Why aren't my
How to figure out what is wrong with your telescope when the
stars appear distorted
All images and text Copyright Mike Lockwood, 2013
you've had a telescope for a while, or just gotten a new one, and you
notice consistent or inconsistent distortion in the star images.
To put it another way, stars aren't round in the
inside or outside of focus, or maybe even at best focus. This
may be intermittent, or may always be the case.
How do you figure out the source of the distortion?
this article is helpful in leading you down a path that will lead to an
There are many possible causes of distorted, non-circular star shapes.
You may even be seeing more than one
distortion on top of
another one or two! This can make deciphering
the puzzle a
difficult, but hopefully this article will allow you to identify one
fairly easily, and this will then allow you to track down the other.
In order for this article to make sense, first we have to set some
ground rules and satisfy a few conditions before we can move into
diagnosis of the problem(s).
For all of the
advice to follow, please put the star in the center of the field of
view and keep it there.
This is important for determining if the issue is due to
collimation, and reduces the chance of some other eyepiece or corrector
aberration being confused with another issue.
Before you do anything else, determine if you have
astigmatism in your eyes.
This is fairly simple - if you turn
your head, astigmatism in star images that is caused by astigmatism in
your eyes will rotate with your head.
You may even already know from your eye doctor that you have
astigmatism. However, if you don't rule out this
problem first, you may drive yourself crazy
trying to figure out what is going on!
Next, take a look at what
power you are using - if it is fairly low, the distortion may be coming
from the pupil of your own eye vignetting the light cone.
Move your eye side to side while looking at a focused and defocused star - if the
changes or the secondary shadow appears to move around, your own pupil
is blocking some of the light and may be
causing the strange shape you are seeing. In this case, raise
power and/or get more dark adapted so your pupil is larger and does not
block the light cone. If you are already using high power, you
wish to try low power to see this effect for yourself and know what it
Fourth condition: If it's
early in the evening, understand that your mirror will
likely be cooling off, and strange things can happen.
currents can form, cooling parts of the mirror faster than others.
A breeze can set up asymmetric airflow in the telescope's
mirror box, also cooling some of the mirror faster than the rest.
Both of these are a recipe for strange star shapes that go
after a bit of cooling. This goes for the primary mirror
the secondary, especially if it's fairly large.
Fifth condition: For this article,
we are assuming that your mirror cell is properly designed and set up,
including the edge support of the mirror. If
your optics are
glued to the cell, then you cannot easily rotate them to help rule out
some possible causes of non-round stars. Gluing a mirror (other
than a small secondary) to a
cell made of something other than glass will often cause it to warp as
the cell shrinks by a different amount and pulls on the mirror,
creating optical problems. I do not recommend
mirrors to their cells/holders, as it will often cause non-round stars.
If your primary mirror and or large secondary mirror is glued
something, I recommend finding a way to mount it without glue, and then
coming back to this article if problems remain.
Now, understanding the possibilities given above, let's begin.
is an optical aberration. Mirrors with astigmatism produce
oval-shaped, or in terrible cases, line-shaped stars! The
line rotates 90 degrees as the eyepiece is moved to the other side of
focus. In mild cases, stars still generally appear round at
focus, though diffraction rings, if they are visible, may appear
distorted. In severe cases, the star will appear cross-shaped
coma in a Newtonian telescope
far the most
common reason for non-round stars in a Newtonian telescope is poor
collimation. This produces an optical abberation called coma.
So, we'll discuss this first, then move on other less common
IF YOU SPEND AN HOUR OR TWO LEARNING THE COLLIMATION
DESCRIBED BELOW, YOU MAY IMPROVE YOUR TELESCOPE'S IMAGES
WITHOUT HAVING TO SPEND A CENT ON NEW COLLIMATION TOOLS.
also have a sharper eye for collimation than most other observers.
NOTE: Lasers can
easily go out of alignment, and this will
seriously throw off telescope collimation. Put
your laser in the
focuser and push the top of the laser against the top of the focuser.
I don't usually tighten the thumbscrews, because they often
tilt the laser within the focuser. Now rotate the laser in
the focuser (you can also do this in a v-block). If the laser
spot moves in a circular
path on the mirror, then the laser needs to be aligned.
