AA-SPIT

Peter John Smith (North Queensland, Australia) and Dan Cassaro (Florida, USA) are pleased to announce the commencement of a new ATM project, the somewhat-whimsically named "AA-SPIT" (Australian/American Super Planetary Internet Telescope).
AA-SPIT is an 11" triplet refractor with an f/15 focal ratio, optical design by Peter John Smith, optical and mechanical construction by Dan Cassaro. The primary design goal is a superb visual planetary/high magnification instrument for suitable for permanent mounting.
Peter has kindly agreed to post the design specifics on his web pages, and I've posted some of the history of this project and construction details here.

The History

Ever since the appearance of Roland Christen's article in Sky and Telescope (October, 1981) describing the superior performance of triplets for planetary observing, I've had a strong desire to build one, perhaps in the 8" size range. The rather steep costs of optical glass in these sizes, coupled with the general un-availability of the abnormal dispersion flint required (KZFS-1) kept this project in the "someday" pages of my ATM wish list.
Fast forward to the recent past, and the ATM group, where Guy Brandenburg offered for trade some 11.5" disks of BAF4 and SK4. A bit of horse-trading later, the disks were mine, and Guy had enough 12" Pyrex to keep him busy for a while....
I traced a few classical doublets for refractors in this size, and it soon became clear that secondary spectrum would be severe in any doublet of non-ridiculous focal length. Following the path of the Christen design, a number of attempts were made to design a triplet with either one or both of these glasses plus KZFS1. Third order theory yielded designs with excellent color correction and no spherical aberration, but with OSC well above tolerance. It became clear that I needed help to create a successful triplet design.

A call for help went out over the internet to Peter Smith, who very kindly offered to assist. A few iterations of Zemax later, Peter completed a flint-leading design, using Ohara's BPM 51 (very similar to Schott's KZFS-N4) as the middle element. His design had a number of excellent features:

    Some analysis showed that there was a definite gain in going past the original targeted f-ratio of f/12. For mechanical reasons, f/15 was selected as the maximum practical focal length for AA-SPIT. The blanks from Guy for the front and rear element were marked as BAF4 and SK4, along with some cryptic inscriptions which were assumed to be melt identifiers. The kind folks at Schott were nice enough to search their records, even including the ones at Jena in Germany, but no matches were found. The exact determination of the indices of these blanks is yet unresolved (see Identifying unknown optical glass). While melt data is not an absolute requirement, building without it is a bit dicey.

Dan gets lucky

    This left only the acquisition of some BPM51 (or KZFSN4) to complete the "gathering of the glass" phase of this project. And thereby, as The Bard said, hangs a tale. Schott's quote was $10,000+. Ouch. Hoya was unable to quote on piece this size. Ohara quoted less than Schott, but still a very large number. Things looked fairly grim, as the quotes were beyond the budget available for glass. And then life took one of those bizarre turns that keep things interesting....
By an incredible stroke of luck a large piece of glass marked "613442" had been surplused by the government a few years earlier, and by an equally incredible stroke of luck, I located the adventurous soul who had purchased it from a surplus dealer. Some initial tests by the then-owner led him to believe it was BPM51, but he could not be certain. After getting in contact, I agreed to buy the glass, and a bit later it made its way to Florida.  As you can see, that is one big chunk of glass!

Using some "crude but effective" tests, I determined the approximate index of refraction and specific gravity of the glass. The index and SG, coupled with the obvious staining led me to believe that it was indeed a KZFS-type glass. I arranged to have the piece cut by water jet by Perfect Edge in Atlanta into disks suitable for AA-SPIT, and a variety of other sizes. 

Here is the disk being cut at Perfect Edge.  

    The abrasive waterjet uses 120 mesh garnet, at 55,000 PSI pressure, and will cut just about anything, from thin lightweight foam to 8" stainless steel!  The end result was the set of cylinders, below.  The ruler shown for scale is 24", not 12".

A small sample was cut and sent to Schott for exact determination of the refractive indices. Some very nervous weeks passed awaiting the results, but the answer arrived exactly as hoped for: the glass was almost identical to BPM51. It's index was very slightly higher, but the partials were the same, and its the relative partial dispersions that are important for color correction.
    The cylinders were shipped via UPS to United Lens in Massachusetts, and they sliced the cylinders into disks, which I then sold to my fellow ATM's.   My 12" section had a flat spot, (which you can see in the picture) the result of a small mishap while being cut by the water jet.  

Machining the glass

This triplet design, like most, relies on accurately knowing the refractive indices of all three glasses.  I knew the glass type of the crown and flint, but I need the melt data.  I cut some small samples, and sent these off to Schott as well for index determination. 

 Here you can see where the blade has made a very thin cut.  I then scribed along the edge, and broke off the slice.  Some further machining gave me the 20x20x5mm sample size required by Schott.  See Cutting Glass with a Shoptask for more details.

All three disks were different diameters, and while it might be possible to create a custom cell to allow for this, I decided to go with the more traditional approach and edge to a common diameter, about 11.4".  To do this, I used some diamond wheels I purchased as surplus, and made some mandrels to mount them in the mill head of my Shoptask.  For horizontal work, they chuck directly into the lathe.

The edge of the KZFS was very rough from the water jet cut, with a rippled edge on the order of a few tenths of an inch deep.  My first thought was to mount this on my 8" rotary table, but I was worried about all of the glass dust and water that would be flying around, so I decided to build a simple rotating jig out of a big bearing and some wood.

That left the problem of how to mount the disk to the bearing.  I couldn't see any easy way to clamp it, and I was temporarily out of pitch, which is the traditional method for mounting glass.  I had some industrial strength Velcro from McMaster-Carr, so I decided to test it out.  What I found was very surprising.  One square inch of Velcro stuck on the glass would support the entire weight of the disk, about 20 pounds!  So, I stuck some Velcro strips on the top of the bearing, and put six pieces on the glass.  A bit of wiggling, and I had it centered fairly accurately.

I edged the disk with the setup shown in the photo.  For rough edging, I feed the disk horizontally into the diamond wheel mounted in the lathe, using about 500 rpm and taking passes of about .010".  It went fairly slowly, but I was in no hurry.

As the center of the disk was below the  centerline of the lathe, the result was a tapered cut, with the bottom of the disk slightly smaller than the top.  To fix this, I used another wheel, mounted in the mill head, and fed the disk in about .005, spun the disk on the jig a bit more than twice around, and then fed the wheel down about .100".  It too, was slow going, but the results were excellent.   The disk is right on diameter, with less than .001" taper.

The Next Step

Grinding begins!

back to the main page....