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Q7 vs C8 on planetary?

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#1 Darren Drake

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Posted 13 November 2012 - 06:52 PM

Has anyone ever compared a Questar 7 to an excellent C8? I would think that while the C8 has a little more secondary obsstruction % (I think) that its increased aperture would make up for that on planetary viewing. Any actual experiences out there?

#2 rmollise

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Posted 13 November 2012 - 09:04 PM

The Q7 would blow the C8 out of the water if it ever cooled down. But it never does. ;)

#3 bremms

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Posted 13 November 2012 - 11:26 PM

Yes.. I've used both side by side. The Q7 I looked through had the finest optics of any scope I've used. That Q7 was better an AP 6". Really in a different league. The one we were using was a special order 1/15 wave low RA system for a non astronomy application.. I can't imagine how much the purchase cost was to the government. oops. Looked through another a couple years later and it was about the same.
Q7 is a really special scope and you would have to find a one in a thousand C8 to give it a real run.

#4 iluxo

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Posted 14 November 2012 - 05:18 AM

On planetary my skywatcher 180mm f/15 easily out-resolves all the C8 and Meade-8's that I've tried side by side, so a Q7 definitely will - once it cools.

You do have to allow an hour for cooling down with the tube vertical.

Looking at stars at high power, In essence the difference is that the f/15 maks will give a good Airy disk in conditions where the SCT's just give a blur. It's not just a matter of the central obstruction, I got the impression the correction of commercial SCT's on the whole is pretty marginal at best. Obviously there must be a few good ones, if you have the chance to hand - pick one.

#5 azure1961p

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Posted 14 November 2012 - 07:38 AM

id have thought the highrr quality c8s coming out of china would have made the q7 in a dead run if only bevause no matterhow good the 7 it still hss less angular res.. too a c8 that shows a blurr of a diffraction pattern sounds like a garbage scope. it almost reads like the slighting of thec8 is factoring in the terrible samples produced in the usa padticularly circa halleys comet.

pete

#6 Darren Drake

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Posted 14 November 2012 - 09:10 AM

Okay so if all the C8 and other scts all basically are inferior optically to the almighty Qs what weaknesses or optical aberrations do the scts generally suffer from? Perhaps some may come from a slightly bigger obstruction. Is it persistant surface roughness or slight chromatic aberration? Maybe perfect spherical correction is virtually never achieved? I suspect based on Wolfgang Rorhs many posts that these explanations may be on tghe right track. He never did however test a Questar so its hard to say....

#7 Eddgie

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Posted 14 November 2012 - 09:50 AM

"Weakness" is in my opinion an bad word.

I mean after all, couldn't I just as easily say the the Q7 is a "Weak" desing because it limits me to relativly high powers?

Today, for many observers, a telsecope with a narrow field is a huge "Weakness."

And of course there is the price.. OMG!

I think that saying that these are different "Design Compromises" is a better way to look at it.

The design compromises of the modern SCT is that it uses a large central obstruction to keep the OTA short and light, and all spherical optics to keep it inexpensive to manufacture and hence "Affordable!".

How can it be said that it is a "Weak" design when it is smaller, lighter, and more affordable (per inch of aperture) than an MCT???

The Q7 is a beautiful telescope but it is (for a 7" apeture) heavy and expensive to produce, and has a very confining true field capability.

To call the SCT a "Weak" desing is to basically dismiss the fact that different designs have different performance characteristics, and that they often provide this becuase of consumer demand.

Consumers wanted larger aperture scopes of compact design and reasonable cost.

The modern SCT is far from being "Weak." It is a huge market-place success.

Can you get better performance from other types of the same or slightly smaller aperture? Sure. But this brings its own comprimse. Cost, size, or weight.

Want a better planetary scope than a Q7? Buy a C11.

But I don't think it is far to call the SCT a "Weak" design. It is perhaps one of the most successful telescope designs ever made.

