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New AT 6" and 8" Classical Cassegrain

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#851 MennoB

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Posted 04 November 2019 - 09:04 AM

I think someone in the Netherlands has. I thought I red about it a while ago. I'll try to look it up.

 

Anybody tried the 10"?

 Looked it up, but saw it was just about purchasing one, sorry...



#852 davebl

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Posted 04 November 2019 - 11:06 AM

December issue of Sky & Telescope has a review of the 10".
Dave
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#853 MennoB

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Posted 04 November 2019 - 11:15 AM

Then again; you'd have a focal length of over 3 meters!



#854 quilty

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Posted 04 November 2019 - 02:03 PM

I doubt that. Got a better Idea now. Found out with a short achromate that the telescope's focus is quite where the eyepiece stops at the top of diagonal. My eyepieces seem to to so at least. Then the distance from telescope bottom to focus is 260 +-3 mm. Saw the moon for a split second. So the question is how to get the telescope's focal length from that. And what distance have other 6 inch casses.



#855 MennoB

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Posted 04 November 2019 - 02:41 PM

Sorry Quilty,
I was referring to the 10" truss version.

#856 macdonjh

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Posted 04 November 2019 - 03:59 PM

Wow!  That is quite a large change for such a small adjustment.  For what it’s worth I’ve experienced something similar on my 12” Cassegrain—it seems more sensitive to secondary position than one would expect.  I don’t know what the optical explanation for this could be.  Maybe because the curvature at the edges of the secondary is greater than the average?

 

JimC

Back focus for Cassegrains varies with the square of the secondary mirror's amplification.  So, for an f/12 system that has an f/4 primary and a secondary with amplification of 3x, every mm you change the separation between the primary and secondary mirrors changes the back focus by 9mm.

 

Something to keep in mind: any deviation from the mirror separation designed for the scope introduces spherical aberration.  If you've moved your mirrors closer together or farther apart than they were and you have soft images, now you know why.

 

I have been thinking of buying one of these scopes, but have been put off by the seemingly constant need to change spacers to get long and short focal length eyepieces to come to focus either in a 2” or 1.25” diagonal.  
 

As far as the brightness differences mentioned when comparing these CC scopes to others, at nearly same magnifications, the exit pupils should be nearly the same as well.  This would be a valid way to compare the brightness.  If the refractor is using a lower magnification eyepiece compared to the CC, then the image may be brighter in the somewhat smaller refractor or any other scope.  But sounds like the scopes perform pretty well otherwise.

 

Bill

Changing diagonals has a large impact on the amount of back focus you need.  A 2" diagonal has a 100mm optical path length, a 1-1/4" diagonal has a 50mm optical path length (if I remember correctly).  So if you have yo scope set up with a 2" diagonal and then substitute a 1-1/4" diagonal you'll likely need to also install a 2" spacer to make up for the differences in OPL.


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#857 quilty

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Posted 05 November 2019 - 04:04 AM

sorry Menno, you're right. But I'm sill interested in what backfocus lengths other 6"-casses have. And how to get the telescope's true focal length from that. And, this is the point where I joined, work the 8 inch and 10 inch casses at their full or a reduced size as my 6 inch does, for it is a true 5.5 inch cas. 

And Skylab, how do you know do you know a f/12 cas has an f/4 primary and a 3x secondary amplification. I think, the 6inch cas has an f/3.16 primary



#858 quilty

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Posted 13 November 2019 - 06:41 AM

"Something to keep in mind: any deviation from the mirror separation designed for the scope introduces spherical aberration.  If you've moved your mirrors closer together or farther apart than they were and you have soft images, now you know why."

