The observatory formerly housed a Celestron Compustar C14 f/11 Schmidt-Cassegrain telescope. The Compustars were the first mass produced "GoTo" telescopes. It is permanently mounted on a Le Sueur Astro Pier that is anchored in 1 ton of concrete in the ground. The pier passes through an opening in the deck that supports the observatory. The Compustar C14 is attached to the pier via a Le Sueur Aurora adjustable wedge. This wedge adjusts the the altitude and azimuth of the heavy Compustar C14 for accurate polar alignment with ease using paired timing belts. The polar alignment is locked in place with multiple, no-shift locking mechanisms.
The Compustar C14 has since been sold to a Celestron collector in Kansas and the Le Sueur Astro Pier and Aurora wedge have been sold to an amateur astronomer in Virginia.
This image was published in the October, 2020 issue of Astronomy magazine.
I use an SBIG STL 11000M self-guiding CCD camera with an internal filter wheel. The filter wheel contains Baader Planetarium LRGB and H-alpha filters. I usually use a focal reducer to increase the photographic speed of the system. For years, I used an Astro Physics 0.75x .focal reducer. It yields a focal ratio of f/8.3 but is only a reducer, not a corrector, and leaves the coma inherent in Schmidt-Cassegrain telescopes in the image corners. Recently, I began using a Starizona SCT Large Format (LF) reducer/corrector. It yields a focal ratio of around f/7.5, depending upon what else is in the imaging train (see below) and corrects the SCT coma all the way to the corners of the STL's large format chip. You can tell which reducer may have been used for a particular image based on the f/ratio listed after the image's exposure data.
When possible, I use the SBIG AO-L adaptive optics device. It corrects guiding and seeing errors up to 10 times per second, yielding sharper images but it will typically achieve only 3 to 8 corrections per second. The Compustar mount has a worm gear drive with substantial periodic error which would yield trailed stars but, unlike most modern drives, has no provision for periodic error correction. The rapid corrections of the AO-L permit guiding that is equivalent to that obtained with the most expensive precision mounts. However, It requires bright guide stars (the brighter, the faster the correction rate achieved), which are sometimes not available for a particular target. That is more of a problem when using the H-alpha filter, which dims guide stars considerably.
Schmidt-Cassegrain telescopes focus by sliding the primary mirror foward and backward over the central baffle tube. This design gives them a problem known as mirror flop. As the telescope moves across the sky tracking an object, the mirror can wobble on the central baffle. This is particularly true as it crosses the meridian when the primary mirror goes from leaning east to leaning west. It is also particularly true for a C14, which has a very heavy primary mirror that is prone to flop. Mirror flop can affect the telescope’s collimation, resulting in coma. The other big issue for astrophotography is that it changes the illumination pattern on the CCD chip, resulting in flat field errors during image calibration. There are a variety of remedies, such as imaging mostly to the east of the meridian and taking flats for different positions in the sky. Another is the use of Flop Stoppers. They are two special bolts on long plungers that can be screwed into the rear of the primary mirror in 2 places through special locking collars which can be attached to the rear cell of the telescope. The plungers slide in and out through the collars during focusing, but once focus is achieved, they can be locked in position with set screws. The focusing mechanism itself, on the opposite side, then acts as a third point of triangular fixation. They are beautifully machined by Thomas Esmerelda in Hawaii. While they don’t completely prevent mirror flop, they help greatly.
My former observatory was a Technical Innovations Pro Dome 10 fiberglass observatory. It was 10 feet in diameter and was supported on a base ring and three extra wall rings, each one foot high for a total wall height of four feet. This observatory has since been sold to a fellow amateur astronomer in the Seattle area.
