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GSO 8-inch TRUE CASSEGRAIN


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               GSO 8-inch TRUE CASSEGRAINIAN

                                                     By Lawrence Carlino

 

Decades ago, I acquired my first telescope with the performance and tracking ability to undertake serious planetary observing.  The Cave Optical 8-inch, f/15, true Cassegrainian boasted excellent optical quality, but it was compromised by its hefty weight and puny 1.25-inch focuser that made any rich-field viewing absolutely impossible.   However, the scope did enlighten me with its sharp image, high contrast, and the amazing sight of spokes in Saturn's rings before they were widely known.

 

As I moved through a series of larger Newtonians, SCT's, Mak's, and APO refractors,  I had pretty much forgotten the virtues of the true Cass design.

 

Recently, Guan Sheng Optical of Taiwan (GSO) re-introduced the parabolic primary- hyperbolic secondary design in 6, 8, and 10-inch apertures.  I opted for the 8-inch scope with a glossy white tube [it's also available with a gloss black tube] as it seemed to the best combination of capability, weight, and price. 

With an advertised aperture of 8 inches and an f/12 focal ratio with an efl of 2436mm, the instrument is configured with a 2-inch, dual-speed focuser and spacer rings to bring it to focus with a variety of star diagonals and imaging cameras.  The tube interior is graced by numerous light baffles that nicely suppress stray light.

Both Losmandy and Vixen-style mounting rails are attached to the tube to permit easy mounting, and a Synta dovetail is provided for finder or RDF positioning.

 

Overall fit and finish is excellent with no obvious flaws or inferior materials.  The focuser, though it's not exactly a FeatherTouch, is quite smooth with no detectable wander or backlash.   The GSO looks and feels like a much more expensive telescope.

 

Collimation of the telescope was found to be almost perfect right out of the box, and only high power made a bit of adjustment necessary.  With the fixed-position primary mirror, the scope can be dialed in with the three recessed screws on the secondary mirror housing.  Five minutes of tweaking resulted in dead-on alignment.

 

 

I mounted the scope on a simple Universal Astronomics Unistar alt-az mount with a 2-inch diameter leg Celestron tripod.  This combination proved to be very steady at anything below about 250x and was light enough to be carried about as a single unit.  I weighed the scope itself with a 50mm finder, 2-inch star diagonal, a 2-inch 30mm eyepiece in place (using the somewhat crude ”bathroom scale differential method,”) and it registered 19 pounds – lighter than a Celestron 9.25 but considerably heavier than a C8.

I suspect that an AVX equatorial would suffice for visual observing, but a beefier mount might be advisable for ccd imaging.

 

                                                    UNDER THE NIGHT SKY

 

 

In a strange quirk of fate, the usually uncooperative weather in the Buffalo, NY, area provided a string of clear nights that permitted a series of observations with the GSO and direct comparisons with APO refractors, a C8, and (twice) a Sky-Watcher ( Synta) 7-inch Mak-Cass.

 

 

With Jupiter near the meridian at sunset,  I put the telescopes to work in observing the giant planet – to me, at least, one of the most definitive tests of an instrument's optical quality.  With high-quality dielectric and prism star diagonals in place, I tried to equalize magnifications by using a complete set of Paradigm ED (Starguider) and TeleVue Nagler Type 6 eyepieces.  Powers varied from about 145 to 252x, and close enough with the various telescopes to allow valid comparison. 

I fully expected the excellent 150mm, f/8, ED doublet refractor to sweep the test, but the GSO Cass surprised with its beautiful rendition of delicate festooning in the (now yellowish) Jovian Equatorial Zone, high-contrast image, and true, pure color saturation.  The Great Red Spot was a strikingly intense oval with its interaction with the SEB easily visible.  The amount of detail seen was dead equal to that in the refractor with about the same overall image brightness.

In comparison, an excellent Celestron 8 produced a noticeably brighter image, but with lesser contrast of elusive detail in the polar areas and EZ.  Certainly very good, but not quite up to the crisp, high-contrast views provided by the GSO Cass and refractor.