On axis performacne is not limiting factor to me, because the aperture is so inexpensive that for the price, I can just buy a bigger SCT.

My planetary scope is a C14. I have a 6" APO. As much as the refractor people will talk negativly about SCTs with there big central obstruction, if you were to look though my two scopes side by side, you would quickly conclude that the C14 is a better planetary scope.

You can fix a lot of ills with a couple of extra inces of compact, inexpensive, light weight aperture.

And for the "Yes, but the SCT is bigger and heavier than the 6" APO" crown, I would point out that the moutn required to hold my 6" refractor steady at 300x is actaully bigger and heavier than the mount required to hold my C14 to the same level of steadiness.

When you factor in the mount, it takes almost as much time and energy to set up the 6" APO as it does to set up the C14.

Oh, it does take a "Little" more energy to set up the C14, but my reward is that I see a "Little" more planetary detail.

If I could not, I would not use the C14 for planets.

So for me, the SCT design is in no way "Weak." It is simply compromised towards packaging an price, but my preferred plaentary scope is an SCT.

Oh, maybe it would be a 12" MCT if I win the lottery.

#8 vahe

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Posted 14 November 2012 - 10:18 AM

Q7 vs C8 on planetary?

If the discussion is on the planetary performance of these two then we can discount the large co. issues since both have them.

Cooling will be a problem with both since they do not have an active cooling.

The C8 has aperture advantage but its not a massive advantage when compared to a 7” aperture.

The Q7 and for that matter all high end Maksutovs have one major advantage over SCT’s and that is the SMOOTH optics, this is the single most important quality for any planetary scope.

Narrow field of view is also a major advantage for any scope that is optimized for tiny objects that require high powers and large image scale.

And finally, the OP question is limited to these two scopes, not larger SCTs or Maksutovs.

Vahe

#9 Eddgie

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Posted 14 November 2012 - 11:49 AM

this is the single most important quality for any planetary scope.


I do not know where this myth comes from but it is not true in any way, shape, or fashion.

A small amount of surface roughness has only a small amount of impact to the image. The roughness has to be quite pronounced to have a meaningful impact, and this is not usually the case with Celestron SCTs.

Far more damaging are spherical abberation and zones.

Sadly, the case with many SCTs over the decades has been that they did not have one serious defect, but rather a combination of minor defects (true of most mass produced optics).

There will typically be a bit of surface roughness, a bit of zonal error, and a bit of spherical abberation.

Ad a bit of this and a bit of that to a scope with a large obstruction and it takes a toll.

But Rohr at Astro-foren dwells on rougness, but if you look at the tests, they are rarely responsible for lowering the Streal of the telescopes tested more than .01 or .02.

Star testing is extremly sensitive to surface roughness, and a little rougness will easly show in a star test.

1/20th wave of surface roungness looks pretty ugly in a star test, but this is about the minimum amount that needs to be present to lower the image contrast to the point that a skilled observer doing careful comparisons will be able to see it.

I have never personally owned a commercial SCT with surface rougness this bad.

Typically, the surface roungness on SCTs that I have owned has been more in the range of 1/40th, and I would estimate that my C14 and EdgeHD are both at least this good and perhaps better.

1/40th wave of surface rougness lowers the contrast by about 2 percent.

No observer will be able to see this small loss in contrast.

People need to get over it. SCTs do not have optics as smooth as you will find in an MCT that costs twice as much.

But that doesn't mean that the level of surface roughness is a major issue with SCT performance.

In fact, it is the least serious error that I usually observer.

More common is spherical abberation and a little of this is much more damaging than a little surface roughness, but even here, all other things being equal, SA has to fall to about 1/6th of a wave before the impact is detectable.

This is why people reporting using the "Standard" MCTs Intes Micro scopes report that they can't tell the difference between the "Premium" Intes micro scopes.

This is a myth and nothing more.

