224 replies to this topic

[Alph]

Again, I think the concept of 'cells' gets in the way of understanding.

Would it help if you were told that the cells (or the distortion in a star image due to seeing) change(s) at a rate of 100 times a second? Think of the cells as lenses of different refracting indexes moving across the sky. It is just a model. Have you ever heard about Bohr model of atom? Atoms do not look like Bohr envisioned in his model, yet his naïve model turned out to be useful.

This is why I assert that the 4" typical 'cell' diameter is more a result of statistics. Under the conditions typical of the average clear night at most places,

Again, a 4 inch (or rather 10 cm) cell is NOT typical. It corresponds to 1 arc-sec seeing.

[Asbytec]

I dont recall hearing the 3R0 rule either, I stumbled on it crunching the numbers. Turns out, long exposure FWHM improves rapidly with aperture then improves only slightly in larger apertures. Short exposure FWHM improves with aperture until beyond 4R0 when the speckle pattern is more fully developed. (Hardy, 1998)

As for 4" statistical seeing, when you look through a 4" aperture, what do you see? If you see a nice, intact pattern then R0 ~ aperture and about 8/10 Pickering. I normally get 8/10 in the tropics.

[Cotts]

The concept of a "4-inch cell" (or any other size) is misleading even to the point of being erroneous. These cells are naturally variable phenomena both at a given instant and over time, and therefore there will be a natural distribution of cell sizes between your scope and the stars. Not just 4-inch cells. The mean size might be 4-inches but to think that this is the only size at any given moment is ludicrous. The central tendency of the cell sizes around the mean and the skewness of the distribution (which tail of the curve is 'larger') are also variable, causing even less chance that a certain cell size hovers over your objective lens.

A cell size of 4", if that were the only size, might lead to 1" seeing if the cylinder between your scope's objective and the upper atmosphere contains only this size. But it is absolutely not the only size at any point in time and the cells don't line up nicely over your objective and they are moving across your line of sight.

Too many variables by far to state flatly that a "cell size" "corresponds to" a "certain level of seeing".

Dave

[Geo31]

Look, hype aside, and I think we can all ID the hype, I'd love to have one. I wouldnt pay the price for a new 50th Anniv edition, but a reasonable price for a used one? You bet. Screw the (unnecessary IMHO)GOTO, it's a great instrument that I'm sure I would enjoy using regularly. I'd want. A larger instrument when warranted, but for casual observing, I think a Q would be a joy to own.

[freestar8n]

The Fried parameter, r0, should not be viewed as something inherent in the atmosphere, but it can be viewed as a property of the wavefront entering the telescope: the wavefront will comprise independent patches with diameter approximately r0. You can then use that r0 value to characterize the seeing - and this is done today at professional observatories with devices such as a DIMM. An example is here.

There are many other examples and Fried's work continues to be cited. Note that the temporal behavior of the wavefront is separate from the spatial behavior, which is defined by r0. The temporal behavior depends on the speed of the winds and can vary - but the speckle pattern can usually be seen by the eye at high power, and captured by video at a relatively low rate. The appearance of speckles is a direct indication of those "seeing cells" - in terms of independent patches of wavefront entering the telescope.

So there is nothing "hype-y" about referring to r0 = 4" - but I agree it is misleading to say there are 4" blobs in the atmosphere that cause it. Instead there is a turbulent atmosphere that ends up producing independent patches across the entrance pupil that are about 4" diameter.

But even if you agree that the Fried parameter is a useful characterization of seeing, it is still incorrect to conclude that the best you can do is with an aperture equal to r0. The theory doesn't say that at all - so for an ad to imply that such a small aperture is best would just be incorrect, rather than hype.

Frank

[azure1961p]

Look, hype aside, and I think we can all ID the hype, I'd love to have one. I wouldnt pay the price for a new 50th Anniv edition, but a reasonable price for a used one? You bet. Screw the (unnecessary IMHO)GOTO, it's a great instrument that I'm sure I would enjoy using regularly. I'd want. A larger instrument when warranted, but for casual observing, I think a Q would be a joy to own.

No ones saying its a bad instrument or even sub par. Its unfortunate when the topic strays there but Im a fan as much as anyone.

Pete

[Asbytec]

Frank, I get the feeling Questar boasts about R0 = 4, because their 3.5" aperture remains diffraction limited at that seeing level. Larger apertures are not, therefore, Questar is the better choice. While true, as I understand it, it is misleading because, as you correctly point out, 3 to 4xR0 still out resolves the smaller aperture and has better light grasp.