The 180mm Synta Mak-Cass generated images similar to those of the C-8: very pleasing but hard to hold in exact focus as the telescope seemed to have some difficulty holding thermal equilibrium and suffered from a bit of focal shift.

 

Trying higher magnifications in the 250x range showed no apparent breakdown of the image in any of the telescopes, but the traditionally poor seeing conditions in the Western New York area made true comparison almost impossible. 

 

Overall, the victory here goes to the ED refractor and GSO Cass, as they provided the best image quality - something that I had not expected.

 

Saturn, its rings almost fully open to our line-of-sight, was glorious in all of the telescopes.  Here ,again, the refractor and GSO rendered the sharpest overall images, but the extra brightness of the Celestron 8 made the “C” ring easier to see.  The inner moons were also more easily visible in the C-8 as it seemed to have more light gathering power than the other telescopes.  Details in the planet's North Equatorial Belts and north polar region, because of their inherently low contrast, slightly favored the refractor and GSO.  As one might expect, however, Saturn looks good in any high-quality telescope.

 

Observing the moon from waxing crescent to near full again confirmed the fine optical quality of the GSO.  The absolute lack of false color in the all-mirror system provided deep black shadows and stark contrast.  At 162x, with a Paradigm 15mm eyepiece in play,  the Cass showed marvelous detail in the craterlet chains near Copernicus and in the delicate rille system near Triesnecker.  Craters in the lunar south stood out in bold relief, and lunar domes were remarkably easy to discern when near the terminator.  The 150mm ED refractor was very slightly sharper and handled variable seeing conditions better, but it showed no additional detail. 

 

 

                                                         DOUBLE STARS

 

 

On several nights with reasonably good seeing conditions, I ran the GSO through a series of challenging tests of resolution on a variety of multiple stars.  The instrument proved to be very effective and pleasing in its delivery of authentic color and clean separation. 

To some observers, the unavoidable presence of diffraction spikes from the secondary mirror's 4-vane spider could be an annoyance, but it did little to interfere with the view.  I, personally, actually enjoy the effect.

 

The fairly difficult Delta Cygni, with its large magnitude difference, was barely resolved at 128x using a TV 19mm Panoptic, but became laughably easy at 203x with a 12mm Paradigm ED in the drawtube.

Compared to the 150mm ED refractor, the image in the GSO was not quite as clean, but its pinpoint rendition of both the primary and secondary stars was impressive.

 

A similar result was achieved with Epsilon Bootis and RasAlgethi with the colors of the stars being very true and accurately intense.  Again, the refractor produced a somewhat cleaner image, but with a bit of chromatic aberration as the seeing conditions pushed the image in and out of focus.

Iota Cassiopeiae, the neat triple system, was a beautiful sight at 162x using a 15mm Paradigm ED.

The very sharp and concentrated stellar points with plenty of dark sky between nearly equaled the refractor view and fell just a bit short because of the larger first diffraction ring in the obstructed GSO configuration.

The frequently elusive companion to Antares popped in and out of view at 203x under rather indifferent seeing conditions, again aided by the tightly concentrated star images.

Overall, I found that the Cass is fully capable of resolving to the Dawes' limit with its excellent definition and almost refractor-intense images.

 

One unexpected virtue of the telescope was its ability to hold thermal equilibrium despite quickly falling night time temperatures.  The scope was stored in an unheated garage, so I can't comment on how it would fare in a winter excursion from a warm house into frigid air.  However, I never encountered any major air circulation problems or out-of-focus “spikes” coming from the secondary mirror.

 

In addition, despite some fairly intense dew formation that rendered the finder useless, the optics of the GSO never suffered from any condensation – one of the obvious advantages of an open-tube design.   

 

 

                                                 DEEP-SKY PERFORMANCE

 

 

An f/12 telescope is obviously not the best choice for deep-sky observation, as its long f/ratio makes true rich-field observing impossible.  Nevertheless, the GSO acquitted itself well by generating pinpoint stars, color purity, and fine contrast.  The Double Cluster in Perseus just squeezed into the field-of-view with a 2-inch, 30mm wide-angle eyepiece at 80x.  A lovely sight.  However, the image seemed to no brighter than that produced by the 150mm refractor and was FAR dimmer than the C-8's image.  The same lack of image brightness was confirmed when viewing M13, M27, and another dozen or so of globular clusters and planetary nebulae.