 

macdonjh: Is that the whole truth? What about Maksutov or SC designs, where focus is adjusted by a primary shift by some mm or two thus changing the distance between primary and secundary? Can't imagine the principe is that different between the designs. Latest focal length reading at full moonlight: 25.8 +-3 mm distance from telescope's buttplate to focus at GSO 6inch Cass



#859 Axunator

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Posted 13 November 2019 - 06:56 AM

"Something to keep in mind: any deviation from the mirror separation designed for the scope introduces spherical aberration. If you've moved your mirrors closer together or farther apart than they were and you have soft images, now you know why."

macdonjh: Is that the whole truth? What about Maksutov or SC designs, where focus is adjusted by a primary shift by some mm or two thus changing the distance between primary and secundary? Can't imagine the principe is that different between the designs. Latest focal length reading at full moonlight: 25.8 +-3 mm distance from telescope's buttplate to focus at GSO 6inch Cass

It DOES change spherical correction in SCTs and MCTs as well. It is not super sensitive though (that’s why they get away with moving primary mirror design), but it does happen, and for best results one should aim for back focus distances that are not too far away from optimal design specs.

Edited by Axunator, 13 November 2019 - 06:57 AM.

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#860 Joe1950

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Posted 13 November 2019 - 10:03 AM

A big factor is the SCTs and MCTs use spherical primaries and secondaries. The SA correction is handled by the front corrector lens. There is a correction change as you get away from optimum measurements, but it is insignificant for a couple inches or so. 

 

I think that’s how it works, anyway. 


Edited by Joe1950, 13 November 2019 - 10:05 AM.

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#861 Peter Besenbruch

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Posted 13 November 2019 - 06:22 PM

I was debating between the 6” and 8”. But then I found this CC on eBay! 

Does that ever bring a lump to the throat.


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#862 quilty

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Posted 14 November 2019 - 08:30 AM

It DOES change spherical correction in SCTs and MCTs as well. It is not super sensitive though (that’s why they get away with moving primary mirror design), but it does happen, and for best results one should aim for back focus distances that are not too far away from optimal design specs.

 

Does this apply just to off-axis imaging or to imaging power on the optical axis too? May I remind you that there are DKs, classical CCs, and RCs  with spherical, sub-parabolic, parabolic, hyperbolic and further devices with ellipsoid mirrors out there all of which claim to provide optimum spherical correction. And some of them claim to do better on the axis others off-axis. Maybe a mirror distance shift changes the optimum imaging area from axis to the rim or to opposite direction. Which mirror shape is most sensitive to distance shift if there's one?



#863 Axunator

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Posted 14 November 2019 - 03:36 PM

It DOES change spherical correction in SCTs and MCTs as well. It is not super sensitive though (that’s why they get away with moving primary mirror design), but it does happen, and for best results one should aim for back focus distances that are not too far away from optimal design specs.

Does this apply just to off-axis imaging or to imaging power on the optical axis too? May I remind you that there are DKs, classical CCs, and RCs with spherical, sub-parabolic, parabolic, hyperbolic and further devices with ellipsoid mirrors out there all of which claim to provide optimum spherical correction. And some of them claim to do better on the axis others off-axis. Maybe a mirror distance shift changes the optimum imaging area from axis to the rim or to opposite direction. Which mirror shape is most sensitive to distance shift if there's one?

I can’t answer all your questions, but the answer to the main point: yes, it does affect spherical correction on axis as well. However, e.g. in C8, you have to extend the back focus (by moving the primary forward) almost up to 300 mm (i.e. a LOT further than the design optimal of appr. 100 mm of back focus), before the increased SA worsens the performance to be no longer diffraction limited on axis (this estimate is based on Ken Hutchinson’s ray trace analysis, and of course presumes an optically perfect specimen - in real life the limit may be reached earlier).

Edited by Axunator, 14 November 2019 - 03:39 PM.

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#864 luxo II

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Posted 14 November 2019 - 03:53 PM

...put off by the seemingly constant need to change spacers to get long and short focal length eyepieces to come to focus either in a 2” or 1.25” diagonal.

As one who has had 2 big MCT with fixed mirrors I can say that is not true - if the backfocus is properly adjusted, and your focuser has 25mm travel, this is sufficient. I use a 2” diagonal and 2” and 1.25” eyepieces from 38 to 5mm.