Consult a knowledgeable
astronomer or the manufacturer to get help with this.
3) NOTE: Using
only collimation tools - laser, barlowed laser,
cheshire, autocollimator, etc., I almost never achieve perfect
collimation. That is, I can almost always detect
some coma in a
star image using the technique described below. So, I always
finish up collimation using a star image, and that is what I will
First, go through your normal collimation procedure, paying particular
attention to making the secondary mirror alignment as close to perfect
as you can get it.
If you need help with this, seek help elsewhere or from a
If you can nail the secondary alignment, the instructions
below will then help you get close to perfect primary collimation.
with a coma corrector in the focuser if the telescope is f/5 or below,
and a fairly high-power eyepiece
in it, put a star in the center of the field of view and keep
there using a drive or by hand-tracking.
Defocus the star slightly inside then outside of focus. Not
the star shape. Let's
say the star appears to "bulge" on one side on the inside of focus, and
the "bulge" is still on the same side as you look at the image outside
of focus. In this case, you are likely seeing an issue with
collimation that needs to be addressed by tilting one of the mirrors.
Pinched optics, or other distortions of the mirrors will
on the other side of focus.
stars appear comet-shaped (a bright point near one corner with a tail,
somewhat triangular), this is almost certainly an aberration
called coma that is normal in mis-collimated Newtonians. With no
coma corrector in place, a star placed precisely in the center of the
of view should
This is because a Newtonian telescope will produce a
round star image at the center of the field if it is collimated
properly. However, if the aim of the telescope (without coma
corrector) is adjusted so
star is moved to the
right of the center of the field of view, you will see the star start
to bulge or it may appear to grow a tail that points toward the right.
Likewise, on the left of the center of the field of view,
will point to the left. In fact, no matter where the star is,
bulge/tail will always
away from the center of the field, except at the center of the field,
where the tail does not exist.
If you don't know what coma looks like, it may be a good idea to remove
the coma corrector, put in a high-quality eyepiece, move the star
around the field, and see coma for yourself.
Putting a coma corrector in the focuser will help correct the coma IF
the telescope is collimated properly. So, with a corrector
the focuser, if the aim of the telescope is again adjusted so that the
star is to the right of the center of the field of view, the star will
still appear mostly round, and will either not grow a tail or not grow
nearly as large of a tail as it did when there was no corrector being
used. This is because the coma corrector is reducing or
eliminating the coma.
But here is another important point - if the primary mirror
is not collimated properly, residual coma due to
mis-collimation will still be seen even with the coma corrector in the
For the collimation method I describe below, I actually
leaving the coma corrector in the focuser, because the coma due to
mis-collimation can still be seen.
The above observations and information lead to a collimation
method I have used for years: With the star
centered in the eyepiece and a coma corrector in
the focuser, observe the shape of the star in focus. It
fairly round. (If not, read the rest of this article.)
Now defocus the star very slightly. We're not
talking much, just
enough to expand the star slightly - a fraction of a turn on a
fine-focus knob. When you do this, you may see an obvious
grow, indicating significant coma. In that case, you have a
of coma, so skip the paragraph below and follow the directions in the
If coma is not obvious, don't just look at the shape of the star after
you defocus it - observe how the star expands. That is, as
just barely start to defocus the star, watch to see if the star image
expands first in a particular direction, or "bulges" in that direction.
In doing this, we're looking to see HOW the star expands, not
what it looks like when it's expanded. It is subtle at first,
more obvious with practice. Doing this very close to focus
increases the sensitivity of the test.
To help with this, note
the location of the star at best focus, and stare at that spot.
Now defocus inside of focus and see if the star simply
all directions at once, or if it "bulges" or "pulls" in one direction,
Repeat going outside of focus. If you see the bulge
same direction as going inside of focus, repeat by defocusing in both
directions a few more times to confirm that it is consistent and is not
a strange air current or moment of bad seeing. If it is
consistent, chances are it is coma.