 

With the advertised high-reflectivity mirrors, the GSO should have roughly the same magnitude penetration and image brightness as the C-8.  I contacted the dealer, Agena Astro, to explain the situation, and they immediately queried GSO  for the technical details.  The response was detailed and quite interesting.

The design of the telescope is a compromise that gives priority to high-power lunar and planetary performance.  With the relatively short-focus parabolic primary mirror, a large secondary that would degrade image contrast would not be desirable.  The solution: Use a smaller secondary [still a roughly 33% obstruction] and reduce the effective aperture of the scope to 7.34 inches(186.5mm).  According to the factory specs, the 100 percent illumination circle is 15mm.  The sparkling planetary views are the result, but light grasp obviously suffers.  This is reminiscent of the situation with the Synta 127mm Mak-Cass that, from numerous reports and measurements, has a clear aperture of about 118mm. 

I, for one,  would appreciate some truth in advertising here.  ANY telescope should be defined by its clear aperture and not the diameter of its main optic.

 

Nevertheless, deep-sky performance of the scope is generally satisfying because of its tight star rendition and dark background, but the configuration is not optimal if maximum light grasp is needed.

 

                                                                 IN SUMMARY

                                                        

 

GSO Cass can be regarded as somewhat of a specialist instrument.  Its excellent overall optical quality, fine lunar and planetary performance, reasonable size and weight, and bargain price make it a fine choice where sterling deep-sky and rich-field capability are not a priority.  It is a good alternative to much pricier 5 to 6-inch apochromatic refractors for both visual and ccd work.  In the price- to- performance ratio, I don't see anything in its price range that comes close (except, perhaps for a high-quality long-focus Newtonian [but try to find one!]) as a dedicated lunar and planetary instrument.

One could only wish that it were a TRUE 8-inch telescope.

 

Clear and steady skies!

Larry Carlino

 

Disclaimer:  I have no affiliation with or financial interest in Guan Sheng Optical or any of the manufacturers or dealers mentioned in this review


  • Sol Robbins, nirvanix, stanislas-jean and 16 others like this


35 Comments

Nice review. I think this scope fills a niche of color-free lunar/planetary/doubles work without having to take out a second mortgage.

    • Venetia2004 and EverlastingSky like this

Good review, thank you.  However, the claimed effective reduction in aperture would not account for noticeable differences between it and the C8, especially since the C8 has  two more optical surfaces.  It's likely the mirror reflectivity is less than claimed, or less than implied at least.  It does sound like a good scope though.

    • Live_Steam_Mad, jgroub and mikeDnight like this

got one

good scope real snap focus

but very sensitive to any alignment errors 

imho more so than sct's

    • nirvanix likes this

Having read the Dec 19 S&T test report of the 10 inch cass, I'm wondering if it has the same reduction in aperture issue as the 8 inch.  The review does mention a "small annulus of light around the secondary mirror."  So is this because the secondary is too small?

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project nightflight
Dec 16 2019 05:31 PM

Thank you for your review! I have been waiting for some time for a decent review of the GSO 8" Cass. Your comparison of the OTA with other instruments is especially appreciated.

 

BTW, there is a report on CN that the focuser of these GSO scopes comes with a tilted tube: https://www.cloudyni...egrain-focuser/

Can you confirm the focuser problem or was this merely a single flawed unit?

 

Again, thank you, and all the best.

One thing that never seems to get any notice is that these scopes apparently come with quartz mirrors (both primary and secondary). That's often a very expensive option and it should aid with cool down and help to prevent focus/figure change with temperature.

 

I've not found a single mention of this on CN nor in any online reviews. However, both Orion (who second sources this same scope) and a few other retailers mention this in the specs (including Astronomics, for their rebranded Astro-Tech CC).

