Test Your Optics by Splitting Binaries?
Binary or multiple stars provide their own unique pleasures for an
observer. I particularly enjoy such systems when they exhibit
contrasts either in colour or in magnitude. However, many observers
sometimes try to split closer binaries in order to evaluate the seeing
conditions, or during evenings of good seeing, the optics on their
telescopes.
Here is a list, ordered from wide to narrow separation, of some of the
binaries I use to test my scope for splitting binaries.
Binary Separation Combined Mag.
Castor 4.0" 1.6
Epsilon Lye 2.3" 4.6 (closest double-double)
33 Ori 1.9" 5.5
Pi Aql 1.4" 5.7
Gamma Vir 1.3" 2.7
Zeta Can 0.9" 5.0
Zeta Her 0.8" 3.0
Eta Cbr 0.7" 5.1
One thing I have found to be fairly useful for me in testing the
collimation of my SCT-8" is to observer progressively closer binaries
to if I can split them. Again, you have to be aware of the level of
seeing on a particular evening for this kind of test to make any
sense, otherwise, it is much too easy to evaluate the seeing instead
of your collimation.
Note that splitting binaries is only one of the practical tests one
can do to see if your optics have been properly aligned and are
performing according to specification. I prefer a star test as a much
more telling method of testing optics. When the gas planets are out,
I will often try to see the amount of detail I can see. A third
option is to see how many small craterlets I can detect on the Moon
(down to approx. 1 km diameter - under ideal conditions, with my 8").
For example, on an 8" scope, "The Backyard Astronomer" states that I
should be able to split a pair down to about 0.58 arc seconds. That's
under ideal conditions. Usually, if I split any doubles under 1" I'm
fairly happy. But remember, you have to have good seeing to begin
with for this to work. The best I've been able to definitely split
with the SCT-8" thus far is 0.7" for Eta Corona Borealis (J2000).
But for the beginner, splitting a double star is one of the easiest
tests to perform, with easier to interpret results - you either split
the star or not. For this kind of thing, it's usually better to
choose binaries of approximately similar magnitudes. Start with wider
pairs and then work your way narrower. This might also give you an
idea of the seeing conditions for the evening. If you can't split a
4" binary, then forget about fine details on the surface of Mars!
Again, it's not the only possible test for seeing or collimation, but
I like the fact that I can do it quickly, without to much additional
equipment, and get some useful information about my optics or seeing
conditions. Also, when my already cooled scope shows signs of poor
collimation using the star test, I like splitting binaries, if
conditions permit, to verify the quality of my collimation after I've
worked on it.
If you are going to do this, please understand that you must magnify
quite a bit to split very close binaries. I often use 300X or even
400X to split the closest sub-arc second doubles.
So... don't do as I did as a neophyte, and put a 52mm eyepiece in your
scope, and then wonder why the map called Epsilon Lyra a double-double
when you can only see two widely separated stars. Best of luck
splitting doubles - as with everything else, practice makes perfect!
You can also get lists of additional double star data online in many
instances, or in almost every sky map in software or book form.
Good hunting!
Roland Prevost
