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Bob Abraham
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Reged: 05/17/05

Loc: Toronto, ON, Canada
A tale of two Questars new
      #4569486 - 05/08/11 01:34 PM

A few weeks ago an irrational compulsion to get another small telescope came upon me. My original idea was to get something like a TV-85 but a nice Duplex Questar (1986 model, with a Zerodur mirror) came up for sale on Astromart and I decided it might be fun to get another Questar. I sold my old Questar a couple of years ago (a 1978 standard model with a pyrex mirror, also bought used). I sold the old Questar because I have now got myself a backyard observatory and an ultraportable little telescope with a fairly small field of view seems a little pointless. But I confess I missed that little Questar, for reasons I can't quite explain. Anyway, the one I just bought is sufficiently different from my old one (duplex vs. standard, zerodur vs. pyrex, somewhat newer etc) that I thought others might find the comparison between the two interesting.

I've only had the new scope out for a couple of nights but have already formed an impression about how it fares relative to the old Questar. This isn't a proper review by any means, and the short version of this post is that the new Questar seems just a little better than the old one. Perhaps the real message of this post is that even if you've got an observatory with an 8" Mak in it a little Questar is still tremendous fun. For example, last night I meant to go out for 15 minutes to check it out, but stayed out for 3 hours.

I was a little surprised that the OTA of the 1986 Questar was quite a lot more modern-looking than the 1978 model, since only 8 years separates the two (which is peanuts compared to the long timescale over which these have been made). The secondary seems to have a machined light baffle affixed to the meniscus instead of the painted baffle on the front of the older model. In fact the whole meniscus cell appears different, extending out a little more. In retrospect both these things are obvious from pictures of the scopes if one looks closely. Also the screw-in lens cap on the newer scope is more substantial and better made than the old one.

As with my older Questar, the newer Questar has really good optics. I was worried about some of the sloppiness in SCT scope manufacturing around Halley Mania perhaps also affecting Questar too, since the newer scope dates from 1986. But fortunately the optics seem first-rate. The collimation looks 99% spot-on, and it may be 100%. In-focus stars show the first diffraction ring as a complete circle though with the barest hint of one side of the ring being ever so slightly fainter than the other side. This may well be my imagination, and even if it isn't, it's not at the level where I would adjust things, even if I could adjust them (and I am anal about these things). The cool-down behaviour of the Zerodur mirror struck me as perhaps slightly unusual relative to my experience with small Maks with pyrex mirrors. In the latter case my experience is that before cooldown the images start off as horrible aberrated messes with a mix of rapidly churning aberrations but mostly dominated by spherical aberration which more-or-less smoothly goes away as the mirror cools and tube currents settle down. With the Zerodur mirror my impression was that the image starts off better with the main aberration being some astigmatism that pops in and out from a tube current that gradually goes away with cooldown. In other words, the non-equilibrated Zerodur mirror seems dominated by a single aberration rather than the usual crazy mix of aberrations. I'll have to monitor the cooldown behaviour over a number of seasons to see if this initial impression turns out to just be my imagination.

The duplex configuration is terrific! It makes the scope much more versatile. However the semi-cylindrical duplex mounting bracket has the disadvantage of easily blocking the finder's field of view when the tube is rotated. This is not a big deal as the eyepiece position was generally comfortable even without rotating the tube, and in one or two cases where it wasn't I just lived with the awkward position until the object was lined up in the finder, then rotated the tube.

A couple of other differences between the older and newer scope:

The view through the finder in the new scope seems considerably brighter than the view through the old one was. This seems to make a big difference. I'm in a big city with a typical limiting magnitude around 3.5-4.0, so star-hopping is hopeless, and I use setting circles or GoTo to find most things with my scopes. With the questar, after a cursory polar alignment, I use the circles to get me close and zero in with the finder. In the old scope it was sometimes a challenge to find stuff in the dim finder image to line things up in the main scope, particularly faintish double stars. But with the new scope this does not seem to be an issue at all. For example, last night I had some fun with doubles in Bootes, and I went through Mu, Xi, Epsilon, Pi, Zeta (unresolved), Delta, Kappa and Iota Bootes in short order. Even the fainter Bayer-lettered doubles were easily visible in the finder, and I didn't miss goto at all. (I was mainly using an 11mm Plossl so the field in the finder wasn't particularly large). My conclusion is that one might want to keep an eye on the reflectivity of the finder mirror on older Questars... I suspect this is the first optical surface that shows signs of degradation because the finder mirror is quite exposed.

