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GSO 6inch F12 Classical Cassegrain It’s A Winner!

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#76 AhBok

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Posted 03 May 2019 - 10:24 PM

I just received my 6” GSO CC and was delighted to find it in perfect collimation. Looks like some early morning viewing and imaging coming up for Jupiter and Saturn.
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#77 JP-Astro

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Posted 07 June 2019 - 07:02 AM

I just received my 6” GSO CC and was delighted to find it in perfect collimation. Looks like some early morning viewing and imaging coming up for Jupiter and Saturn.

Please report on your results.



#78 AhBok

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Posted 07 June 2019 - 09:13 AM

I’ve had limited observing time with it due to the clouds plaguing many of us, but here is my experience so far. First, this scope does not disappoint in terms of magnification. I have relatively steady Bortle 5 skies here in Tennessee. My goto ep so far has been 6mm (204x) for Jupiter, the moon and M57. A 6mm (306x) eyepiece was also useful on the moon and a couple of close doubles. Jupiter easily showed the Neb/Seb and several other of the darker bands as well as the white (ammonia?) bands. There were plenty of swirls and festoons. I have not viewed any shadow transits yet, but I’ve seen enough to know I will enjoy looking at them.

So far, I would rank my 6”CC as about equal to the best 6” C6s I’ve seen and the better mass-produced Maks. If I had a good 150mm Mak, I’d keep it, but if I was in the market for a new 150mm F12, the CC offers some nice advantages including no need for a few shield or heaters and fast cool down. Also, the build quality and focuser are surprisingly robust at this price point. I could not be happier with a $500 OTA.


Edited by AhBok, 07 June 2019 - 09:15 AM.

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#79 Thandal

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Posted 08 June 2019 - 11:24 AM

I’ve had limited observing time with it due to the clouds plaguing many of us, but here is my experience so far. First, this scope does not disappoint in terms of magnification. I have relatively steady Bortle 5 skies here in Tennessee. My goto ep so far has been 6mm (204x) for Jupiter, the moon and M57. A 6mm (306x) eyepiece was also useful on the moon and a couple of close doubles. Jupiter easily showed the Neb/Seb and several other of the darker bands as well as the white (ammonia?) bands. There were plenty of swirls and festoons. I have not viewed any shadow transits yet, but I’ve seen enough to know I will enjoy looking at them.

So far, I would rank my 6”CC as about equal to the best 6” C6s I’ve seen and the better mass-produced Maks. If I had a good 150mm Mak, I’d keep it, but if I was in the market for a new 150mm F12, the CC offers some nice advantages including no need for a few shield or heaters and fast cool down. Also, the build quality and focuser are surprisingly robust at this price point. I could not be happier with a $500 OTA.

 

I had almost the exact same experience with my new Orion CC6 (which I understand is made by the exact same manufacturer, just painted differently.  wink.gif )

 

See my report (not quite a real "First Light" one) over on the Orion 150mm Class Cass forum.



#80 Stephen Kennedy

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Posted 08 June 2019 - 04:29 PM

I had almost the exact same experience with my new Orion CC6 (which I understand is made by the exact same manufacturer, just painted differently.  wink.gif )

 

See my report (not quite a real "First Light" one) over on the Orion 150mm Class Cass forum.

Yes, all of these Classical Cassegrains and the similarly priced Ritchey-Cretiens are made by GSO in Taiwan.  Making and testing a convex hyperboloidal mirror  is extremely difficult and it appears that even making and testing a concave hyperboloidal  mirror is not that easy as was demonstrated by the HST when it was first launched.  This is one of the reasons that other makers of RCs charge very high prices for their telescopes without necessarily better image quality than GSO.

 

GSO has made concave paraboloidal mirrors for some time now which are used in both CCs and Newtonian reflectors.  Somehow, they invented a process to make and test concave hyperboloidal mirrors used in RCs and convex hyperboloidal mirrors that are the secondary mirrors in both RCs and CCs, that produced high quality images but could be mass produced at much lower cost.  The fact that GSO has no competition for affordable well made CCs and RCs indicates that they have been very good at keeping how they do this secret.  


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#81 quilty

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Posted 19 November 2019 - 04:39 AM

Spacedude4040 wrote:

George you are right. I think the 6-8" are teaser scopes and the excitement is in the 10-12" scopes and maybe more. The 6" falls into a group with a lot of good offering in the market but the 10-12" might be in a league of their own in price/quality.

For me it’s all about the STREHL thru put of the scope because I'm a planetary nut. All things being equal the Classical Cassegrain should up the game in the Cassegrain area. Here's a little info on that   http://www.whichtele.../benchmarks.htm      without changing the direction of this thread.

 

Good info, What is the result for strehl when you apply the benchmark info on the telescope's 42% linear obstruction?


