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#576 clamchip

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Posted 26 November 2018 - 11:32 PM

The Ritchey 24 inch was a very important telescope being the prototype for the Mt. Wilson

60 inch and 100 inch.

At the time G. Ritchey was the optician for the Yerkes Observatory and built the 24 inch with

a incredible for the time f/4 primary. The scope is Newtonian and Cassegrain focus.

It now resides in the Smithsonian.

Robert

https://commons.wiki...eyTelescope.jpg

http://adsabs.harvar...ApJ....14..217R

 

RitcheyTelescope.jpg


Edited by clamchip, 26 November 2018 - 11:43 PM.

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#577 clamchip

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Posted 27 November 2018 - 06:12 PM

I'm working on a telescope that also came out of the Yerkes Observatory, and you can see the

family resemblance.

The one I'm working on was built in the 1960's, parts of it may be older. The mirrors were ground

in the sixties.

8 inch, Cassegrain and Newtonian focus, and not quite finished.

I'll do a report on it soon.

Robert

 

IMG_8881.jpg

IMG_8882.jpg


Edited by clamchip, 27 November 2018 - 06:13 PM.

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#578 Jim Curry

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Posted 27 November 2018 - 07:55 PM

Has this been inspected by Mindy?


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#579 Garyth64

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Posted 27 November 2018 - 08:09 PM

I am looking forward to seeing more of this build!


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#580 clamchip

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Posted 28 November 2018 - 11:13 AM

I bought the scope from a retired University of Washington professor.

He did graduate studies at the Yerkes Observatory in the mid sixties and got his PhD in Astronomy.

He taught Astronomy at the UW before changing to research professor Atmospheric Sciences and held

this position until retiring from the UW.

The telescope he built in the Yerkes shops with the help and guidance of the observatory optician.

They ground and polished, tested, and aluminized the primary all at Yerkes.

The skeleton tube came from Yerkes.

The primary is figured parabolic and the secondary is to be figured hyperbolic but he ran out of time

and didn't finish it.

 

Robert


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#581 clamchip

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Posted 28 November 2018 - 04:23 PM

Here's the unfinished secondary, and next to it is a newt diagonal from my '79 Cave 8" f/7 I have

adapted to fit the secondary holder. I made new screws with hex heads because the hub will not

allow a screwdriver in to adjust these. And I made the 3/4" shaft for the hole in the hub.

Both of these have a 3/4" shaft you plug into the secondary hub and tighten a set screw.

The newt diagonal is 2", too big for my Cave anyway.

I have a feeling the Cass/Newt was intended, and actually The Professor mentioned it. I'm only

using existing holes and not modifying any of the original structure.

Eventually I will need to read up on figuring and testing a convex Hyperbolic, that should be 

interesting because that's a place I've never been and they don't accept American Express!

Robert 

 

IMG_8883.jpg

IMG_8879.jpg


Edited by clamchip, 28 November 2018 - 04:49 PM.

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#582 DAVIDG

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Posted 28 November 2018 - 04:36 PM

 To test and figure the convex hyperbolic secondary you need a Hindle sphere. http://mirrorworksho...phereBench.html

 

               - Dave 



#583 Garyth64

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Posted 28 November 2018 - 05:03 PM

Robert

I've reused my 8" Gregorian tube, to hold a 8" cass set I purchased a couple years ago.  Right now that cass mirror is in the tube, and I've been using it as an f/4.5 RFT.  That scope is my avatar.

 

But I laid out the optics to use both the diagonal, and the cass secondary.  I adjusted the housing of each so that all I have to do is take one out of the spider housing and replace it with the other.

 

This winter, I will be working on getting the cass secondary in place.


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#584 DAVIDG

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Posted 28 November 2018 - 05:19 PM

 You also need to calculate and install baffling on the primary and secondaries for the Cass configuration or the contrast will be very low.

 

                 - Dave 


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#585 Garyth64

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Posted 28 November 2018 - 05:35 PM

Yes, I've done that with the Gregorian, I just have to some tweeking for the Cass.

 

mirror cell 001b.jpg


Edited by Garyth64, 28 November 2018 - 05:43 PM.

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#586 DAVIDG

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Posted 28 November 2018 - 05:48 PM

 If I was making a Cass/Newt I would design the secondary system so it was magnetic and keyed with a simple safety catch/pin so the secondaries can be switched easily yet hold alignment. Also I would make sure that the spider is strong enough not to come out of alignment when you swap the system from one configuration to other. Both fast Newtonian and Cassegrains require critical collimation to work well so if every time you switched the configuration you have to break out the screwdrivers and laser collimator it won't get switched from one to the other very often. Poor mechanical design of the spider/secondary holders that require a fair amount of playing  around with them is the reason why most duel systems rarely get switched  back and forth. That and the fact that most Cassegrains secondaries are poorly figured so the image is not very  good so you find them being used as a Newtonian instead.

