Adventures
with the Refractor
Jack Eastman, DAS Chief
Observer
 |
In the continuing
saga of one person's attempts at the perfect telescope,
I offer the following. Having gone on at length about
the Newtonian in part 1, I now go to the Refractor. This
story is a bit circuitous, as this telescope evolved
over several incarnations, starting with a small lens,
then to a Schmidt Cass, from which the mount was born,
finally to the acquisition of a really fine 5-inch lens,
and the fabrication of the 5-inch tube assembly. This
whole process was spread over about 10 years, as opposed
to the 6 or 7 months for the 12.5-inch. |
I had always wanted a refractor of some size.
I still had, and have to this day, the 60mm Polarex, and a
couple of other 60mm 'scopes, but they are 60mm. A real
refractor is a 4" or more. As I said earlier, except for the
Knotts telescope, a Newtonian, I was raised on refractors. There
was the 12" Zeiss at the Griffith Observatory, and a beautiful
6" Brashear/Warner & Swasey at Mt. Wilson, which I was
privileged to look through early on, and finally gained the use
of on a nearly unlimited basis. There was an identical 6" at the
student's observatory at UCLA. Remember, from Part 1, my driving
my parents batty about a bigger telescope, I kept howling about
Unitron: "Look a 4"! and on an equatorial to boot!!" Never mind
the equivalent price, in today's dollars, of something like
$8,000! This all passed, sort of, with the completion of the
12.5", but, still, a 4- or so inch refractor would have been
nice.
I had the delusion of trying to make a lens.
This seemed, at the time, out of reach. For one thing the math
involved looked formidable. Maybe after I had algebra... That
came and went, maybe Trig... No, how about Calculus... By the
time I bit the bullet, I discovered all the high powered math
wasn't needed. A little algebra maybe, and a whole lot of
arithmetic. The gory details of the design process really is
beyond the scope of this discussion, however, these design
methods are described in Amateur Telescope Making (ATM) II, by
J. R. Haviland (pp 212--) and in ATM III by Alan E. Gee, (pp
208--), Charles L. Woodside (pp 565--) and James H. Wyld, (pp
581--). Then there was the problem of glass. Mirror blanks were
plentiful and cheap, optical glass wasn't. Today none of it is.
Cheap, that is!
Somewhere along the line, I think I was just
finishing up at UCLA, we were rooting around at the venerable
old C&H Sales in Pasadena when I came across what appeared to be
4" lens blanks. Fat biconvex and deep plano-concave pressings,
which if made into lenses, would have been very short, maybe f/3
or f/3.5. I don't recall if the index and dispersion numbers
were on these or if we just assumed they'd be "ordinary" crown
and flint. I bought a couple of sets of these things, for a
quarter each, and thought seriously about making the long sought
after 4" lens. Using the formulas for color correction and
assuming the properties of these chunks of glass I set about to
regrind the curves to make an f/15. It turned out these things
weren't 4" but something less, leading to a 3.6" aperture. I
thought if I make the crown equi-convex, and match the concave
side of the flint, I could test the concave with the knife edge
test, and match the convexes to that by interference fringes. If
all looked good, I could then test the whole lens and,
supposedly any error remaining would be on the back of the
flint, which could then be dealt with accordingly. After
polishing I was somewhat gratified that the focal length seemed
close to the design, and with an eyepiece the color correction
was reasonably good. I tried to figure these glasses with
limited success, finally noting the lens was terribly
astigmatic. A test with crossed polarizers showed the dreaded
colored "Maltese Cross" in the crown element, indicating a large
amount of strain in the glass. Amazingly enough, this was the
crown element. The flint was fine. I checked the other blanks I
had, and they too were strained. That put this telescope on the
back burner.
