A guide to collimation.

What is collimation?

Telescopes whether refractors, reflectors or compound types like Schmidt Cassegrain, Maksutov, etc. all use optical elements (lenses and mirrors) to gather available light and to bring that light to a point of focus.  In order to do this without causing distortion, the optical elements have to be correctly aligned that is in line, centred and parallel with respect to each other.  The process of aligning the optical elements is called collimation. 

Although all types of telescope need to be in good collimation, refractors and compound telescopes are normally pre collimated during production and quality checks and if used and handled carefully should hardly ever need attention.  This guide will specifically cover the collimation of Newtonian reflectors.

Collimation Aids

There are several devices available to assist with collimation of a reflector and they vary in complexity, ease of use and of course, price.

Sight tube or collimation cap

A sight tube is the simplest device and consists of a cylindrical object the same size as a standard eyepiece with a small hole drilled precisely through the centre axis of the cylinder.   A collimation cap is a flat version of this but works on the same principle.  This forces the eye to be placed centrally and makes alignment possible by ensuring that all the images of mirrors seen through the sight tube are both circular and concentric.

A simple collimation cap can be made from an old 35mm film canister.  Remove the cap and discard, examine the base of the canister, there will be a pip produced from the moulding process that marks the centre of the canister.  Remove the pip carefully with a scalpel or similar sharp knife, use a pin or similar to mark the centre (push it through if you can) and then use this marking to carefully drill a 2mm hole.  Finally, remove the rim of the canister if it has one and you should find the remaining canister is a good fit in your eyepiece receiver.

Cheshire eyepiece

Strictly speaking, this is not an eyepiece at all but rather a more sophisticated sight tube with fixed crosshairs and an opening set at 45 degrees to the light path to allow ambient light to illuminate the brightly painted oblique surface which is then reflected in the image to give a clear target.  The Cheshire is an all in one device – no other aids are required to use it.

A Cheshire eyepiece will only be usable if the primary mirror has a centre spot.

Laser collimator

A laser collimator uses the same principle as a Cheshire eyepiece but the crosshairs are replaced by a precisely centred laser with a beam parallel to the sides of the collimator.  The spot falls on the secondary mirror, is reflected to the primary and then back up to the secondary and finally to a target marked by concentric circles on the oblique surface set in the side of the sight tube.

As with a Cheshire eyepiece, a laser collimator requires the primary mirror to be marked with a centre spot.

It’s important to note that a laser collimator is only as useful as it’s own collimation is accurate.  Before you consider buying a laser device, you should ask if it can be collimated – there are usually three grub screws set around the outside of the device to achieve this.  On some devices, these screws may be covered with a sticker or locking compound. 

Autocollimator

This is the most expensive option of all.  An autocollimator consists of a perforated mirror mounted inside an eyepiece.  It’s used after a rough collimation is performed with a Cheshire or similar and a view through the autocollimator pupil shows several reflections of the primary mirror center spot. Typically, four reflections are seen and the collimation is finely adjusted so that two of the images are centred on the reflection of the autocollimator pupil.

An autocollimator usually requires a special type of centre spot to be fixed to the primary.

So, what are the pros and cons of each type?

Sight tube/Collimation cap

Cheshire eyepiece

Laser collimator

Autocollimator

It’s worth mentioning that a laser collimator can be used in conjunction with a barlow lens.  This produces a spot in the shape of the centre marking and can be very accurate if used correctly.  The same pros and cons apply as for a simple laser collimator.

My personal recommendation is the Cheshire eyepiece.  It’s simple, fairly cheap, there’s little to go wrong with it and it works every time!

Basics of collimation

Collimation is a very methodical process that follows a set procedure.  We’ll start with the assumption that both mirrors have been completely removed from the telescope and will both need alignment.  Most of the time, it won’t be necessary to touch the secondary mirror  – just the occasional fine adjustment of the primary mirror.

