Rebuilding a 12.5″ f/5 Truss Tube Dobsonian

12.5" Dobsonian
Some time ago my friend Bill Lofquist bought a dobsonian telescope from Roger Ceragioli. Roger had built the ‘scope to provide a home for a beautiful 12.5″ f/5 mirror he had made. The mirror is gorgeous, as is typical for Roger who is one of the best opticians I know. I have an APO triplet of Roger’s that is a prized possession.

Mechanically the scope had a few problems. The truss tubes were attached with separate hardware top and bottom, so that setup required over 20 minutes of sorting through screws and futzing with eight separate truss tubes waving around the whole time.

The elevation bearings had been set about 1/8th inch off from each other leading to a side to side twist when the scope was moved in elevation. This was not a major problem when using the scope visually but would make the use of digital setting circles impossible as DSC’s require orthogonal axis in the scope.

The ground board was a bit undersized, making the scope prone to tipping when used at low elevation.

The rebuilt scope is essentially finished with the usual tweaking and small adjustments remaining. Things are coming out very well and a few of the changes are worth passing along to the ATM community. In the sections below I will concentrate on practical details in hope of conveying some of the finer points in telescope making.

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Violet Haze, A 90mm f/13 Apochromat

Violet Haze
Violet Haze, a 90mm f/13 apochromatic telescope
I had wanted a high quality APO refractor for some time. Mostly for photographic use. Opportunity presented itself when Roger Ceragioli offered me a 90mm telescope he had finished the year before and was willing to sell. Working for the Steward Mirror Lab, Roger normally grinds very large optics, things like secondaries for six to eight meter telescopes. But as a hobby he makes somewhat smaller telescopes. This particular lens set had won him a merit award at RTMC in 2002. I had previously seen this telescope and after some negotiation we settled on a price.

The lens triplet is exquisite, providing absolutely perfect airy disks at high power. The photo below shows an example of the out of focus image of Antares taken with the telescope. Pulling out my copy of Suiter’s Star Testing Astronomical Telescopes shows nearly identical images for the ideal diffraction pattern. No wonder the ‘scope won a RTMC merit award.

Photographically it has proven to be almost perfectly free of color, corrected across the spectrum. There do not seem to be any detectable UV or IR halos around bright stars. This is partly a result of good design, and aided by the long focal length of f/13. No field flattener is required, with pinpoint stars across the focal plane.

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A Backyard Telescope Pier

Complete Pier
The completed pier with a telescope atop
Have you ever wanted to have a place to set up your scope easily in the backyard? with instant polar alignment? no tripod legs in the way? Even for someone with little handyman experience a pier is an easy weekend project that can be completed for around $60. Add the cost of a wedge for your scope, about $125-$400 new, less used, and you have a usable pier. A few bags of concrete, a little rebar and a sonotube will do the job. I know, we ATM’s usually use sonotube for telescope tubes, but this is what it is really meant to do, cast concrete.

A pier is also the first step in a real backyard observatory, build the pier first, then a building around it. The process shown here works for any pier and can be scaled as needed for larger scopes. The pier shown in the photos is intended to hold an eight inch SCT.

The plans and photos shown here have been used for several piers here in the Tucson area and have been refined with the experience. Feel free to improve on what is here, and if your idea works well send photos!

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Improved Rocker Box Pads

A small design detail in a dobsonian telescope is a method to restrain the mirror box in the center of the rocker box, to keep it from sliding side-to-side in the elevation bearings.

Rocker Box Pad
A standard carpet pad embedded into the wall of the rocker box
A common solution is to use a couple carpet pads to provide a lateral support that keeps the wood from rubbing. The pads do not add any friction that would keep the scope from tracking smoothly at high power. These pads are available in most hardware stores in both sheet form as well as pre-cut circles, usually one inch in diameter.

I have had trouble with these pads in Deep Violet. They do not stay put! Sometimes when inserting the heavy mirror box into the rocker I would catch a pad and simply shear it away from the wood. I ended up using a larger pad and using small wood screws to secure it instead of the adhesive.

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Every astronomer has a first telescope, mine is a 6″ f/5.1 Newtonian I first built as a teenager. As life progressed I was forced to dismantle the telescope and it dwelt for a time as a pile of parts in a box. Eventually I had an opportunity to rebuild the telescope, but as both my technical capability and my financial means had increased I was able to do a substantially better job.

Primero setup at TIMPA outside of Tucson
The result is the instrument you see to the right, Primero, or simply ‘first’ in Spanish. The original mirror was used but little else from that first telescope was reused. A completely new mount, a new tube and all new fittings. The only purchased parts are the focuser, the Telrad and the secondary mirror. Several parts of the mount were removed and reworked from a previous mount, this includes both the bearings, shafts, counterweight and saddle. The entire tripod, tube, mirror cell, spider and secondary holder were produced by hand for this scope.

The optical design of the telescope is standard Newtonian with a f/5.1 primary mirror of 6.0 inches giving a focal length of 777mm. The mirror was hand ground when I was a teenager and thanks to expert help during figuring is an excellent mirror.