Other than these small things, the experience using two Questars were pretty much identical, as one would expect.

In more general terms, in case anybody is thinking of getting a Questar after not having owned one for a while, my experience over the last couple of nights has been very positive. I think the scope has a nice niche as a ultra-portable tracking scope with its particular strength being the moon and doubles. Doubles, in particular, are wonderful with this small scope, with beautiful textbook diffraction patterns that are aesthetically very appealing. Last night Saturn was nice too with good detail (e.g. obvious Cassini division, obvious banding with hints of detail in the bands), but I felt the small scope was hurting for aperture nonetheless. On the other hand, I find that bright doubles in a small scope are often as pretty (or prettier) than in a large scope. And the moon was really fabulous.

My overall conclusion: the little Questar is a great scope and I'm very happy I've got one again. While not cheap, used ones seem to me to be reasonably good value for money if you like small well-made Maks, particularly once you factor in the cost of a mount.

Bob

P.S. Does anybody prefer TV plossls to Brandons on their Questars? I suspect I'm alone on this, but I do. The TV plossls I've tried all come to focus in the finder (in fact because of my near-sightedness the Brandons tend to lie at the very end of the diopter adjustment unless I'm wearing my glasses). The contrast in the plossls seems just as good as with the Brandons to me, and the FOV is of course wider. The Brandons are nice too of course.


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justfred
super member
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Reged: 09/24/10

Loc: Birmingham, Alabama, USA
Re: A tale of two Questars new [Re: Bob Abraham]
      #4569522 - 05/08/11 01:59 PM

"... for reasons I can't quite explain." "... tremendous fun."

Some things can't be quantified. Your words have described this one.

Great post, Bob.

Fred


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EddWen
Pooh-Bah


Reged: 04/26/08

Re: A tale of two Questars new [Re: justfred]
      #4569727 - 05/08/11 03:32 PM Attachment (39 downloads)

Yup !!

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astro_que
sage
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Reged: 11/11/09

Re: A tale of two Questars new [Re: EddWen]
      #4569867 - 05/08/11 04:47 PM

Bob,
Your 1976 Questar may not be a 1976. It could be a year or two earlier. From your description of the meniscus cell, it has an R1 optical system. In the R2 system, the meniscus cell is noticeably deeper. This was done to give better protection to the corrector.

While the meniscus cell is the tip-off to an R1 optical system, other differences are not as easy to identify. In the R1, the silvered secondary is on the OUTER surface of the corrector, right under the painted spot. Compared to the R2 system, light travels through an extra inch or glass. According to Jim Reichart, the immediately noticeable characteristic of the R2 is increased brightness.

But that's not all. Because of the placement of the secondary in the R1, which was due to a patent dispute, the optical design is not optimal. No matter how perfectly made R1 optics are, a well made R2 scope is simply sharper, because the optical design is theoretically correct.

Your comments may be the first side-by-side comparison by an owner of the R1 and R2 optical systems. Hopefully, it will dispel the notion that the two are equivalent instruments. They are not. Individuals who contemplate a used Questar should be aware of these differences, and make the appropriate adjustments for value.

The machined baffle on the corrector is not a big deal. It can be installed on any R2 Questar at nominal cost as part of the service package. The change was made with the discovery that the Dupont paint, which is no longer available, is susceptible to solvents and might eventually fall off anyway. Questar replaces the paint with a metal plate that is attached with a little adhesive. The last time I checked, the cost (if a service was being performed), was an additional $25.


