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New AT 6" and 8" Classical Cassegrain

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#1076 quilty

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Posted 11 September 2020 - 05:03 AM

CC6 in cart. Nice size & weight, great for test drive. Viewed with Evo 8 - hmmm - CC8 in cart, CC6 out. But I have the Evo 8 and it's wonderful & no spider - hmmm - CC8 out, 180 Mak in. Oh, yeah, wanted fixed mirror. Mak out, CC6 back in - nice size & weight for test drive of a classic cass... see if I like Classic Casses. <click> done.

Surely me to. This is quite a good summary of this thread. Guess I'll go for a 8 inch Meade ACF or a classical C8, for they weigh no more than the CC6. But before buying I'll try to have a look through to avoid any surprise. By the way, is there a word for a bad surprise, in German there is?



#1077 Thandal

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Posted 11 September 2020 - 08:24 AM

 By the way, is there a word for a bad surprise, in German there is?

 

In English that thought is more commonly expressed through intonation:

"Sur-pr-EYE-z", said with a rising (and sometimes a drawn-out "Sur-pr-eye-EYE-z") emphasis on "eye" and a slightly sarcastic edge.  smile.gif


Edited by Thandal, 11 September 2020 - 09:25 AM.


#1078 AstroKerr

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Posted 11 September 2020 - 10:28 PM

Surely me to. This is quite a good summary of this thread. Guess I'll go for a 8 inch Meade ACF or a classical C8, for they weigh no more than the CC6. But before buying I'll try to have a look through to avoid any surprise. By the way, is there a word for a bad surprise, in German there is?

I use "Booshnell Ro'try" - epitome of 'bad surprise'. 

 

I don't think you can go wrong with an 8 - all around goodness in manageable packages. Had the Evo 8 on an LX65 for a few nights. Handled it well, but a just little slow to settle, right at the 14~15lb main saddle limit. But that combo was quite manageable physically - grab the 65's handle and scoot.  Just plain fun. 

 

CC6 came in today, but can't 1stLight til Sontag. Mmrrph.


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#1079 quilty

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Posted 12 September 2020 - 03:59 AM

ok, looking forward then to hearing how you think the CC6 does compared o the Evo 8. Hope there's no Booshnell Ro...


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#1080 AstroKerr

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Posted 14 September 2020 - 03:14 AM

Well, I won't say it's equal to an Evo 8 - that's a Kippers and Squid comparison - different designs, different apertures. I will say that I am quite pleased with the CC6. Was going to do an out-of-boxing, but I got off graves and slept til 6ish. It was the GSO dark version.

 

  • It came double boxed - not Astronomics et al box-filler-box, just thick box-thick box. Well done, though.
  • Came with requisite 1-2" and 2-1" extensions - you will need them.
  • Focuser is nice. I prefer R&P, but this appears to work very well.
  • All machining, fit, finish is top shelf.
  • Collimation - I did star test @ 8, 5 & 3.2, looked for any miscollimation on Vega, Capella, Altair, Aldebaran, etc. Nothing negative I can see - really nice - it travelled well.
  • Diagonal was Astronomics 2", used a Swan 40mm, ES 11 82°, Sgs/Paradigms, Baader Zoom. All required the 2" extension until I tried the 2" (ugly) Antares 1.6x Barlow, which made it to focus barely with a 1" ext and it would have preferred no ext. 
  • The views are good for a 6" with mmhmm CO. Bright enough for now. I'll want a 10" later. I'll do a full FOV nasties survey later, but I didn't notice any real diff with Mar, Saturn, stars at the edge vs centered - but I wasn't really looking hard for anything - just obvious stuff. I was hustling for a bit - finder issue - and I missed Jupiter. Waiting on Moon, Venus, ought to look for Uranus.
  • Details for Mars, Saturn below 8mm were mushy - but that's seeing, not scope.
  •  

  If I had this experience under my belt already, I may have done the CC8. But, I now know I want a 10 later, so I saved a little, will learn a lot. The extensions are necessary, just a little annoying. If you get the right eps, you won't have to switch out. The 'feeling' of scope - it's operation - is definitely not SCT/Mak moving mirror. I quite like it - nice change. To have it arrive bang-on collimated was a nice surprise, but now I'll have to create a collimation experience - because one needs that.

