Tag Archives: cassegrain

12-inch Ealing-Made Ritchey-Chretien Telescope for Sale

20 Friday Sep 2019

Posted by in flat, Hopewell Observatorry, optical flat, Optics, Telescope Making

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We have a 12-inch Casssegrain optical telescope assembly for sale at an extremely attractive price: just two hundred dollars (or any reasonable offer). You pay for shipping.

The full-thickness primary mirror alone is worth much more than that as a raw piece of unfinished Pyrex! (United Lens charges $450 for an equivalent, 12.5″ diameter, roughly 2″ thick, raw, unfigured, disk of Borofloat!)

The telescope was part of a package (mount-cum-telescope) that was purchased from the Ealing company back in the 1960s by the University of Maryland. The scope itself never gave satisfactory images, so the UMd observatory sold it off in the early 1990s, and it ended up at the Hopewell Observatory about a decade before I became a member. Hopewell kept the mount, which still works quite well, but removed the telescope and replaced it with a 14-inch Celestron Schmidt-Cassegrain.

I recently examined the telescope itself (the one we are selling) and found that it indeed has a hyperbolic primary with a focal length of about 4 feet (so it’s f/4). Presumably, the convex secondary is also a matching hyperboloid, to create a Ritchey-Chretien design, but I don’t feel like perforating a large spherical mirror to create a Hindle sphere to test it properly. In any case, using a 12-inch flat, I was unable to produce decent Ronchi images.

As you may know, figuring and collimating a Richey-Chretien require a LOT of patience, more than I have. My suggestion would be to refigure the primary into a paraboloid, procure a standard flat, elliptical diagonal, and repurpose this as a Newtonian. Refiguring this mirror a task that I don’t feel like taking on, since our observatory already has a 14″ Newtonian, a 14″ SCT, and I already have built a 12.5″ Newtonian of my own. Plus, I am finding that figuring a 16.5″ thin mirror is plenty of work already.

So, our loss could be your gain! Make an offer!

I attach a bunch of photos of the OTA from several viewpoints, including a ronchigram. The mirror has been cleaned off since these picture were made; the little electronic motor was for remote focusing of the secondary.

 

Difficulties in Using the Matching Ronchi Test on a 12″ Cassegrain Mirror

08 Saturday Sep 2018

Posted by in astronomy, flat, Hopewell Observatorry, optical flat, Optics, Telescope Making

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The other regulars and I at the DC ATM group at the CCCC have been trying to test a 12 inch Cassegrain mirror and telescope manufactured nearly 50 years ago by a company called Ealing and currently owned by the Hopewell Observatory, of which I am a member. It hasn’t been easy. I discussed this earlier on Cloudy Nights.

Reports from several people, including Gary Hand and the late Bob Bolster, indicated that the optics on this mirror weren’t good at all. Apparently the folks at the University of Maryland’s observatory were sufficiently unhappy with it that they either sold it or gave it to National Capital Astronomers, a local astronomy club, who in turn gave it or sold it to Hopewell Observatory.

With a plain-vanilla Ronchi test, we could see that the mirror was very smooth and continuous, with no turned edge, astigmatism, or bad zones. With the Foucault/Couder zonal test (aka “Foucault” test) , I got results indicating that it was seriously overcorrected: some sort of hyperboloid, rather than the standard paraboloid characteristic of classical Cassegrain telescopes, which have a parabolic primary mirror and a hyperbolic secondary mirror.

However, I have begun losing my faith in my zonal readings, because they often seem to give results that are way out of whack compared to other testing methods.

So we decided to do some additional tests: the Double-Pass Auto-Collimation (DPACT) test used by Dick Parker, as well as the Matching Ronchi test (MRT).

The DPACT is very fiddly and exacting in its setup. We used (and modified) the setup lent to us by Jim Crowley and illustrated by him at his Astro Bananas website. Our results seem to show that the mirror is in fact NOT parabolic, rather, overcorrected, which confirms my Foucault measurements. If it were a perfect paraboloid, then the ronchi lines would be perfectly straight, but they definitely are NOT: they curve one way when inside the focal point, and curve the other when the tester is outside the focal point.

We also tested the entire setup on a radio tower that was about half a mile (~1km) distant. We found that the images were somewhat blurry no matter what we did.