Anyone that reads the section of Suiter's Book on star testing and that has tested a bunch of SCTs will quickly realize that while few SCTs have optics that are as smooth as some expensive MCTs, fewer still have enough surface roughness to degrade the image sufficiently that it will be detecatble at the eyepiece.

1/40th is common, but too small to be an issue.

There are a lot of SCTs that Rohr has tested with a Strehl of .95 or higer.

I personally am very skeptical of anyone that says that they can see the difference in performance between a scope with .95 Strhel and one with .97 Strehl. It is far to small for the human eye to perceive.

But even here, I have seen Rohr test some Celstron scopes with .96 and .97 Strehl.

And almost all of these show a bit of surface rougnness.

It is just not enough to matter.

#10 Darren Drake

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Posted 14 November 2012 - 12:17 PM

Okay but does strehl even take into account surface roughness. Does the interferometer even detect slight roughness? I've seen plenty of.95 strehl systems that look a bit messy on the null test and its hard to believe there isn't more impact on the image.

#11 Eddgie

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Posted 14 November 2012 - 12:36 PM

Regarding my post above.. Here is a perfect example.

Here is a C11 with a minor amound of surface rougness. My C14 is about like this, and my EdgeHD is better than this.

Notice that even though the surface is not as nearly as smooth as you would see with some high end MCTs, the Strehll of this sample is a clearly excellent .967!

SCT with 1/40th surface rougness and .967 Strehl

Another one. A C9.25. Star test shows maybe somewhere between 1/20th and 1/40th, and the Strehl is still .967!

And this scope shows more roughness that is typical (in my own experience) with Celestron scopes!

C9.25 with less than 1/40th smoothness, but still. .967 Strehl!

Astro-foren is loaded with examples like this.

My opinion is that what makes Rohr so critical of surface rougness in many SCTs is because it is present so many times that it keeps a scope with almost perfect optics from being perfect.

And that is too bad. To have a scope with .97 Strehl that could be .98 or .99 with a bit better polish seems so sad!

But at what cost? And would the additional cost make it worth it to the consumer?

These two scopes are perfect examples of what I was saying earlier. A bit of surface rougness is almost always present in commercial SCTs, but it is rare to have it present in sufficient quantities to make a visible difference on the in-focus image....

#12 EddWen

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Posted 14 November 2012 - 12:38 PM

The most important advantage a Q7 has is the lack of mirror focus shift/flop found in all common SCT designs.

A scope that mounts the mirror on a baffle tube which can move +/- 0.002" will not remain in collimation pointing in various directions.

The Q7 mirror is mounted on a baffle tube that rides on pre-loaded ball bearings, hence, always staying in collimation and not subject to focus shift.

Yes, there a 'work arounds' for an SCT. My NX11 has the mirror locked in place and uses a Crayford style focuser.

#13 Asbytec

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Posted 14 November 2012 - 12:55 PM

The Mak and the SCT handle SA differently. The meniscus has a higher power and a steeper curve than the Schmidt corrector. The meniscus targets the primary's lower order spherical more strongly, but in the process adds much more HSA than a Schmidt corrector.

So, while a Schmidt has a 5th order term that nearly eliminates HSA, the lower order terms on the primary are still less affected by the weaker corrector. They are certainly diffraction limited or better, but not as strongly corrected as a meniscus (and the remaining surfaces.) In fact, as I understand it, the Mak absolutely must be balanced or corrected with an aspheric term (depending on aperture and the resulting HSA induced by the meniscus.)

The act of balancing HSA (in lieu of aspheric term) from the meniscus necessarily reduces primary aberration by adding an amount equal and opposite in sign. The SCT simply eliminates this term with an aspheric and leaves LSA relatively untouched (I think, since there is no longer any need to add any LSA of opposite sign.) This lower order surface approximation reduces MCT overall SA by some factor - by minimizing the higher order in the system as a whole (as all spherical surface radii are affected as well as mirror spacing and meniscus thickness. All of these things come together in a balanced scope with highly corrected SA.)

