Sky & Telescope - November 2021 - 68

80 Years: Where We Are
The Secondary Mirror
I have a mirror-making friend who says he doesn't hate anyone
enough to teach them how to make their own secondary
mirror. Grinding and polishing a truly flat mirror is way
harder than making a parabola. So, most amateur telescope
makers simply buy them.
The size of the secondary depends upon the primary diamNEWTONIAN
ESSENTIALS The primary and secondary mirrors are
the heart of a reflector telescope.
The Primary Mirror
The heart of every reflecting telescope is its primary mirror.
All else is, well, secondary. And it's pretty much a given that
if you're going to make your own telescope from scratch,
you'll want to grind your own primary.
To start with, you need a piece of glass. The cheapest
blank I can find online is from Got Grit (gotgrit.com) and
goes for $55 when it's in stock. That's for ¾″-thick plate
glass, which is perfect for an 8″ mirror. In the old days the
rule of thumb was that your glass should have a thickness
of one-sixth of its diameter to avoid sagging due to gravity,
but that has proven to be overly cautious; ¾″ is plenty for an
8-inch mirror.
An 8-inch grinding kit from Got Grit provides you with all
the grit, pitch, and tool supplies you'll need, and it currently
costs $64. So you're out $119 to get started.
Anywhere from 40 hours (if you've done this before or if
you're lucky) to 100+ hours later (if you're fussy and/or overzealous
in the final figuring like I am), you'll have a parabolized
mirror ready to be coated. Several different companies
offer vacuum-deposited aluminum coating services, which
is the standard telescope mirror coating nowadays. (A list
of companies that provide the service appears in our March
2019 issue, page 38.) Their prices for an 8-inch mirror average
about $100. Shipping both ways is about $50 more. The
adventurous might consider silvering their own mirror for a
bit less (S&T: Jan. 2020, p. 74), but let's assume you're having
yours aluminized. So now we're up to $269, and we have a
shiny primary mirror in hand.
Interestingly enough: You can buy a completed 8″ f/5 mirror
for about $240. Add in shipping costs and it's a wash. Arrgh!
uOPTICAL TUBE ASSEMBLY Left: Cardboard concrete forms make
excellent, inexpensive telescope tubes. Middle: You can buy a secondary
spider, but as John Dobson himself showed, you can easily make one
from materials as basic as cedar shingles. Right: The primary mirror cell
can be as simple as three bolts sticking up through a slab of plywood,
padded with squares of Masonite held in place using cardboard from a
cereal box.
68 NOVEMBER 2021 * SKY & TELESCOPE
eter and the focal ratio of the scope. F/5 is a very common
focal ratio for an 8″ scope, so let's assume that's what we've
ground our primary mirror to. The diameter of the secondary
mirror is also heavily dependent upon how far from the
optical axis your focuser sticks out, but it's common practice
to add 3.5″ to the radius of the mirror (4″ in this case) to
account for the focuser's average travel. A little math (secondary
diameter = centerline-to-focal-plane distance divided by
focal ratio) tells us we need a 1.5″ secondary mirror (3.5 + 4 /
5 = 1.5). You can buy secondary mirrors from many vendors.
The average price I've found for a 1.5″ is about $40.
We're now at $309.
The Optical Tube Assembly
You need something to put these mirrors in. Most 8″ f/5
scopes go in tubes rather than in truss assemblies, because
tubes of that size are easier to deal with, they protect the
optics from dust when you cap the ends, and they provide
excellent light baffling. The easiest way to get a tube is to buy
a cardboard concrete form, often called a Sonotube (even if
it's made by someone other than Sonotube). For an 8″ f/5
scope you'll need about 3.5 feet of tube. You'll most likely
http://www.gotgrit.com
Sky & Telescope - November 2021

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