 

I mightn't have tried a Classic Cass were it not for all the threads full of info - many thanks for providing a well-lit path to follow!  waytogo.gif


Edited by AstroKerr, 14 September 2020 - 03:16 AM.

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#1081 quilty

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Posted 14 September 2020 - 05:07 AM

Yesterday night was good seeing conditions. Planets quite still, saw Cassini gap and a bit of cloudy ring on Saturn for my first time, Jupiter with two moons, one was hidden the other did a real sharp spot of shadow like I never saw before. Mars again my first direct seeing of icecap.

 

But, everything thru the 5" Bresser Mak. Never saw anything near this thru the 6" cc. sorry I already said so, things don't change unless you do.

 

And yes, I'd like to compare and I'm looking for the best possible direct vision through the least (the smallest, shortest, most lightweight and least expensive scope) which will be supported by a NEQ3 mount. And thought it could have been the 6"cc but was, here's the word, quite easy, got disappointed, thought wrong. A whole year of trying yielded less success than the last 4 weeks through the Mak again. And intention was to improve the Mak's performance at any regard. The only improvement is the now fixed mirror shift. Still like  the cc scope's design in general. Think it might be worth a try for a second series with less weight, alloy tube and a mirror combination that provides full aperture and less CO. But just repeating..


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#1082 glend

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Posted 14 September 2020 - 05:18 AM

Yesterday night was good seeing conditions. Planets quite still, saw Cassini gap and a bit of cloudy ring on Saturn for my first time, Jupiter with two moons, one was hidden the other did a real sharp spot of shadow like I never saw before. Mars again my first direct seeing of icecap.

 

But, everything thru the 5" Bresser Mak. Never saw anything near this thru the 6" cc. sorry I already said so, things don't change unless you do.

 

And yes, I'd like to compare and I'm looking for the best possible direct vision through the least (the smallest, shortest, most lightweight and least expensive scope) which will be supported by a NEQ3 mount. And thought it could have been the 6"cc but was, here's the word, quite easy, got disappointed, thought wrong. A whole year of trying yielded less success than the last 4 weeks through the Mak again. And intention was to improve the Mak's performance at any regard. The only improvement is the now fixed mirror shift. Still like  the cc scope's design in general. Think it might be worth a try for a second series with less weight, alloy tube and a mirror combination that provides full aperture and less CO. But just repeating..

Typical Cloudy Night commentary, and why i generally avoid looking in here as a rule. Why do threads like these clog up with so many posts talking down whatever subject they are discussing. The Negative Nancies just love a good whinge.  And frankly, a Bresser has never been a benchmark for anything.


Edited by glend, 14 September 2020 - 05:22 AM.

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#1083 AstroKerr

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Posted 14 September 2020 - 06:24 AM

I've found every post helpful in some way, truly have. I like to see both sides. I like folks tearing into scopes physically and reporting what they find and how they interpret the results. Discrepancies. Inaccuracies. It's good to knock the gloss off something when you have one in hand - I find it commendable and of good value.  

 

Back to the Moon =)



#1084 macdonjh

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Posted 14 September 2020 - 07:41 AM

By the way, is there a word for a bad surprise, in German there is?

I've thought about this for the past week.  "Shock" is the only English word I've come up with so far.  Perhaps I should find my thesaurus? 



#1085 quilty

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Posted 14 September 2020 - 12:47 PM

Yes, isn't it just "disappointment"? though not that schocking. In my case, "disappointment" will do I guess, but thanks for help!



