Auburn Astronomical Society's
Guide to Using the 12.5-inch f/7 Loaner Scope

This is the flagship of the AAS loaner telescope fleet.  Due to the size and weight of the 12.5-inch, we'll require that a minimum of two able-bodied members will be available to load, transport, unload, and set-up the scope.  A van or truck (with a padded floor) will be needed to transport this scope.  It will not fit into a small SUV.  Expedition/Suburban class vehicles should be large enough.  You should have a 6-foot step ladder if you plan to observe near zenith.

It's also suggested that if this will be your first time using a Dobsonian telescope, that you get your feet wet with the 8-inch Loaner before using the 12.5-inch.

Weights and Measures

OTA Length 80 inches (6'-8")
OTA Diameter 16.5 inches
Cage Diameter 27 inches
Rocker Box 39.5" high x 27" deep x 25.5" wide
Eyepiece zenith height 80 inches (6'-8")
Total height at zenith 8 feet
Rocker Box weight  
OTA weight  


Aperture 12.5 inches 317.5mm
Focal Length 87.5 inches (7.29 ') 2222.5mm
Focal Ratio  f/7  
Eyepiece Magnification Exit Pupil
32mm 69 4.6
10mm 222 1.43

Tom McGowan with the Cave 12.5 in its Dobsonian configuration

Ray Kunert with the rocker box

Scope in the rocker box

Loaded in the van

This is how much space it takes up on your sun porch

        Setting up the Scope

Select a level place to locate the rocker box.  Because of the longer than usual focal ratio, the scope at azimuth will be a little more top-heavy that shorter f/ ratio scopes.

When lifting the OTA, use the handles on the altitude bearing with one hand and secure the octagonal OTA cage with the other.  If both lifters use their left hand on the handle, the right hands will be supporting opposite ends of the OTA.  Tom balanced the OTA assuming a 2-inch eyepiece, so the mirror end will be slightly heavier when the eyepiece is not in place.

The accessory case should contain two eyepieces and the Telrad.  The Telrad should be attached to its base by inserting the two tabs on the focuser side of the base, lowering the tabs on the far side, then tightening the thumbscrews on the far side of the base.  Exposing the Telrad to direct sunlight could destroy the reticle. 

Check to be sure that the Telrad, and the Optical Tube Assembly (OTA) are aligned.  This is best done during  twilight on a distant terrestrial object such as a street light or a radio tower strobe.  First center your alignment target in the main scope, then adjust the finder.  Should the Telrad need to be aligned, this is accomplished by adjusting the three small screws on the back of the unit.  Although the Telrad may look a bit like a right-angle finder, it is not.  You must place your cheek on the OTA and sight straight up along the OTA through the Telrad until you see the 3 concentric rings superimposed on the clear plastic.  The on-off switch/ brightness adjustment is a small lever on the side.  It's best to keep the circles as dim as possible when location objects in the night sky.  Remember to turn the switch off when you're done for the evening. If you forget, please leave a fresh set of AA batteries for the next user.  When looking through the Telrad to locate your target, keep both eyes open

The 2-inch focuser uses the rack and pinion for coarse focusing, and helical focusing (where the eyepiece meets the focuser) for fine focusing. 

Someone else had the same idea:


See the AAS History page, for the years 1983-1988. 

1983:  Attorney, Charles Floyd, of Phenix City, offered to donate his 12.5-inch f/7 Cave-Astrola telescope to the fledging Auburn Astronomical Society. 

The 12.5-inch f/7 Cave-Astrola in its original configuration

1984:  The primary mirror was sent to Richard Fagin, of Miami FL, to have it refigured and recoated.  In September our Moore's Meadow Observatory was completed with the 12.5-inch f/7 Cave-Astrola telescope as its primary instrument.

The telescope served us well through Comet Halley's 1985-1986 apparition. 

1988:   We lost access to the Moore's Meadow property ergo, the observatory and telescope.  Jim Chesnutt volunteered to keep the scope in storage under his house until we could find another location.

Fast forward two decades.

When Ray Kunert learned that AAS had a 12.5-inch, f/7 Cave Astrola telescope languishing in storage, he started a campaign to resurrect the beast and get it back into service.

On Saturday, July 8 2006, Ray Kunert, John Burns and Russell Whigham, drove to the home of Jim Chesnutt in Auburn to pick up the society's 12.5-inch f/7 Cave Astrola telescope.   Jim had kept the optics stored in his house and the non-optical parts of the telescope in the crawl space since we had to give up our Moore’s Meadow Observatory in 1988. 

Ray Kunert digs through the rubble to liberate the slumbering giant. At right, the OTA with its observatory pedestal.

As we have no observatory to keep it permanently mounted, and given that the massive german equatorial mount precludes it from being easily moved, a decision was made to mount the original tube assembly on a Dobsonian mount and that it be used as a loaner scope.  Ray has since reconditioned the hardware and done an excellent job of putting a new finish on the tube.  The primary mirror was in pretty good shape, but the secondary needed to be realuminized.  William Baugh took care of that for us.  At the June 2007  meeting, William returned the newly aluminized diagonal.  A big “Thank you!” to William for doing that for us. 

Meanwhile, we had contacted our resident telescope maker, Tom McGowan, about making a rocker box.  Tom called the Sunday following our June 2007 meeting and said that he would have time to work on the rocker box during the next few weeks.  Tom said that he would need the optics in the tube for balancing.  He recommended a Telrad rather than a finder scope.  He'll see if we can incorporate the rotating rings into the mount.  The rings aren't really necessary with the Dobsonian configuration, but Tom determined that using them as a saddle would be the easiest way to mate the tube assembly to the rocker box.  Tom suggested  a traditional stained/polyurethane finish on the birch plywood for the rocker box.

We also needed to decide on where to keep the scope when it's finished.  One possibility would be to keep it at the W. A. Gayle Planetarium when not loaned out.  Rick Evans has a small scope at the entrance to the planetarium now.  If we could put this one there, perhaps with "Auburn Astronomical Society" and "" nicely lettered  on the tube, it would provide some excellent publicity.  This would also make a wonderful addition for our annual Astronomy Day events, there.  Rick has welcomed the idea, and offered to “accommodate you in any way we can”.

Tom has warned that the finished product will be quite large.  It will require a good size van or pick-up truck as well as two people to transport and use, but should provide magnificent images.

August 2007  The Telrad that we ordered from OPT has come in and has been mounted. 

We had two offers of eyepiece donations from members -- exact size and focal length to be determined later.  We also need a small accessory case for the eyepieces and the Telrad.

The cost of materials used in Tom's construction of the rocker box and OTA cage came to $200.00.  Tom has donated his considerable time and know-how.  Thanks, Tom!