Barlowed Laser Collimation

Maintaining and Getting the Best from Equipment

An observatory is the principal and most practical means of organising and maintaining your valuable astronomical equipment. In this chapter I will address a few of the main issues concerning optimising and maintaining observatory equipment at its best, and mention a few products and methods I have found to be particularly useful.

Collimation is the accurate adjustment of your telescope optics. No telescope will perform at its best without perfect collimation, though errors will be most noticeable at the highest magnifications or image scales, and will always be more noticeable with shorter-focus instruments. I do know distinguished observers who hardly ever collimate their telescopes. If they work in fields such as astrometry, photometry and survey astronomy, it may hardly be necessary. For all types of imaging, and studies where high resolution is required, such as double-star work, it is crucial, however. A well-collimated telescope is also more satisfactory to look through.

Refractors

Small refractors are almost never provided with a means of collimation: they are set permanently on a lathe when they are made. They will be either right or wrong, with nothing you can do about them. Some large and short-focus refractors are provided with adjustments on the objective cell. The collimation of a refractor is only half as critical as that of a reflector of the same focal length, because of the basic laws of ray optics. If the angle of a refractive surface is changed, the ray emerging from the other side will be deflected by the same angle, whereas if the angle of a mirror is changed, the emergent ray will be deflected through twice the angle. This is also why lens surfaces only need figuring to half the accuracy of mirrors, to perform as well.

The essence of refractor collimation is that the objective needs to be square on to the axis of the tube. This is tested using a small light source, or better, a laser, placed at the drawtube, with a piece of glass between it and the telescope. The light is reflected off the glass surfaces, and, with adjustment of the objective cell, the reflection off the objective can be made to coincide with the reflection off the test surface.

Newtonians

Newtonian reflectors are far more difficult, particularly if they are below f6. The classical, and simplest, method is to use a drawtube stop, generally an old eyepiece with the lenses taken out, or a 35 mm film canister with the base sawn off and a small hole drilled in the lid centrally. This is used to centre the eye on the drawtube. All the reflections are brought into concentricity, firstly by positioning the drawtube, secondly by adjusting the secondary, and thirdly by adjusting the primary. For f8 reflectors, this is sufficient, and no further tools are necessary. For shorter reflectors, higher precision is necessary, and various accessories can help. The method I recommend for short-focus Newtonians is the method of the Barlowed laser. This was developed by Norwegian amateur telescope maker Nils Olof Carlin. I believe this to be the most accurate method, and my experience is that it is possible to use it to collimate an f4.8 Newtonian to absolute diffraction-limited performance, a feat that is almost impossible by any other means.

The Barlowed laser collimator is a gadget that can either be bought ready made, or produced by mating a standard laser collimator with a standard Barlow lens. If the drawtube is 31.5 mm (1.25 in.) fitting, the laser and Barlow need to have this fitting. If the drawtube is 51 mm (2 in.) fitting, an adaptor can be used, or the larger-fitting laser and Barlow can be obtained, though these are much more expensive, and do not deliver greater accuracy by this method in practice. The other accessory that is necessary is a stop with a hole in it, made from an opaque material, such as wood or plastic, that fits exactly in the lower end of the drawtube. Some experimentation will be necessary here, as the lower end of the drawtube may not have a standard eyepiece diameter. It is vital that this stop can be pushed in to the drawtube from the inner end, and that it will stay in, and not fall onto the mirrors. This stop is drilled centrally with a small hole. I made this stop for my reflector out of plywood, cut out of a sheet with a hole saw, which also produced the central hole. I then glued on an edging of felt (Fig. 10.1).

To use this method, the centre of the mirror must be marked. People tend to get a bit agitated about this, thinking it might damage the mirror, but in fact

Figure 10.1. Tools for precise Newtonian collimation: drawtube stop (film canister), laser collimator, Barlow, and drawtube inner-end stop.

it does no damage at all. This spot is, of course, completely shadowed by the secondary, and plays no part in the telescope's performance. The best object to make the spot from is a paper binder reinforcement ring. Simply measure the mirror as accurately as possible with a plastic ruler (which stands no chance of scratching it), determine the centre point by measuring diameters at right-angles to one another, and stick the binder ring down. Now, with the telescope re-assembled, it is possible to collimate.

First, use the film-canister-type stop in the top of the drawtube to get a rough alignment of the optics visually. The outline of the secondary should be centralised in the focusing tube, by moving it up and down the main tube and rotating it. If it cannot be centralised, the focuser may not be perpendicular to the tube, and may have to be collimated itself, possibly by shimming it. The reflection of the primary is then centralised in the secondary by adjusting the secondary collimation screws. Often, these three screws all bear on the secondary cell simultaneously, the mechanism is not sprung in any way, and it is necessary to keep the adjustment tight by loosening two screws whenever the other is tightened. Better secondary collimation systems avoid this by being spring-loaded. The primary collimation screws are nearly always spring-loaded.

