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Brain fart...what’s the configuration called? Newtonian where the light goes through a hole in the primary?

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#1 MiguelStrongEye

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Posted 22 April 2019 - 09:43 PM

Yeah...

what’s it called where a Newt is set up with a flat like a Cassegrain?


Edited by MiguelStrongEye, 22 April 2019 - 09:51 PM.


#2 Keith Rivich

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Posted 22 April 2019 - 10:02 PM

Classical Cassegrain maybe?


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#3 MiguelStrongEye

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Posted 22 April 2019 - 10:27 PM

Classical Cassegrain maybe?

That’s got the convex secondary...



#4 Richard O'Neill

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Posted 22 April 2019 - 11:40 PM

folded optical system



#5 JoeInMN

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Posted 23 April 2019 - 12:12 AM

Does any Cassegrain type have a flat secondary? The secondary would have to be enormous.

 

There's also the Gregorian, which has a concave secondary.


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#6 Kipper-Feet

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Posted 23 April 2019 - 12:22 AM

MiguelStrongEye, kindly check your Personal Messenger Inbox.



#7 MiguelStrongEye

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Posted 23 April 2019 - 12:22 AM

There is a Newtonian with a flat secondary piped down the center...can’t remember the name though. Like a gregorian or a classical cass but with a flat secondary.


Edited by MiguelStrongEye, 23 April 2019 - 12:33 AM.


#8 luxo II

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Posted 23 April 2019 - 05:05 AM

Well it would just be a folded newtonian - this assumes a primary mirror with a very long f-ratio so that it can be tilted slightly to direct the reflected beam off-axis to a folding flat without introducing too much coma, the result being a scope with a long focal length folded into a box roughly half that length. 

 

if you think more carefully about that configuration it somewhat resembles a cassegrain chopped in half down the centre, or a folded refractor (Unitron made one but it wasn't popular).

 

This then leads to other unobstructed off-axis variations such as the two-mirror and 3-mirror Schiefspieglers and Yolo.

 

Or are you perhaps thinking of a Nasmyth cassegrain, or possibly the Loveday folded newtonian which is a special case...


Edited by luxo II, 23 April 2019 - 05:17 AM.


#9 25585

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Posted 23 April 2019 - 08:24 AM

https://www.oldhamop...om/astronomical


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#10 JoeInMN

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Posted 23 April 2019 - 10:05 AM

As I visualize it, a flat secondary in a Cassegrain-type system of any f/ratio would have to be huge... In order to put the focus out through the hole in the primary to where an eyepiece or camera could reach it, it would have to be located a bit less than halfway up the image path, and be slightly more than half the diameter of the primary. The hole would be a bit bigger as well. I don't see that it would work well at all.



#11 MiguelStrongEye

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Posted 23 April 2019 - 11:08 AM

As I visualize it, a flat secondary in a Cassegrain-type system of any f/ratio would have to be huge... In order to put the focus out through the hole in the primary to where an eyepiece or camera could reach it, it would have to be located a bit less than halfway up the image path, and be slightly more than half the diameter of the primary. The hole would be a bit bigger as well. I don't see that it would work well at all.

The advantage would be in resolving power...You’d lose ligh but gain diameter.

that said, since they aren’t built, it clearly doesn’t work.



#12 Sean Cunneen

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Posted 23 April 2019 - 04:03 PM

Are you thinking of Coude? Nasmythe?
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#13 macdonjh

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Posted 23 April 2019 - 07:03 PM

I have read about one amateur- owned, large aperture Newtonian that uses a large flat secondary to "feed" a diagonal tertiary mirror in a Naysmyth configuration so the focal plane is at "standing height" instead of ladder height. Apparently the owner hated ladders more than gigantic secondary mirrors.

#14 25585

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Posted 24 April 2019 - 06:20 AM

I have read about one amateur- owned, large aperture Newtonian that uses a large flat secondary to "feed" a diagonal tertiary mirror in a Naysmyth configuration so the focal plane is at "standing height" instead of ladder height. Apparently the owner hated ladders more than gigantic secondary mirrors.

 The Nasmyth maybe, or a Folded Newtonian. See my link.



#15 Jon Isaacs

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Posted 24 April 2019 - 08:20 AM

Yeah...

what’s it called where a Newt is set up with a flat like a Cassegrain?

 

I think it's called a Cassegrain.  The Cassegrain focus is through the primary mirror, the Newtonian focus is at 90 degrees to the optical axis of the primary.

 

Jon



#16 macdonjh

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Posted 24 April 2019 - 08:23 AM

 The Nasmyth maybe, or a Folded Newtonian. See my link.

Perhaps, I am traveling and on my phone, so I didn't follow you link.



#17 KLWalsh

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Posted 28 April 2019 - 10:11 PM

There have been large telescopes like this built.
They are ‘astrometric reflector’ telescopes.