Still, its an enticing bit of pleasing hype and probably good marketing. I can understand it, having normally very good seeing for my own aperture. It allows full use of the smaller aperture, but is merely pleasing compared to larger scopes.

Alph mentions 2" seeing as more common than not. Thats not entirely my experience.

Id still like to understand their claim of 1/10th P-V at the focal plane. It takes more than 1/50th wave on each surface. The Q will have SA, unless aspheric in such a small aperture, the radii and focal length play a role, too. So, it takes more han extremely smooth, nearly perfect spherical surfaces to get that good correction. A parabola with 1/50th P-V, sure, but a spherical system is different fundamentally.

Do they asperize or use a secret, highly efficient set of radii? What makes them that good to claim 1/50th puts up such a good wavefront? Sounds like they are hyping P-V from a reference sphere. To get 1/10th at best focus probably means a little better than 1/4th at Gausian focus. I imagine thats doable at f/13 without doing the math.

[Paul G]

I have used a Q3.5 yes. Planetary images comparable to my Televue refractor. The spindly legs - ok nicely machined spindly legs - are for tabletop use which in itself is a bad idea. Its a rare table that'll support a telescope with any solidity and heaven forbid you put your DL ows on the table as u observe!

At anyrate having a huge data base for a Q in a GOtO alt-az fork mount would really bring that scope up to speed a d leave the undersized finder for initial star align like I do with my C6 GOTO.

Seriously JJK they need to GOTO that fork mount already. That finder demand it actually.

Alas I don't want to start nitpicking a scope I love. That wasn't the intent of the thread. But man I man - is it that hard to encode the axi and such.

The workmanship is beyond reproach. The neglect if a GOTO capable Q especially the anniversary edition is clearly the work of a company facing backwards with blinders on.

I know - its expensive as it is without it - yes but this is standard technology by now so the R&D's been carved out already sans the finer custom details.

Pete

I hate to say it, but sometimes things are perceived by many to be fine, just as they are. Adding GOTO might possibly be seen by some as actually cheapening what, to them, is an otherwise fine instrument! Kind of like tagging the Mona Lisa, so that it would appeal to a wider audience.

Besides, they've already made DSC encoders, and a HC available as an option to facilitate "PUSH-TO." The scope may not slew itself to the object, but at least you only need to use the finder once per session, in order to align it!

If Questar just went ahead and incorporated GOTO, they might find that they had invested a lot of time, and money, re-engineering the drive system in order to add a feature that their target demographic not only doesn't want or need, but might even alienate them to some degree as a result! On the other hand, by not adding GOTO they actually appear to display a commitment to maintaining the exclusive nature of the scope, an exclusivity that their customers desire! By not adding it, they show that they haven't succumbed to a "me too" kind of mindset, one that their target customers probably abhor!

In their minds, the Questar has always been, and shall always be, a fine optical instrument that stands head, and shoulders above the rest, setting a benchmark standard of quality, and excellence that others can only hope to aspire to! You just do not add "features" to something like this merely because everyone else is doing it. After all, this is a serious instrument, not some toy telescope that's here today, and gone tomorrow!

Yeah, I know how that sounds. But, in the end, I think that's what it really boils down to. Not just for those needing to "keep up with the Jones'" but also for those people who truly believe that the Q really does stand above the rest, because they just need to know that the instrument they're using is the best they can get, and that the level of quality they've come to expect will never be compromised!

Is that a bad thing? It would be cool if other manufacturers would at least lean in the general direction of that attitude towards their products! Is it wrong? Well, they should at least make sure that their products consistently live up to the hype. Otherwise, eventually people are just going to laugh at them, and walk away! That being said, though, they've managed to remain above reproach for the most part, so I don't believe for a moment that they're in any danger of anyone laughing, or walking, any time soon.

Anyway, this is just a thought, and I could very well be wrong! And if it turns out that I am, I can live with that. However, you have to wonder because, if GOTO was seen by Questar as being a significant selling point, and seen by their target customers as being a worthy addition, then you can bet the farm that Questar would have already added it by now. And not only that, but they would have done it a whole lot better than anyone else! After all, they have a reputation for quality, and innovation, that they have to live up to!

I think it stands to reason... YMMV.

Company 7 discusses Goto for the Q here:

Company 7 Questar info

More C7 stuff on Q

[Alph]

but I agree it is misleading to say there are 4" blobs in the atmosphere that cause it. Instead there is a turbulent atmosphere that ends up producing independent patches across the entrance pupil that are about 4" diameter.

