16" Jones-Bird

In January 2009, a new telescope was installed at the Broemmelsiek park astronomy center in St Charles Missouri. The optical tube is a 16" F5 using a Jones-Bird optical configuration on a computer driven Alt/Az mount. This page documents a bit of the history of this scope.

One thing that makes this scope particularly unique is the short length of the optical tube for a 16" F5 telescope.

The length of a normal 16" f5 using a standard Newtonian configuration will be on the order of 6 feet plus. When mounted in an Alt/Az mount, the eyepiece will be nearly 6 feet off the ground when pointed near the Zenith. This optical tube is slightly more than 4' 8" and the eyepiece is only 5' 6" off the ground when pointed straight up.

In March 2010, I used this scope to complete a Messier Marathon. I was able to see 106 of the 110 objects with no problem at all (the remaining 4 were lost in early morning twilight and city skyglow to the East). This could not have been done with a smaller scope as the astronomy center is located in a suburban area with moderate light pollution.

From October 2010 to June 2011, I used this scope to complete the Astronomical Leagues' Herschell 400 club. 380 objects were observed with this scope under moderate to heavy light polluted skies (the remaining 20 were observed with a 7" newtonian scope so all 400 objects in this list are easily within reach of this scope). Certificate #460 was awarded on July 6th 2011. The observing log can be found at this link: "H400 Log extra b.pdf ".

2014 Update

A major overhaul and refurbishment of this scope took place between 2013 and 2014. That effort is detailed at this link.

History

Original Johnsonian intent

In the early 60's, St Louis Astronomical Society (SLAS) member Herb Sassenberg set out to build a 16 inch telescope using a Johnsonian design. This design used two mirrors with an elliptical mirror opposite the focuser:

The primary was completed but the secondary was broken during the final phases of polishing.

Buchroeder configuration

Following the accident, Herb decided to change the design to a Buchroeder. This configuration is much like a Cassegrain but without the forward corrector and with the addition of extra optics following the secondary:

(This schematic shows the correcting optics too close to the secondary. In reality, the optics are located within the central hole of the primary.)

This is the reason why the primary in the resulting telescope has a hole in the center.

Final Jones-Bird design

At some point, Herb changed the design again. This time he settled on the Jones configuration.

Initially conceived by Jones in 1957, the optical design uses all spherical elements. The use of lenses between the primary mirror and the diagonal eliminates the spherical aberrations from the mirror as well as acting as a low power Barlow. Typical mirror F ratios are on the order of 2 and 3. The lenses in the diagonal holder effectively raises the f ratio to 5 and 6. The resulting tube length is much shorter than an equivalent pure Newtonian design

Refinements by Bird results in the elimination of nearly all chromatic aberrations.

Herb never finished his scope but the primary was obtained by SLAS member Jim Melka. Jim finalized and obtained the corrector optics. He worked with Bill Davis to complete the optical tube assembly and mounting collar.

Design correspondence

A few letters between Bill Davis and Robert Cox.

The original Spider did use a radial wire system but was replaced when frequent vibration induced oscillations occurred. In conversations with Bill Davis, it appears that the oscillations were at a natural frequency determined by the length and tension of the wires with the magnitude and susceptibility determined by the larger weight of the secondary assembly.

Bill indicates that this oscillation seemed to be omnipresent and never appeared to dampen out. In retrospect, he indicates that a radial system of this nature is naturally vulnerable to torque induced oscillations. In fact, a system like this will convert any mechanical vibration into radial oscillation.

This would make an excellent project for a mechanical engineer using modern day analysis tools.

Subsequent applications of the wire spider method in other telescopes confirmed Bob Coxs' recommendations not to use a radial approach. Furthermore, the use of very thin music wire results in no detectable diffraction spikes.

An example of a successful non-radial wire spider

(I'm told that several weeks after heeding Bills' advice, Bob's teeth returned to their natural white state.)

Completed Optical Tube assembly

As Initially completed and mounted on a temporary fixture.

Bill Davis: the mechanical engineer who designed and built the OTA and mounting collar.

Completion of full up telescope

In the early 2000's, Jim Roe from the Astronomical Society of Eastern Missouri (ASEM) acquired the OTA. Following a grant from the Enterprise Leasing corporation, he had Bob Kirschenman complete the mounting of the OTA to a computerized base. The completed telescope was initially tested and located in Moscow Mills Missouri before it was relocated to its' current home at Broemmelseik park in St Charles County Missouri. It is currently used for public outreach events as well as by members of ASEM.

The business end showing the diagonal & optics holder:

In September, a solar panel was added to keep the battery charged. Another feather in the cap: The Jones-Bird has gone green.

Performance

Obviously, on site performance will be subjective in nature. Personally, I think this scope excels in two areas:

1] Low magnification, deep sky work.

2] Accessibility to eyepiece at all angles > 10 degrees from the horizontal. Consider that a normal 16" F5 Newtonian (like the Meade series) puts the eyepiece more than six feet up in the air when pointed to the zenith! This scope puts it right at five feet six inches. Perfect for regular use during long observing sessions. Not to mention usage during outreach events.

It might also be a very excellent scope for planetary work but the views I have seen through it at high magnification were a bit soft for my tastes. This may be due to the normally soft seeing we experience here in Missouri.

Jim Roe has performed a few digital imaging tests using this scope and his images were very good for short exposure stacks without using flats or other advanced imaging techniques.

Alignment

Please go to this link for initial details on optically aligning this telescope.

Info request

I would be pleased to hear from anyone with information on this design - implemented or otherwise. I can be contacted here (grantmartin2000 at Hotmail dot com)

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