Some Collimation Myths and Misunderstandings
At last updated per march 27, 2015 – after ten years
It seems that the
collimation of Newtonian reflectors is a subject full of widespread myths and
misunderstandings - this webpage attempts to put some of them in their proper
perspective - to the best of my ability.
myth: You have to square
the focuser very accurately
I'm not quite sure of even
what "square" is supposed to mean - likely it means set perpendicular
to the tube, or possibly to the optical axis - or both, always assuming you
have made them coincide. There is nothing wrong with doing it, of course, but
the secondary is optically flat, and the angle of reflection is not critical.
Most secondaries are made to look circular when
tilted 45 degrees (to reflect 90 degrees), but if the angle deviates from this
by a few degrees, the only consequence is that the secondary will appear
slightly elliptic - it won't affect the image.
The important thing is that
the focuser and the secondary are lined up, as seen from the focus. If not, you
should adjust either, as appropriate - if the secondary seems to be off in a
direction from or towards the primary, you can usually move the bolt that holds
it in the spider. If the error is "sideways", and the secondary is
indeed well centered in the optical tube, the first thing to check is the
rotation of the secondary holder. If rotated some, you can still tilt the
holder to reflect the focuser axis to the center of the primary, but the holder
will look tilted. Correct the tilt and rotation – if still not centered, it may
be the focuser that is off to one side and needs shimming. If this error is
left alone, the penalty is that the fully illuminated field at focus won't be
centered - a small error here is no disaster, but the center of the field
should always cover the center of the field of view, and best with a small
margin to avoid the effects of a possible narrow turned-down edge on the
secondary. Check with a sight tube in daylight when the telescope is collimated.
When the peephole of the sight tube is close to the focal plane (top of the
drawtube at a "normal" position), the whole edge of the primary
should be visible in the secondary. If the focuser is tilted a little
"down" or "up" the tube, this will be compensated for when
adjusting the tilt of the secondary - the only consequence is that the fully
illuminated field will be slightly elliptic - a thing you will never notice.
myth: The primary mirror
must be exactly centered within the tube
No harm in it, of course.
But unless you also offset the secondary correctly (and I believe not many do),
the optical axis won't be centered in the upper end of the tube, anyway. What
is vitally important to good collimation, though, is that the mirror stays in
place, centered or not. The common 180 deg sling,
properly designed, will do a fine job of balancing the forces that might cause
astigmatism by "potato chipping" of a thin mirror. But such a sling
will not prevent the mirror swinging "sideways" when the tube is
moved - and with a f/4.5 mirror, 1 mm of swing will
cause 1 mm of miscollimation at the eyepiece.
This is enough to affect performance noticeably with critical viewing. (For the ATMer, I have a modification that takes care of this
myth: The secondary
mirror must (or must not) be offset
There are indeed two
separate “offsets” of the secondary, and both are really centering in two
directions – even though the distances of offsetting are equal.
“Offsetting” towards the
primary mirror is done to center the fully illuminated field. By centering the
secondary mirror as seen in a suitable sight tube in the focuser, this “offset”
is automatically done, and you may not even be aware of the fact. Without such
a sight tube, collimate the focuser axis (using a laser or crosshairs
combination “Cheshire” tool). Now, the primary is by definition centered on the
focuser axis, so look in a peephole device such as a collimating cap to see if
the secondary is centered on it. If so, the secondary is correctly placed (“offset”)
along the tube. If not, move it and re-try.
If you want the optical
axis (that is aiming at a star that you see in the center of the field of view)
to be centered in the tube, at the level of the focuser, you need to offset the
secondary away from the focuser. This offset may even be built into the
secondary holder. If not, you can safely ignore it, the optical performance is
not affected, and the aim is usually within a single degree from the tube axis.
myth: A laser collimator
is the most accurate collimating tool
It is easy to be impressed
by the accuracy of laser beams in e.g. construction work, and jump to the conclusion
that the laser is as accurate for collimating telescopes. I agree that it is
the most convenient tool for adjusting the tilt of the secondary, and you can
center the beam in the donut opening to within a millimeter or a few - far
better than really needed. But when it comes to adjusting the primary mirror, thismay not be good enough - the collimating error is in
fact half the vector (i.e directions considered) sum
of the errors at the primary and at the laser faceplate where the beam returns.