The result is a relaxed radius on one or more surfaces and a longer focal ratio and a tighter caustic pattern. This is due to both a longer focal length and minimizing HSA by adding lower order of opposite sign. Basically, extending the primary's focus closer to it's center of curvature where the spherical primary would be perfectly corrected. Not quite there, but closer than the primary's infinity focus and slightly more so than a Schmidt corrector.

The residual higher order SA is of little consequence as it is only a small portion of the wavefront. And since a tiny amount of lateral aberration (HSA) means that light is spread out very thinly at best focus (transverse aberration.) The resulting blur size and RMS are quite good in a balanced system. Not perfect, though, that requires an aspheric surface.

This balancing act is what makes MCTs real performers, albeit with a smaller FOV and longer focal ratio that result. MCT's are - they have to be - well balanced or corrected to perform above the level of a spotter scope. All thanks to the steeply curved meniscus and it's inherent HSA. In fact, APOs also use steeply curved spherical surfaces and correct for their higher order SA in much the same way: balanced SA or aspheric terms. Take away the CO, and you really do have an APO-like scope.

Not bashing SCT's at all, just applauding Dimitri and his design. They are sweet scopes, even the cheap ones (provided some care is taken to calculate the surface figures and balance HSA and grind accurately to spec.) There is no doubt Intes basic scopes perform quite well with regard to their deluxe models. They simply must perform that well or become an uncorrected spotter scope. There is no doubt any Synta Mak is a great scope, too, at least as good as Intes basic models. I guess that's a bold claim and gonna ruffle some feathers...but they are.

#14 Eddgie

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Posted 14 November 2012 - 01:48 PM

My points are simple ones.

The standard SCT design is not "Weak." but rather a "compromse."

All optical design is a compromise of one kind or another.

Per inch of aperture, the SCT is near the bottom of the ladder for contrast performance, but that is only per inch of aperture.

Whatever compromises the SCT requires to make an affordable and compact design still allows one to own a very large SCT with a mount and eyepeices for less than a much smaller MCT OTA alone would cost.

And aperture is the cure for most optical comprimises that are made with the standard SCT.

This is why the SCT basically caused the extinction of the f/15 achromat.

So while an 8" SCT might not have the contrast performance of a botique 7" MCT, an 11" one can, and at a fraction of the price.

But if getting the most on-axis performace in the smallest possible package (another kind of compromise) the MCT makes great senes.

But not economically for may buyers.

The OPs quest though is a typical example of the kind of comparison that takes place on this forum.

Here we have a telescope (Q7) that is miticuously hand crafted and sells for $7000, and we compare it to a telsecope that is mass produced and seels for 1/1oth of that price, that is lighger and has a wider field of view, and more light gathering...And someone calls the SCT "Weak" because of this?

And actually, neither of these would be my first choice for planets. Both lack the clear aperture to show more than the most obvious surface detail with enough clarity and brinhtness to make them interesting. The view very quickly starts to look repetative with the same major features always available but lacking in the nauance of fine, low contrast, ever-changing detail that a bigger aperture delivers due to a brighter image, more contrast transfer, and better angular resolving power.

I would only buy the Q7 it packaging was my highest priority, and for that, it is a great choice for someone with a lot of money to spend.

If I wanted a better view of planets, I would get a good 10" Newtonian over either of these.

My dismay is rooted in the fact that people woudl call the SCT a weak design. It is in fact, a brilliant design and a marketplace winner.

And it only take a bit of extra aperture to make up for the small irregularities in optical quality and the obstruction.

Again, the 8" SCT single-handedly caused the almost absolute demise of the long focus 4" achromat and has become one of the top selling commercial successes of all times.

Can you do better per inch of aperture? Well, frankly, it is hard to do worse per inch of aperture.