Now, if you see coma, either a lot of it or a little, use the primary's
collimation adjustments to move
the star toward the tail of the coma or the
This may easier if you have a friend helping you by turning the
collimation bolts. Using the
primary's collimation adjustments to move the star tilts the
mirror so that the optical
axis (the spot
where a star will appear round even with no coma corrector being used)
gets closer to the center of the field of view of the eyepiece.
By making successive adjustments, you can put the
coma-free spot in the center of the eyepiece's field.
NOTE: If you are working alone,
recommend first turning each bolt in both directions and making a "map"
how each one moves the star in the eyepiece. Then you can be
of turning the right bolts to move the star where you need it.
Keep making adjustments until the star expands uniformly in
all directions, and does
not "bulge" or "pull" in one direction. At
this point your telescope will be fairly well collimated in terms of
primary's tilt, and you're ready to observe. Check
collimation by this method throughout the night to see if it drifts,
and tweak if necessary. It is important to note that the
collimation of the telescope may shift
subtly or obviously as the scope is used throughout the night and/or as
it is moved up and down in altitude. These adjustments may be
required more than once each night to ensure optimal telescope
If you master this collimation technique, please tell your friends
where you saw it, and teach it to someone else.
optics means that the mirror is being squeezed by something.
Generally, since three points can locate a mirror laterally,
the effect of the "tightest" three screws are what is
seen, and that means a three-sided shape is created by the mechanical
distortion, which of course is a triangle. However, by
squeezing the mirror on two opposite sides, I have seen highly
astigmatic images created by pinched optics.
optics have to be pretty bad to make a star look like a triangle, but
More than likely you will only see a hint of triangularity
basically circular shape. Chances are that it is being cause
an optic that was pinched during
manufacturing or that is being pinched in the telescope.
may seem minor, you will want to track it down because it may become
worse at various temperatures, since metal and glass expand at
different rates, and the glass may be more severely pinched at
inspect the mounting of the primary mirror. All clips and
retaining screws should not contact the mirror, but they should come
close. If you find a screw tightened against the mirror, even
small amount, back it off and re-test. You should be able to
"rattle" the primary in the cell. It should not be "snug" in
the cell, it should be slightly loose in all directions.
If no optics appear to be
pinched, then rotate the primary. If the "points" of the
rotate with the mirror, chance are the mirror was pinched during
manufacturing. In this case, let the mirror cool after it is
rotated and observe the star shape an hour or two later. If
points of the triangle go back to the original orientation, then the
triangularity is likely being cause by cooling, such as by three fans
or three ventilation holes.
the distortion does not rotate with the mirror, then it is possible
that the primary mirror's cell is distorting it. If the issue
poor edge support, then the problem will likely disappear if you point
the telescope straight up and ensure that the primary is not "stuck"
against the edge supports. Giving the scope a quick shake
(thereby "rattling" the mirror in its cell)
often cure that. Moving the scope back down near the horizon
would then cause the problem to return.
mirror cells have only three edge supports, and if the mirror is heavy
enough, it can effectively become wedged between the two bottom
supports when the telescope is pointed low, and this squeezes the
mirror. (If the mirror is 10" or larger, I don't recommend a
three-edge-support cell.) This will cause triangular star
images and/or astigmatism as the telescope is pointed low in altitude,
and it may improve if the scope is pointed high. The mirror
may also get "stuck" a little bit and may remain pinched until it is
"rattled" in its cell.
Some mirror cells may actually pinch the mirror from time to time, due
to friction/stiction. That is, if the mirror sits on the cell
a long time, some of the contact points (plastic, fellt, cork) may
actually adhere to the mirror. The solution is simply to move
mirror in the cell, or rotate it slightly, just enough to break the
stiction, and get it "un-stuck". This may result in a
improvement in the star test, and I have seen it cure a major problem
in a couple of cases.
It is possible
to have the secondary cause triangular distortion, but it is not
common. As a last resort, examine its mounting and see if it
being pinched. Like all optics, it should be held gently.
mirror astigmatism - polished in, and thermal
you notice astigmatic star images, the first step is to note the
orientation of the "ovals" on the inside and outside of focus.
