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New AT 6" and 8" Classical Cassegrain

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#51 AlienRatDog

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Posted 07 July 2018 - 03:25 PM

I wish there was a correct Dall-Kirkham in the mix!  I suppose GSO went with classical cassegrains due to tooling. They already make hyperbolic secondary mirrors (from their RC line) and can obviously make parabolic mirrors.  The Dall-Kirkham would require ellipsoidal secondaries and the design for a sub aperture corrector (which would have a high upfront cost that they may or may not make back).  Just a thought...



#52 Phil Barker

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Posted 07 July 2018 - 04:59 PM

These scopes will have a distinct advantage over Dall Kirks ie mewlons.  Better edge of field less coma.  More field curvature however in the CC.

 

Having used a classical Cass many times I can attest to its potential in term of planetary performance and deep sky at higher powers.  The scope I refer to was a 14.5 inch f17 classical cass. 

 

I feel cooldown could be an issue if  non low expansion glass is used.

 

33% isn't small in terms of obstruction but the shorter tube convenience of having eyepiece behind the primary really makes it user friendly.

 

generally f15 with 250mm of back focus needs 30% so its not too bad.  f20 you may as well have the dall kirk which can be around 25% obstructed. 

 

Re the 6 inch version I have a  sw150P newt and a quality barlow effectively makes it a winner for planets etc.  31% obstruction tube not too long could'nt see an advantage in the CC.  My sw180 is a fine scope but cooldown is an issue  could be keen on an 8 inch to put on the eq6az.


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#53 Ain Soph Aur

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Posted 07 July 2018 - 05:12 PM

It appears that the visual back is the same thread as the 8” RC. That is great news, I can re-purpose the Moonlite focuser and OTA adapter from the 8” RC when doing planetary and lunar imaging.


Edited by Ain Soph Aur, 07 July 2018 - 05:53 PM.

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#54 Astronomics

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Posted 07 July 2018 - 06:54 PM

The AT website specifies that the mirrors will be fabricated from optical glass (BK-7).  Why?   Why not Pyrex?  Or fused quartz?  Or is this an error?

We were just informed Friday they will be quartz.  

 

BK-7 has been the glass on the RC6 since it was introduced, as well as the steel 8" RC.  No issues with it at all.


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#55 Astronomics

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Posted 07 July 2018 - 06:56 PM

I wish there was a correct Dall-Kirkham in the mix!  I suppose GSO went with classical cassegrains due to tooling. They already make hyperbolic secondary mirrors (from their RC line) and can obviously make parabolic mirrors.  The Dall-Kirkham would require ellipsoidal secondaries and the design for a sub aperture corrector (which would have a high upfront cost that they may or may not make back).  Just a thought...

The classical was our idea.  Jim had mentioned doing a DK when I was there in 2012.  However, I told him it needed to be corrected.  After that conversation the push for truss systems came out as well as the wish to do bigger scopes.


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#56 Ain Soph Aur

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Posted 07 July 2018 - 07:00 PM

We were just informed Friday they will be quartz. 

Nice, thanks for the update!


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#57 Bill Barlow

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Posted 07 July 2018 - 07:13 PM

Does the classical Cassegrain like these have an oversized primary mirror like the Maksutov Cassegrains?  I assume they don't since there is no front corrector lens/meniscus.

 

Bill


Edited by Bill Barlow, 07 July 2018 - 07:14 PM.


#58 JamesMStephens

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Posted 07 July 2018 - 08:01 PM

Does the classical Cassegrain like these have an oversized primary mirror like the Maksutov Cassegrains?  I assume they don't since there is no front corrector lens/meniscus.

 

Bill

Bill,

 

I think you answered your own question.  The mirror is only oversized with respect to the corrector, which defines the entrance pupil.  

 

Jim


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#59 Phil Barker

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Posted 07 July 2018 - 08:43 PM

so its f12 33% obstructed and low expansion glass  sounds good

 

Here's an idea why not a dall kirk f17 25-27% obstructed into the mix even if its only in a larger size.  Why well roughly 30% plus less obstruction by area leaner sharper stars and planetary images.  

 

clearly not an allrounder but a specialist scope.



