Progress on a New Telescope...

Late last August, I wrote a post about a "found" telescope - Dean Koenig of Starizona gave me a tube assembly that he had obtained from the estate of Tom O'Hara.  Since he wouldn't be able to sell or use a partially-finished telescope, especially a home-built, he passed it on to me several years ago to recycle if I could.  Thinking it had a 12.5" mirror in it (a standard size), I found that it was, in fact, a 14.25" mirror, a non-standard size evidently bridging the 12.5" and the 16" standards.  I've rarely heard of one this size, but is perfect for me (about the biggest that can still be considered portable) and my new upgraded AP1200 mount that is now in operation.

 

That post was also poignant for me because it was the last comment that my blogger buddy Dave Harvey made - he died 3 days after commenting that he "can't wait for first-light!".  Well, he will certainly be in my thoughts when that day comes!

 

So it is time for an update on what I've been doing to the re-purposed piece of glass.  As shown in the picture above, it had a way-too-short focus on it.  Testing showed it to be a good sphere, as it would need in a Schmidt camera, and of about 20 inch focal length (was part of a 10" F/2 camera).  To be useful for me, it would need to be something closer to F/3.5, about 52" focus.  That meant the depth of the curve, which was over 5/8" deep had to be reduced to something closer to 1/4" deep - way too much labor to be contemplated by hand.  In addition, I wanted to go to some effort to lightweight the mirror.  The solution was to use a diamond curve generator.   We don't have one at the Mirror Lab, but back in December before the Christmas shutdown, I scheduled a bunch of generating at the Optical Sciences Center's optics shop to use their generator, sneaking in work on this mirror with some work projects. 

 

One method I'm fond of advocating is a double-arch lightweight - as shown at left, if you have a right circular cylinder, as most mirrors are, and plan to support it on a ring at equal radius, you can remove weight be removing glass that is not in direct line of supporting the front surface.  It is easy enough to make with a diamond curve generator - after first making the front curve more shallow to 104" radius of curvature, I waxed down that surface on a mounting plate to make the curves on the other side.

 

The result is shown here at left, while still waxed down showing the double-arched rear surface.  It is modified a bit from the ideal above, as I needed a way to hold it while polishing the front surface.  With all the generating this blank had done on it, it needed some post-generating work to relieve stress - generating puts stress into a surface, which can be relieved by acid etching, or by grinding and polishing.  To make it pretty and show it off a bit, I decided to polish out the rear curves.  The photo at right shows two of the rear surfaces inked up to show when enough material has been removed (usually repeat a few times to assure enough glass removed).  Oh, and the weight?  Originally about 25 pounds when started, feeling pretty heavy, it is currently just under 15 pounds, so a good weight savings!

 

On this mirror, there is a flat land about 2 inches wide - the outer diameter is 12 inches so that it can be fastened to 12" tooling for holding while working the front (shown below).  At left you can see a flat tool used to grind, then polish the flat surface.  Then at right you can see the small tool that was used to first grind the surface with tiles, then the tiles were faced with polyurethane pads to polish the inner surface.  The outer surface, outside the flat land is convex and can't be conventionally worked because it is adjacent to the land, so was worked by hand with small tools of diamonds and tile to minimize generator damage, then use polyurethane and felt tools to shine it up.

 

Here is how that flat land with the 12" diameter is used to mount the mirror for work on the front.  Since the flat was ground and polished with the cast iron tool that it sits on here, it holds it pretty stress-free.  Just to make sure it holds it uniformly, every hour or two of polishing it gets rotated and retaped to randomize any stress that the cast iron plate might introduce.  The bottom picture shows the pipe tape that holds it securely during polishing.  At right is the current view of the mirror with the pitch tool being used to polish it.  While it is 14.25" diameter, the tool is 12", which is fine.  From the generated surface, a tile layer was built-up on a pitch substrate, then a new pitch tool was made for the polishing.  The lab has been using zirconium oxide for polishing lately (a much cheaper alternative to cerium oxide), and I've been using the same in our small optics lab for this project.

 

So here is how the mirror currently looks - looks like a weird lens with the 3 different curves refracting its own set of images!  The sharp-eyed among you might have noticed the little goober in the substrate - it is visible in the left image just above right of center.  Shown in close-up at right, it might have been a fracture when Tom made the original sphere.  In the original blank, you can see where he started putting the curve into it, then stopped and worked the other side, so perhaps the fracture occurred when work started.  I etched it with HF acid to open the fracture and keep it from propagating.  It appears some polishing compound is currently in it, but since it is in the rear surface and treated, it should be just fine with no effect on the performance. 

 

Currently the mirror is within a couple hours of being polished out, so another few evenings or a weekend of work remains before I'll start worrying about converting the near-spherical surface to a parabola.  The focal length is currently 51" so I'm letting the polishing tool linger longer on the outside to lengthen it slightly to my desired 52".  But I'm making good progress from that Fall post - hopefully halfway to that first-light observing session I'll share with memories of Dave Harvey...