 

Oddly, however, this scope seems to be missing from the GSO website, so there is no way to confirm this from the actual manufacturer.

 

Could the quartz mirrors just be a false report that has been repeated by a few of the retailers?

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nateman_doo
Dec 27 2019 05:22 PM

So can I ask a simple question.... why do you call it a true cassegrain? Its an R/C scope.  

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Larry Carlino
Dec 27 2019 09:09 PM

An R/C design uses hyperbolic primary AND secondary mirrors.  The true Cass has a parabolic primary and hyperbolic secondary.

LC

    • Venetia2004 and Live_Steam_Mad like this

The solution: Use a smaller secondary [still a roughly 33% obstruction] and reduce the effective aperture of the scope to 7.34 inches(186.5 mm).

 

Seems like that makes it an F/13.0, since the literature gives an explicit focal length of 2436 mm.

    • jonbosley likes this

Great review!

    • Venetia2004 likes this

Thanks for posting. Excellent, well balanced and objective review.

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EverlastingSky
Jan 01 2020 02:31 PM

Good review of this interesting instrument waytogo.gif 

 

Just my observation: It appears the GSO 8" Cassegrain has the venerable C8 as its main rival to "overcome" so to speak. And the thing that is off-putting more than reduced brightness is the weight. I sure wish GSO could have made the weight of their 8" Cassegrain more competitive with the C8 which can be as little as 11 pounds bare OTA. I imagine that maybe all those baffles and huge focuser are adding many pounds to the weight?

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Live_Steam_Mad
Jan 02 2020 08:27 PM

I see that the Teleskop Service rebranded version of this scope has 99 per cent reflective Dielectric mirror coatings instead of 96 per cent Enhanced Aluminium. I'd like to see a test of the TS 8" CC versus the Celestron C8 XLT or Meade 8" ACF UHTC to see how "dim" or not the 8" CC is of TS. Also I am not keen on the non-moving primary mirror focus in this CC that won't allow large changes in back focus to accomodate a Binoviewer or and Atmospheric Dispersion Corrector (ADC) for planets viewing. The weight of this CC isn't a concern to me until the aperture gets to 10" and over, then the Celestron SCT seems to be much lighter.

    • eq3d likes this
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Charles Kirk
Jan 08 2020 02:26 PM

Having read the Dec 19 S&T test report of the 10 inch cass, I'm wondering if it has the same reduction in aperture issue as the 8 inch.  The review does mention a "small annulus of light around the secondary mirror."  So is this because the secondary is too small?

I don’t think so, it looks like they kept the secondary baffle size quite small to reduce the secondary obstruction to improve contrast on fine lunar and planetary detail. If the specification is for 100 percent illumination over a 15mm circle that implies no aperture reduction other than that implied by having a secondary mirror and baffle in the light path.

    • eq3d likes this
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David Knisely
Jan 16 2020 03:53 AM

I think the Cave Cassegrains were actually Dall-Kirkhams rather than the pure Classical Cassegrain design.

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Wildetelescope
Jan 16 2020 10:02 AM

Thank you for the interesting comparison.  What is the advantage to the classical cassegrain over the Mak and the SCT, if the central obstruction is of a similar size and the Focal ratios are more or less comparable (f10-F15)?   It has always been my understanding that what made the CC of interest was that you could get Focal ratios of 15-20 in a relatively short package, enabling the use of a smaller central mirror.  Is there something inherent in the design of the cassegrain optics themselves that you believe is responsible for what you saw at the EP? Or is it just superior execution of the design relative to the Synta  Mak, which SHOULD have the longest Focal ratio at f15, an smallest CO of the compound scopes you tested.   Just curious!  Really appreciate the review.

 

JMD

    • Venetia2004 likes this
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Larry Carlino
Jan 16 2020 10:46 AM

According to my 1974 Cave Optical catalog, the 8-inch Cass was a true parabolic/hyperbolic design. I owned one of these many years ago. The 10, 12, and 16-inch scopes were of the DK configuration with true Cass optics being an extra-cost option.

