How to figure out what is wrong with your telescope when the stars appear distorted
All images and text Copyright Mike Lockwood, 2012
So
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.
How do you figure out the source of the distortion? I hope
this article is helpful in leading you down a path that will lead to an
explanation.
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 from being confused with another issue.
Also,
understand that you may be seeing more than one distortion on top of
another one or two! This can make deciphering the puzzle a bit
difficult, but hopefully this article will allow you to identify those.
If it's early in the evening, understand that your mirror will
likely be cooling off, and strange things can happen. Air
currents can form, cooling parts of the mirror faster than others.
A breeze can set up asymmetric airflow in the telescope's tube or
mirror box, also cooling some of the mirror faster than the rest.
Both of these are a recipe for strange star shapes that go away
after a bit of cooling. This goes for the primary mirror and/or
the secondary, especially if it's fairly large.
First,
before you do anything else, try to determine if you have
astigmatism in your eyes. This is fairly simple - if you turn
your head, astigmatism in your eyes will rotate with your head. If you don't do this at 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 your own pupil.
Move your eye side to side while looking at a star - if the shape
changes, your own pupil is blocking some of the light and may be
causing the strange shape you are seeing. In this case, raise the
power or get more dark adapted so your pupil is larger and does not
block the light. (If you are already using high power, you may
wish to try low power to see this effect for yourself.)
Now, having ruled out the possibilities given in yellow above, let's dive into the other possible causes.
Collimation and coma in a Newtonian telescope
Let's
say the star appears to "bulge" on one side on the inside of fucus, but
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 "flip"
on the other side of focus.
If
stars appear comet-shaped, this is almost certainly an aberration
called coma that is normal in mis-collimated Newtonians. With no
coma corrector in place, a star in the center of the eyepiece's field
of view should appear round. However, if the 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.
On the left of the center of the field of view, the bulge or tail
will point to the left. In fact, the bulge/tail will always point
away from the center of the field.
This leads to a collimation
method I have used for years - with the star centered in the eyepiece,
if you see coma, move the star toward the tail of the coma or the
bulge. This 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 figure out which
collimation screws move the star in a particular direction, and you can
put the coma-free spot in the center of the eyepiece's field. At
this point you are going to be fairly well collimated in terms of the
primary's tilt. (I assume that the secondary mirror is adjusted
properly. For instructions on that, I'd recommend reading up more
on collimation in detail.)
Pinched optics
Pinched
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.
Pinched
optics have to be pretty bad to make a star look like a triangle, but
it happens!
More than likely you will only see a hint of triangularity over a
basically circular shape. Chances are that it is being cause by
an optic that was pinched during
manufacturing or that is being pinched in the telescope. While it
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
temperature extremes.
First,
inspect the mounting of the primary mirror. All clips and other
retaining screws should not contact the mirror, but they should come
close. If you find a screw tightened against the mirror, even a
small amount, back it off and re-test.
If no optics appear to be
pinched, then rotate the primary. If the "points" of the triangle
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 the
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.
If
the distortion does not rotate with the mirror, then it is possible
that the primary mirror's cell is distorting it. If the issue is
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 will
often cure that. Moving the scope back down near the horizon
would then cause the problem to return.
Often
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.)
Lastly, 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 is
being pinched. Like all optics, it should be held gently.
What is astigmatism?
Astigmatism
is an optical aberration. Mirrors with astigmatism produce
oval-shaped, or in terrible cases, line-shaped stars. The oval or
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 at
focus.
Primary mirror astigmatism - polished in, and thermal
If
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 telescope's
optical axis, and not the ground. That is, note the orientation
of the ovals with respect to the tube of the telescope, not the ground.
Now,
rotate the primary mirror. If the astigmatism rotates with the
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, caused
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 the
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 around
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 change or go back to its original orientation.
If the astigmatism always follows the rotation of the primary on
all nights and over many hours, it is being caused by the primary
mirror itself.
Mirror-cell induced astigmatism and bad secondary mirrors
Let's assume now that you have checked, and the astigmatism does not rotate with the primary mirror.
If
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 fairly
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 mirror
with the sling or push at incorrect places on the edge, squeezing or
bending the mirror. The astigmatism should change with telescope
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.
If you
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.
Primary mirror strain-induced astigmatism
If
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.
When
exposed to a temperature change, especially a rapid one such as
bringing a scope outside into the cold, the mirror may go astigmatic.
The
good news is, as the mirror temperature equalizes, the astigmatism
should go away. Additionally, the strain should rotate with the
primary mirror and stay with its rotation over time as it slowly
disappears.
However,
if you like to observe shortly after bringing the telescope out, this
is not good news. You will be stuck with astigmatic images until
the mirror cools sufficiently.
Poorly aligned focuser or corrector
The
last cause of astigmatism that I have encountered is a poorly aligned
focuser or corrector. This can be a real problem for a faster telescopes
(those with lower f#s) and when coma correctors are used.
If all
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 a
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 have
found your culprit.