross reference back and forth. 

You too? lol.gif

 

Let that absorb in! Take 1/4 wave seeing...look through it with a 1/4 wave optic...the image produced will be 1/2 wave!!

 

If it were only that simple we'd not have volumes of deep chit to cross reference. smile.gif

 

“and optical quality adds it’s own aberrations to the resulting image”!

 

Almost have to concede or ponder that point. Still, one slope of the disturbed wavefront still focuses somewhere else, it's just looks prettier when it misses the Airy disc. 


Edited by Asbytec, Today, 08:29 AM.


#232 k5apl

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Posted Today, 08:28 AM

For several years we have had a lighted resolution chart to test our optics, placed on a hill top, at the Texas Star Party.  I estimate it is about 1 mile away from the observing fields.  I was "shocked" about how poor the Seeing was when trying to evaluate my telescope optics.  Just 1 mile and the image was dominated by poor seeing.  Sometimes

it was better later on in the night. Sometimes it was better for a minute or two.  Sometimes it did not get better.  Like

observing Jupiter, I had to wait for those breaks of stability, to really see the resolution of my optics.

 

I did notice that the Seeing affected all scopes to some degree.  Still waiting for a cloud burning, seeing stabilizer telescope objective.


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#233 t.r.

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Posted Today, 08:46 AM

The best statement so far I can reference at t-optics.net is in 5.1.2 Seeing Error: OTF...

Now the discussion is on obstructed optics but this statement is clear...go down to “Combined atmospheric and telescope contrast transfer is given by the product of their OTF’s. It is the final OTF in the image plane at the objective.” I therefore imply that the optics other aberrations are also multiplied into the wobbly stack to derive the final OTF in the image plane. Abberations are aberrations whether they be obstructions, SA or total strehl.

#234 peleuba

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Posted Today, 09:00 AM

Maybe so and then again maybe not but one thing is for sure, we average guys, we astronomers sure as h*ll get to see some great things on those dark nights, it gives you the feeling, that urge to get out and the best is we get to keep a lot of our money ! And thats another contention, guaranteed !

 

I find your comments amusing and mildly disturbing.   Why the focus on money?  Its a hobby and you should enjoy it as you see fit.  No one is criticizing you.     The conversation is about answering this question:  what is it that makes you believe you really do have good optics?  And its evolved into telescope performance as a function of seeing conditions.  

 

The fact is that good optics trump poor optics in all types of atmospheric seeing.  And, yes, good optics are usually more expensive then average/poor optics.  That's just life. 

 

Don't get unhinged because someone prioritizes spending their money on expensive optics.  


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#235 bobhen

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Posted Today, 09:03 AM

That's true. hmm.gif

 

That's the key question that requires some out of the box thinking to answer. I've been of the school of thought that it's not the fine image(s) formed by the optic that matter, what matters is the seeing affect on the image formed by the otherwise fine optic. That affect should be pretty much the same in either with similar aperture. This is consistent with folks who say it's the better seeing where the fine optic makes a difference. Just debating because it's interesting topic and it is cloudy tonight. 

Of course good seeing is where the differences are best seen because they are "easier to see". As the seeing gets really bad, differences "are there" they are just "harder to discern". It just becomes harder for observers to see the difference between a really really bad image and a one that is just really bad.

 

Even a perfect optic in really bad seeing (say 1 wave distortions) is still not going to deliver great images. Why? Because optics have NO impact on the seeing and because a perfect optic plus 1 wave (really bad) seeing will form a 1 wave image and although that is not close to diffraction limited it will "still be better" than starting with a ¼ wave optic even though the difference will be harder to discern.

 

The differences become more obvious as seeing improves (to average and better) between a great optic and one of lesser quality. And that is "exactly" when you want to employ that excellent optic so that you can take advantage of the improving seeing without adding any aberrations in your telescope to those already in the seeing.

 

Bob


Edited by bobhen, Today, 09:04 AM.


#236 Asbytec

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Posted Today, 09:07 AM

"Now the discussion is on obstructed optics but this statement is clear..."