Thanks one and all for your kind comments on my GSO Cass review.

Larry C.

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Charles Kirk
Jan 16 2020 04:08 PM

What is the advantage to the classical cassegrain over the Mak and the SCT, if the central obstruction is of a similar size and the Focal ratios are more or less comparable (f10-F15)?

I think that there are two main advantages, the first is there is no corrector plate to dew up.

The second is that there is very little coma compared to the contentional SCT.

At these f ratios the Classical Cassegrain performs as well as or better than an RC http://www.dreamscop...04/ccvrc-04.htm.

Maks in the Gregorian configuration are limited to a maximum aperture of around 7 inch / 180mm.

Of course there is one disadvantage which is the presence of diffraction spikes from the secondary support.

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David Knisely
Jan 22 2020 12:13 AM

According to my 1974 Cave Optical catalog, the 8-inch Cass was a true parabolic/hyperbolic design. I owned one of these many years ago. The 10, 12, and 16-inch scopes were of the DK configuration with true Cass optics being an extra-cost option.

Thanks one and all for your kind comments on my GSO Cass review.

Larry C.

Actually, some of their optics were made, not by Cave, but by 3B Optics, and those may have been the Dall-Kirkhams that were put out.  We had a fairly nice 1960's vintage 8 inch f/15 Cave Cassegrain at Hyde Observatory on long-term loan from Nebraska Wesleyan University in Lincoln, NE, and it was the first scope I ever saw the companion to Sirius in (the 2nd was our C14).  We eventually replaced it with an 11 inch Celestron NexStar SCT, which outperformed it notably.   Clear skies to you.

    • Jon Isaacs likes this

Great review! Thanks for posting. 

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Upstate New Yorker
Feb 16 2020 10:00 AM

This is a clear, authoritative review that explains the technical specs of the revived classical Cassegrain telescopes, as well as their strong and weak suits.  The discussion of the resistance to dew by these scopes was an added bonus, one that was especially helpful for those of us living east of the Mississippi.  Thanks.  

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CGAstronomer
Mar 05 2020 08:26 PM

I reviewed this same telescope in the December 2019 issue of Astronomy Technology Today (Vol 13, Issue 5).  I did not see the lack of image brightness with the scope I purchased that Lawrence described in his review.  The secondary mirror obstruction in the GSO is 33%, approximately the same as in an 8-inch SCT.  Any reflector is going to have an effective light gather diameter smaller than the primary mirror due to the secondary obstruction.  I found the view in the GSO crisper and better contrast than an 8-inch SCT.

    • edl, Live_Steam_Mad, eros312 and 1 other like this
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Live_Steam_Mad
Mar 06 2020 06:26 AM

I reviewed this same telescope in the December 2019 issue of Astronomy Technology Today (Vol 13, Issue 5).  I did not see the lack of image brightness with the scope I purchased that Lawrence described in his review.  The secondary mirror obstruction in the GSO is 33%, approximately the same as in an 8-inch SCT.  Any reflector is going to have an effective light gather diameter smaller than the primary mirror due to the secondary obstruction.  I found the view in the GSO crisper and better contrast than an 8-inch SCT.

Did you try the GSO brand one specifically and not a rebranded one?

 

Regards

AG

That review was good reading thank you.

I have recently bought one of these from AstroNZ here in North Auckland, New Zealand. Only managed to use it once so far but am very happy. The only problem I have at the moment is that I haven't been able to put my 2" optics on it from my Dob. Have been using my 1.25" diagonal and eyepieces. Had a talk to AstroNZ and they say I will be able to use the 2" stuff once I buy a 2" diagonal. I have put the larger supplied spacer in before the focuser to achieve focus for my general use, but couldn't get near achieving focus with all the supplied spacer rings when trying to use my 2" eyepieces when I plugged them straight into the back with no diagonal.
Regards Roger
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CGAstronomer
Mar 24 2020 01:40 PM

Did you try the GSO brand one specifically and not a rebranded one?

 

Regards

AG

Yes mine is the direct GSO brand model.



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