As most of the readers of these pages will know, collimation is the secret to getting good performance from
a Newtonian telescope. The Celestron Collimation Eyepiece is a tool which allows the amateur to collimate a telescope
quickly and accurately, and in my experience, markedly better than collimating by eye alone or with a sighting
The process of collimation is, on paper at least, quite easy. The idea is to align the optical axes of the two
mirrors with the eyepiece holder. In practice, the problem is in being able to judge when this is the case, particularly
in being able to line up the optical axis of the eyeball with that of the eyepiece holder. To solve this problem,
many amateur astronomers use a 'sighting tube' a cylinder with an opening through its centre that plugs into the
eyepiece holder. By peering through the opening in the sighting tube, it becomes easier to accurately align the
eye with the optical axis of the eyepiece and the secondary mirror beneath it. A simple sighting tube can be made
from a 35 mm film cartridge, which luckily is just the right diameter.
Having bought a Celestron 114 mm Newtonian telescope (see Review elsewhere
on this site), I was keen to get the best out of the optics, which allowing for the aperture are really very
good indeed. Using a sighting tube, I managed to get consistently reasonable performance, splitting Epsilon Lyrae
without much trouble, for example. I like double stars, but some which should have been split, like Epsilon Bootis,
were not. I figured that the collimation was off, and that I needed to get something a bit more accurate that the
home-made sighting tube. I picked up the Celestron Collimation Eyepiece from my local astronomy shop, David Hinds
in Tring, Hertfordshire, England, for 27 UK Pounds. It appears that identical devices are sold under different
brands, e.g. Orion (US).
The actual process of collimation was not quite as simple as the supplied instructions would suggest, and after
some trial and error I have found what seems to be one way of doing it well.
What follows may seem a little severe, involving taking the telescope apart. But it isn't difficult to do any
of the things described here, and the only tools needed besides a collimation eyepiece (or equivalent) are the
Allen key and screwdriver that came with the telescope, a sheet of paper, some string, adhesive tape, a ruler and
and a small piece of sticky paper. Moreover, if the mirrors of the telescope are seriously misaligned, this method
should return them to their proper positions.
Step 1 : Remove the primary mirror
Tilt the telescope so that the aperture points steeply downwards and set the locking clamps. Use the screwdriver
to remove the three small screws and put them somewhere safe. Lift off the primary mirror housing and place it
carefully on a table. It is probably best to put it mirror downwards so that dust doesn't settle on it (the screws
on the mirror clips will prevent the mirror actually touching the table).
If you wish, you can mark the centre of the primary mirror with a small (2-3 mm width) piece of sticky paper.
This helps adjustment of the primary
mirror later on.
Step 2 : Make a reference mark using string over the end of the optical tube
First undo the clamps and turn the telescope optical tube upwards until it is horizontal, then re-tighten the
clamps. Cut two pieces of string longer than the width of the optical tube. Using some adhesive tape, attach them
to the end of the optical tube to make a cross. This is a convenient reference mark for adjusting the secondary.
Looking from the back of the optical tube, where the primary mirror normally is, the view should be something like
Step 3 : Correct the position and angle of the secondary mirror
Carefully put a sheet of white paper inside the optical tube directly behind the secondary mirror and across
from the focuser. This results is a clearer view of the position of the secondary relative to the focuser tube,
and it should be very obvious whether or not the secondary mirror is centred under the focuser. Looking down the
focuser (without any eyepieces!) you should be able to see the inside of the focuser tube, the secondary mirror,
and the piece of paper on the other side of the optical tube; the view should be something like one these (simplified)
On the left is how the secondary mirror might look if it is not directly under the focuser, or angled toward
the primary mirror properly. The first error is shown by the asymmetry of the white space around the mirror; the
second by the reflection of the string cross not being centred. On the right is how it should look. Note the uniform
distribution of white space around the mirror and the centred reflection of the strings.
Corrections to the secondary mirror are made using the screwdriver and the Allen key. Use the central screw
to adjust the mirror forwards or backwards and the three hexagonal screws to adjust its tilt. Once done, remove
the sheet of paper carefully so as not to disturb the secondary mirror.