Let’s start with some of the basic terms used: 

Primary mirror

This is the large parabolic mirror at the lower end of the telescope tube.  It’s supported by a cell that permits it to be tilted in three directions – there will be three adjustment screws at the base of the telescope and in some instruments these may be paired with locking screws to prevent movement once an adjustment has been made.

If your primary mirror isn’t centre spotted, then it’s a good idea to add one.  It’s a fairly simple task to do this and comprehensive instructions are given further on.

Secondary mirror

This is the small elliptical mirror that can be seen through the focusing tube.  It’s supported on a carrier that in turn is attached to a spider support – this may be one, two, three or four thin arms that are in turn attached through the telescope tube.  The mirror carrier can be rotated on its axis by loosening the main attachment screw, and it also has three screws that permit it to be tilted in three directions.

Before you start the collimation process, a few simple rules:

1. Check before you adjust anything.  Use your collimation tools to check whether adjustment is necessary. 
2. If you have to use tools to adjust the secondary or anything inside the telescope tube, move the tube to the horizontal first. That way if anything is dropped, it won’t cause expensive damage to the primary.  
3. Make sure you have the correct tools for the job.
4. Don’t overtighten screws and bolts.

Adjusting the spider

NB:  This is not normally needed – if you’ve removed the secondary for any reason or otherwise disturbed the spider, then you may need to do this.

The spider is adjusted by loosening and tightening opposing nuts in order to move the assembly towards the centre of the tube.

With three vane spiders, loosen the opposing two nuts and tighten the third.

To determine where the centre of the tube is, you can use one of two methods.

Draw a circle the same size as the tube on a piece of card or stiff paper and carefully cut it out.  Find the centre and make a clean hole.  If you haven’t got the centre marked then you can find it by folding the circle in half and then quarters.  The centre is where the folds meet.  Place the circle carefully over the end of the tube and look through the centre hole.  The centre screw of the secondary holder should be directly under the hole.  If it isn’t, adjust the spider as described.

Or

Measure the diameter of the tube carefully and make a note of it.  Measure along each spider arm from the edge of the tube to the centre screw of the secondary holder.  Each measurement should be equal.  If it isn’t, lengthen or shorten the arms as needed using the method described above.

Be careful not to overtighten the nuts and always loosen the opposite before you tighten to avoid distorting the tube.  If the tube is distorted, you won’t necessarily find the correct position.

Aligning the secondary

This is the trickiest stage of collimation and can take the most time to complete.  However, unless the secondary has been removed or disturbed or the tube has taken a knock or been severely shaken in transit then it shouldn’t need attention once it’s been set correctly.  Always check carefully before you touch anything!

Secondary alignment is done in two stages.

The first is to get the secondary mirror directly in line with the focuser tube and appearing as a circle rather than an ellipse.  This is achieved by a combination of lowering and raising the holder, rotating it and tilting it as necessary.  You will need to use either a collimation cap or a sight tube for this stage. 

It’s quite hard to make sense of the multiple reflections seen through the focuser tube but there’s a simple and elegant method to get around that.  You will need two pieces of thin card or stiff paper one white, one a colour of your choice but not black or white.

Set the telescope tube to be horizontal and lock it in that position.  You will not be moving it from this position until collimation is complete.

Take the white card and carefully pass it through the spider vanes by bending it or rolling it (not folding).  Position it between the secondary and primary so that it obscures the primary completely – it should hold position in the tube easily.

Now place the coloured card into the tube, again passing it through the spider vanes, and place it against the tube wall opposite the focuser.

This will simplify the view through the collimation tool.   What you will see now is the secondary mirror and its holder superimposed against the coloured card.

In the diagram below you can clearly see the coloured card, in this case red, with the secondary mirror in the centre and the secondary holder (the black section).

The aim is to get the secondary mirror to appear perfectly circular and concentric with the outer circle of the collimation tool.