#60 kcb

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Posted 08 July 2018 - 11:12 AM

I was thinking of the skywatcher 7''  f/15 maksutuv-cassegrain for portable lunar studies visually  but now I see the astrotech 8'' f/12 classic cassegrain , can anyone  help with my choice ? , thanks



#61 jgraham

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Posted 08 July 2018 - 11:37 AM

The Skywatcher 7" Mak is a known quantity with a good track record. As nice as the AT classical Cass's sound we won't know what they are like until the first production units ship. If you're not in any hurry you can wait until this fall (time enough for the first units to ship and to get user reports) or go ahead and get an available scope. Optical quality aside, it would be nice to hear from anyone who has used both. I have two Meade 7" f/15 Maks and I am very impressed with their flat, coma-free field. I am curious how the classical Cass design will compare.


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#62 Achernar

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Posted 08 July 2018 - 12:05 PM

They would be mainly visual telescopes. I do not recommend  them for imaging. Classical Cassegrains suffer from coma more than SCT's that are not aplanatic do. Hence the advent of Celestron's EdgeHD, which is a big improvement over classical SCT's for imaging. They also have open tubes, which means the optics are exposed to the elements. That is both a good and a bad thing. The large secondary mirror does lower contrast. These facts were by the way that lead to the development of the Ritchey-Cheretin Casegrain. The long focal lengths and narrow fields of view make them best for medium and high power observing of objects suited to them.

 

Taras 


Edited by Achernar, 08 July 2018 - 12:09 PM.


#63 Guest_djhanson_*

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Posted 08 July 2018 - 12:23 PM

They would be mainly visual telescopes. I do not recommend  them for imaging. Classical Cassegrains suffer from coma more than SCT's that are not aplanatic do. Hence the advent of Celestron's EdgeHD, which is a big improvement over classical SCT's for imaging. They also have open tubes, which means the optics are exposed to the elements. That is both a good and a bad thing. The large secondary mirror does lower contrast. These facts were by the way that lead to the development of the Ritchey-Cheretin Casegrain. The long focal lengths and narrow fields of view make them best for medium and high power observing of objects suited to them.

 

Taras

Not true for all types of imaging.  Classical casses are great for planetary imaging and excel over SCT's there.  (I've owned both)   And really, the EdgeHD wasn't developed to improve over the classical cass.  (I highly doubt Celestron had this in mind when it improved their design here)  It was developed to improve over the standard SCT and they had wide field imaging in mind here of course.  There's a big difference between planetary vs wide field imaging.  DJ



#64 TG

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Posted 08 July 2018 - 08:11 PM

Classical Cassegrains suffer from coma more than SCT's that are not aplanatic do.
Taras


This is not accurate. Classical Cassegrains have coma but not as bad as the D-Ks which have about the same coma as an (uncorrected) SCT.

At f/12 and up I would not expect coma to be a problem at all since the true field is not going to be wide enough to show it.

Tanveer
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#65 Phil Barker

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Posted 09 July 2018 - 01:01 PM

You are right however classical cassegrains have more field  curvature than a Dk of the same specs.  

 

I made an 8 inch f19 dk a few years back wasn't as successful as I hoped must have another go one day.

 

Overall howver CC superior to a DK or uncorrected sct.

 

 

This is not accurate. Classical Cassegrains have coma but not as bad as the D-Ks which have about the same coma as an (uncorrected) SCT.

At f/12 and up I would not expect coma to be a problem at all since the true field is not going to be wide enough to show it.

Tanveer



#66 TG

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Posted 09 July 2018 - 04:48 PM

You are right however classical cassegrains have more field  curvature than a Dk of the same specs.  

 

I'm not disputing this may have been the case for your scopes due to differing configurations but as far as I understand the field curvature is a function of the (apex) curvatures of the two mirrors and not their eccentricities/conic constants. For an equivalent f/ratio, assuming equal magnifications provided by the secondary and equivalent "f/ratios" of the primary, a CC and a D-K should have similar very similar field curvatures.

 

If I'm wrong, a pointer to a reference would be appreciated.

 

Tanveer.