We also attempted the MRT on the same mirror. However, requires very accurate photography and cutting-and-pasting skills in some sort of graphics programs. What you are inspecting is the curvature of the Ronchi lines. Here is the result that Alan T and I got last night:

matching ronchi for 12 inch cass

In black is the ideal ronchigram for this mirror, according to Mel Bartels’ website. The colored picture is the one we made with either my cell phone or the device I finished making earlier this week, shown in my previous post. Here are the two images, separated rather than superimposed:

IMG_1337

ideal ronchigram 12 inch cass ealing

The mirror’s focal length is 47.5″ and the grating has 100 lines per inch, shown somewhat outside of the radius of curvature. The little ‘eyelash’ on the lower left is simply a stray wire that was in the way, and doesn’t affect the image at all. The big hole in the middle is there because the mirror is a cassegrain.

I don’t know about you, but I don’t really see any differences between the real mirror and the theoretical mirror. Do you?

Conclusion

So, what does this all mean?

  • One possibility is that the mirror is in fact perfectly parabolic (as apparently shown by the MRT, but contrary to what I found with Foucault and DPACT) but there is something wrong with the convex, hyperbolic secondary.
  • Another possibility is that the mirror is in fact NOT parabolic, but hyperbolic, as shown by both my Foucault measurements and the DPACT (and contrary to the MRT), which would mean that this telescope was in fact closer to a Ritchey-Chretien; however, since it was marketed as a classical Cassegrain, then the (supposedly) hyperbolic secondary was in fact not tuned correctly to the primary.
  • The answer is left as an exercise for the reader.
  • A star test would be the best answer, but that would require being able to see a star. That hasn’t happened in these parts for quite some time. In addition, it would require an eyepiece holder and a mount of some sort. Or else setting up an indoor star…

Calculations with a Curious Cassegrain

08 Sunday Oct 2017

Posted by in astronomy, flat, Hopewell Observatorry, optical flat, Optics, Telescope Making

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I continue to try to determine the foci of the apparent hyperbolic primary on the Hopewell Ealing 12inch cassegrain, which has serious optical problems.

My two given pieces of information are that the mirror has a radius of curvature (R) of 95 inches by my direct measurement, and its Schwarzschild constant of best fit,(generally indicated by the letter K)  according to FigureXP using my six sets of Couder-mask Foucault readings, is -1.112.

I prefer to use the letter p, which equals K + 1. Thus, p = -0.112. I decided R should be negative, that is, off to the left (I think), though I get the same results, essentially, if R is positive, just flipped left-and-right.

One can obtain the equation of any conic by using the formula

Y^2 – 2Rx + px^2 = 0.

When I plug in my values, I get

Y^2 + 190x -0.112x^2 = 0.

I then used ordinary completing-the-square techniques to find the values of a, b, and c when putting this equation in standard form, that is something like y^2/a^2 – x^2/b^2 = 1

Omitting some of the steps because they are a pain to type, and rounding large values on this paper to the nearest integer (but not in my calculator), I get

I got

y^2 – 0.112(x – 848)^2 = – 80540

and eventually

(x – 848)^2 / 848^2 – y^2 / 248^2 = 1

Which means that a is 848 inches, which is over 70 feet, and b is 284 inches, or almost 24 feet. Since a^2 + b^2 = c^2, then c is about 894. And the focal points are 894 inches from the center of the double-knapped hyperboloid, which is located at (848, 0), so it looks a lot like this:

cass equations

Which of the two naps of this conic section is the location of the actual mirror? I suppose it doesn’t make a big difference.

Making that assumption that means that the foci of this hyperbolic mirror are about 894 – 848 = 46 inches from the center of the primary mirror. I don’t have the exact measurement from the center of the primary to the center of the secondary, but this at least gives me a start. That measurement will need to be made very, very carefully and the location of the secondary checked in three dimensions so that the ronchi lines are as straight as possible.

It certainly does not look like the common focal point for the primary and secondary will be very far behind the front of the secondary!

Bob Bolster gave me an EXTREMELY fast spherical mirror that is about f/0.9 and has diameter 6 inches. I didn’t think at first that would be useful for doing a Hindle sphere test, since I thought that the focal point in back of the secondary would be farther away. But now I think it will probably work after all. (Excellent job as usual, Bob!) (I think)