If you know this (the laser manufacturers won't tell you how important it is!)
and you can center the beam accurately enough, fine - but I
suspect with many telescopes it is difficult or impossible to get both
adjustments close enough to get this most critical part right. The good old
Cheshire eyepiece shows only the error of the primary, and so does the Barlowed Laser - you have a better chance of getting
close to perfect with either (for a better description of the Barlowed Laser, see Sky&Telescope,
manufacturers tend to claim that their products have very small angular errors
of the laser beam (relative to the housing). This is not quite as critical as
they believe - the angular error will cause the collimation of the focuser to
be slightly off, but it won't affect the critical collimation of the primary
enough to matter.
myth: If the laser isn't
the most accurate tool, then surely the autocollimator is
In the 4th
edition of "Perspectives of Collimation", Vic Menard (with Tippy D'Auria as co-author) writes: "
before we decided to write this book, the autocollimator was without
question the least understood of the collimation tools".
Since then, Vic has written
a 5th edition, greatly revised and improved: see http://www.vicmenard.com/telescopes/
See my analysis of the reflections that can be seen in the
autocollimator to find out what it shows – in particular, with Jason D’s
Note that to see the
reflections, you need a bright, illuminated (and reflective) center spot on the
primary – such as Jason D’s HotSpot.
For both, see http://www.catseyecollimation.com/
Used together with a
Cheshire (including Catseye’s BlackCat)
or Barlowed laser to set the collimation of the
primary, it can be used to some advantage in checking and adjusting the
collimation of the focuser axis - in particular, if this was initially set
using a sight tube, but possibly also with a laser. This may not really make a
visible difference, but if you like to use one, I suggest this is the best way
to use it:
1. Do a reasonably close collimation
with the sight tube/laser and the Cheshire/Barlowed
2. Use the Cheshire/Barlow to check,
and if necessary adjust, the primary (unless this is done accurately,
the reflections in the autocollimator won't mean anything useful!)
3. Adjust the secondary, or better the
focuser if it is adjustable, to make the reflections "stack". When
they do, go back to 2 and check - if the primary is still accurately
collimated, as it will be within a few iterations, stop there!
Thus, the Cheshire defines
the critical collimation of the primary, while the autocollimator finishes the
less critical collimation of the focuser axis.
What you shouldn't bother
to do or try is:
the darkening of the autocollimator's reflection as a criterion of collimation
(as suggested on Tectron's instruction page). It
takes gross miscollimation to "open the light
path" - far more than would be left even by a casual collimation as in 1
solely on adjusting the secondary - if the optical axes are not coincident
here, no adjustment will bring them together. You need to alternate by
adjusting the secondary and the primary, as described by steps 2 and 3 above.
myth: You should always
finish collimation by star collimation
At the focal plane of a
paraboloid mirror, there is one point where coma, the one-sided aberration, has
a minimum. This point is, by definition, the focal point. Locating it can
really only be done by star collimation - using a real star, or a sufficiently
distant artificial one, collimating the primary mirror for a symmetric star
image (high magnification, and image close to focus for best sensitivity) at
the center of the field of view. If you put a small peephole at the true focal
point and look, you will see a distant reflection of the peephole itself (if
its inside is illuminated), as if far behind the mirror surface. The spot on the
mirror that the peephole seems to lie behind is - again by definition - the
optical center of the main mirror.
The common thing to do is
to place a spot at the geometric center of the primary mirror and use it with a
Cheshire eyepiece, centering the reflection of the Cheshire behind the spot.
Normally this is close enough (I am still waiting to hear about anyone with
a mirror where the optical center is significantly offset from a well centered
spot!). Using the Cheshire is a lot easier, quicker and better reproducible
than star collimation is - particularly with less than excellent seeing.
However, if you find the one-sided asymmetry of coma on a star at the center of
the field, it can only mean one thing - the center spot isn´t at the true
optical center. The best thing is to move it or put a new one, centered in the
reflection of the Cheshire (the next best is to make a note of the direction
and distance of the offset), for later collimation. Now you know you have the
true optical center marked.
Thus, you should take the
trouble and do a careful star collimation under good seeing to ensure that the
primary is accurately marked - but when this is done once, collimating with a
Cheshire or Barlowed laser is easier and more
reproducible. There is no point in star collimating every time. Recently, Jason
D, in a thread on Cloudy nights forums, has added
valuable knowledge: start with the star centered and focused, then carefully
defocus outward to see where the diffraction rings first emerge. The more
common way to see the centering of the emerging secondary shadow is far too
last updated Mar 27, 2015 /Nils Olof