Oh, well, no. Actually the model assumes the existence of blobs or rather isoplanatic patches as they call them nowadays. Aside from length or diameter (Fried’s parameter R0), the patches also have the property of height or altitude which leads to the concept of isoplanatic angle defined as the angle over which the index of refraction can be considered uniform. Isoplanatic angle = 0.314 r0/h where h is the turbulence characteristic altitude. This angle is on the order of few arc-seconds, really small. Isoplanatic angle and coherence time (10 ms or so) are very important concepts in adaptive optics.

[Asbytec]

I am sure Frank was differentiating between a model and the atmosphere. Theres no dipute here.

The isoplanic angle and temporal properties become problematic with larger aperture beyond 4R0. In fact, its the reason aperture suffers from seeing, adaptive optics exist, and for Questar's hype.

[freestar8n]

I don't know how you could be familiar with Fried's work if you had never heard of his key conclusion about short exposure imaging, that resolution peaks at D = 3-4r0 - as shown in his Figure 1, 1966 JOSA paper.

This is getting off topic, but since several people have doubts about 4" cells in the atmosphere - Fried's theory does not assume anything like that about the atmosphere. The only thing assumed about the atmosphere is that it has Kolmogorov turbulence over a range of scales from millimeters to meters, and that turbulence leads to isotropic changes in refractive index. After light propogates through that turbulence, the resulting wavefront entering the telescope will consist of patches with diameter r0 - but that r0 has no direct connection with any structures (bubbles, cells) in the atmosphere. r0 results from the statistics of passing through that turbulence and encountering small variations in refractive index along the way.

The rate of change of the wavefront depends on the dynamics of the atmosphere, and one approximation, Taylor's hypothesis, is that the wavefront is frozen and moving across the aperture at some rate - and that rate will depend on the lateral speed of the winds above.

It may be that the people writing the ads knew something about Fried's parameter and wavefront patches - but they did not know that the same theory pointed to optimal aperture well above r0. Even today in CN there is a lot of confusion and arguments over optimal aperture. All I'm doing is pointing at the relevant theory to say that the theory itself doesn't say the optimum is at D=r0 - even though it does say the wavefront will be broken into independent patches of diameter r0. And it certainly doesn't say there are bubbles in the atmosphere with a certain size - and they show up as speckles in a star spot.

Frank

[JJK]

I don't know how you could be familiar with Fried's work if you had never heard of his key conclusion about short exposure imaging, that resolution peaks at D = 3-4r0 - as shown in his Figure 1, 1966 JOSA paper.

This is getting off topic, but since several people have doubts about 4" cells in the atmosphere - Fried's theory does not assume anything like that about the atmosphere. The only thing assumed about the atmosphere is that it has Kolmogorov turbulence over a range of scales from millimeters to meters, and that turbulence leads to isotropic changes in refractive index. After light propogates through that turbulence, the resulting wavefront entering the telescope will consist of patches with diameter r0 - but that r0 has no direct connection with any structures (bubbles, cells) in the atmosphere. r0 results from the statistics of passing through that turbulence and encountering small variations in refractive index along the way.

The rate of change of the wavefront depends on the dynamics of the atmosphere, and one approximation, Taylor's hypothesis, is that the wavefront is frozen and moving across the aperture at some rate - and that rate will depend on the lateral speed of the winds above.

It may be that the people writing the ads knew something about Fried's parameter and wavefront patches - but they did not know that the same theory pointed to optimal aperture well above r0. Even today in CN there is a lot of confusion and arguments over optimal aperture. All I'm doing is pointing at the relevant theory to say that the theory itself doesn't say the optimum is at D=r0 - even though it does say the wavefront will be broken into independent patches of diameter r0. And it certainly doesn't say there are bubbles in the atmosphere with a certain size - and they show up as speckles in a star spot.

Frank

Theory is a good guide, but where I live, there have been nights when my 175 mm or 180 mm AP apos did no better visually (resolution-wise) than my 105 mm Traveler. That said, I still bring out the big guns whenever possible, because if the seeing makes a turn for the better, the larger scopes win handily.

There are several ways to interpret the intent of the 4" cell oversimplified model described in the Questar ads. 1) Q was trying to deceive the public. 2) Q used a simple concept (4" cells) to describe what in fact can occasionally be observed (see above), but suggesting the Q 3.5 bests the larger scope is not in my realm of experience. 3) The folks at Q were ignorant. Take your pick, but your guess will just be a guess. Ask the folks at Questar, if you really need to know why they made the ads in question.

