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Old 18-01-2020, 11:25 AM
glend (Glen)
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GSO 10" Truss tube Cassegrain about to land in Oz

Some of you may have seen Dennis di Cicco's review of the new GSO 25cm f12 Cassegrain scope in the January issue of Australian Sky and Telescope. This prompted me to ask Luke at Andrews about when this scope might make it to Oz. Up until this past week Andrews has been awaiting their next shipment of the 6" and 8" GSO Cassegrains, which are standard closed tube based models, and he could not confirm the 10" Truss model, presently for sale in the US (Agena Astro). But lo and behold, the Andrews website has now been updated to show that yes, the 10" Truss Cassegrain is in the next shipment. No price as yet, but the Agena Astro USD price is $2795, so you can estimate the AUD if interested.
Note you will need a mount that can swing maybe 19kg, the scope alone weighs 17kg, and at a focal length of over 3000mm, it will need a pretty solid base I would think.
I was about to pre-order the 8" closed tube version, but I might hold off until they can confirm the 10" price.
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Old 18-01-2020, 12:08 PM
Wavytone
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That’s surprisingly heavy - my 10” mak - closed tube and rings - is 14kg despite the massive corrector at the front...

10” is a significant increment on 8”.
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Old 18-01-2020, 12:29 PM
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In simple terms please, what is the difference between a classical cassegrain & a Richey Creighton design scope..

I understand the difference between these & say an SCT or Mak but, haven't been able to find out the main difference between the Classic Cat & an RC..

Not that I want one... I'll be dead if I buy anything else astro related...

Just curious in expanding my knowledge...
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Old 18-01-2020, 01:05 PM
glend (Glen)
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Quote:
Originally Posted by Outcast View Post
In simple terms please, what is the difference between a classical cassegrain & a Richey Creighton design scope..

I understand the difference between these & say an SCT or Mak but, haven't been able to find out the main difference between the Classic Cat & an RC..

Not that I want one... I'll be dead if I buy anything else astro related...

Just curious in expanding my knowledge...
Well a couple of things: an RC is more of an imaging scope, it is faster at f8, but has a larger central obstruction (say 40%), the hyperbolic mirror set is designed to be coma free (well third order in theory, and free of spherical aberation).
The Cassegrain has a smaller central obstruction ( say 30%), and is slower at f12 (ie longer focal length, and not as concerned with coma at the edges.
The Cassegrain is the original folded design which the RC grew out of, in a sense.
That's my take on them, one RC is an astrograph design and the pure Cassegrain is visual mainly but can be used for planetary imaging.
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Old 18-01-2020, 01:23 PM
Saturnine (Jeff)
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In basic terms the Classical Cassegrain has a parabolic primary and hyperbolic secondary mirrors and is more suited for higher magnifications. The Ritchey Chretien design is a hyperbolic primary and hyperbolic secondary mirror. The RC design is chosen to eliminate coma over a wider field, to make it more useful for astrophotography.
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Old 18-01-2020, 01:25 PM
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Quote:
Originally Posted by glend View Post
Well a couple of things: an RC is more of an imaging scope, it is faster at f8, but has a larger central obstruction (say 40%), the hyperbolic mirror set is designed to be coma free (well third order in theory, and free of spherical aberation).
The Cassegrain has a smaller central obstruction ( say 30%), and is slower at f12 (ie longer focal length, and not as concerned with coma at the edges.
The Cassegrain is the original folded design which the RC grew out of, in a sense.
That's my take on them, one RC is an astrograph design and the pure Cassegrain is visual mainly but can be used for planetary imaging.
Quote:
Originally Posted by Saturnine View Post
In basic terms the Classical Cassegrain has a parabolic primary and hyperbolic secondary mirrors and is more suited for higher magnifications. The Ritchey Chretien design is a hyperbolic primary and hyperbolic secondary mirror. The RC design is chosen to eliminate coma over a wider field, to make it more useful for astrophotography.
Thankyou gentlemen, I learned something more about optics..

Cheers
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Old 18-01-2020, 06:39 PM
Wavytone
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Carlton they can be good but don’t forget the OTA is open, unlike your mak. This make a huge difference to the longevity of the mirror coatings, for one thing, and the Rumak (yours and mine) have zero coma (when collimated) and flat fields.

And I’d be very surprised if GSO produced one that overall was 1/8 wave PV.
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Old 18-01-2020, 07:01 PM
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Quote:
Originally Posted by Wavytone View Post
Carlton they can be good but dont forget the OTA is open, unlike your mak. This make a huge difference to the longevity of the mirror coatings, for one thing, and the Rumak (yours and mine) have zero coma (when collimated) and flat fields.

And Id be very surprised if GSO produced one that overall was 1/8 wave PV.
Not withstanding that I am not permitted to purchase anymore telescopes or, for that matter.. mounts...

Nah, I wasn't looking... I was just curious what the differences were...

I am extremely content with the scopes I have now & my newly acquired AZEQ6 GT...