Yes, there does seem to be a larger seeing induced FWHM with an obstruction. This is probably due to the diffracted light into the rings. A similar effect probably happens with some aberrations that are field independent and increase ring brightness, like SA which is the same across the field as at the center. Maybe a few others to some degree, including inherent off axis abberations in some scope designs. Good point. It didn't go unnoticed earlier in this thread. Lemme review that reference, too. Been a while since I read it fully.

Edit : The obstruction above is D/2 which is pretty severe equivalent SA.

To confuse things, er I mean inform, here's what Damien had to say. :lol:

"Other issues such as collimation accuracy, focus accuracy, thermal equilibrium and above all astronomical seeing are really of much greater concern than the small performance differences that actually exist between telescopes under "lab test" conditions."

Edited by Asbytec, Today, 10:03 AM.


#237 John Fitzgerald

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Posted Today, 09:08 AM

Say you have optics that are .9 Strehl at total equilibrium. When cooling down, they could be .85 or .95, or the same, or higher or lower, depends solely on luck, the way I see it. On an aberrated optic, seeing usually degrades it further, but theoretically could improve it for short periods. YMMV. Bottom line, with seeing and cool down, it’s an inexact science.

#238 Bomber Bob

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Posted Today, 09:20 AM

True, the seeing acts on an optic equally but that does not mean that optics act on the seeing equally.

 

That’s what makes the difference. It’s not the seeing it’s the optic. Or it maybe something else along the “wobbly stack”.

 

Perfect seeing on a perfect optic equals a perfect image.
Perfect seeing on an optic with ¼ wave aberrations equals an image with ¼ wave aberrations.

 

Bob

Okay, in plain talk:  You just bought a new refractor -- it's an unknown quality factor.  Your local seeing stinks for planetary.  DON'T ATTEMPT PLANETARY - & - DON'T ASSESS HOW GOOD IT IS ON A LOUSY TURBULENT NIGHT.

 

There.  Really, refractor folks can spin a piece of thread into a 5 mile long tapestry...

 

If you live under a jet stream, go DOB & DSO.  If you have 5 perfect planetary seeing nights a month, and can afford a $12K Tak 150, buy it.  The vast majority of folks on this Forum are between those extremes, hence the OP's original question...

 

what is it that makes you believe you really do have good optics?


Edited by Bomber Bob, Today, 09:21 AM.

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#239 bobhen

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Posted Today, 09:26 AM

Say you have optics that are .9 Strehl at total equilibrium. When cooling down, they could be .85 or .95, or the same, or higher or lower, depends solely on luck, the way I see it. On an aberrated optic, seeing usually degrades it further, but theoretically could improve it for short periods. YMMV. Bottom line, with seeing and cool down, it’s an inexact science.

Seeing cannot improve or degrade optical quality (if that is what you meant). If the seeing gets better the views will improve because the seeing has improved not because the optic has improved.

 

Whatever your lens quality Strehl is, that is a fixed number that cannot be changed by seeing.

 

Bob


Edited by bobhen, Today, 09:51 AM.


#240 John Fitzgerald

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Posted Today, 09:29 AM

Strehl can be temporarily changed by cooling, but usually is worse. The larger the optic, the more pronounced and longer lasting the effect.

#241 FirstSight

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Posted Today, 09:31 AM

When Al Nagler comes over to you at a Star Party, and wants to spend a few minutes looking through your scope, it means one of two things:

1) you have excellent optics; OR

2) OTOH if your optics are other than Televue, he might just be checking out the competition.


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#242 bobhen

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Posted Today, 09:53 AM

Strehl can be temporarily changed by cooling, but usually is worse. The larger the optic, the more pronounced and longer lasting the effect.

Sorry, yes you are correct. I meant when stable.

 

Bob



#243 Asbytec

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Posted Today, 10:14 AM

Just read telescope optics.net. Gonna give this horse a break before it gets dead from all the beating.

The one formula prevalent in the literature seems to be Lambda/R0. RMS and PV are mentioned, but usually related to D/R0 in some way. Both indicating seeing affects the image.

It's intuitive better optics can survive seeing, I gotta admit. That's why 1/4 pv seeing added to 1/4 pv optics equals mush and such a thing makes intuitive sense.

Need a break...


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