With the collimation tool inserted, rack the focuser out to give you a clear view of the secondary and the coloured card as above.

You’re unlikely to be lucky enough to see a perfectly circular and centralised secondary mirror from the outset and some adjustment may be needed.  To do this, assess the position of the secondary with respect to the top and bottom of the tube.  If it isn’t perfectly central then you’ll need to adjust the height.  The adjustment screws are shown in the picture below:

To move the mirror up towards the top of the tube, carefully tighten the centre screw.  You may need to loosen the three tilt screws in order to do this as they bear against the mirror holder and may prevent it from moving.  Do not apply excessive force!  If there is resistance, loosen the tilt screws before proceeding further.  Tighten the centre screw until the top and bottom of the mirror appear centred in the view.

To move the mirror down towards the bottom of the tube, loosen the centre screw until the mirror appears centred.  You may need to tighten the three tilt screws evenly to stop the mirror from moving about.

You should now have your secondary mirror centred with respect to the top and bottom of the tube.

Next, carefully rotate the mirror until it presents a symmetrical and roughly circular centred shape.

Now use the tilt screws to tilt the mirror until it appears perfectly circular in the view.  The tilt screws should all be in contact with the secondary holder – if you’ve loosened them in a previous stage then ensure that they are lightly screwed back in.

If all has gone well, you should now have a centred and circular secondary mirror that appears similar to the drawing above.  If it appears to have moved off centre, then repeat the stages until you’re satisfied with the alignment and then lightly and evenly tighten the tilt screws to lock the assembly in place.

Finally check it again.

The second stage of secondary alignment is to align it with the primary mirror.  This is the trickiest part of collimation and it may take a few attempts to get it right.  Be patient and methodical.

Carefully remove the coloured card and the white card from the telescope tube.  Now look through the collimation tool and you should see the primary mirror reflected in the secondary.  Take your time and sort the visual information out.  The primary can be identified by the three retaining clips that can be seen around its edge.  At this stage, you may not be able to see them all – it depends on how well the first stage of alignment went.

You should see something similar to this – the circles may be in different places and you may not see all of them completely at this stage.

The aim of this stage is to adjust the tilt of the secondary to bring it into alignment with the primary mirror.  Again, we are aiming for perfectly concentric circles and this is where the Cheshire comes into its own. 

To align the secondary to the primary, carefully adjust the tilt screws on the secondary holder until the primary drifts into the centre of the image and the crosshairs of the Cheshire are centred on the centre spot.

You should now see something similar to this:

Once you have everything centred as above, perform a visual check on the secondary holder to make sure that it is not tilted excessively and that the adjustment on the tilt screws is more or less even.  You can use a small flat mirror to help with this if necessary.  If you find that the tilt is excessive repeat the process from the first stage and realign.

Aligning the primary

This is the final stage of collimation using the Cheshire.  It’s the stage that you will use the most frequently and the stage that should be checked regularly. 

The aim of this stage is to correct the tilt of the primary mirror and align it fully with the secondary

On the bottom of the telescope tube is the primary mirror cell.  This will always have three tilt adjustment screws or knobs, and may also have three locking screws to prevent movement after collimation.  There may be an access plate covering these if so, remove it.

Loosen the three locking screws and look through the Cheshire (see view above).  You now have to adjust the tilt screws carefully to bring the reflected Cheshire image into the middle of the image and centre the reflected crosshairs onto the actual crosshairs. 

Turn one of the tilt screws a small amount and see which direction the reflected Cheshire moves.  This will give you an idea of which screws need to be adjusted to complete the collimation.  Remember that the tilt screws can be loosened as well as tightened.  Ideally you should have the tilt screws set in the middle of their range of adjustment when you’ve finished.  If you find a screw tightening up, then loosen them all off by an equal amount and start again.  If you find one screw too loose, then tighten them all by an equal amount and start again.

