Sky and Telescope - June 2015 - 74

Gary Seronik
Telescope Workshop

Secondary Scheming
The optimum diagonal size depends on facts and preferences.
Like collimation, the correct size of a Newtonian

GARY SERONIK

reﬂector's secondary mirror generates a lot of discussion
in online forums. Much of the resulting confusion arises
from the fact that for a given telescope there's no single
right answer. So, if you're trying to sort out what size is
ideal for your scope, the best way to proceed is to separate
fact from opinion and consider each independently.
Let's begin with the facts. First, there is a certain
minimum size your scope's secondary mirror has to be.
If it's undersized, you may ﬁnd your 8-inch scope working like a 7-inch or smaller. Second, if the secondary is
too big, it obstructs incoming light needlessly. That's
undesirable because the image will be very slightly dimmer, and diﬀraction eﬀects potentially reduce ﬁne, lowcontrast detail. And a bigger mirror will also cost more.
Third, the larger the secondary mirror, the larger the
region of the focal plane that receives light from the primary mirror's entire surface (the area referred to as the
fully illuminated ﬁeld). Fourth, diagonal mirrors come in
a limited number of sizes, which means you may not be

This small mirror causes big debates. But once you know some basic
facts, it's easy to choose the optimum diagonal size for your scope and
observing needs.

June 2015 sky & telescope

able to get one that precisely matches the size you desire.
The ﬁrst issue to settle is the minimum size your
telescope requires. The smallest diagonal you should consider using is just big enough to deliver 100% of the light
gathered by your scope's primary mirror to the center of
the focal plane. You can calculate this by dividing L by f,
where f is the focal ratio of the primary mirror, and L is
the distance (length) from the center of the telescope tube
to the focal plane. In the case of an 8-inch f/6 mirror, L
is often about 9 inches. Dividing 9 by 6 yields a diagonal
size of 1.5 inches. Now you have to decide if the minimum
secondary is the right choice, or if a bigger one might be
better. This is where we enter the realm of opinion.
In the good ol' days when most reﬂectors were f/8
or f/6, a popular rule of thumb stated that you should
choose your diagonal to fully illuminate the center
½-inch of the focal plane. So if the minimum secondary size for your scope is 1.5 inches, you can satisfy that
condition with a 2-inch diagonal mirror (1½ + ½). That
approach still works, but for many of today's mirrors,
which are often f/5 and faster, it will usually result in a
secondary mirror that's bigger than necessary.
In my opinion, rather than worrying about the size of
the fully illumined ﬁeld, it makes more sense to choose
your diagonal based on the amount of dimming at the
edge of the ﬁeld of a low-power eyepiece. When I make a
telescope, I usually let images there lose about ½ magnitude - an amount that's far from obvious, especially as
it's conﬁned to the extreme ﬁeld edge of only my lowestpower eyepiece. For all my other eyepieces, the drop oﬀ
is much less, and sometimes even zero. How do you
calculate the edge-of-ﬁeld illumination? Easy. You can
ﬁnd several calculators online, but the best one is on Mel
Bartels's website (bbastrodesigns.com/diagonal.htm).
Let's try an example: an 8-inch f/5 reﬂector with a
2-inch focuser and an L value of 9. Using the formula
discussed earlier, we know that the secondary mirror for
this scope should be no smaller than 1.8 inches. If we use
the ½-inch rule, we would choose one with a 2.3-inch
minor axis. Since secondary mirrors aren't commercially
available in that size, you'd use the next size up - a 2.48or 2.6-inch diagonal, depending on the vendor. However,
if we allow the edge of ﬁeld of a 2-inch, low-power eye-

http://www.bbastrodesigns.com/diagonal.htm

Sky and Telescope - June 2015

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