But with SCT aperture being so cheap (as compared to other types) that hardly matters. Bigger is better, especially for planets.

#15 Mirzam

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Posted 14 November 2012 - 06:35 PM

Doesn't the spherical aberration change as the mirror position is moved in SCT's? Does this also happen in Maksutovs?

(I want correction to be perfect when using my binoviewer).

JimC

#16 GeneT

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Posted 14 November 2012 - 06:41 PM

In this horse race, my money is on the Q7. :grin:

#17 bremms

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Posted 14 November 2012 - 06:49 PM

A good 10 inch newt will be better on planets than a Q7. Mine was, but it did have a a superb mirror. My recently resurrected C11 gave a nice image of Jupiter the other night. Good detail, typical SCT contrast.

#18 Mirzam

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Posted 14 November 2012 - 06:50 PM

Well I've never seen a Q-7 do this:

http://www.cloudynig...5519006/page...

JimC

#19 azure1961p

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Posted 14 November 2012 - 08:03 PM

id. be inclined to believe you needn't require a ten inch reflector to outdo a Q7 but especially now with actice cooling boundary fan and rear exhaust my 8" might be up to it. I think at the least it'd be a tie. The only cass I've seen it bested by was a C11. Particularly with large sct like the 11 anx Eddgies 14 the jump in angular resolution is just too huge to vault when the differences are that good and the optical quality is at leást good. Id love to do the Q7 challenge but I think the eight'd be ok.

this is an interesting thread. Norme and Edd K enjoyed your points.

Pete

#20 Erik Bakker

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Posted 14 November 2012 - 08:23 PM

Yes. The Q7 is in a class of it's own in this aperture size :bow:
The smoothness is unsurpassed, as are the alignment of the optics and mechanical quality of the scope. CO is comparable. My Q7 had stunning spherical correction as well, comparable to what Bremms describes.

And Uncle Rod, please be careful in spreading the wrong myth. The Q7 does cool down :foreheadslap:
At least where I live. First hand experience over many years of observing with the Q7.

#21 Eddgie

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Posted 14 November 2012 - 09:32 PM

In fact it does, but you have to get about 300mm of back focus before the change in correction enough to be easily visible in the image.

And if the scope is slightly undercorrected at stated focal length, a lot of back focus can actually improve the correction.

But if the scope is perfectly corrected at stated focal lenght, 300mm of back focus can cause about 1/5th wave of spherical abberation (as I recall... Rohr did this measurment on Astro-Foren once and has a graph for back focus on the C11... I seem to recall that it had to be about 300mm before there was enough SA that the average observer could easily see).

I am sure that you can find it if you check Astron-Foren.de.

#22 Asbytec

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Posted 14 November 2012 - 10:02 PM

Eddgie, all your points are well taken. The SCT is quite possibly the most successful or popular design around. Like most folks, I have owned my share and been quite happy.

I have yet to put my finger on the reason the Mak has reinvigorated my love for observing, especially planets. There is something about the design that offers them that niche at a given aperture and CO. So far, the best answer seems to be the way they treat SA and and other aberrations, including their smoothness. It might well be the best mass produced scope I have owned in terms of optical quality and resulting crisp images.

No doubt a 10 and 11" SCTs would have prevailed. And an 8" should prevail, too. But, a very well made 7" Mak might just hold it's own with an 8" scope in terms of planetary contrast. It's a very close call, especially if the Mak MTF curve is not further degraded by any notable aberration.

With both scopes having roughly the same CO, any aberration advantage in either might well equal or overwhelm any gain on 1" in aperture through most of the spatial frequency range. That would depend on the care taken during manufacture, one would have to be very well corrected and smooth and the other average or less. At this point, the MTF curves defining resolution above the Dawes limit could be about the same. Even if one were slightly better, that small improvement likely would be not seen at the eyepiece.