The RFT design is deliberate and has proven to be a good choice, particularly with modern eyepieces that perform so well in short focal length scopes. With a 35mm Tele-Vue Panoptic eyepiece the scope provides a 22x image with just over a three degree field.

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Afocal Photography

When doing any sort of public astronomy, showing folks the beautiful sights available to a telescope, I often hear the question “Can I take a photo of that?” The person asking the question is usually holding the ubiquitous compact digital camera. They are often surprised when my answer is “Yes”.

Afocal Photography
Taking a photograph of the Moon using afocal photography

It is indeed possible to manage hand held shots of bright astronomical objects by simply holding the camera up to the eyepiece. There are a few tricks to making it work, but nothing that can not be demonstrated in a minute or two. The resulting photographs can be quite pleasing, definitely worth showing to friends and family along with the rest of the Hawai’i vacation shots.

The method of positioning a camera with a lens in front of an eyepiece is called afocal photography, or sometimes digiscoping. Afocal has been around for a while, but was not considered a practical photographic method by most. The advent of common digital cameras without removable lenses has changed this.

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Keck in Motion Scene Guide

I have been getting a few questions about the video. To answer a few of them I have compiled a guide to the scenes. Some quick explanations to what you are seeing, information on the camera used as well as the exposure information.

The video is a combination of two techniques. Many scenes were filmed as standard video then accelerated during editing to allow the motion to become clear. Examples of this are scenes of telescopes slewing and the interferometer delay lines moving.

Slower subjects, such as clouds or the stars moving across the sky, were photographed as time lapse. Here a large number of still images were taken. These are then processed and converted to video using Photoshop CS5 before loading into the video editing software, Adobe Premiere Elements. To construct the time lapse sequences sometimes required thousands of separate images, quickly filling memory cards and exhausting batteries. After dark it is long exposure time lapse that is used, with individual exposures often 15 seconds to one minute long.

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Polishing an 8m Mirror

My friend Dean Ketelsen has posted another great bit of video from the Steward Mirror Lab where he works. This time it is time lapse of polishing one of the eight meter GMT mirrors.

Somehow the video does not do justice to the sheer size of an eight meter mirror, it looks smaller than it really is. Be sure to read his description for all of the technical details.

An Evening with a New Telescope

The Astro-Tech 6″ RC is a wonderful little telescope… A true Ritchey–Chrétien design, at a size well matched to DSLR astrophotography. It was get one now or never, these were the last of the production run, and now they are gone.

Best of all, Astronomics was letting them go at a fire sale price… Less than $300 each!! It may have taken months on a waiting list, but I finally received one. Then it took another two months of waiting for some necessary accessories to make it work! No problem with the wait, I was patient… mostly.

With the final parts, the extension tubes, in hand. I determined to spend part of my four day Thanksgiving weekend getting the new ‘scope into service.

The Orion Nebula
NGC1976 or M42, the Great Orion Nebula, sum of 113 frames at 35x5min, 38x1min, 20x20sec, and 20x5sec. Taken with the Canon 20Da and a AT6RC telescope.
It took hours to rearrange the setup, un-bolting and re-bolting telescopes to and from the plates of aluminum that hold everything. Carefully aligning each of the four items in the setup… Main telescope, guide telescope, the finder CCD camera and a Telrad. Finding the best focus, recollimating the 6″RC, re-balancing the mount, fixing a computer power supply issue, rearranging and tying up all the myriad of cables, etc. etc… Astrophotgraphy really is the art of endless details. Finally, late into the evening, I took the first test image.

A few images later and things were not looking all that bad, maybe even functional. Better yet, my venerable ST-4 autoguider seemed to be tracking well. Maybe take a real picture? What to shoot? Something easy… M42 was rising over the garage… Why not? Just a test for the new setup, a fair chance something will go horribly wrong.

AT6RC atop an iOptron ZEQ25 mount
Result? Not too bad. This is despite many shortcomings… I forgot to take raw images, thus I had to process from the JPEG’s. I didn’t get any decent calibration frames. There seems to be substantial flexure between the guide ‘scope and the imaging ‘scope, but it is slow and does not effect individual frames. Actually the registration drift over the hours helped me process out the hot pixels and other image artifacts by creating an effective dithering. I did lose a few frames to vibration, the mounting could be stiffer, and I must be careful to walk softly on the concrete slab of the driveway during exposures.

But still, not bad for a first real attempt.

The stars look nice across the frame, showing that the collimation is decent, always an issue with the RC design. I suspect the optical quality of the telescope is quite acceptable. The brighter star images are a bit “fat” but that is due more to the mediocre seeing over Waikoloa. After all of these years shooting with a refractor, I had forgotten how pleasing diffraction spikes can be. Better yet, with the scope positioned for north up on the tube, the spikes are neatly at 45° to the cardinal directions. I am looking forward to some more imaging sessions with a new telescope.