Make sure that you record this with respect to the
optical axis, and not the ground. That is, note the
of the ovals and which side of focus you are on with respect to the
tube of the telescope, not the ground.
rotate the primary mirror. If the astigmatism rotates with
mirror, then the primary mirror is causing the astigmatism. HOWEVER, you must now
observe the astigmatism over several hours to see if it changes or goes
back to its original orientation.
If the astigmatism goes back to its previous orientation over
time, then chance are that you are seeing THERMAL astigmatism, likely
by unevenly cooling the mirror.
Thermal astigmatism may appear or
disappear only on windy or still nights, so more than one evening of
testing may be required. If you see a pattern, look at how
cooling fans are positioned or the air flows through the mirror box due
to wind. Having only the bottom of the mirror box open at the
back may cool the bottom half of the mirror more, causing uneven
temperature. Having fans in a line rather than distributed
the mirror is also a usual suspect.
In all cases, though,
thermal astigmatism will appear to rotate with the primary at first,
but then it will eventually change or go back to its original
If the astigmatism always follows the rotation of the primary
all nights and over many hours, it is being caused by the primary
One form of thermal distortion that does not rotate with the primary
mirror is the effect of an air current. It will typically
one side of a star image, but the effects can range from minor to
severe depending on the temperature differences involved. It
should be recognizable in out-of-focus star images as a slowly moving,
distorted area on one side of the image. In some cases, it
much worse. A closed tube will make it worse, and pulling up
shroud on a truss-tube dob may help alleviate it and aid in primary
induced astigmatism and bad secondary mirrors
Let's assume now
that you have checked, and the astigmatism does not rotate with the
the ovals (defocused star images) appear roughly aligned with the tube
of the telescope, that is the oval on one side of focus is aligned with
the tube and on the other side of focus it is perpendicular to the
tube, then the likely culprits are a bad secondary mirror or the edge
support of the primary mirror.
A bad secondary mirror will have
a curve to its surface, and this cause astigmatism because it is
mounted at a 45-degree angle. The astigmatism should be
consistent for all telescope altitudes.
A mirror cell that is
causing astigmatism usually has inadequate edge support, such as a bad
sling or poorly selected contact points. It will pinch the
with the sling or push at incorrect places on the edge, squeezing or
bending the mirror. The astigmatism should change with
altitude, and if you point the telescope straight up and give it a
"good shake" to make sure the mirror isn't hung up on anything, then
the astigmatism should all but disappear.
A properly-positioned cable
sling should introduce a small amount of astigmatism in a primary
mirror, but not a large amount. If this is not the case, you
should make sure the cable is positioned properly and that the mirror
is not bumping into something else due to a slack cable.
have astigmatism that actually seems to get better as the telescope is
pointed lower, then you may have a mirror cell with triangles or
supports that are bound up. I have seen this happen more than
once. Make sure the mirror is sitting on all of the support
points, and that the triangles or pivots can move freely.
If, after adjusting the cell, rotating the primary, shaking the scope,
or even giving the mirror box a good whack with your first, images
suddently become good and stars round, you have freed something that
was bound up.
Don't be surprised if the fix is as easy as bumping some part of the
scope to free it up. The moral of the story is this - you
make stars round accidentally - you make them round by fixing problems.
Primary mirror strain-induced
the primary mirror is made from poorly annealed glass, it may have
strain. Strain is internal stress within the glass, and it is
more likely to be found in cheap mirrors made from cheap glass.
exposed to a temperature change, especially a rapid one such as
bringing a scope outside into the cold, the mirror may go astigmatic.
good news is, as the mirror temperature equalizes, the astigmatism
should go away. Additionally, the strain should rotate with
primary mirror and stay with its rotation over time as it slowly
if you like to observe shortly after bringing the telescope out, this
is not good news. You will be stuck with astigmatic images
the mirror cools sufficiently.
aligned focuser or corrector
last cause of astigmatism that I have encountered is a poorly aligned
focuser or corrector. This can be a real problem for a faster
(those with lower f#s) and when coma correctors are used.
other possible sources of astigmatism have been eliminated, check the
squareness of the focuser and make sure that the eyepiece and coma
corrector are in the focuser squarely. One time I experienced
case where the setscrew of a focuser pushed the coma corrector of to
one side and caused it to tilt slightly, introducing astigmatism and
other aberrations into the images of an f/4 telescope. I
quickly fixed the problem.