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#82 Stephen Kennedy

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Posted 20 November 2019 - 05:28 AM

Quilty, I think you made a good point about the 6-8" RCs and CCs that GSO manufactures being "teasers" for their larger models.  I had a 210 mm Newtonian that I had been using for 30 years when I purchased the GSO RC6.  I have developed a strong interest in imaging very  distant galaxies such those in the Virgo and Coma super clusters and came to realize that I needed more aperture with a fairly long focal length.  Since I do not really care for SCTs I believed that an RC would be the best choice but before I spent a lot of money on a 12" (305 mm) F/8 or larger RC I wanted to see how well GSO RCs actually performed.  For the modest sum of $399 one can get a new 6" (150 mm) F/9 RC produced by GSO and be able to make the determination of whether or not I should spend at least $3,500 to order a GSO RC12.  I think the RC6 I purchased has been an excellent performer, as well as being a very well made telescope which makes me much more comfortable about my current plans to order an RC12 and hopefully have it set up in time for Galaxy Season in the coming Spring.



#83 quilty

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Posted 20 November 2019 - 09:38 AM

Hi Steven,

I had to decide between 6inch RC and CC and I am not totally happy with the latter. Could you provide exact data about telescope length and secondary diameter including cup? Do you know the primary's f numer or focal length? It schouldn't be too hard to read, just see where the moon is displaying in front of the telescope close by secondary and read the distance to the primary. Maybe I'll change to RC. Maybe at moment GSO have optimized their RCs and consider their CCs sort of by-products with less care. The simplest guess was primary f3 and secondary f3 for RC and secondary f4 for the CC. But my primary has a focal length of 480mm instead of 460 which turns me down a bit.



#84 Stephen Kennedy

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Posted 25 November 2019 - 05:17 AM

Quilty,

 

The measurements I was able to perform showed the tube of my RC6 to have a length of 384.2 mm with the distance between the primary mirror and secondary mirror being 304.8 mm and the diameter of the secondary including cup is 75.6 mm.  According to the specifications  the concave hyperboloidal primary mirror is F/3 while the convex hyperboloidal secondary has power of three to bring the system focal ratio to F/9.  The advertised system focal length is 1,370 mm.

 

While I do have a Foucault tester which I use to make primary paraboloidal mirrors for Newtonian reflectors, I do not know if it can be used to measure the focal length  of mirrors on an RC, I am inclined to believe it certainly would not be useful on convex mirrors.  I am not sure if it is known outside of GSO how GSO tests its RC mirrors.  This is the most demanding step in making an RC and for the past ten years they have continued to turn out ever higher quality RC optics and can sell them for about one third the price anybody else making RCs can.  It was not until about two years ago that they started selling CCs, probably assuming that their uncanny ability to make and test convex hyperboloidal secondaries would be a major competetive edge.  However, it appears that they had more difficulty than anticipated in making the short focal length paraboloidal primary mirrors for the CC and production was delayed.

 

In a way, I do not understand why GSO bothered to make the CC.  George Ritchey and Henri Cretien developed the RC to overcome the shortcomings of the CC such as off-axis coma.  Since the latter part of the 20th century nearly every major new observatory built has used the RC design.  If you lack the ability to make a good RC then it makes sense to settle for making CCs which are less demanding.  But over the past ten years GSO has perfected the art of making quality RCs at extremely competitive prices.

 

GSO may have thought that there was an unmet demand in the market for CCs and that they were leaving money on the table by not using their expertise in making RCs to also make CCs but I think it has not gone as smoothly as they expected.  I am planning on purchasing another, larger RC from GSO but would certainly not buy a CC from them which they amazingly charge more for than an RC.

 

Here is an image of the third quarter moon that I obtained with the GSO RC6 on a 32 year old Vixen Super Polaris GEM:

 

IMG_3507 (2).JPG

 

Stephen 



#85 quilty

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Posted 25 November 2019 - 08:40 AM

Hi Stephen,

 

thanks a lot for supplying all the data. First of all I wouldn't want to complain at all but your pic gives no evidence for a a supreme performance of the RC design. All my three telescopes display way better visually (127/1900 Mak Bresser, 152/1848 CC and even AR 102/460 xs, Bresser). For I am unable to take such pictures at all I don't know what is the optical loss in the imaging process.

 

Back to subject: Your tube length of 384.2 mm without focuser I take now for granted which is 50 mm less than my CC's length. The RCs are a bit shorter. What is strange: You read a mirror distance of 304.8 mm, I do 288 mm. I guess you took the secondary cup bottom whereas I took the true secondary surface for reading. Then your true mirror distance could be 238 mm., which makes sense since your secondary is then in a position quite half at the focal length of (supposed) 460 mm of primary and with 75mm diameter in the appropriate size. I don't know neither how to measure the secondary's focal length but the primary's is easy (+-3mm): Just see, at what distance from primary the moon displays sharply in front of the telescope just beside the secondary. (I also use the CC now as a razor mirror, just that it serves for a purpose :-) and then the distance between my sharp eye and pirmary is 990 mm. But dont't know how to get the focal length from that). 