[ColoHank]

Do they asperize or use a secret, highly efficient set of radii? What makes them that good to claim 1/50th puts up such a good wavefront?

Questar has acknowledged that the 3.5's optics are not spherical throughout, but the company does not divulge which surface is aspherized. I suppose, though, that someone could take a Questar apart, do some measurements, and make that determination. In a Rutten Mak (or Rumak), the separate (and sometimes adjustable) secondary is commonly aspherized. The secondary on a Gregory Mak like the Questar is integral to the R2 surface. If that spot is aspherical, it would take a mighty deft hand to figure it, methinks. I've often wondered if the disc applied to the R1 surface is meant to mask the fringes of an aspherized secondary spot. Cumberland Optical supplies Questar's correctors and mirrors.

[Asbytec]

Thanks, Hank.

[freestar8n]

I can't keep track of which Maksutov's explicitly say they rely on a big asphere, but my personal view, after looking at the tolerances on spherical maks, is that the good ones tend to involve final figuring in an autocollimator, which involves retouching one of the surfaces to null out any remaining errors. As a result, I think any good Maksutov effectively relies on an asphere - albeit a minor one that is nearly a sphere. Even Maksutov's original papers, and Gregory's writings on spot Maksutov's - all refer to final figuring with an autocollimator.

So I think it's a myth that Maksutov's can be made easily and well because they involve only spheres. Even if the design assumes spheres, the tolerances on the radii are so tight that final corrections would be needed - and they require an autocollimator and manual work - but I don't think it's a lot of work for a skilled person.

I don't know of any ads for Maks that say, "Our final product is great because it can be made from perfect spherical surfaces." They generally don't say much about the figures. The Meade Mak and the AP Mak are exceptions - they involve big aspheres and - for the Meade - a big secondary. I think the Questar 12 is explicitly asphere also.

I know some Maks do involve a different radius on the secondary spot - and Questar may be that way but I can't find a reference handy. Yes - I think that would be hard to do well - but it avoids a separate glued hunk of glass and associated thermal issues that might affect a Rumak.

Frank

[Edit - corrected name error]

[Asbytec]

Hi, Frank. Its an interesting aspect of the MCT. From my study, the larger MCTs (>6" at large relative apertures) begin to require an aspherical surface to handle large amounts of HSA from the meniscus. So, the Meade 7" would employ one. Their ETX line probably does not. Smaller apertures either apply balanced SA or are left as is.

Im not familiar with the retouching you mention using an autocollimator, but balancing higher order SA does involve a slight tweak to one radius. Most likely the correction is applied to the primary. Of course, an asphere can be applied to another appropriate surface such as the secondary, if feasable.

In any case, as Hank says, that Q uses an aspheric term on such a small aperture is an amazing thing. Otherwise, uncorrected, its probably no better than diffraction limited at best focus and slightly better (1/6th P-V, ~ 0.040 RMS) if the balanced form is applied.

Thats a guess guess to spur discussion, its something Id like to understand better. Im not sure how well corrected the system would be at gausian focus and f/13.

[freestar8n]

Both Maksutov's early writings, and Gregory's later writings, say that Maks are good for amateurs because they only involve spheres - but at the same time they say to do the final figuring with an autocollimator. To me that was eye opening because the guys who were encouraging amateurs to make them were themselves relying on autocollimators - and making final adjustments to the "spherical" surface to get a good result for the system. On the other hand, most amateurs diving into it didn't have a flat to use for an autocollimator - so they were going "blind" just relying on the spheres to be exactly the needed radii. And that would be hard and take skill.

Whether or not spheres will work depends on the tolerances - and the tolerances are not easy to calculate without a Monte Carlo search over the parameters - which I have done. With a thicker meniscus and a slower primary - and slower overall f/ratio - I think it's reasonable without an autocollimator. But these fairly short f/15 systems would be harder to do.

I doubt that anyone who went to the trouble to make a different radius for the "spot" would also have aspherized that spot. I think that's extreme. But just allowing that radius to be different offers a much needed degree of freedom to the design - and would help a lot even if it were sphere. A normal, spherical spot maksutov only has three parameters in its optics: 3 radii and one thickness - with the spacing then adjustable. That is incredibly constrained, and if anything is slightly off the spherical aberration would not be balanced.