When the primary is collimated, you should see something similar to this:

Fast and slow telescope differences

Slow telescopes (F5 and above) should all show symmetrical collimation images similar to those shown in this guide.  However, some fast telescopes (below F5) may have their secondary mirrors offset slightly.  The procedures are the same but you must be aware of the difference.  Fast telescopes are far less tolerant of collimation errors than slower ones and accuracy must be maintained when collimating to prevent distorted images.

An offset collimation pattern will appear something like this:

As can be seen, although the secondary appears to be offset, the centres are still over the primary centre marker and everything is in line.  Take care with faster scopes and note how things look before you start to collimate.

Star tests

The final proof of good collimation is a star test.  Collimation errors lead to distortion in the viewed image, an error may cause a bright star to appear to have a comet like shape.

A start test will prove good collimation and is also a useful guide to other errors.

To do this test, pick a bright star preferably high in the sky – Polaris is ideal.  The telescope must be properly cooled to ambient temperatures and the seeing must be good.  If there is obvious atmospheric interference you won’t get good results and may be mislead.  Also make sure that the telescope is as far away as possible from local heat sources (houses and buildings, concrete masses etc.) to avoid thermal distortion.

Fit a low powered eyepiece and get the star centred and in focus.  Now switch to the highest power eyepiece you have (not barlowed), centre the star again and get it into the sharpest focus you can achieve.  You should see the star as a bright point of light and you may also see diffraction spikes caused by the spider.  Now defocus very slightly.  You should see an Airey disk which consists of concentric light circles – any distortion from circular and concentric indicates an error.

Atmospheric turbulence (poor seeing)  looks like this:

You cannot perform a star test if the seeing is poor.

Adding a centre spot to your primary mirror

You’ll need a fine pointed black indelible marker, a cotton bud, a pair of tweezers, some self-adhesive ring reinforcements and a sheet of thin card or paper.

First, remove the mirror cell.  Remember that it’s a heavy item and that the mirror surface is very fragile.  Be careful when removing it and keep your fingers and any tools away from the mirror. 

Next, measure the diameter of the mirror.  Now you can proceed in one of several ways here, you can either remove the mirror from the cell, place it carefully on the sheet of card or paper and draw around it, or you can use a compass to draw a circle of the correct diameter on the paper (this has the advantage of giving you a perfect centre to start) or you can use a computer program (CAD or similar) to draw a circle of the correct diameter with a centre marking and print it out.

Whichever method you choose, carefully cut the circle out using either scissors or a scalpel.

If you’ve just drawn around the mirror onto the paper, you can find the exact centre by carefully folding the circle in half, creasing the card or paper to get a sharp line, and then folding and creasing again so that it’s folded into quarters.  Where the folds meet is your centre. 

If you’ve used a computer program or a compass then you’ve got the exact centre marked already.  Make a small hole in the exact centre with a sharp needle or similar and carefully enlarge it so that the fine tip of the marker can pass through it.   It is vital that the hole is exactly in the centre of the circle.

Now place the circle carefully over the reflective surface of the mirror, making sure that the edges of the mirror and the circle line up properly.  This will place the centre hole directly over the centre of the mirror. 

Holding the circle in place firmly but carefully, take the marker and place the fine tip through the centre hole.  Press down gently until you feel the tip contact the glass.   Remove the marker and circle and you should now have a black dot exactly in the centre of the primary.

Use the marker pen to colour one of the ring reinforcements solid black and allow to dry.  Using the tweezers, peel the reinforcement off of its backing and very carefully position it just above the surface of the mirror so that the black dot is exactly in the middle of the ring.   Once you’re happy with the position, lower the ring until it’s touching the mirror (keeping the metal of the tweezers off of the mirror surface) and press it into place with the cotton bud until it’s firmly stuck down.

Replace the mirror in the cell (if you removed it) and very carefully refit the cell to the telescope tube taking care to keep the mirror itself away from the edges of the tube.