 

75,6 mm secondary diameter is quite a lot, as much as 50% CO. This is the downside of RC design (and ultra-short newtonian design). I don't know what exactly this does to strehl and contrast and I think in general it is the carefullness how the mirrors are shaped, polished, coated and assembled rather than the design principes which yield good optical performance. Why is the production of RCs more demanding than CCs? 

Quilty



#86 Stephen Kennedy

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Posted 25 November 2019 - 05:10 PM

Hi Stephen,

 

thanks a lot for supplying all the data. First of all I wouldn't want to complain at all but your pic gives no evidence for a a supreme performance of the RC design. All my three telescopes display way better visually (127/1900 Mak Bresser, 152/1848 CC and even AR 102/460 xs, Bresser). For I am unable to take such pictures at all I don't know what is the optical loss in the imaging process.

 

Back to subject: Your tube length of 384.2 mm without focuser I take now for granted which is 50 mm less than my CC's length. The RCs are a bit shorter. What is strange: You read a mirror distance of 304.8 mm, I do 288 mm. I guess you took the secondary cup bottom whereas I took the true secondary surface for reading. Then your true mirror distance could be 238 mm., which makes sense since your secondary is then in a position quite half at the focal length of (supposed) 460 mm of primary and with 75mm diameter in the appropriate size. I don't know neither how to measure the secondary's focal length but the primary's is easy (+-3mm): Just see, at what distance from primary the moon displays sharply in front of the telescope just beside the secondary. (I also use the CC now as a razor mirror, just that it serves for a purpose :-) and then the distance between my sharp eye and pirmary is 990 mm. But dont't know how to get the focal length from that). 

 

75,6 mm secondary diameter is quite a lot, as much as 50% CO. This is the downside of RC design (and ultra-short newtonian design). I don't know what exactly this does to strehl and contrast and I think in general it is the carefullness how the mirrors are shaped, polished, coated and assembled rather than the design principes which yield good optical performance. Why is the production of RCs more demanding than CCs? 

Quilty

Quilty,

 

Here is a better processed image of the third quarter Moon taken with the RC6

 

IMG_3509 (3).JPG

 

RCs are usually more difficult to make than CCs because the convex curvature of the secondary is more extreme in the RC.  Also, the concave hyperbolic primary is harder to make and test (just ask Perken-Elmer who made the primary for the HST) than the concave parabolic used in a CC.

 

Stephen



#87 Stephen Kennedy

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Posted 25 November 2019 - 06:32 PM

This is a 30" exposure of the star Vega imaged with the RC6 on a 32 year old Vixen Super Polaris GEM. 

 

IMG_2890 (6).JPG

 

As you can see, the diffraction spikes are very prominent.  I like them but I know there are people who do not care for them.  I would suspect that the CC6 at F/12 would have similar spikes but would not be as long when using it visually compared to a 30" exposure on an F/9 RC.



#88 quilty

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Posted 26 November 2019 - 06:07 AM

Hi Stephen,

 

thanks fpr the pics. I don't care for the spikes. Moon shows good contrast, yet, visually I see sharper through my CC. What are the Vega neighbouring stars. Strange, they look like sunlit moons or Mars and Mercury, but Vega cannot feature the sun, light is shining from left side above. Guess when you turn the image by 180° impression might be quite different. 

What is the true RC mirror distance, surface to surface, or how exactly did you read the distance? Is it about 238 mm?

 

Do you write the sun with small but Vega with capital letters? Even every modest planet will be written in captials. Isn't that strange? What about  Moon and Earth? I'm getting confused.

When I'm not mistaken, we're living at the only place in the universe, where the central star is so familiar, what lucky incident. For us Germans the sun is even female. 

 

Quilty



#89 quilty

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Posted 26 November 2019 - 06:14 AM

Has anyone except me tried to measure the focal length of his 6 inch CC?



#90 checcocpb

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Posted 26 November 2019 - 07:22 AM

Just bought the 8"CC, not yet tested under stars, first impressions are of a very well executed telescope, all metal construction, double dovetail, both vixen and losmandy, and a lot of razor blades inside to prevent scattered light. The shop owner tested it with me at the optical bench, the scope delivered a very good diffraction image.

Cheers


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#91 quilty

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Posted 04 April 2020 - 04:55 AM

seems to be dead ad moment.

Got an idea how to simply assess the long focal length of a cc and similar.

When you have a scope of a certain focal length, baybe a refractor then you can read the focusser position difference to focus at 100m and infinity to be for instance 10 mm.

Then you do so with your cc scope and find for instance 40 mm focus shift. Can't we just assume then that the cc focal length is just 4 times the focal length of the refractor?

That would be not too exact but fast and easy.




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