You know, thinking about it, asking a question like this is really one of those probably too close to call or to make a blanket statement that generally holds true. Comparing a 7" SCT of average build would be easier to say one way or another (but then there would be no question, probably.) There is just something sweet about a Mak on planets, I'm still trying to put my finger on what it is.

Yes, the Schmidt is a fascinating design, too. The shape of the corrector does neat things to the wavefront, less correction near 70% zone, and progressively more toward marginal and paraxial rays. The meniscus is much simpler and does not provide this level of zonal correction. I suppose, though, that once the Schmidt surfaces are corrected to within the diffraction limit they are set and have no significant higher order spherical. By virtue of it's steeply curved meniscus and very significant HSA, the Mak has to be better corrected (balanced or aspheric.)

This seems to suggest a Mak has tighter tolerances and less slop in the surfaces, and this level of tolerance might account for their /perceived/ consistency across samples. Maybe that's the edge, and maybe that edge can put a Q7 on par with an 8" scope in terms of planetary.

Anyway, really don't like these comparison threads, but the Mak design interests me.

#23 azure1961p

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Posted 14 November 2012 - 10:22 PM

Interestingly, what would seem to bypass this comparison discussion is F
ccd imaging. I ve seen some 7" apo Jupiter images and they can't hold a candle to a 10" f/4 reflector. Imagers seem to sidestep the visual contrast querks and nuances and beeline it straight to the angular resolution advantage and design be damned. The 6" apo details in ccd images - despite wonderful inherent contrast have details that seem stubby and smoothed compared to a C11s. In the electronic imaging realm the contrast is adjusted with sweeping control in processing that seems to not care a bother for CO and SA. It creates an interesting paradox of sorts about modern imaging and cutting edge contrast sensitivety. On one hand its arguably better than human vision on the other hand it seems callously indifferent between those finer contrast we observe even casually between differing Scopes. Angular resolution seems king while contrast resolution is *negotiable*.

It'd seem like a better imagery would indeed seperate the Q7s from the C8s but the processing software seems to over power this, even handedly.

Pete

#24 orion61

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Posted 15 November 2012 - 12:01 PM

Very interesting post here thank you for sharing, that C11 looks like it would be a pretty good performer.
I have had over a dozen SCT 8's go through my hands over the years. Fairly good is the majority of the quality with Celestron generally winning the overall optical tests.
This is why I have been for the ability to have these tested and graded, (think of baseball cards)This way you know exactly what you are getting.
Intes has been doing this and the better ones fetching beter prices.
I have finally settled on an old Celestron Pacific Orange tube.
As I got them in I had kept the best one, I found this one a number of years ago and havent had one out perform it yet.
I have had a bit brighter ones, prettier ones,
I have also had some come through that left me scratching my head wondering how in the world it ever made out of QC?
I also know 1st hand Celestron used to have a few held back and when Dealers took a tour they were offered these for sale as a personal scope. This story came to me from a Dealer who still has his that he picked up at the factory.
It is indeed a better than average tube.
Heck the one I have doesn't even have multicoatings but I have kept it over Starbright and one XLT tube.
I can say tho the optics coming out of China are better in a more consistant basis than any others I have seen.
I have a C6se that is just killer.

#25 azure1961p

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Posted 15 November 2012 - 11:12 PM

You know I just may get an old orange tube c-pacific. I HAD one that was actually pretty good and I let it go. The mechanics on the pacifics I love though the image shift can be a little wonky.

I'm glad you like your c6se mine is terrific too. I honestly can't fault the beast. The C11 is probably one of the best lunar planetary Scopes out there and for allthe reasons eddgie mentions. Apparently the 9.25 has a strong fan base too.

Back on the Q7 though I do recall a review in the review section under man's comparing a Q7 to a Meade 7mak. The Q7sounded like it needed itscoatings renewed and enhanced.

Its projection to be sure but I don't think a good C8 would suffer as bad as might be expected. Just a hunch.

Pete






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