#1086 quilty

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Posted 18 September 2020 - 04:30 AM

Never ending story

Night from Wed to Thu was long. First Jupiter, Saturn then the autumn stuff like M13, Struve 2470 and 2474, M27, M57 then Andromeda and finally Mars entering the scene. Was really fine watching through the 6" CC and finding northern icecap and cassini gap. My question now adresses open refractor optics in general

Though the vision was one of the best I ever had is it true that open reflector scopes provide a vision that is more changing and unstill compared to closed optic designs? there were for split seconds double images, the images were like flattering, throgh the Mak they're just wabbling.

Collimation was perfict as far I could see. Epsilon Lyrae showed four Airy spots surrounded by a (flattering) symmetric array of side maxima.

And yesterday night under a totally clear sky everything was gone. Nothing worth to observe. Night was quite cold but can open reflector scopes be more sensitive to atmospheric seeing conditions than closed scopes not only on a short term view (flattery) but also concerning the seeing in the best moments?

Felt yesterday like if the primary was loose again. Don't want to believe so. Ther're always such (shocking) experiences which draw me back from the CC when I start to get a liking.



#1087 Thandal

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Posted 18 September 2020 - 08:06 AM

@quilty;  With open tube instruments, thermal equilibrium is achieved fairly rapidly (for a given aperture and temperature differential.)  In most situations a 6" CC should be cleared of tube currents within very few minutes (10-15).  A closed tube takes much longer to stabilize.

 

It's not the differential that's the direct problem for observing, it's the air movement inside the tube as the heat dissipates to eliminate that differential.  There are methods for assisting closed tubes with this issue by delaying that, such as wrapping it with an insulating material like "Reflectix".  There are also methods of speeding things along for open tubes (especially very large aperture ones) such as adding fans.

 

So... No, an open tube design is NOT more prone to "wavering" than a closed tube one.  The opposite, in fact.

 

The wavering of the image you observed was most likely due to atmospheric eddies, and therefore would have been the same for any instrument of the same size.

 

And unless there was a manufacturing defect, (or you actively loosened it) it is impossible for the primary mirror of a classic cassegrain to be loose at all.  It's a "fixed mirror" design.


Edited by Thandal, 18 September 2020 - 08:14 AM.


#1088 jgraham

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Posted 18 September 2020 - 08:46 AM

I haven't checked my GSO CC8 yet, but I did have a problem with mirror flop in my GSO/Astro-Tech RC8. I am the 3rd owner of this scope and I had to work with it a bit to get it aligned and performing well. The last issue was a problem with the image drifting in a manner consistent with a severe case of mirror flop. I was puzzled by this since the mirrors in the RC8 and CC8 are supposed to be fixed. After researching the design of the mirror cell online I found that it is _very_ easy to check. The mirror in the RC8 and CC8 are attached to the cell via the bore-hole in the primary using a fixture that also carries the light baffle. In the case of my RC8 I _very_ carefully reached past the secondary, gripped the baffle, and _gently_ turned it. Sure enough, it was very loose, almost a full turn! After snugging the baffle/mirror mount my image shift dropped to zero, right where it should be. I had a similar problem with a C11 from 1996 production which was also fixed by snugging the lock collar around the light baffle (also loose by at least a full turn). I plan on checking the baffle on my CC8 as soon as I get a chance.

 

Soooo, if you are seeing any signs of mirror flop in a GSO CC, check the light baffle and see if it is snug, not gorilla tight, just snug.

 

Food for thought.


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#1089 Spacedude4040

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Posted 19 September 2020 - 06:26 AM

Ok a fixed mirror only means it doesn't move during focusing!!!

 

Yes you can get mirror flop with these scopes and the proper way to fix it would be to remove the rear cell from the tube and there is a collar that the baffle attaches to. Look for the small locking screw and loosen it. Now see if you primary will spin, if so tighten the ring down till it doesn't. Do not over tighten or you will cause mirror issues. Tighten the set screw and reassemble the scope and collimate.

All these scopes will have issues of mirror flop over time as there is a thin o ring under that collar that over time will loose its shape, shrink or even crack and  disintegrate.

Mike



#1090 Thandal

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Posted 19 September 2020 - 07:24 AM

@Spacedude4040;  Again, this can only occur if there were a manufacturing defect, or the user deliberately loosened the primary.  "Mirror flop" just isn't a thing with CCs.