 

    Happy Holidays,

        - Dave 


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#587 clamchip

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Posted 28 November 2018 - 07:56 PM

The primary in the above telescope is a odd 7-5/8" diameter, it was a blank Yerkes had on hand.

I measured the focal length at 43.25" and with a diameter of 7.625" the focal ratio is f/5.67.

I'm hoping I can come up with a simpler way to grind and figure the secondary than the Hindle.

Although the Hindle gives very precise results.

There is very little curve on this secondary, and it will not focus so I guess I'm starting from scratch

with it.

I thought I read I can figure to a sphere and use the concave tool to interference test the convex

I didn't finish reading but I wish I could work with the tool to get the proper figure on the convex

if its even possible. Or the Norman test because I think the back of this secondary is polished.

Robert

 

IMG_8880.jpg

 

 


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#588 Garyth64

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Posted 28 November 2018 - 10:10 PM

 If I was making a Cass/Newt I would design the secondary system so it was magnetic and keyed with a simple safety catch/pin so the secondaries can be switched easily yet hold alignment. Also I would make sure that the spider is strong enough not to come out of alignment when you swap the system from one configuration to other. Both fast Newtonian and Cassegrains require critical collimation to work well so if every time you switched the configuration you have to break out the screwdrivers and laser collimator it won't get switched from one to the other very often. Poor mechanical design of the spider/secondary holders that require a fair amount of playing  around with them is the reason why most duel systems rarely get switched  back and forth. That and the fact that most Cassegrains secondaries are poorly figured so the image is not very  good so you find them being used as a Newtonian instead.

 

    Happy Holidays,

        - Dave 

Hopefully, that will  not be the case with mine.  I set mine up first as a newt  to see how the mirror is. So far so good.  Next will be to put the secondary in at the correct spacing.

My secondary holder is very solid:

 

2.25 diagonal.jpg

secondary holders.jpg


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#589 DAVIDG

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Posted 29 November 2018 - 09:52 AM

 You can make a concave test plate that needs to be of the exact same radius as the convex secondary, then figure the concave test plate to the exact hyperbolic figure as what is needed on the secondary. Then using that test plate figure the convex secondary to match.

   The tools you'll need are a  good sphereometer to measure the radius on the secondary and use it to match the concave radius on the test plate. An optical flat to zero the spherometer. An accurate method to test and figure the concave test plate to the hyperbolic figure and a monochrome light to test by contact interference the test plate against the secondary. All doable just a bit of work. 

 

              Happy Holidays,

                - Dave 



#590 clamchip

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Posted 29 November 2018 - 10:54 AM

I like that idea Dave, it sounds like just as much work as the Hindle method though.

Maybe instead of my first glass pushing project being a convex hyperbolic secondary I should

start with a small lens grinding kit like this one:

http://www.kenpress....lescopekit.html

Or I guess I could just go ahead and dive in.

 

Robert



#591 DAVIDG

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Posted 29 November 2018 - 11:34 AM

 Nice kit put it doesn't teach anything about testing. Your secondary is already ground and polished so what you need to understand is the figuring and testing part. 

   Did  you get any design specs on the optics ? The first step is measure the radius on the secondary. Once you know that, you can calculate the spacing needed and what the strength of the hyperbola you need. 

  By the way the reason why it most likely won't come to focus is that the spacing is wrong between the primary and secondary. So the focal plane is either too far in or out vs the travel of the focuser. I'm sure you know that the since the secondary magnifies, a small change in the position of it makes a big shift were the image comes to focus.

  You can use the Sun to find were the focal plane is located behind the primary and move the secondary so it is in the range of the focuser.  Point the scope at the Sun and  since the secondary is uncoated,  project  the image of the Sun  onto a card behind the focuser. If your worried that the light hitting secondary might be to intense then just stop down the primary with an off axis mask with a hole around 2" diameter. Then you can move the secondary so it is in range of the focuser. If you measure that spacing I can back calculate the radius on the secondary.  The back focal ie distance behind the primary for a Cassegrain is usually designed to be around 10" so you have enough to use a star diagonal.

 

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#592 clamchip

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Posted 29 November 2018 - 12:09 PM

The Telescope didn't come with any specs, and he could not remember how far along the secondary is.

Good idea using the sun to find the focal plane, I'll see if it focuses to a point, measure, and report back.