#67 Exnihilo

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Posted 10 July 2018 - 12:26 AM

They would be mainly visual telescopes. I do not recommend  them for imaging. Classical Cassegrains suffer from coma more than SCT's that are not aplanatic do. Hence the advent of Celestron's EdgeHD, which is a big improvement over classical SCT's for imaging. They also have open tubes, which means the optics are exposed to the elements. That is both a good and a bad thing. The large secondary mirror does lower contrast. These facts were by the way that lead to the development of the Ritchey-Cheretin Casegrain. The long focal lengths and narrow fields of view make them best for medium and high power observing of objects suited to them.
 
Taras

The Vixen VMC line are “corrected Cassegrain” much like the Celestron Edge are “corrected SCTs.” I briefly considered a VMC, but very soon decided I wasn’t interested. But I’m actually interested in these classical Cassegrain.

Edited by Exnihilo, 10 July 2018 - 12:26 AM.

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#68 tonyt

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Posted 10 July 2018 - 01:13 AM

The RC's are claimed to have dielectric mirror coatings. If that's true will these have the dielectric coatings? I'm interested simply from a durability standpoint.



#69 Astronomics

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Posted 10 July 2018 - 09:40 AM

Dielectric coatings are no longer available on the RC scopes.  They are now enhanced with 96% reflectivity.  The Cassegrains will be the same coatings.



#70 Bill Barlow

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Posted 10 July 2018 - 10:40 AM

I see the TS version shows 99% reflectivity on the mirrors.  Would think GSO would only offer one type of coating.

 

Bill



#71 tonyt

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Posted 10 July 2018 - 07:41 PM

Dielectric coatings are no longer available on the RC scopes.  They are now enhanced with 96% reflectivity.  The Cassegrains will be the same coatings.

Thanks for the clarification. Do you know if they changed coatings due to technical difficulty of applying the dielectric coating to the curved surface?  



#72 Astronomics

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Posted 10 July 2018 - 08:22 PM

I see the TS version shows 99% reflectivity on the mirrors. Would think GSO would only offer one type of coating.

Bill


They have not updated their information. It is indeed 96% reflective according to the owner of GSO.
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#73 Astronomics

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Posted 10 July 2018 - 08:22 PM

Thanks for the clarification. Do you know if they changed coatings due to technical difficulty of applying the dielectric coating to the curved surface?


I think it was cost related really.

#74 Achernar

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Posted 11 July 2018 - 08:41 AM

TG, on 08 Jul 2018 - 8:11 PM, said:

This is not accurate. Classical Cassegrains have coma but not as bad as the D-Ks which have about the same coma as an (uncorrected) SCT.

At f/12 and up I would not expect coma to be a problem at all since the true field is not going to be wide enough to show it.

Tanveer

Then why was the 200-inch Hale telescope the last professinal observatory telescope to be built as Classical Cassegrain and virtually every other observatory and space telecsope built as a Ritchey-Cheretin? The point is visually the views are okay, photographically the coma is a problem. The argument between Hale and Ritchey started anf ultimately led to them becommimg bitter enemies had apparently not gone away.

Taras
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#75 Axunator

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Posted 11 July 2018 - 09:07 AM

Then why was the 200-inch Hale telescope the last professinal observatory telescope to be built as Classical Cassegrain and virtually every other observatory and space telecsope built as a Ritchey-Cheretin? The point is visually the views are okay, photographically the coma is a problem. The argument between Hale and Ritchey started anf ultimately led to them becommimg bitter enemies had apparently not gone away.

The main reason is that Ritchey-Chrétien exchanges off-axis coma to off-axis astigmatism, which is less problematic for scientific astrometry (the central point of astigmatic star can still be measured accurately, unlike that of comatic star). The highly curved, astigmatic field of fast RCs can also be corrected for astrophotography by a relatively simple field flattener.

 

However, since we are talking about amateur scopes, it's worth mentioning that classical Cassegrains have approximately same amount of coma as Newtonians of the same focal ratio. I.e. an f/12 Cassegrain should have about same diameter of diffraction-limited coma-free field as an f/12 Newtonian. That's pretty good, IMO. And much, much better than Dall-Kirkhams or conventional (non-Edge, non-ACF) SCTs...

 

P.S. No need to get any bitter enemies over these matters, it's a hobby for us for god's sake


Edited by Axunator, 11 July 2018 - 09:21 AM.

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