By Donald S. Lynn

How To Build It

More than 30 years ago I was attending the Riverside Telescope Makers Conference and Celestron was selling scrapped telescope parts at their booth. They were probably cleaning out their factory of stuff that customers had broken and brought in to be repaired. Looking at this assemblage of bits, I began wondering if I could buy a whole telescope and build it up piece by piece. The prices were so cheap that building a whole telescope this way would add up to a fraction of the cost of a new, fully assembled one. So I bought a corrector plate with an attached secondary mirror of an 8-inch Schmidt-Cassegrain, and began a long search for the rest of the parts.

The major manufacturers of Schmidt-Cassegrains make their optics near perfect by the final figuring of the secondary mirror. This counteracts any substantial imperfections in any of the optical components (corrector lens, primary mirror, or secondary mirror). Ideally it’s be best to get a matching set of the three optical components, but a complete set is rarely seen in this type of sale. If a customer broke something, it’s most likely one of the glass pieces, so at best two of the three optical pieces will be left intact. I had heard that the primary mirrors were pretty close to perfect spheres as they are made by grinding and polishing machines designed to produce spheres, and most grinding procedures tend to produce a nearly perfect sphere anyways. Thinking this, I thought it was safe to buy the corrector/secondary matched set without the primary.

It turns out I was wrong, but I didn’t know it for decades.

Another year I found a primary mirror that I liked. It was a factory reject that had an obvious chip out of the edge, that looked to me like it would not affect the optical performance. The chip was even carefully indicated by marking pen, apparently by an inspector at the factory. Though not aluminized, it was completely polished to the best of my inspection.

Over the years I found and bought an empty tube, a fork-mount base, a focus knob, and the focusing ring and screw. I was especially impressed with the mount base, since it was manufactured during the years that Celestron used Byers gears, which are famous for their precision. I think I paid $25 for it. It was the most expensive part. A friend who was upgrading her Celestron 8 to use a super focuser gave me the old focus mechanism – the only part I got for free.

After several years of not finding the remaining parts, I went to a dealer of used telescope parts and bought the corrector retaining ring and the primary mirror retaining ring. After that I was missing only some bolts, which I found in a hardware store. I now had parts from eight different telescopes, plus the primary mirror, which apparently was never put into a ‘scope.

In order to avoid the bother and cost of carefully packing and shipping the fragile primary mirror, I carried it into Newport Glass Company, which coats mirrors with the reflective aluminum. The mirror-coating expert there saw that this was a factory reject mirror and warned me that I really should get a matched set of optics. I told him I would take my chances and to go ahead with the aluminizing. It cost fifty-some dollars.

I was finally ready to assemble my telescope. I carefully cleaned and lubricated all the parts that needed such and put it all together. I then collimated the secondary mirror.

First light was a star test of the optics, comparing the pattern seen inside and outside of focus. The first image was less than perfect; it clearly had a fair amount of astigmatism. Either my primary was not a perfect sphere, or the secondary or corrector were not their perfect shapes. I repeatedly rotated the corrector, along with the attached secondary mirror, with respect to the primary to see if I could improve the image quality. After a series of trial an error attempts, I found a rotation that had nearly perfect star test images. Two wrongs sometimes do make a right!

Altogether, the completed telescope probably costed about $130 for the parts and mirror aluminizing.

A fellow amateur astronomer, observing that I had cobbled together the telescope’s body parts and brought them to life, started calling my rig “The FrankenScope.” Regardless of the name, I am happy with it. I have used it for several outreach star parties in addition to my regular star gazing.

Why Not To

Though a fun, if lengthy, project, there are reasons that it likely wouldn’t work again. Firstly, I have not seen Celestron, or any other manufacturer, sell scrapped telescope parts for several years now. The era of being able to buy a working telescope part by part is probably over.

Secondly, I got lucky that the primary mirror and the corrector/secondary assembly had nearly the same astigmatism so they could be rotated to cancel out each other’s imperfections. That probably would not happen if tried again.

Lastly, the telescope was obsolete as soon as it was built because it took so long to complete. The mount was built back when such telescopes were motorized only on the Right Ascension axis, without declination motor, and had no computer control. I have to find objects in the sky the old-fashioned way, moving the ‘scope by hand to the desired coordinates, without the aid of GoTo capability. I also have to polar align the R.A. axle, which is not necessary for computer-controlled telescopes. I don’t mind; that’s the way I learned to do it.