How to test, compare barlows?

What's the best way to compare and judge the quality of barlows?

I've ended up with several that are similar mag factors (2x, 3x) and would like to run my own comparative "shoot-out".

Are there particularly good test objects--planets, certain DSOs, the Moon? Particular aberrations to look for?

Should I only test on pristine nights? Would it matter which scopes I'm testing on--or would the results transcend type of scope?

To pose this question a different way: How did you decide which barlows to keep and which ones to sell?

One method suggested by Ed Ting is to aim your favorite eyepiece at a uniformly, moderatetly lit light source and slip a barlow over the end of the eyepiece as you look through the eyepiece. You may have to do this several times to get it to work but you will notice how the barlow dims the image a bit. Some barlows dim the image more than others or add some color to the image. In comparing my Orion apo shorty ultima clone to my 2" GSO barlow from Owl Services, I find a bit more dimming with the Orion barlow versus the 2" GSO barlow.
Your mileage may vary...

Ideally the test would be done on an optical bench with
a collimator and scope of your choice, but if that is not
an option, I would try to do a star test on a bright
star like Vega. Use a good quality scope.
Do the test with the star high in
the sky to minimize air turbulence effects.

First test only the scope with a good say 5mm eyepiece.
Look at the patterns the star makes just inside and
just outside best focus. If the seeing is good, the star
will form patterns of concentric rings on both
sides of focus. Now put in the Barlow with some
other eyepiece that comes out to about the same power
(i.e. like 10mm for a 2x Barlow). And repeat the
test. What changes do you see in the patterns
inside and outside focus? Ideally there would be no
change in the patterns (but I have yet to see a Barlow
that good).

Things to look out for are:

Chromatic aberration --
is there a color halo or color patterns in the star test
rings that was not there before? Is there a
violet halo that was not there before?

Spherical aberration -- are the ring patterns
identical on both sides of focus? Ideally the rings
inside and outside focus will be identical.
Or is there now a distinct difference between
the patterns inside and outside focus that was not
there before? Fast scopes (F/4 or F/5) will
bring out spherical errors in Barlows more than
slow scopes (F/12 or F/15).

Lateral color -- one error I sometimes see in
Barlows is that the light becomes spread out
into tiny spectrums. This is caused either by
wedge in one of the Barlow elements, or bad alignment
of the elements. In the milder case, star
images might be red on one side and blue on
the other. (This might be easier to see on
Jupiter, but you'll have to wait a while to do
that.) If you think you see some of this spectrum
effect, try rotating the Barlow -- does the
effect rotate with the Barlow? Make sure the star
is in the field center for this test.

A related test is to rotate the Barlow while
looking at a star. Ideally the star should
stay in the same place. But if the star moves in
a circle as the Barlow is rotated, then there is some
wedge or collimation problem.

Ghosts and reflections: if there are bright
or shiny surfaces inside the Barlow you can
get ghost images and stray light. This would
be most easily tested by pointing the scope
just off the moon and moving the scope around.
Do you see funny reflections or stray light filling
the field of view that come and go as you move the
scope near the moon? Make sure to test without
the Barlow also.

Some barlows may match certain scopes better than others,
so you might also think which scope you will use it
most in, and test with that. Also Barlows can match
some eyepieces better than others, so give some thought
to which eyepieces you will use the most.

Good luck!

Take your favourite telescope and look at your farourite objects on your average night with different eyepieces and with/without different Barlows.

Dmitri

Well, I agonized over whether my barlow should be a 1.5X or a 2X, a 1.25" or 2",then I calculated what the magnifications would be and finally realized that for the price of the 2" barlow I could buy 3 Televue Plossls. Guess what I decided.