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Celestron Collimation Eyepiece
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 tube.
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 this:
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) diagrams:
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 being "pinched".
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.