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New AT 6" and 8" Classical Cassegrain

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#1 Ain Soph Aur

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Posted 05 July 2018 - 11:41 AM

I see AT just announced new 6" and 8" f/12 Classical Cassegrain GSO OTA's. I am not very familiar with a classical Cassegrain, but initial reading show these are designed more for  visual use. I was wondering what the secondary obstruction size will be? Since this is designed for visual work, I suspect it should be alot smaller than a SCT?


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#2 Loren Gibson

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Posted 05 July 2018 - 12:07 PM

I just inquired with AT about secondary obstruction size. They don't know all of the final specifications (including secondary obstruction size) at this moment, but they should have full specs within a couple of weeks. Production has started.

 

Loren



#3 danielguo

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Posted 05 July 2018 - 12:20 PM

I saw a new TS-Optics 8" f/12 Cassegrain coming. If it's the same one, then it'd have 33 % secondary obstruction.



#4 spongebob@55

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Posted 05 July 2018 - 01:01 PM

I just inquired with AT about secondary obstruction size. They don't know all of the final specifications (including secondary obstruction size) at this moment, but they should have full specs within a couple of weeks. Production has started.

 

Loren

I'm awaiting more information too.  But interesting with f/12 like a Mak, but no heavy front glass, a rear 2 speed focuser, and 8 baffles, etc.   

But I have to laugh, that 'production has started' yet they don't have any specs.  Yeah, right.

SB



#5 JamesMStephens

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Posted 05 July 2018 - 01:16 PM

I was about to start a thread, but you've all beaten me to the punch!

 

https://tinyurl.com/ClassCass


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#6 desertlens

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Posted 05 July 2018 - 01:25 PM

And a link to Astronomics web site:

 

https://www.astronom...escopes_c2.aspx


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#7 jgraham

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Posted 05 July 2018 - 01:49 PM

Some might find this description of different Cassegrain designs useful...

 

http://www.rfroyce.com/cassegrains.htm


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#8 mclewis1

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Posted 05 July 2018 - 01:55 PM

But I have to laugh, that 'production has started' yet they don't have any specs.  Yeah, right.  SB

That's easily possible and even likely. Starting production only means that some part of the product process has been finalized and has started. That could be anything from ota cutting, getting the primary mirror cell from casting or just ordering things like the focuser or other fittings from other parts of the company. There could easily be some additional items that haven't been finalized or more likely at this late hour just not signed off on. 

 

I'll bet GSO has some form of control over releasing final specs to resellers that probably requires upper management sign off. While they have certainly produced prototypes (and could simply go over a measure the CO or look it up in the CAD system) they might not have yet formally finalized all the specs and therefore won't release any.

 

It's been seen a number of times that Astronomics hasn't received final specs from a manufacturer even while the first production units are in shipment to the US. Acceptance of any specs is likely part of any contract details between the manufacturer and Astronomics and that won't mean any risk to us potential customers unless of course you absolutely have to be one of the very first to order one ... in which case you'll just have to trust Astronomics.


Edited by mclewis1, 05 July 2018 - 01:59 PM.

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#9 Astronomics

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Posted 05 July 2018 - 02:36 PM

I'm awaiting more information too.  But interesting with f/12 like a Mak, but no heavy front glass, a rear 2 speed focuser, and 8 baffles, etc.   

But I have to laugh, that 'production has started' yet they don't have any specs.  Yeah, right.

SB

No need to laugh, they are under production right now.  So please, don't essentially call us liars.

 

From GSO:

"We will mass production of the 6” and 8” within July, start shipping on August. Also, coming next is the 10” and 12” model will be ready in September."


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#10 jgraham

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Posted 05 July 2018 - 02:45 PM

Thinking back to my ATM days, the tricky bit with the classical Cassegrain was always the secondary. To do it right you needed a flat the same size as the primary. I suspect that modern production and testing techniques have made it reasonable to produce the design on a commercial scale. For me, I chickened out and went with a 10" f/30 Gregorian. smile.gif

 

Neat stuff.

 

P.S.

 

I am going to be sorely tempted... I have always wanted a classical cass. I also actually like observing and imaging with long focal length scopes and this could be very interesting.

 

Resistance is futile...


Edited by jgraham, 05 July 2018 - 02:59 PM.

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#11 Axunator

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Posted 05 July 2018 - 02:49 PM

I saw a new TS-Optics 8" f/12 Cassegrain coming. If it's the same one, then it'd have 33 % secondary obstruction.

From GSO:
"We will mass production of the 6” and 8” within July, start shipping on August. Also, coming next is the 10” and 12” model will be ready in September."

This may turn out to be a very interesting product line for visual observers. And good news for us Europeans if TS will offer them here as well. Exciting times ahead...

Edited by Axunator, 06 July 2018 - 11:35 AM.

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#12 desertlens

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Posted 05 July 2018 - 03:32 PM

I saw a new TS-Optics 8" f/12 Cassegrain coming. If it's the same one, then it'd have 33 % secondary obstruction.

I've always felt that specifying CO as a percentage of aperture diameter was a bit unrealistic. I'm aware that the presence of any CO will reduce contrast somewhat but it strikes me that a more reasonable approach would use percentage of area. As an example, the typical 8" SCT would have an obstruction of ~30% by diameter and ~10% by area. The latter is more accurately suggestive of light loss. Just thinkin'.


Edited by desertlens, 05 July 2018 - 03:34 PM.

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#13 WadeH237

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Posted 05 July 2018 - 04:15 PM

The latter is more accurately suggestive of light loss.