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astro_que
sage
*****

Reged: 11/11/09

Re: A tale of two Questars new [Re: astro_que]
      #4569874 - 05/08/11 04:50 PM

Bob, a note about the TV Plossls. They are not optically ideal for this scope. The Brandons are. Perhaps your eyes have an optical defect (all eyes have some) that are compatible with the TV Plossls. In general, however, the Plossls add aberrations.

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Bob Abraham
sage


Reged: 05/17/05

Loc: Toronto, ON, Canada
Re: A tale of two Questars new [Re: astro_que]
      #4569932 - 05/08/11 05:16 PM

Hi, sorry, I think you must have misread my post. My earlier Questar was from 1978 and my new one is from 1986 (verified by serial numbers). Neither is from 1976, and in fact neither is an R1 Questar. We had a couple of those at work and I've inspected them and the front portion around the meniscus looked a bit different to what I saw/see around my scope(s).

As for TV plossls adding significant aberrations... well, I'm very skeptical. Eyepieces generally contribute just a tiny component of the wavefront error budget, and on-axis and at f/15 they're really not working very hard. If this were off-axis and f/5 things might be different of course.

Bob



Quote:

Bob,
Your 1976 Questar may not be a 1976. It could be a year or two earlier. From your description of the meniscus cell, it has an R1 optical system. In the R2 system, the meniscus cell is noticeably deeper. This was done to give better protection to the corrector.

While the meniscus cell is the tip-off to an R1 optical system, other differences are not as easy to identify. In the R1, the silvered secondary is on the OUTER surface of the corrector, right under the painted spot. Compared to the R2 system, light travels through an extra inch or glass. According to Jim Reichart, the immediately noticeable characteristic of the R2 is increased brightness.

But that's not all. Because of the placement of the secondary in the R1, which was due to a patent dispute, the optical design is not optimal. No matter how perfectly made R1 optics are, a well made R2 scope is simply sharper, because the optical design is theoretically correct.

Your comments may be the first side-by-side comparison by an owner of the R1 and R2 optical systems. Hopefully, it will dispel the notion that the two are equivalent instruments. They are not. Individuals who contemplate a used Questar should be aware of these differences, and make the appropriate adjustments for value.

The machined baffle on the corrector is not a big deal. It can be installed on any R2 Questar at nominal cost as part of the service package. The change was made with the discovery that the Dupont paint, which is no longer available, is susceptible to solvents and might eventually fall off anyway. Questar replaces the paint with a metal plate that is attached with a little adhesive. The last time I checked, the cost (if a service was being performed), was an additional $25.




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astro_que
sage
*****

Reged: 11/11/09

Re: A tale of two Questars new [Re: Bob Abraham]
      #4569973 - 05/08/11 05:37 PM

Bob, it is a known fact that the Plossl design is sharpest on an annulus, not at exact center. This has been discussed recently; for your convenience, I'll repost here:

There are many designs that are called Plossl, but are actually variants. The results you have seen are possible. Simply adding a glass type provides an additional degree of freedom. The Brandon superficially resembles a Plossl, but, as it uses four different types of glass, including lanthanum, Chester Brandon was able to create a design more similar to the Abbe Orthoscopic, with some advantages.

Chris Lord's exacting methodology is demonstrated in this article,
"COMPARISON TEST OF THREE PLÍSSL EYEPIECE TYPES BRANDON, TELEVUE, GSO REVELATION" at
http://www.brayebrookobservatory.org/BrayObsWebSite/HOMEPAGE/PageMill_Resources/Comparison%20test%20of%20TVPlossl%20vs%20Brandon.pdf

In this article, Lord's procedure successfully discriminates between eyepieces intended for fast and slow instruments. Figure 37-28 provides curves of Plossl aberrations, obtained from "Handbook of Optical Systems Herbert Gross Wiley-VCH Berlin 2009:" The curves show correction zero-crossings that are displaced from the center.