I have no more organs or body parts to sell....
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Old 18-01-2020, 09:01 PM
ariefm71 (Arief)
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Larry Carlino did a report on CN/AM on the 8" version and found that the effective aperture of the scope is only 7.3" due to the smaller secondary used (still 33% CO). The image was far dimmer than a C8. I wonder if the 10" version has this issue.

Arief
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Old 20-01-2020, 10:31 PM
ausastronomer (John Bambury)
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Quote:
Originally Posted by glend View Post
The Cassegrain has a smaller central obstruction ( say 30%), and is slower at f12

I don't know about the GSO Classical Cassegrains, but a properly designed Classical Cassegrain will usually have a Central Obstruction under 30%, which would be a maximum sized Cassegrain Secondary when an F3 Primary was used. Classical Cassegrains historically have used about an F4 parabolic primary which normally would utilise a Secondary with a 20% to 25% CO. F12 would normally be about as fast as you would find them also, unless specifically designed for imaging. Although things are changing in this regard with more and more scopes being designed faster and around imaging. Many Classical Cassegrains historically were built around F5 primaries with small central obstructions and these made exceptional planetary visual and imaging scopes having about an F20 overall focal ratio.

Parks have sold a series of scopes called "HIT" for years, which are Classical Cassegrains which also have a Newtonian focus. They use an interchangeable secondary mirror to achieve this. You just change out the secondary and you go from a Classical Cassegrain to a Newtonian in a few minutes, so you have 2 telescopes in 1. Optically they are very good, but they are also very heavy. They used to make them in several apertures but only do the 16" now. This is a 16"/F4 Newtonian and a 16" F/18 Classical Cassegrain. The OTA alone weighs 118kg so it needs a serious mount, which is included in the $USD31,999 price tag.

I put a summary together years ago on the different attributes and design parameters of different compound telescope designs:-

The Classical Cassegrain Telescope (CCT) uses a parabolic primary and a hyperbolic secondary. It is an excellent design as it offers very little coma, the same as a Newtonian of the same FINAL f-ratio (so an
f20 CCT has the same coma as an F20 Newtonian). The field curvature
is however dependent on the focal ratio of the primary mirror, and
can be quite bad for designs with very fast primaries (< f3). An F20 system with an F5 primary makes an excellent system, convertable to wide field Newtonian views and high magnification Cassegrain views.

The Dall-Kirkham Cassegrain (DK) telescope has an elliptical primary and a
spherical secondary. The on-axis images are superb, but off-axis they
aren't great. They have about the same coma as a Newtonian with the same focal ratio as the PRIMARY MIRROR, not the FINAL f-ratio, as in a
classical cassegrain. So a DK with an f5 primary and a F20 final
focal ratio has the same coma as a f5 Newtonian. Which isn't bad, but
when you start getting down to f3 primaries you have a telescope better suited for narrow-field work only.

The Ritchey-Chretien Cassegrain (RC) telescope has a hyperbolic primary
and a hyperbolic secondary with the curves of the two mirrors
carefully chosen to simultaneously eliminate spherical aberration
and coma (to third order approximation). They have slightly more
astigmatism and field curvature than the classical cassegrain, but as
these are relatively minor annoyances, you get a wider diffraction
limited field than any reflector design (only the Petzval refractor
significantly betters it).

Another design called the Press Carmichael uses a spherical primary as in (SCT) and an ellipsoidal secondary. These aren't common and exhibit greater aberrations than the 3 forementioned designs.

As a Visual telescope a Classical Cassegrain with an F4 to F5 Primary, a small secondary obstruction and an overall F-Ratio of F16 to F20, is clearly the best choice. As an all round imaging scope an F8 to F12 Ritchey Chretien is the best choice.

Cheers
John B
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  #11  
Old 21-01-2020, 03:35 PM
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Allan_L (Allan)
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Thanks Glen for the heads up.
I missed reading that article and your post inspired me to go back and have a look.
I thought it a good review (not that I'm technically qualified) but in that it inspired me to get back out with scope and just have a look.
I think some of us just need that kick in the pants to keep the enthusiasm alive sometimes...(IMHO)
Guess I'm getting old and cranky and sky deprived.
Cheers Buddy
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Old 21-01-2020, 06:34 PM
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Its always good to see new and interesting telescopes come on to the market. I have not had a chance to see the review Glen mentioned.

Given that the classical cassegrain is considered good for high magnification visual observing, what advantage would the GSO 10" f12 cassegrain have over the GSO 10" f4 Newtonian? Especially as it is not at the optimum f15-f20 focal ratio.

Andrews has the truss 10" f4 newtonian at under 2k$. Even if you added a TV Paracorr you would be under 3k. I guess the proof will be in the pudding so to speak.
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Old 22-01-2020, 08:35 PM
ariefm71 (Arief)
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Great question Andy, I wish I know the answer both will have more than 30% CO but this video shows what a very good 10 f/4 newt can produce https://www.youtube.com/watch?v=9LKkxTTJgB8
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