The scene now switched to my first job, right
out of school, with Valor Electronics. Tom Johnson, the owner,
was fooling around a new type of telescope. My first encounter
with Tom was at a star party, where he showed up with an 18"
Cassegrain. (See cover, Sky & Telescope, March 1963) Remember,
this was the mid '60s, and aperture fever stopped at 12.5",
maybe a 16 or two. This 'scope didn't perform all that well, and
Tom tried some redesign and, skipping over the details, his
fiddling led to serious breakthroughs in the manufacturability
of the 4th order aspheric plates for the Schmidt telescope. This
fooling around led to what we know today as the Celestron
Schmidt Cassegrain. I helped Tom develop this system and he was
able to keep me out of the army. Clearly a fair trade! One
optical system we did while I was there was an 8" f/12, using a
very fast f/1.6 primary. I made an extra set of optics as a
backup, and when the order was delivered I was able to keep the
spare set.
In the first part I talked about the machine
shop education. By now my dad and I had the lathe, drill press
with X-Y and rotary table and all. Also, the L.A. area is a
paradise for tinkerers, as there is a major scrap metal yard
seemingly on every street corner! Also, even though this was
California, USA people, back then, weren't as damned "sue-happy"
as they are today, so the managers of these places let us go
tramping around and pick up what we needed, weigh it, pay up and
go. What came of all this was the fabrication of an equatorial
mount, for the 8" SC, patterned, functionally, after the Warner
& Swasey at Mt. Wilson. This one was all machined from aluminum
with 1.25 thick walled tubing for the shafts, threaded and set
up at 90 deg to the saddle plate and bearing housings. Again,
ball bearings (some lessons never get learned!) but this time
there was adequate damping in the design of the clamps and the
bearings could be preloaded to some extent. The drive is a 6.25"
100 tooth brass worm gear, a 16:1 reduction and a 1 RPM motor.
But wait! That's 1600:1! Thanks to the slot car hobby I obtained
the gears for a 20:18 gear train, and voila! 1440:1. The motor
is mounted on the baseplate, the power is transmitted to the
gearbox by a short shaft and universal joints. This system
allows a little flexibility as the latitude is changed. The slow
motions are both tangent arms, the declination being the
equivalent of a 691:1 gear reduction, the RA being 534:1. the
Declination is clamped to the bearing housing, the R.A to the
hub of the drive gear. This way, using the RA slow motion
doesn't affect the drive rate, and therefore the RA circle
carried on the gear doesn't get out of time. Since the lead
screws on the tangent arms straight and the motion is along the
arc of a circle, the screw mounts and the nuts they engage must
be able to swivel to avoid jamming. The worm on the main gear is
spring loaded to eliminate backlash, and allow for any
"out-of-round" of the main gear. I was complemented on this
design at one of the star parties and it was strongly suggested
I get this patented. I said I really can't. I stole this entire
concept from an 1880's vintage Warner & Swasey mount! The ball
bearings are retained by threaded rings in the axes, which can
be tightened to provide preload to the bearings. The cover over
the lower polar axis bearing is also a 3.5-inch hour angle
circle. Since the shafts and housings are both aluminum, there
is no trouble with temperature changing this preload. This
equatorial head is supported by a stout old transit tripod. The
telescope this mount was built for was the 8" SC for which I
subsequently completed the tube assembly. The details of this
telescope will be told later.
While at Valor I remembered my failed 3.6"
lens and asked our glass supplier if he had any suggestions. He
said to bring the lens and maybe his place could reanneal it or
something. When I got the lens back, it looked OK, but when I
put it together the performance really stunk. Close examination
showed a sort of "fire polished" appearance on one side. It
needed to be reground. As I refine-ground the bad surface it
ground in an hour-glass pattern. After this I again reassembled
it and took a look. The astigmatism was much worse. Rats! I
needed to regrind the other side as well. When the glass was
reannealed it sprung into a potato-chip shape. After all this
consternation I finally got the lens to a reasonable condition,
but it never did work very well. A Clark or Zeiss this is not!
This telescope is now serving as a guide telescope on a 4"
astrograph made from one of the old f/6 Aero Tessars. This
camera, with the 60mm Polarex as a guide 'scope, is pictured on
the cover of the Griffith Observer for Feb. 1961.
It seems I have strayed, digressed and
wandered from the purpose of this article, my 5" refractor, the
one in the accompanying photo. Believe it or not all the
previous baloney is leading up to this 5".