Excuse my misreading of your post. The optical designs are identical. You did not mention whether your 1978 has a broadband coating. However, the only reasons for difference in brightness are:
1. Deteriorated broadband coating.
2. A 1978 which has an aluminized coating.
This is also sugggested by your report about the finders. There is no difference in the optical design of the finders whatsoever.

A change has recently been made to the corrector cell: it has been eliminated! The formerly discrete cell has been replaced in the newest units with a cell integral to the barrel. This was made possible by advanced CNC techniques. Since machining of the barrel is now highly automated, there is little chance for error. Hence it became unnecessary to divide the job into two discrete pieces.


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Bob Abraham
sage


Reged: 05/17/05

Loc: Toronto, ON, Canada
Re: A tale of two Questars new [Re: astro_que]
      #4570135 - 05/08/11 07:31 PM

Hi,

Interesting that Questar have redesigned the meniscus cell fairly recently. Unfortunately my curiosity about the new design will have to wait a long time since used Questars are all I can afford!

I've also heard the claim from time to time that the Plossl design is sharpest off-axis, and I confess I've always been rather mystified by the claim. I've never seen a ray trace that shows this, including the ones in the PDF you just sent a link to. I think you should take a look at the appendices in this document again. Perhaps you are confusing the height of the zero crossings in the longtitudinal spherical aberration plot with the angular distance from the center of the optical axis? I'm sure I don't need to tell you these are very different things. Please check out the actual ray traces (the rightmost sub-plot in each set of plots in the appendix) and I think you'll agree each design is obviously sharpest on-axis, including the Plossl. (Also look at Ch. 16 of Rutten and van Venrooij).

As far as I can tell, a legitimate critcism of the Plossl is that it can have worrying ghosting properties, but I think these days coatings are so good this is not really an issue (at least it isn't with the eyepieces I've examined). And I'm pretty sure that issues of manufacturability (e.g. cleanliness, scratch/dig ratio etc) dominate over other factors for all these designs on-axis at f/15 (f/30-ish with the Questar barlow).

Anyway, eyepiece likes and dislikes are pretty subjective and I know what works for me!

Regards,

Bob


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astro_que
sage
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Reged: 11/11/09

Re: A tale of two Questars new [Re: Bob Abraham]
      #4570207 - 05/08/11 08:15 PM

Bob,
The distortion of the Plossl is zero on axis, but that is not the same as sharpness. Spherical aberration, which destroys sharpness, is not zero on axis.

I refer you to
http://www.brayebrookobservatory.org/BrayObsWebSite/BOOKS/EVOLUTIONofEYEPIECES.pdf , page 31. To quote,

"However, unlike the Abbe Orthoscopic and its derivatives, where the longitudinal spherical correction is zero on axis, the asymmetric form of the Plossl leads to a zonal correction and the sharpest imagery does not occur on axis but some 30% towards the edge of the field of view. At low to medium powers this is of no consequence, but it is noticeable at high powers (exit pupils less than 1.5mm)."

The Televue Plossls use plain old crown and flint glass. They are Plossls, and they are desirable for fast scopes. Chester Brandon's design is optimized for slow scopes, and they use four different types of glass, including lanthanums.

Every once in a while, someone expresses a preference for an eyepiece design that is not matched to the scope. I speculate it is due to peculiarities of that mushy globe of water that constitutes the human eye. Anyone who is serious about getting the best performance out of a Questar should at least have the Brandons available to make a direct comparison. Time and distance make reliable comparisons impossible.

For terrestrial use, I treasure my Televue Panoptics, where the wide field of view makes a Questar far more practical. But in searching for faint objects, the economical design of the Brandons wins.


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Bob Abraham
sage


Reged: 05/17/05

Loc: Toronto, ON, Canada
Re: A tale of two Questars new [Re: astro_que]
      #4570453 - 05/08/11 10:33 PM

Hi, thanks for steering me to that quotation. I think this might be a case of "don't believe everything you read on the internet" (even from me!) but more likely Chris is simply referring to a particular aspect of an asymmetric Plossl design that isn't the present symmetric one. And in any case, ray traces don't lie! I just checked a couple of other sources, including this very nice one:

http://www.telescope-optics.net/eyepiece_aberration_2.htm

The plossl is sharpest on-axis in every reference I've seen. I can cajole one of my grad students to ray trace one as an exercise next time he's sitting in front of ZEMAX but I don't seriously doubt what the answer is going to be.