The one thing I have noticed though is that it does matter which type of eyepieces you are using. All of mine seem to be pretty nearly parfocal with a 2”/1.25” adapter ring in place on the smaller ones, alternatively the focuser will accomodate the difference.

But Ethos are set up for a focal plane in a completely different position which would require a long spacer. TBH I don’t use them anyway.

And as one who has actually adjusted the backfocus by 30mm it made absolutely no discernible difference to the correction - and this is in a scope that is corrected to 1/11 wave PV.

There are a lot of armchair theorists here. I am not one of them.

Edited by luxo II, 14 November 2019 - 03:59 PM.


#865 quilty

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Posted 15 November 2019 - 05:22 AM

me neither, but I'd like to understand the principles. Well, maybe we can agree that a mirror distance shift of some mm yielding an inch backfocus shift (roundabout) will not spoil the optics coniderably. I'm thinking about that no more to get a full 6 inch device for this turned out to be no solution but in order to get the heavy 2 inch stuff closer to the mount for two inches or so. That would mean mounting the secondary 3mm more outwards wich is easily done by just drilling four additional holes into the front ring which catch the four spider arms.

Neverheless, why have so many SC and Mak designs the primary shift design? Just to avoid long focuser travel or backfocus adjustments. The longer focal length the more need for movement on the axis as soon as the length of light path is altered. And when we want a small device with a long focal length an exceeding 2 inch focuser length would spoil the design.



#866 quilty

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Posted 15 November 2019 - 06:19 AM

another question, mennoB wrote:

 

Hi Quilty,

 

Yeah, about that first subject I understood from Mike as well. The exit pupil is one of the main things, if I understand correctly. About your correspondence with TS; smirk.gif . Yep, they aren't the manufacturers so it seems. Good thing the scopes perform well anyway!

 

Has anyone experience in addressing GSO directly? Did anyone get useful answers to such questions? I think a good provider should do and in case he's unable he should be able to tell who will do. Thought it was just a German disease, this beware-of-customer-thing. I'm waiting for an answer from GSO for weeks now. Some years ago I had a traumatic experience with the Samsung company which wouldn't provide anything but vain advertising. stuff.



#867 macdonjh

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Posted 15 November 2019 - 08:46 AM

I was debating between the 6” and 8”. But then I found this CC on eBay! 

 

 

Does that ever bring a lump to the throat.

Sure does.  I was looking at the ad, thinking, "Yeah, right, another fake or piece of junk" until I got to the end and saw who posted it.  


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#868 George Methvin

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Posted 15 November 2019 - 10:06 AM

So what I am taking away from all this is if you want a scope the does really well on planets then you might want to go with a nice 5 to 6 inch apo refractor or a 8 inch Classical Cassegrain.  The 8 inch Classical Cassegrain in side by side views with a 8 sct seem to give a some what better view of the planets but a some what dimmer view of deep skies objects, plus If size and weight are a factor then a 8 inch Sct is smaller and lighter then the 8 inch Classical Cassegrain so you may need to look into getting a bigger mount if you go the 8 Classical Cassegrain route, seem there's always a trade off lol.   Now as far as price goes that's a matter for each person pocket book if you are rich then you get what you want if not rich then there are compromise to be made. All in all these scope are good at what they seem to be made for it up to each person to decide what they want or feel they need. Clear skies and happy shopping.


Edited by George Methvin, 15 November 2019 - 04:32 PM.


#869 Peter Besenbruch

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Posted 16 November 2019 - 03:45 PM

So what I am taking away from all this is if you want a scope the does really well on planets then you might want to go with a nice 5 to 6 inch apo refractor or a 8 inch Classical Cassegrain.

I'd throw in a 7" Maksutov. The good part is if you did throw it in there, it would stay in collimation. ;)


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#870 Dzukielis

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Posted 23 November 2019 - 09:35 AM

 Very nice photos and review of  8"   is going on here:

 

https://stargazerslo...2032436-mm-ota/

 

 This scope is definetly number one for planetary visual work   to add up to my  TS 6"  refractor.


Edited by Dzukielis, 23 November 2019 - 09:35 AM.

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