Hodgepodge: The Mishmash Mount

I have previously published a description of Hodgepodge, the mish-mash mount I used for the solar eclipse. While I wrote about the mount, and posted some photos, I did not really cover the construction. Now for some details…

Hodgepodge and the Eclipse
Hodgepodge set up at Grant’s Spring to photograph a total solar eclipse

A few months back I rebuilt the drive unit of an old Celestron mount, installing a drive corrector into the base unit in the process. I have used this sidereal drive for time lapse, but it can also be used to mount a telescope again. Better yet, with a few minutes reconfiguration, it can do either.

With the solar eclipse looming on the calendar I realized I needed a tracking mount to allow photography. Tracking would allow me to keep the Sun and Moon in the field without wasting precious seconds framing the image during the eclipse. It would also enable longer exposures without motion blurring the image.

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Oregon Star Party 2017

Due to lucky happenstance the location for Oregon Star Party, the same location used for decades, was within the path of totality for the 2017 total solar eclipse. This provided an opportunity to both attend the star party again, and to view the eclipse.

An assortment of telescopes wait out the day at oregon Star Party 2017
An assortment of telescopes wait out the day at oregon Star Party 2017
I do enjoy the large star parties, something we do not have on the island. I had attended OSP a few years ago, the eclipse made the opportunity to attend once again very tempting.

Registration for the star party was an issue. Due to the eclipse attendance was going to be very good, so good that registration was closed within two hours of opening! I got the announcement email, then waited until I got out of a meeting to register, only to find out I was too late! I put my name on the waiting list and hoped.

With a month to go I received word that my waiting list position was opened for registration. By this time my family already had plans to camp in the Ochoco Mts. for the eclipse, no reason not to do both!

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In the end it is a Celestron C8 telescope drive and forks containing custom electronics, a Meade heavy duty wedge from an LX-90, a hand-made tripod, holding a Televue 76mm APO telescope, using a Vixen style dovetail base, with a Baader film solar filter.

It is tempting to call it Frankenscope after the similarities with the classic monster.

Hodgepodge setup on the side of Mauna Kea with the TV-76mm and Telrad on the plate

To further add the the Mary Shelley plot similarities, much of this was revived from the junk pile. I very nearly decided to toss the drive and fork, they were that bad, peeling paint and rusty bolts. A great deal of work was necessary to re-animate these components.

The wedge came from a telescope that was killed in an unfortunate incident with an aquarium heater. The heater was used to de-humidify the OTA and prevent fungus on the optics. Note: Aquarium heaters are not made to operate out of water.

The Hodgepodge mount assembled for the first time

Several new aluminum parts were machined from scraps, some of which were scavenged when the observatory shop was being cleaned out and a lot of metal stock was tossed.

Many of the electronic components used to build the drive corrector were also scavenged parts from dead electronics, this includes the 1.8432MHz crystal that forms it’s beating heart. This heartbeat keeps the mount turning at exactly sidereal rate.

Yeah, you could call it Frankenscope.

But I will call it Hodgepodge.

Old School Drive Corrector

Telescope clock drives from the 1980’s or earlier often used AC synchronous motors. These commonly available AC motors are used to power timeclocks, record player turntables, and telescopes, anyplace a motor needed to run at a very accurate speed.

Sidereal Drive Platform
A Celestron telescope drive configured as a sidereal rate tracking camera platform

The speed of a synchronous motor is set by the frequency of the powerline, in North America and many other places this is 60Hz. As the frequency must be synchronized for every power station on the grid the frequency is quite accurate, a feature exploited by clockmakers and telescope builders. Once found everywhere these motors are less common, but are still around.

Drive Correctors

It was the common use of these motors in telescope drives that led to the invention of the drive corrector, a device that was once a required piece of kit for serious amateur astronomers. Drive correctors like this were needed when operating from a battery at some remote location, generating AC from a 12Vdc car battery.

You also needed a drive corrector for guiding while doing astrophotography. The corrector could speed up or slow down the telescope drive a bit to correct the telescope drive speed and stay on target, something not possible with the fixed 60Hz of the mains supply. Thus the term drive corrector.

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ATM Gallery

The Dobsonian Telescope

In restoring a 20″ Obsession telescope I found myself pulling a book from the shelf that had not been opened in a while. David Kriege and Richard Berry’s The Dobsonian Telescope – A Practical manual for Building Large Aperture Telescopes is a book I once read cover to cover.

The Dobsonian Telescope
My copy of The Dobsonian Telescope by David Kriege and Richard Berry
The information here was critical in the success of my building Deep Violet, my 18″ telescope. Within the pages of this book are plans and drawings of the important bits as well as detailed discussions of what does, and does not work, when building a telescope.

The Dobsonian Telescope is the primary reference for those building large amateur telescopes. This book, along with the design revolution that went with it, put large telescopes in the hands of countless amateur astronomers. These telescopes extended the capabilities of amateur observers immensely, allowing spectacular views of deep space objects that were only fuzzy smears in the eyepiece before. Want to see the spiral arms of galaxies? A 20″ telescope can do that!

As I perused my well thumbed copy I was surprised to find bits of my own telescope plans used as bookmarks. There was dust on the top of the pages, but I still remembered where I disagreed with Kriege in the dimensions of the mirror box, or how to place the truss tube clamps. I may have deviated from the plans shown here in some aspects, but in other parts of the design I directly used the dimensions shown in the book.

So many old memories, good memories. Pursuing an art that has been around for four centuries, combining bits of wood and glass to make an instrument that can reveal our universe. Sure you can simply buy a very good modern telescope. But it is hard to overstate the pleasure of building one yourself. This is an art that can still be done in a garage, with tools available at the local hardware store, with results that can rival or even surpass something purchased from a catalog.

Restoring an Obsession

Obsession Telescopes are something of a standard in the astronomy community. David Kriege was one of the first to start building truss tube dobsonian telescopes commercially, bringing portable large aperture telescopes to the astronomy community. These telescopes were a bit of a revolution in the pursuit, with sizes unreachable only a decade before, when a 10″ or 12″ telescope was considered big. When I built my 18″ it is David Kriege’s book I used for much of the design, following in the footsteps of so many amateur astronomers.

20" f/4 Obsession Telescope
The restored 20″ Obsession telescope set up at hale Pohaku on the side of Mauna Kea

A 20″ f/4 Obsession donated to the observatory has presented a challenge and an opportunity. The telescope was the prized possession of Bob Michael having been ordered new directly from Obsession. The telescope is serial number 004 with a manufacturing date of June 1st, 1990. As David started Obsession Telescope in 1989, this is a very early example of his work. For many years Bob and his wife used this telescope to observe, completing the Herschel 400 and other observing projects. Unfortunately he was forced to give up astronomy due to age and glaucoma, donating his equipment to the observatory.

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