As for spherical aberration being zero... well... if you get the aberrations in the ray trace down to the point where the spot distribution is way below the airy disc size, physics says you're done, and that seems to be the case for the plossls on axis (and I suspect it's true for the Brandons too). Getting good on-axis performance out of an eyepiece isn't hard, particularly if your beam is slow, at least not on paper (or in a computer). Of course getting a design manufactured without ruining the performance in some way is a different matter, but that's a separate thing from the optical design.

That said, when all is said and done you're right the human eyeball is a big part of the equation and people have preferences on that basis. The Brandon is a nice design and I can of course understand somebody having a preference for one... especially because there's a lot more to an eyepiece (and an eyeball) than a ray trace. I'll stick with my plossls, you stick with your Brandons, and everybody should be happy.

Bob

Quote:

Bob,
The distortion of the Plossl is zero on axis, but that is not the same as sharpness. Spherical aberration, which destroys sharpness, is not zero on axis.

I refer you to
http://www.brayebrookobservatory.org/BrayObsWebSite/BOOKS/EVOLUTIONofEYEPIECES.pdf , page 31. To quote,

"However, unlike the Abbe Orthoscopic and its derivatives, where the longitudinal spherical correction is zero on axis, the asymmetric form of the Plossl leads to a zonal correction and the sharpest imagery does not occur on axis but some 30% towards the edge of the field of view. At low to medium powers this is of no consequence, but it is noticeable at high powers (exit pupils less than 1.5mm)."

The Televue Plossls use plain old crown and flint glass. They are Plossls, and they are desirable for fast scopes. Chester Brandon's design is optimized for slow scopes, and they use four different types of glass, including lanthanums.

Every once in a while, someone expresses a preference for an eyepiece design that is not matched to the scope. I speculate it is due to peculiarities of that mushy globe of water that constitutes the human eye. Anyone who is serious about getting the best performance out of a Questar should at least have the Brandons available to make a direct comparison. Time and distance make reliable comparisons impossible.

For terrestrial use, I treasure my Televue Panoptics, where the wide field of view makes a Questar far more practical. But in searching for faint objects, the economical design of the Brandons wins.




Edited by Bob Abraham (05/08/11 11:15 PM)


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astro_que
sage
*****

Reged: 11/11/09

Re: A tale of two Questars [Re: Bob Abraham]
      #4570542 - 05/08/11 11:13 PM

Bob,
I have just realized, and possibly not for the first time, that Al Nagler calls his eyepiece a Plossl, while in fact, it isn't the original Plossl design. Thus my specfic criticism of Nagler's eyepiece does not apply. Chris Lord did not make a mistake; I did, though if Nagler had been a little more revealing with his choice of name, the error would have been avoided. Of course, usage evolves, so I'm not saying that Nagler's choice is wrong. It's just confusing.

That said, I direct your attention to "Abbe", the diagram next to "Plossl" in link you supplied, http://www.telescope-optics.net/eyepiece_aberration_2.htm. The off-axis characteristics of the Abbe are far superior, even at a modest 7 degrees.

Putting aside my deprecation of Nagler's eyepiece on the wrong grounds, and whether or not it should be called a Plossl, the "Abbe" diagram shows why orthoscopic designs are the best choice for a Questar. The critical aperture for orthoscopics is about F7. Above that number, they are the eyepieces of choice. Below that number, Plossl related designs are superior.

In case it seems I'm flogging Brandons, the Abbe, which is available from several souces, including University Optics, is a suitable choice. For high eyepoint, the Televue Radians might be excellent, though I have not tried them.

To those who contemplate eyepieces for your Questar, be sure to include a good orthoscopic design in your comparison. If you see no difference with at Televue Plossl at 7 degrees off axis, bully!


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