This is about to be my last posting on this issue, I hope so. After my impressions from Tuesday and Wednesday I tried to directly compare the two of the scopes 6“ CC and 5“ Mak. Had therefore to switch from CC to Mak and to CC again, for I own just one mount. But to me things are clear enough to have a stop here.
I had the impression of sudden failure of CC similar to the one caused by the loose primary mirror. Yes that occurs on new instruments no matter if you call it a production defect or just a routine to get started. Mine needed a half turn to be fixed. And similar shocking disappointments turned me down towards the scope. But there wasn’t any second failure, my experince can be explained by three observations.
Brightness. As said at my very beginning the 6 inch CC displays significantly brighter than the5 inch Mak, it does still when you consider the 14% plus light (aperture minus CO). This , to me at least gives the impression of reduced contrast. Jupiter was just too bright für my eyes at 225x. As soon as clouds dimmed Jupiter to some extent, contrast improved. So an appropriate filter might help a lot.
Seeing. Wasn’t aware what seeing conditions might involve. Didn’t know at all that seeing under a clear sky where stars and milky way shine way brighter than usual, seeing can be very poor especially when watching at high power. The decrease at seeing from Tuesday to Thursday though Tuesday’s sky was brighter was to an extent I wouldn’t have thought possible. And the days before hadn’t been very hot so there was no point to expect warm air to bubble up to the sky all night long.
Sensitivity to seeing conditions, especially air inhomegenity near ground and tube seenig. The direct compare of both scopes gave evidence like this: Vision through the Mak was wabbling (changing within seconds) even when watching near zenit. The CC displayed a flattering vision (changing in split seconds to seconds, sometimes double images). In the best moments vision through both scopes was similar but those monents were quite short in case of CC. And it’s worst moments were worse than the Mak’s. For me it is plain to see now that open mirror designed scopes are more delicate to ground air inhomogenities than closed scopes as Maks or refractors. This is just, because that much changing and exchangeable air fits between the two mirrors, and I‘m afraid a truss construction wouldn’t help anything.
This three things all work the same direction and did enough for my repeated sudden disappointment. So, guess I’m done with the subject, yet not with the CC scope. Open mirror disigned scopes are great when it’s about telling bad seeing conditions.
When you’re interested you find below a mail to GSO. Don’t know how they get it or if they consider at all, my experience is negative to this regard. But got a positive answer, maybe just courtesy, yet a bit of hope.
GSO 6 inch Classical Cassegrain
Considerng the resonance that the new GSO classical cassegrain telescopes caused in the last year, since their introduction they seem to address an overall and far flung demand for such a simple, classical scope design. The smallest 6 inch device seems to find general positive acceptance which I can’t totally join in. Though I was one of the first to purchase one. What are the issues, apart from my liking of the design in general?
1. Data provided. They’re all from GSO but various providers advertise them as: 154/1848, 152/1836 or 150/1800 mm telescopes with quartz mirrors of 99%, 96% and 94% reflectivity. Schall we really believe in those exact numbers? Aren’t they in fact all the same? The exact-in-mm numbers of focal length are just due to the stated primary mirror diameter and the f/12 ratio.
2. The data aren’t telling the truth. I dismantled my scope to find the (aluminated) primary diameter as exact 150 mm unlike the advertised 154 mm. The advertised central obstruction of 33% increases to 44% at a closer look, which is considerable. The reason for this discrepancy is that the obstruction is not controlled by the secondary baffle (58 mm) but by the primary fixing ring (61 mm) and, which is more severe , the fact that effective aperture is not 150 but 139 mm, just this smaller area seen or catched by the secondary mirror.
The real thing is a 5.4 inch scope with 44% CO with (assumed) 94% reflecting mirrors. May I assume that this alone would have prevented many fans from purchasing a device? To me this applies. What do I own now? A medium gifted, quite heavy scope which performs way below to its potential! What improvements are possible without introducing any higher material or machining quality? I think a good redesign could turn this scope into an inevitable powerful displaying, refractor-challenging standard as well as beginner’s grab-and-go scope to which all others would compare. Things I’d improve:
Weight could be reduced by 1 kg to then 4,5 kg. Replacing steel tube by an alloy tube thus reducing weight and improving collimation stability. The vixen rail connects buttplate and end ring like a bridge and might shift the scope not just out of focus but out of collimation for it is mounted at one side. This is no problem when the tube is alloy. The weight reduction by 20% yields a reduction of swinging inertia to the same extent. Fixing the Vixen rail directly to the tube yields 10 more % of swinging inertia reduction (distance from swinging axis comes with square power). All in all there’ll be a 30% reduced swinging inertia, not to forget the reduction of counterweight. This is considerable.
Slight adaption at mirror focus. The 150/3 primary focal length turns out to te too long for the mirror distance. This is why the effective aperture is just 139 mm. When adapting primary design why not doing the whole job? The scope with its dimensions would take an 160 mm primary mirror, which then would be an f/2.8 thus yielding a focal length of 450 mm which is necessary to completly catch the light by the secondary without changing the mirror distance or secondary size. When the f/12 ratio for the scope shall be maintained the secondary power would then be 4.3. Well, who would care care if the result was f/13 instead of f/12?. This means the fans, me at least, are not after a scope that exactly matches an f/12 ratio or a 1.800 mm focal length, we rather prefer a design that provides optimum performance considering size, weight and money to spend.
The result of this quite roughly estimated redesign would be a true 6.3 instead of 5.4 inch classical Cassegrain telescope with true 38 instead of 44% linear CO that weighs 4.5 (10 pounds) instead of 5.4 kg (swinging inertia reduction about 35%) which yields a 0.72 sec. definition instead of advertised 0.90 sec. and outperforms any other scope of similar dimensions. And it wouldn‘t need any additional or high-end material or production refining than the actual CC6.
