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New GSO Classical Cassegrains


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4 minutes ago, dweller25 said:

@John hope its better than the VC200L i had which was the worst planetary scope i ever looked through ?....

Interesting to know Dave. I think I'll cross that one off my "I'd like to have one one day" list.

 

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Looks like it has a decent focuser to begin with rather than the having to buy one after market.

Why do the Celestron SCTs  and Meade SCTs  of this world ( and RC designs)  use the 'moving primary mirror method'  and all the associated flop issues ?

Is it because it gives a wider focus range for terrestrial targets etc ??

Edited by Craney
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20 minutes ago, Craney said:

Why do the Celestron SCTs  and Meade SCTs  of this world ( and RC designs)  use the 'moving primary mirror method'  and all the associated flop issues ?

It enables reaching focus with all sorts of stuff hanging on the back, there isn't enough movement in a separate focuser to achieve this.

I've recently been imaging planets using a DSLR and 2 X Powermate and comets using CCD camera, to achieve focus with both setups requires lots of winding the mirror in / out.

The good thing is you can always reach focus but a separate precision focuser for final focus is required for imaging IMHO.

Dave

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Looks a very interesting telescope and at 399 Euros it seems a very good price for a 6" in comparison to a SCT or Mak Cass. 

When it arrives it would be good if there was a review comparing a 6" SCT, 6" Mak Cass and this scope.

I await in anticipation unless someone obtains one and writes a positive review on SGL. 

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I'm not sure about that one ...

Looks like too much compromises. I would expect classical Cassegrain to be at least F/18, but it seems that GSO opted for F/12 for a number of reasons:

1. Reuse of tube and other components from existing RC model (both 6 and 8 inch) - F/18 or longer would require longer tube

2. Trying to make all rounder rather than specialist scope - bigger market share.

 

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1 hour ago, vlaiv said:

2. Trying to make all rounder rather than specialist scope - bigger market share.

Yes, it does give the impression of being designed by a committee: too many compromises.
Price-wise it is comparable to an 8 inch SCT
And in F-ratio, too.

It doesn't seem to be different enough to attract much market share, given that there are so many SCTs occupying that market segment.

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46 minutes ago, pete_l said:

Yes, it does give the impression of being designed by a committee: too many compromises.
Price-wise it is comparable to an 8 inch SCT
And in F-ratio, too.

It doesn't seem to be different enough to attract much market share, given that there are so many SCTs occupying that market segment.

Remains to be seen. I believe there are several visually oriented amateurs out there (count me in!) who fill find an open tube classical Cassegrain design - even at f/12 - very tempting and interesting... Especially at these prices. 

Whether it will be sufficiently profitable for GSO/TS/Astronomics, only time will tell. At least their RCs have been going on for years, and they are targeted to quite narrow audience as well.

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I see several advantages in these, as compared to standard SCTs:

- easier thermal management compared to closed tube SCTs

- way easier dew management (no corrector plate)

- flatter field due to slower primary

- much wider diffraction-limited coma-free field (standard f/10 SCT has coma-free field of ca. f/5 Newt, whereas f/12 Cassegrain equals ca. f/12 Newt, i.e. very wide diffraction-limited field. EdgeHD and ACF SCTs are of course a completely different story in this regard, but come with a price tag to match... yet still have that dew-magnet corrector plate).

- no mirror flop during focusing (can be avoided with after market focusers in SCTs as well, albeit with a price)

 

Of course, there are downsides as well:

- a bit narrower FOV due to longer focal length (although diffraction-limited field is wider compared to standard SCT, see above)

- diffraction spikes from the secondary spider

- fixed focal length (back focus in 8" is reported to be 150 mm behind the 2" focuser), which may affect the use of some accessories, like binoviewers

- more difficult to collimate because of hyperboloid secondary

 

I guess you can tell that I'm excited ? And yet I'm still quite happy SCT owner as well ?

Edited by Axunator
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2 minutes ago, DaveS said:

I've been looking at their RCs and asking myself if I can justify the cost difference between CFF and GSO.

All I can say from experience is that TS/GSO RC8" is worth the price.

Someone mentioned focuser on this new Cas above, and it is the same unit as on RC 8" model - I ended up replacing mine. Although standard focuser (Monorail 2") delivered with scope is quite usable unit (for visual), there is serious drawback for its primary use (as astrograph) - it does not have threaded connection so tilt due to any mismatch in 2" interface is likely to happen.

But if you just count a set of 8" F/8 RC mirrors - of quite good figure quality (I tested mine to be >=0.94 strehl) - at that price, I believe it is very worth it.

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Dedicated planetary telescopes aren't produced on such scales as your average fast Newtonian so the price gets higher. Add a low volume high quality small company and the price goes even high. f/12 is still good although the aperture could be bit bigger. f/20 can be problematic as planetary cameras get smaller and smaller pixels. There are some Sony big pixel sensors, but those are only in PGR cameras at the moment.

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The slop/flop of the main mirror in an SCT can certainly be improved by running the mirror up and down the baffle tube; by rotating the focusing knob (about 30 turns) each way a few times.
This re-spreads the grease on the baffle tube and reduces the effective gap.

A routine job which should be done by all SCT owners at regular intervals.

Adding a external focuser to the SCT OTA doesn't prevent the main mirror moving.........

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11 hours ago, vlaiv said:

All I can say from experience is that TS/GSO RC8" is worth the price.

Someone mentioned focuser on this new Cas above, and it is the same unit as on RC 8" model - I ended up replacing mine. Although standard focuser (Monorail 2") delivered with scope is quite usable unit (for visual), there is serious drawback for its primary use (as astrograph) - it does not have threaded connection so tilt due to any mismatch in 2" interface is likely to happen.

But if you just count a set of 8" F/8 RC mirrors - of quite good figure quality (I tested mine to be >=0.94 strehl) - at that price, I believe it is very worth it.

Thanks Vlaiv.

Been looking at 12" / 30cm RCs. The CFF one is light enough to go on my DDM60, but the GSO RC will need a heavier mount, probably a DDM85 (I do like DDM mounts!), which is more than the cost differance.

In either case I'd be looking at a FT focuser.

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8 hours ago, Merlin66 said:

What about a quick change over secondary? From Newtonian (F4?) to Cass f12 all in one OTA????

 

I was just thinking about that, if Takahashi could do it with CN212, how hard can it be? :D

But I would still prefer Cassegrain configuration to be with small secondary and long focal length - for planets.

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