The reason for this design is that, traditionally, astrometry (precise measurement of star positions to study parallax, for ex) has been done with large refractors. Other reflector designs had too many optical and mechanical variables that complicated the process of astrometry. The stiffness and simplicity of the design - though requiring a huge, flat, secondary mirror - allowed for telescopes as large, or larger, than the 40 inch Yerkes refractor to be built, at a reasonable cost.

Here’s a link to an article about one:
https://www.scienced...083665666900158
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#18 izar187

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Posted 29 April 2019 - 10:00 AM

Parks sold an either/or newt/cassegrain, I think.

It was an either/or, with two interchangeable secondaries.

Do not know the shape of the non-newt one.

 

These:

http://www.parksopti...Superior System


Edited by izar187, 30 April 2019 - 05:22 AM.


#19 luxo II

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Posted 29 April 2019 - 11:07 PM

Miguel, maybe this crowd is what you were thinking of ? For seriously big scopes they used a big tilted flat secondary to put the focal plane in a safe and accessible location .

https://www.optiques...com/telescopes/

Edited by luxo II, 29 April 2019 - 11:08 PM.


#20 Simon B

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Posted 30 April 2019 - 04:26 AM

Dilworth Relay?



#21 scopeboy42

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Posted 01 May 2019 - 08:03 AM

Parks sold an either/or newt/cassegrain, I think.

It was an either/or, with two interchangeable secondaries.

Do not know the shape of the non-newt one.

 

These:

http://www.parksopti...Superior System

Here is another link to the Parks Optical H.I.T. Series. Here it is referred to as a Classical Cassegrain.

 

http://www.parksopti.../site/hit.htm#1



#22 macdonjh

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Posted 01 May 2019 - 01:13 PM

Parks sold an either/or newt/cassegrain, I think.

It was an either/or, with two interchangeable secondaries.

Do not know the shape of the non-newt one.

 

These:

http://www.parksopti...Superior System

Parks did, I have a 10" Parks HIT.

 

Here is another link to the Parks Optical H.I.T. Series. Here it is referred to as a Classical Cassegrain.

 

http://www.parksopti.../site/hit.htm#1

When I get home, I'll post a photo of mine.  They were very similar to the Takahashi CN-212 and came in 10", 12" and 16" OTAs (I think).  The scopes have a paraboiloidal primary mirror that is cored, and came with both flat 45o and round convex hyperboloidal secondary mirrors.  With the flat secondary the scope is an f/4 Newtonian; with the hyperboloidal secondary the scope is a f/15 classical Cassegrain.  It takes three minutes to switch secondary mirrors.  Because of the two configurations, the scope has two focusers.  Parks scopes were already built heavy, but the HIT are really heavy.  My 10" weighs something like fifty-five pounds.  

 

When I have time to travel to star parties again it will be my travel scope.  It really is pretty cool to be able to switch from f/4 to f/15 in a couple of minutes: wide field one minute (1.8o TFOV with a 22mm Nagler) and high magnification the next (170x and 0.5o TFOV with the same 22mm Nagler).  The only fault I find in the design (it's too heavy, but I'll forgive that) is Parks should have made the OTA longer so smaller secondary mirrors could be used and the risers/ extension tubes for the focusers wouldn't have been necessary.  I have fantasies about rebuilding the scope one day.



#23 macdonjh

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Posted 01 May 2019 - 01:15 PM

OK, I just followed scopeboy42's link.  I don't think I ever saw the 6" or 8" versions of the HIT.  An awfully complicated design to offer in fairly small apertures.  I lusted after that 16", but it was HEAVY and expensive.



#24 macdonjh

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Posted 01 May 2019 - 04:12 PM

Here are some photos of a Parks 10" HIT in the wild.

 

Parks 1.JPG

 

Parks 2.JPG

 

One showing the Cassegrain focuser a little better.  If I ever rebuild the scope, I'd separate the mirrors a bit more, provided that wouldn't introduce too much spherical aberration), to reduce the length of the extension tube some (or eliminate it altogether).

Parks 4.JPG

 

One showing the Newtonian focuser a bit better.  Increasing mirror separation would also reduce the number of shims I'd need under this focuser.  If I got really lucky, I'd even be able to use a smaller secondary mirror.

Parks 3.JPG

 

But that is a project for a few years from now when the kids are grown and my wife doesn't want to talk to me.  Then I'll have a project for the garage where I can talk to myself and nobody will make fun of me.


Edited by macdonjh, 02 May 2019 - 01:11 PM.

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#25 siriusandthepup

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Posted 06 May 2019 - 11:03 PM

OK - is this what you were thinking? wsDK - weak secondary Dall Kirkham telescope

 

If you fold a Newt back through the primary with a flat you are automatically folding over half the focal length, which in turn means that you need a flat that is OVER 50% of the diameter of the primary. You can do that, but it is not pretty!

 

The wsDK is the better version. It compromises a weakly curved secondary to get a 34%, plus or minus, secondary size and utilizes a tertiary to give you the eyepiece height that allows viewing without a ladder. (And no hole required in the primary)


Edited by siriusandthepup, 06 May 2019 - 11:06 PM.



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