#1091 Spacedude4040

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Posted 20 September 2020 - 05:57 AM

@Spacedude4040;  Again, this can only occur if there were a manufacturing defect, or the user deliberately loosened the primary.  "Mirror flop" just isn't a thing with CCs.

 

Mirror flop is when you go from east to west and the mirror shifts due to gravity. Ask all the RC 6-8 owners, same exact system and many had issues. I had to snug my CC-6 up a tad. Have you even taken yours apart?

Foucus shift you will never get with this system as the primary doesn't move!

So mirror flop can be a issue with these scope no defect need, just in the design.



#1092 Thandal

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Posted 20 September 2020 - 07:35 AM

Mirror flop is when you go from east to west and the mirror shifts due to gravity. Ask all the RC 6-8 owners, same exact system and many had issues. I had to snug my CC-6 up a tad. Have you even taken yours apart?

Foucus shift you will never get with this system as the primary doesn't move!

So mirror flop can be a issue with these scope no defect need, just in the design.

 

I'd definitely call any such movement of the primary in a CC 'scope a manufacturing defect.

The primary is fixed and there should be ZERO "flop".  Ever.

 

Certainly hasn't been an issue with either of mine (6" & 8".)


Edited by Thandal, 20 September 2020 - 07:35 AM.

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#1093 Spacedude4040

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Posted 20 September 2020 - 08:15 AM

So how is your primary fixed and have you every checked for mirror flop and how?

#1094 jgraham

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Posted 20 September 2020 - 09:12 AM

Mine has shown absolutely zero shift since I snugged the light baffle. Zero, zilch, nadda. Auto-guiding has become supremely accurate since I fixed this.

 

Wonderful!


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#1095 Spacedude4040

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Posted 20 September 2020 - 10:15 AM

Exactly your good to go!
Rubber and plastic have a tendency to lose memory over time so this may have to be performed every few years. This is more of a maintenance issue if you even have to do it. The best time to check would be when you have the rear cell out to clean the primary, just see if the mirror spins with light pressure. If it doesn’t just clean and reassemble

#1096 Mirzam

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Posted 20 September 2020 - 01:44 PM

If the primary rides on the baffle tube with a clamp in front to hold the mirror in place, there is potential for mirror shift.  Why?  Because the hole in the mirror must be large enough to fit somewhat loosely over the baffle tube, otherwise there may be stress induced by differential expansion and contraction.  It is the same problem that one has in a Newtonian scope, where we are always advised to allow the mirror a tiny bit of clearance from the mirror clips.  What I did on a 12.5” Classical Cassegrain that had issues with mirror shift was to shim between the mirror and the baffle tube with thin pieces of foil.  The goal is to remove most of the play without making things too tight.

 

JimC



#1097 TxStars

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Posted 20 September 2020 - 10:33 PM

For mirror shims I have always used cork..

https://www.hobbylob...ve---12/p/28806

 

I replace it ever year as it does compress over time.


Edited by TxStars, 20 September 2020 - 10:34 PM.


#1098 quilty

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Posted 21 September 2020 - 05:27 AM

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.


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#1099 Spacedude4040

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Posted 22 September 2020 - 11:18 AM

Ok for anybody wondering on how the primary can be loose here’s a reason. The o ring is pressed up on the front face of the mirror.
If the scope is sitting in a box facing downwards at any time during storage or shipping then the weight of the primary will flatten the o ring a little and the rubber may not come back to the original shape. So this can definitely cause the primary to be loose.
Just a thought.
Mike

#1100 jgraham

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Posted 22 September 2020 - 02:12 PM

It's a very easy thing to check and snug if needed. Creep is just a fact of life with any elastomer and you certainly don't want a hard material in this location. If this component ever becomes completely smooshed I may look at a different material, but I'll cross that bridge when I come to it. It just so happens that I'm conducting a study on the compressive stress relaxation of O-rings right now and there are a lot of options.
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