As a last resort, try
rotating the corrector and/or eyepiece in the focuser to see if the
astigmatism rotates with either of those. If it does, you
found your culprit.
thermal effects on optics
let's conclude this article by looking at one issue that involves round
stars. Thermal effects on optics aren't always of a nature
causes stars to be non-round - sometimes they affect the perceived
correction of the optic. Many people star test
help them determine the quality of their optics. However,
often mistake the effect of dropping temperatures on the
for optical problems.
It is completely normal for a well-corrected, high-quality mirror to
appear overcorrected while it cools. This condition is
by obvious rings in the structure of a defocused star on the outside of
focus, and indistinct rings on the inside of focus. As the
approaches air temperature, this condition gradually goes away.
The reason for this is the fact that the mirror doesn't cool evenly -
it cools faster near the edge of the mirror and wherever the miror is
exposed to the cold sky above (obviously the front of the mirror is
exposed to the sky) or to cooling breezes or fans. The cooler
areas physically shrink, changing the shape of the mirror, often
resulting in perceived overcorrection.
For example, let's say the outer part of a 2"-thick mirror is one
degree Celcius (1.8 deg F) cooler than the rest of the mirror.
The coefficient of thermal expansion of Pyrex/Supremax
(borosilicate glass) is 3.25*10^-6 per degree C. If we assume
that the front of the mirror shrinks downward toward the center of the
mirror and the back of the mirror shrinks upward toward the center of
the mirror, then this means that we're interested in the dimensional
change of a 1" thick piece of glass.
So, a one degree lower temperature in the outer part of the mirror
means that the 1" of glass underneath the optical surface shrinks
downward by 3.25 millionths of an inch, or about 1/7 wave on the glass,
2/7 wave on the wavefront. This pulls down the outer part of
mirror, effectively overcorrecting it. The effect is visible
star test and looks like overcorrection. It will affect
A difference of two degrees C, or 3.6 degrees F, results in shrinking
by 2/7 wave on the glass, or 4/7 wave on the wavefront. This
should be very easily visible in the star test, and the effect on
images will be significant too.
For many decades it has been a common practice by some to leave mirrors
undercorrected to help
compensate for this, but that results in an inaccurate mirror when it
gets close to equilibrium. These mirrors are then
undercorrected when optimal observing conditions are experienced, and
will only perform optimally when cooling conditions exactly match the
amount of undercorrection that is figured in, which will be quite rare.
has been suggested by others in the past that insulating the edge of
the mirror could reduce temperature imbalance within the glass, and may
help reduce the temperature-induced overcorrection. With the
lower-quality "light bucket" optics of the past, the effect of
insulating the edge may not have been noticeable or significant.
However, now that we are in
a modern era that features more high-quality, large, fast mirrors, it
may be time to try this. I have not heard of anyone trying it
recently, and I have not tried it myself since my 20" f/3 mirror is
1.25" thick, and the thermal effects are not as significant.
shrinks more than glass, so in some cases, a shrinking mirror cell can
actually pull very slightly on a mirror. This may also
contribute to a
perceived overcorrection. Giving the mirror a "shake" may
help in some
article has been many years in the making. I've experienced
kinds of telescope problems including most of the conditions that I've
described above. I've also refined my collimation technique
I describe above so that I can always be assured of having good
collimation that lets very fast mirrors perform at their best, and
just as well as slower instruments.
So, the problems mentioned above can be very real, as are the
I've been filing them away in my memory, and it was finally
to put them into an article. As an optician, it is also in my
best interest to educate people on what can hurt or help the
performance of telescope optics. Quite honestly, educating
telescope owners reduces
the number of issues that I am blamed for, and allow their owners to
fix them and enjoy the full potential of their often very expensive
I hope this article has helped you understand a bit more the behavior
of telescope optics under realistic conditions. I also hope
it has allowed you to improve your telescope's collimation,
has given you the skills to troubleshoot common problems with telescope
optics. If it has, please ask your friends to read it, and
them what you have learned.
I wish you clear, dark skies, and round, tiny stars.
Lockwood, Lockwood Custom Optics