Some years go by, I switched jobs to Fairchild
Space and Defense Systems in El Segundo, where I was being
taught lens design. I was called in by them to fill an
optician's position, but I wasn't that interested at the time.
The person I went to see said to see "Dick" on the way out.
"Dick" was Dick Heimer, the director of optical design. "Do you
know anything about optical design?" "A little-- telescope
objective.. (the 3.6)" "Do you know anything about computers?"
"No--Besides I'm not all that good at math" "Get your butt in
here Monday-- You'll learn" So, for the next 2.5 years I learned
optical design. After that (FS&DS was closed, all the managers
went back to Long Island, the rest of 'em went to Hughes) I
fulfilled my prime directive-- get out of LA. I found myself
here in Colorado.
I still wanted a "big" refractor. Maybe I
should try again with better glass. I was older, presumably
wiser(?) and it would be interesting to go through the whole
process, hopefully now knowing a little more about what I was
doing. I had approached a very good friend in Tucson, asking if
he knew where one could obtain blanks for a refractor objective.
Given the choice, I decided to go for a 5", thinking that 4s are
nice, but fairly common, while a 6 would be a monster, based on
the experiences with the Mt. Wilson 'scope. Lynn said he'd sniff
around and see what he could find. Tucson probably has more
optical companies and telescope makers (really big telescopes)
per acre than anywhere else on the planet. I had begun to
calculate the curves for a 5" f/15 using Conrady's G-sum method
described in Alan Gee's chapter in ATM III. Somewhat arduous but
the curves supposedly lead to a lens corrected for secondary
color, third order spherical and coma. This method is an
algebraic technique, assumes thin lens approximations, and leads
to a good starting point which should be "tweaked" by rigorous
ray trace techniques. Here is where practicality steps in. One
can tweak and tweak and tweak, but the moment of truth comes
while producing those carefully calculated curves. As the lens
progresses it will need to be tested and figured until all
traces of aberration are gone. It is far less torturous to use
the initial curves from Gee, and then polish until the lens
looks good. Saves a whole lot of wear and tear on the
calculator, (and the brain) besides the indices of the glass
might not be exactly as given and the thicknesses and radii of
the surfaces might be a little off, requiring even more
"tweaking". Making a lens is quite different than a mirror. The
mirror needs only one surface (good) but it must be paraboloidal,
not spherical, and must be made roughly four times more
accurately as the lens surface. (The error on the wavefront for
reflection is twice the surface error, while for refraction it
is roughly half the surface error.) The lens needs four
surfaces, but these are spheres and usually with fairly steep
radii on three of them so the figure is relatively easy to
control. The back surface is generally of a long radius, nearly
flat in some cases, and is a bit more of a problem. One usually
figures this back surface last when finishing up the lens.
Attention needs paid to wedge, the parallelism of the two sides
of the lens element. This HAS to be carefully controlled.
Element thicknesses are not all that critical. in fact these
parameters are ignored in the G-sum calculation, as is the
airspace. Once the lens is close to completion, one can vary the
airspace slightly to try and improve the correction, or to
"tweak" the color correction, then control the remaining
spherical by a tad more polishing.
A large box appears on my doorstep from
Tucson. Lynn's return address on it and "Fragile, Glass" written
all over it. What-?? Then I remembered asking him some months
earlier about telescope glass. I opened the box, dug through a
zillion wads of paper and all and got to the first piece of
glass. I unwrapped it and wow! a pristine, beautiful lens
element! It was even AR coated, edged to 5" diameter and seemed
to have a fairly long focal length, around 800mm or so. My
initial figuring showed the crown element should have a focal
length around 770mm or so. Mixed emotions!
I'd feel terrible grinding into this lens, but
then again, hopefully, it'd be a nice lens after the surgery.