Light loss is rarely the concern when considering the size of the obstruction.

 

The bigger issue is the effect that a large secondary has on how much energy is put into the diffraction rings, and the resulting impact on contrast.  And that is better characterized by the diameter of the obstruction.


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#14 eros312

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Posted 05 July 2018 - 04:32 PM

I might as well start saving now cause the 8" looks to be in my future, lol. Living in South Florida, it'll be nice to not have to deal with dew. Too bad they weren't ready right now for prime planetary viewing. Here's the link for TS. https://www.teleskop...436-mm-OTA.html

 

It looks like the same scope as Astronomics, but TS is using quartz and 99% coatings. There's a little more information there as well. 


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#15 desertlens

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Posted 05 July 2018 - 04:38 PM

Light loss is rarely the concern when considering the size of the obstruction.

 

The bigger issue is the effect that a large secondary has on how much energy is put into the diffraction rings, and the resulting impact on contrast.  And that is better characterized by the diameter of the obstruction.

Help me understand this. Is the amount of "energy" translated to the diffraction rings not directly proportional to the amount of "light" transmitted by the optical system? I don't mean to hijack this thread but I'm curious.



#16 JamesMStephens

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Posted 05 July 2018 - 05:14 PM

Help me understand this. Is the amount of "energy" translated to the diffraction rings not directly proportional to the amount of "light" transmitted by the optical system? I don't mean to hijack this thread but I'm curious.

The point diffraction pattern is modified when comparing an obstructed to an unobstructed aperture.  In the latter case about 85% of the energy is in the Airy disk, the remainder is in the diffraction rings.  With an obstruction less of the energy (less that 85%) is in the Airy disk, more is distributed into the diffraction rings.  The numbers depend on the diameter of the obstruction.  


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#17 waso29

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Posted 05 July 2018 - 06:15 PM

wonder if AT will consider 10" D-K design also



#18 Astronomics

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Posted 05 July 2018 - 06:29 PM

wonder if AT will consider 10" D-K design also


I talked about a DK while in Taiwan in 2012. It didn’t seem like a good fit with the RC at the time. Personally I would prefer the 20” being completed for my institutional customers first, and then consider another design.
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#19 terraclarke

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Posted 05 July 2018 - 06:29 PM

Thinking back to my ATM days, the tricky bit with the classical Cassegrain was always the secondary. To do it right you needed a flat the same size as the primary. I suspect that modern production and testing techniques have made it reasonable to produce the design on a commercial scale. For me, I chickened out and went with a 10" f/30 Gregorian. smile.gif

 

Neat stuff.

 

P.S.

 

I am going to be sorely tempted... I have always wanted a classical cass. I also actually like observing and imaging with long focal length scopes and this could be very interesting.

 

Resistance is futile...

I’ve always wanted one too! The 6” is very tempting. I will need to see how large the secondary obstruction is however- I would think this might be a cool planetary instrument if it’s not to big.


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#20 jgraham

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Posted 05 July 2018 - 07:03 PM

The size of the secondary really doesn't bother me. (Admittedly a personal call.) I have quite a few different Cats with all sorts of different secondary sizes and I only notice a slight brightening of the innermost diffraction ring in one of my scopes with an exceptionally large obstruction. I really like the idea of a fixed primary and fixed primary/secondary spacing. Beside the elimination of mirror-flop, I like the idea of the mirror spacing being what it is supposed to be and staying there. The 6" looks awfully tempting, but the 8" is such a sweet spot. Fortunately, there is time to ponder...


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#21 CHASLX200

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Posted 05 July 2018 - 07:28 PM

Hope it is better than the SCT's being made today.


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#22 Phil Cowell

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Posted 05 July 2018 - 07:37 PM

I talked about a DK while in Taiwan in 2012. It didn’t seem like a good fit with the RC at the time. Personally I would prefer the 20” being completed for my institutional customers first, and then consider another design.

Don’t forget me.grin.gif  Looking forward to the RC20.

Cheers

Phil


Edited by Phil Cowell, 05 July 2018 - 07:39 PM.


#23 JamesMStephens

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Posted 05 July 2018 - 08:12 PM

I’ve always wanted one too! The 6” is very tempting. I will need to see how large the secondary obstruction is however- I would think this might be a cool planetary instrument if it’s not to big.

I used to draw full scale designs for a 6" Cassegrain when I was just out of college, I wanted an f/2.5 or f/3 primary.  That would be a tough paraboloid to make and the hyperboloidal secondary...forget it.  If a nice 6" Cass had been available when I got my INTES Mak I would have opted for the classical design.


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#24 Bill Barlow

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Posted 05 July 2018 - 09:02 PM

I would be interested in the 10" version, depending on the weight and cost.  With spider vanes, would the diffraction spikes on brighter stars be distracting compared to a SCT or Mak with a front corrector/meniscus lens?  I've never viewed through a scope with spider vanes.  Will the 10" be f/12 like the 6"/8" models?

 

Bill


Edited by Bill Barlow, 05 July 2018 - 09:04 PM.


#25 jgraham

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Posted 05 July 2018 - 09:15 PM

Hmmmm, I don't think that the diffraction spikes would be very distracting. I really don't notice them until I get up into the 16" range, and even then they're not overly visible except with very bright stars. A nice feature will be no Schmidt ghosts! No dew shield either.

 

Ugh, the primary in my 10" f/30 Gregorian is an f/3 parabola, and yes, it was a bear to figure. I set it up as Newtonian and used star tests to check the figure. Refiguring it is on my list as a retirement project.




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