SKY_July2014.pdf

70 July 2014 sky & telescope

Quick Optical Evaluation

brighter star, and with larger ones, go fainter. Select a star

high in the sky where the atmospheric seeing is better.

Pick an eyepiece giving an exit pupil of 1 mm or slightly

smaller. A simple way to determine the exit pupil of a

telescope setup is to divide the eyepiece’s focal length in

millimeters by your telescope’s focal ratio. Thus, an eye-

piece with a focal length of 5 mm will yield a 1-mm exit

pupil with an f/5 telescope. For large-aperture telescopes,

seeing may require you to use lower magnifi cations —

longer focal-length eyepieces and larger exit pupils.

Star-testing is easier with a tracking telescope. Other-

wise you have to continuously nudge the telescope along

to keep the star closely centered in the eyepiece fi eld while

watching for moments of good seeing. For those of us in

the northern seats, Polaris is a good target for modest-

aperture telescopes that lack motorized tracking.

If you’re testing a refractor, try adding a green or

yellow fi lter to the eyepiece. Even the best apochromats

may have some color artifacts visible on out-of-focus star

images that veil the diff raction patterns. Refl ective optics

do not require a fi lter, but they can sometimes help you

see what’s going on. Experiment.

For most testing, defocus the star image enough to

see four to eight rings. Move quickly between the view

inside and outside of focus, and carefully compare their

appearances. This is a very sensitive test of optical quality

and you will likely see some diff erences, but minor ones

will not have a noticeable eff ect on the telescope’s perfor-

mance for in-focus observing.

Interpreting What You See

Artifacts in a diff raction image can arise if your telescope

is not at thermal equilibrium. As a telescope with a solid

tube cools, the colder, denser air slides down the lower

side of the tube while warmer, lighter air rises along the

upper side. This variation in air density can alter the out-

of-focus patterns. The eff ects of large temperature diff er-

ences are blatant, appearing as amorphous blobs moving

slowly across the out-of-focus patterns. A nearly cooled

scope will show a dent in one side of the pattern well

inside focus, and a bulge on the opposite side of it when

well outside of focus. Thermal eff ects are oriented verti-

cally. If you change the tube’s orientation, you’ll disturb

them briefl y, but they’ll soon reorient themselves.

If your optics are not quite aligned, the rings in the

out-of-focus pattern will be eccentric and the shadow

of any central obstruction will appear off -center. The

in-focus image will have incomplete diff raction rings

squeezed off to one side. Gary Seronik’s column “No-

Tools Collimation” (S&T: Oct. 2013, p. 64) describes a very

easy way for collimating a refl ector with a star-test.

Bad seeing often blurs the planets and hides fi ne

detail. It can also make star-testing diffi cult. In his highly

recommended book, Star Testing Astronomical Telescopes

(Willmann-Bell, 2008), Harold Richard Suiter explains

that poor seeing makes the outside-focus star image look

as if the dappling of sunlight on the bottom of a pool

is washing across the aperture. Under these conditions

you’ll need to wait for moments when you can separate

the true, unchanging features of the pattern from the

eff ects of seeing. On some nights it may be a hopeless

task, but on most nights patience will prevail.

Inside- and outside-focus patterns that don’t appear

round are signs of astigmatism. A small amount of astig-

matism results in oval rings, and moving the eyepiece

from one side of focus to the other will change their ori-

entation by 90° in the fi eld of view. But astigmatism may

not mean a problem with the telescope’s optics. Turn your

head. If the oval pattern also turns, the astigmatism is in

your eye. You should also rotate your eyepiece to see if it is

a source of astigmatism. In a Newtonian, note the orienta-

tion of the oval pattern and rotate the primary mirror 45°.

If the oval also rotates, then the primary is the problem. In

a Newtonian refl ector, a slightly concave or convex second-

ary mirror may cause images to appear astigmatic.

If the pattern shows rings that are irregular, especially

if they show sharp points, you may have some stress in

the way the telescope optics are mounted. Make sure any

clips holding a mirror in place or retaining rings securing

lens elements are not overly tight.

A common and signifi cant problem with Newtonian

mirrors is what opticians call a turned-down edge, mean-

ing that the very outer part of the mirror is too fl at. In

A quick glance at an out-of-focus

star image is all that’s needed

to tell if a telescope’s optics are

properly aligned (collimated), since

optics that aren’t aligned, such

as those in the 6-inch Newtonian

refl ector used for this image, will

show eccentric diff raction rings.

Astigmatism causes the diff raction patterns of out-of-focus star

images to appear as ovals with an orientation that “turns” by 90°°

when the eyepiece is racked from one side of focus to the other.

The accompanying text explains how to quickly determine if any

visible astigmatism is introduced by the telescope optics, the

eyepiece, or your eye.

Inside focus Outside focus