Maybe I could leave this one alone and match the flint to it,
using other glass tools to shape it. Further digging produced a
weakly negative lens element (the flint) equally as pretty as
the crown. Suddenly it hit me. There is some guy out there
expecting the delivery of a lens and is highly disgruntled at
receiving a couple of blanks! I called Lynn and told him I had
what appeared to be a finished lens and that he'd goofed. When
the customer for the lens called to give him hell, he can
explain the mix-up, and in the mean time I'll get this in the
return mail. Lynn said "no, no goof. You wanted a 5" crown and
flint, that's what you got" "But this is a finished lens!" "So
what, one crown, one flint..." 'but..." "Don't know if its any
good or not. I found it in our scrap glass locker. I apologize
as I don't know which flint it is, F2 or F4. The crown is BK7
for sure. Enjoy..." I took the elements outside and measured the
combined focal length. 1800mm! Just what I was planning to make.
Wow! No grinding, no G-sum process, but was this thing any good?
Scrap glass? I taped them to the end of a long cardboard tube
and looked at Jupiter and a few stars. It was good. Very good.
I obtained a 5" diameter 5 foot long aluminum
tube and some other odds and ends from which to make the lens
cell and rack and pinion focuser. The 5-inch tube was cut off at
90 degrees at both ends and a counter cell machined to fit on
the end. I tested the lens by autocollimation, and determined
the last couple of millimeters of its diameter showed some
error, so the clear aperture was made to be 122mm. I had a
5-inch thin ring, threaded on the outside, so I threaded the
back of the lens cell to match this ring. I cut a shallow groove
in the countercell as clearance and when the lens is mounted on
the tube, this groove prevents any possibility of the cell ring
coming loose and the lens elements falling out. The lens cell is
attached by three sets of push-pull screws to allow for
collimation. The focuser has a brass tube 2.8" in diameter with
a 7" travel and a 2.5 inch holder diameter. I hate refractor
focusers with small travel! Once I got these components put
together the balance point was only about a quarter of the way
down the tube. It worked but looked sort of silly. This thing
needed weight at the bottom end. I solved this by mounting my
60mm Polarex as a guide telescope, which served a useful purpose
and placed the balance near the middle of the tube. Later I
added a Unitron 10X40 finder, purchased from a friend, and a
couple of sliding weights on the tube to rebalance for different
eyepieces and accessories. The tube is fastened to a piece of 3"
channel with stainless steel hose clamps. The channel is bolted
to the saddle plate on the mount's Declination axis
Mount? You may have guessed. The 8" SC mount
talked about earlier was used for the refractor by rotating the
Dec. axis 180 degrees, to bring the slow motion knob closer to
the eye end. The mount could then be used for either 'scope
quite easily. With the design of this mount, with the refractor
on it really looked like a telescope! A freak accident led to
the loss of the 8" SC (a story in itself) after which the mount
was dedicated to the 5" I added extension rods and a flex cable
to the clamps and slow motions so they could be operated easily
from the eyepiece.
Paul Thayer, a long time member of DAS handed
me a big eyepiece one night at a Chamberlin star party. He said
"this thing is a pig! If you think you can use it, it's yours"
It was a 63mm Plossl, war surplus, probably the one described in
MIL-STD 141, Optical Design. I tried it on my 12.5, and it
didn't work well at all. Much too low a power, exit pupil too
big and so on. But what about at f/15. More optical certified
duct tape, and I tried it on the 5" Worked nicely! 28X, 1.8
degree field. Nice. I asked Paul if he really couldn't use it on
his 4" f/10 refractor. He said "no. It's a pig" so I machined up
an adapter for it. One night, New moon, I was in the middle of
Nevada with this setup and was poking through the Southern Milky
way. Dark nebulae, little star clusters, black sky and subtle
contrasts, needle sharp images all the way across the 1.8
degrees! This made a believer out of me and popped the myth that
an f/15 can't be a richest field telescope! With other, more
normal eyepieces this telescope is superb for double stars and
the planets, especially when the seeing is marginal and the 12.5
would need to be stopped down. With a Herschel wedge and #10
welders filter the Sun can be spectacular with all the fine
detail in sunspots and spot groups as well as the granular
appearance of the photosphere. This telescope is primarily used
on the Sun, Moon, planets and double stars, but is a capable
richest field with the 63mm eyepiece. It has fulfilled the
desire for a really nice refractor. It saw first light about
1973, and has been used continuously ever since. A medium sized
refractor is a real pleasure to use, and a 5" seems just the
right size, balancing performance and portability.
FJE