This is also straying off topic - but without interferometry on a bunch of questars over the years, there is no hard data on how well they were made. I personally doubt there was anything exceptional beyond perhaps 1/10th wave as a guess - and I assume that involved some manual refiguring.

Frank

[Asbytec]

No doubt the tolerances are tight and the radii have to be adjusted during design and fabrication. And they are very math intensive. One might imagine once you have a good goodcsetset of radii and thickness, you could mass produce good optics. Better than that seems to need a human touch.

That why I was curious how Q managed 1/10th on the wavefront while quoting 1/50th on the surfaces. Correcting that well involves more than 1/50th wave surfaces alone. Even Intes basic models are advertized at 1/6th wave, while hand figured deluxe are better corrected.

Sure, I doubt they aspherize the spot secondary. That does seem extreme. But something makes the Q good other than their claim of perfectly smooth optics. Since this thread is on hype, I wanted to explore that aspect.

[ColoHank]

Here's a link to a Cumberland Optics site that relates to aspherization of Questar optics. I hope all can open it. I could open it with my iPad, but for reasons unknown, haven't been able to open it with my desktop.

http://cumberlandopt...om/Questar.html

I've run into a couple of other references which state that aspherizing of the primary, R1, or R2 surfaces, but not the secondary in particular, can be employed to correct a Maksutov. I still have no idea which surface Questar aspherizes.

[EddWen]

As I mentioned earlier in this thread, Questar not only claims the optics to be accurate to better than 1/10 Lambda at the focal plane, they provide documentation. Every owner of a 50th anniversary model receives an interferometry report for their scope linked to their serial number. There seem to be >200 delivered to date with a limited production of 250 units. Better order yours now!

My custom Mak was provided with a similar report with a result of 1/6.7 Lambda. Am I disappointed? No! Considering the quality of the glass needed for the meniscus and the figured/polished/coated/mounted surfaces in the larger sizes required, I'm satisfied the craftsmen did well.

With regard to the discussion of "seeing cells", I tend to agree with Glenn. I think a modern analysis of atmospheric non-homogeneity would be based on fractals. The real culprit effecting the refractive index of air is delta T. Not being a mathematician, I can't do it, but I do know that when I go boring into a cloud, there do not seem to be 'çells' of a specific size visible. And, with a minor familiarity with Schlieren imaging, which is based on the refractive delta, where nothing like 'çells' are evident.

So, we have people arguing about decades old perceived claims (not seen any attributions to Questar yet) whilst for the last decade the company claims are discounted?

[EddWen]

I will make an anecdotal claim for a standard Questar being better than any other instrument I've looked through for visual use.

My 1989 Questar 3.5" was bought with a full diameter solar filter. Questar makes their solar filters with 1/10 Lambda flat glass. I use it happily almost every clear day to do observations. Many satisfying hours accumulated ~15 minutes at a time.

When Baader solar film became available, I bought some to use with a good C8 that I had, and my A-P 155EDF. Neither scope provided better views. Or as good as, in my opinion.

Whether seeing conditions that are usually terrible during the day is a cause for this, I can't say. I will say that my Q 3.5 is my go to scope for sunspots, etc.

[frebie]

There seem to be >200 delivered to date with a limited production of 250 units. Better order yours now!

I believe that the number still available may be down to single digits.

[Geo31]

Look, hype aside, and I think we can all ID the hype, I'd love to have one. I wouldnt pay the price for a new 50th Anniv edition, but a reasonable price for a used one? You bet. Screw the (unnecessary IMHO)GOTO, it's a great instrument that I'm sure I would enjoy using regularly. I'd want. A larger instrument when warranted, but for casual observing, I think a Q would be a joy to own.

No ones saying its a bad instrument or even sub par. Its unfortunate when the topic strays there but Im a fan as much as anyone.

Pete

Oh, I think you and I agree, but there did seem to be some bashing going on outside of the hype.

What amazes me is some of the vintage astrophotography that was done with these scopes. That was when long exposure was not counted in seconds. What did they use for drive corrections in both RA and espcially in Dec? I don't recall ever seeing electronic Dec controls for a Q.

[frebie]

Here's a link to the electric dec for the 3.5" Questar: https://www.astronom...inch_p3788.aspx
and one for the 7": https://www.astronom...inch_p5428.aspx

[Geo31]

Here's a link to the electric dec for the 3.5" Questar: https://www.astronom...inch_p3788.aspx
and one for the 7": https://www.astronom...inch_p5428.aspx

Son of a gun. Cool.