The secondary curve is very shallow by eye, it must not have much power, and I would not think it would

with the f/5.67 primary it has. It may be he didn't get very far with it, but it is polished so I think the grinding

stages are done.

 

Robert



#593 DAVIDG

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Posted 29 November 2018 - 12:21 PM

 Robert,

   Taking a guess that the secondary was meant to be 4x and the back distance behind the primary is 10" I get the spacing between the secondary and the focal plane to be  42.6"  and if the back distance was suppose to be 8",  I get 41" from the  secondary to  focal plane. So that give you a place to start.

   If you measure the edge thick and center thickness you can determine the sagitta of the curve and then calculate the radius. Not as accurate as spherometer but it should be close to give you an idea of what the design might be. 

 

          - Dave 


Edited by DAVIDG, 29 November 2018 - 02:25 PM.

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#594 Garyth64

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Posted 29 November 2018 - 02:11 PM

What is the f.l. of the primary?  43.25"?

 

p and p' can be figured out with a simple equation.  But before I give my results, I also want to double check the other parameters.

 

amp = 4x, and the BFL is 8"?

 

With those, I get a separation of the mirrors to be 33", were p = 10.25" and p' = 41"

 

p' = (f.l. - p + BFL).      p' also = 4p    so,   4p = (f.l. - p + BFL)

 

I think all those numbers work.


Edited by Garyth64, 29 November 2018 - 02:14 PM.


#595 DAVIDG

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Posted 29 November 2018 - 02:24 PM

 Oops Gary is correct I calculated p' which the distance from the focal plane to the secondary  So for 10" back focal length the spacing between the front surface of the primary to the secondary should be 32.6 and for 8" back focal length the spacing should be 33". Note how a slight change in spacing makes a large change in back focal length.  

 

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#596 Garyth64

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Posted 29 November 2018 - 02:49 PM

I believe that is what went wrong with my Gregorian.  I deviated from the initial parameters when I remounted the optics in the aluminum tube.  I adjusted the secondary to give me a lot more BFL than originally designed, and the image suffered.  My plan is to correct that some day after I fiddle with the Cass optics.


Edited by Garyth64, 29 November 2018 - 02:50 PM.


#597 clamchip

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Posted 29 November 2018 - 05:47 PM

Thanks for helping me out you guys.

I didn't have the luxury of a sun today but I did set a eyepiece about 10 inches from the aluminized

primary surface. I then moved the secondary through a range of mirror separations 29" to 31-1/4" allowed

by the holder. Mirror separation as received from the professor is 30-3/4".

I also moved the eyepiece through a range of back focus 8" to 14" from front surface of primary.

It will not focus, somethings wrong. I think I'll wait for the sun and try to focus it without eyepiece to a small

point, small enough to burn wood I hope!

Here's some pictures of 'Dem Bones:  

 

IMG_8891.jpg

IMG_8892.jpg

IMG_8888.jpg


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#598 clamchip

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Posted 29 November 2018 - 05:49 PM

I forgot to mention I added the focusers and misc hardware to mount the focusers. 

As purchased it was without focusers.

Robert

 

IMG_8893.jpg


Edited by clamchip, 29 November 2018 - 05:54 PM.

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#599 DAVIDG

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Posted 29 November 2018 - 06:06 PM

As most know in the Cassegrain family of scopes, the secondary has optical power so it position is critical in helping  to fully correct the system. If it is not in the position that it was designed to be at , then the image quality suffers. 

  You may not be able to see a sharp image because there are no baffles and with the secondary being uncoated the  image is very low in contrast. Also you have some where around at least 100" focal length if the secondary is at least 2x. So with even low power eyepiece you still have a good amount of power.  So a very large image of low contrast. 

I don't think your going to be able to burn anything with the Sun's image coming out of the cassegrain focal plane  The reason is that the secondary is uncoated so your losing about 95% of the light off of that surface  and with there being some magnification from the secondary the size of the solar image is about 1% of the focal length. So if the FL at the cassegrain focus is around 170",  the size of the Sun's disk is going to be close 1-3/4" id diameter. Not concentrate enough to light anything on fire. 

  By the way, the yellow color of the secondary means that it could be made from Cervit.  If you shine a laser into the secondary and you can see the beam inside the glass, like you can see the beam on a humid night then it is Cervit.

 

            - Dave 


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#600 clamchip

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Posted 29 November 2018 - 07:41 PM

I just remembered I have a Edmund eyepiece with a 2 inch focal length I can use for a low-power

look. And I still would like to focus the sun to see what's going on.

I shined a laser through the edge of the secondary and yes a can see the beam through the glass like car

head lights on a foggy night. I wasn't sure I understood you so I tried the same with Pyrex and the beam

is invisible.

 

Robert


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