Step 4 : Double check the alignment of the secondary
Put the collimation eyepiece into the focuser. If you look into the eyepiece, the crosshairs in the collimation
eyepiece should line up with the reflection of the string cross at the end of the optical tube (you may need to
turn the eyepiece in the holder). Alternatively, remove the string and look up the optical tube from the back,
and you should see the eyepiece crosshairs reflected neatly in the secondary, like this:
Step 5 : Set the primary mirror
Remove the string cross if you have not already done so. Tilt the optical tube so that the front aperture points
downwards. Carefully replace the primary mirror and put back the screws. Check it is secure, and the tilt the optical
tube back to horizontal. Looking into the focuser, without an eyepiece, you should now be able to see the primary
mirror reflected in the secondary mirror. Since the secondary should be correctly positioned, the reflection of
the primary mirror should be more or less centred in the secondary. The angle of the primary mirror may well be
off, though. With the collimation eyepiece into the focuser, the view will be something like this:
(Note that this has been simplified, with the secondary mirror support, for example, not drawn in.) The aim
of the exercise is to put the reflection of the mirror in the centre of the secondary mirror. This is done using
the three adjustment screws at the back of the telescope. The Cheshire part of the collimation eyepiece helps here.
It casts a diffuse and uniform ring of light onto the primary. Centre this ring of light in the crosshairs. If
you have placed a paper mark at the centre of the primary mirror, this should be in the middle the ring. The result
should look like this:
Step 6 : Test the telescope!
The finderscope will need to be checked and adjusted as necessary. Once this is done it is time to star test
the telescope. Polaris is a good one to start off with, as it practically unmoving. A good reflector should reveal
the much dimmer secondary without much trouble. Concentrate on the primary. At focus, the star should be a point
surrounded by a small number (one or two) faint rings -- these are the diffraction rings, and are an intrinsic
result of the optics in these telescopes. These rings shouldn't be too obtrusive, i.e., they shouldn't completely
obscure a secondary star otherwise within the range of a telescope of a certain size. My own experience is that
Epsilon Bootis and the "Double double" Epsilon Lyrae are good tests. They should split clearly in a well
collimated 114 mm reflector (if atmospheric conditions allow).
A star test is essentially a high magnification observation of a star with a view to seeing how well collimated
the optics are. This is revealed by the diffraction of light by the optics, which manifests itself as a central
disc (the "Airy disc") and a small number of rings of decreasing brightness. The higher the magnification
you use, the
more obvious optical errors becomes, and so the more accurate the star test. A magnification of x 150 to x 200
would be ideal for a 114 mm reflector.
At focus, the star should be a point surrounded by a few faint rings -- these are the diffraction rings, and
are they an intrinsic result of the optics of the telescope. There is also a ray-like pattern running from the
centre of the star off to one side. This is another diffraction pattern caused by the arms that supports the secondary
mirror. The Firstscope 114 has only one arm, but other reflectors can have three or four such arms, and correspondingly
more of these diffraction patterns.
- If the primary is aligned properly, you should see something like the diagram on the left.
- Otherwise, if the central disc and the rings are not concentric, then the primary mirror needs adjusting, as
shown in the centre diagram.
- If you see three points to the rings, as on the right hand diagram, it probably means the primary mirror is
Star test with well-collimated optics (left); and with primary mirror slightly off alignment (centre); and primary
mirror pinched (right)
To centre the rings and disc requires tweaking the primary into alignment.
- Move the primary mirror adjustment screws a tiny amount at a time, and only one screw at a time.
- Each time you do this the star will drift away from the centre of the field of view.
- Use the RA and declination cables to bring the star back to the centre.
- See if the central disc and the diffraction rings are closer to being concentric.
- If they are worse, then undo the damage by reversing the initial adjustment.
- You are trying to tilt the mirror in the opposite direction to the distortion (theory).
- Basically comes down to trial and error (practise).
If the optics are pinched, then ideally you need to loosen the grips holding the primary in place. This is up
to you whether it's worth doing. Sometimes after loosening the grips the primary mirror can slide about every time
you change the orientation of the telescope. There seems to be a balance between having the primary securely held
in place without having the mirror distorted by the pressure of the grips. Pinched optics seem to be more of a
cosmetic than functional flaw, and don't particularly affect optical performance.
Step 7: Use the telescope!
The resolution of the telescope essentially comes down to how small and unobtrusive the disc and rings are. Epsilon
Bootis, for example, can be resolved with a 114 mm reflector on a good night, but the diffraction rings of the
primary do overlap the central disc of the secondary, which makes things tricky. The better the collimation of
the optics, the cleaner the view of each star, and the easier it should be to split a tough double.
High magnification view (x 180) of Epsilon Bootis
After using the Celestron Collimation Eyepiece to collimate the telescope Epsilon Bootis was easily split into
its yellow and green components. Performance on other double stars showed similar improvements, though so far Delta
Cygni has eluded me! Epsilon Lyrae split more cleanly before, with black shy between the components of Epsilon
Lyrae A and Epsilon Lyrae B, where before they looked more like figure-eights.
The bottom line is this: no amateur astronomer with a Newtonian (or any other reflecting telescope) should ignore
collimation, and this tool is a useful aid to getting the job done right.