Once this has been accomplished, the reflection ofthe secondary in the primary in the secondary (the central dark spot) must be centralised on the mirror-centring spot by adjusting the main mirror collimation screws. With one person, this is a tedious job, as one has to keep going from one end of the tube to the other, to find the screws, and then check their effect through the drawtube. Try to keep a note of which action, tightening or loosening, of which screw, has what effect on the central spot. This will make the process less hit-and-miss. Better is to have an assistant at the screws, to whom to give instructions. The result, seen through the stop, should resemble Fig. 10.2.

All this so far constitutes the rough collimation, which will be adequate in itself for an f8 instrument or above. In such a long focus instrument, the accuracy of

Secondary

Reflection of primary in secondary

End of drawtube

Primary centre-spot

Figure 10.2. The view down the drawtube of a correctly-collimated Newtonian.

Primary centre-spot

Reflection of shadow of flat on primary

Secondary

Reflection of primary in secondary

End of drawtube

Secondary holder

Figure 10.2. The view down the drawtube of a correctly-collimated Newtonian.

collimation attained at this stage will most likely exceed the wavefront accuracy of the mirror. That is one of the beauties of long-focus Newtonians (their large range of viewing height is not).

Precise collimation is begun by removing all stops from the drawtube, and placing in it the laser, without the Barlow. This is best done with the drawtube vertical (if the tube is rotatable at all) to allow the laser to rest on the top of the drawtube without being biased to one side by a tightening of the set-screw. If this cannot be arranged, however, it is not a disaster. Check that the laser is itself collimated in its housing by rotating it in the drawtube. The spot made by the beam on the primary should not move significantly. If it does, it may be possible to collimate the laser itself by adjusting hex-head screws around its tube. When this is has been done, the beam should be centred on the centre-mark of the mirror, looking down from the open end of the telescope, by adjustment of the secondary collimation screws. The beam can be seen easily when it strikes the centre mark, but not when it reflects off a clean mirror.

The clever part comes next. The collimator is removed, and inserted into the Barlow barrel, which is then inserted into the drawtube. The "inner end of drawtube stop" or faceplate is then inserted from the inside of the telescope. The laser beam shines through the Barlow and is made to diverge. Part of it passes through the hole in the stop (only a small part, so conditions might need to be quite dark for it to be visible), is reflected off the secondary, the primary, and the secondary again, and returns to the face place, where a shadow of the primary mirror centre mark is cast. Now you will see the purpose of using, ideally, a binder ring as the centre mark. The shadow this casts is itself a ring, which may easily be centred on the hole in the face-plate by adjustment of the primary collimation screws (Fig. 10.3). The primary is then perfectly collimated, assuming the primary has corresponding physical and optical centres. The fact

Figure 10.3. Looking into the side of the telescope tube from the front, this shows the ring-shaped shadow of the centre mark produced by the laser and Barlow almost centred on the drawtube faceplate.

that it may not may mean that a slightly different result would be obtained in star-collimation.

Other methods of using a laser collimator are subject to significant error in determining the primary collimation (which is the critical part of collimation) because they are badly affected by the laser being not centrally directed down the optical axis, due to imperfectly fitting tubes, focuser slack, poor focuser collimation, and inexact determination of the primary mirror centre leading to imperfect secondary collimation. In these methods, a small error in secondary collimation, which is, in itself, not very significant, leads to a bigger error in primary collimation, which is. The method of the Barlowed laser avoids all this, through the way it creates a shadow of the centre spot, which demonstrates the primary collimation independently of other adjustments.

After laser collimation, the mirrors should be viewed through the eye-stop in the drawtube again (removing the stop from the bottom of the drawtube). They should still be almost perfectly symmetrical. If the laser procedure has resulted in the secondary edge being displaced from the primary reflection, it means the secondary is in the wrong place, laterally, in the tube. It should be moved by translation, not by re-angling it, until the reflections are again centred. The whole laser procedure should then be repeated. It is sometimes stated that, following laser or star collimation, if the collimation is then visually tested by looking down the drawtube, and residual asymmetry is still seen, this does not matter. This is wrong. Perfect collimation will satisfy all the tests of collimation, simple and advanced, simultaneously. For optimum results it is necessary re-adjust until perfect.

It may be necessary to adjust the position of the secondary in short focus Newtonians by bending the secondary supports (the spider). This is because spiders often do not take account of the short-focus Newtonian secondary offset. The meaning of this is that, from the point of view of the top of the telescope, in reflectors shorter than f7, there needs to be a noticeable non-centrality of the secondary in the main tube, seen looking towards the primary. The secondary is displaced away from the drawtube, as shown in Fig. 10.4. The view through the film-canister stop in the drawtube, however, must be symmetrical. Many books, including successive editions of Norton's Star Atlas, have this wrong. Getting the symmetry right in the drawtube does give you this offset automatically, and the Barlowed laser helps, but you do have to push the secondary to one side of the tube quite noticeably in f4 to f5 telescopes to get all the collimation criteria satisfied simultaneously.

Cassegrains

Cassegrains vary in the types of collimation they allow. Some are only collimatable by adjusting the secondary, like SCTs, but the best systems also allow adjustment of the primary and the focuser. Broadly, the result that should be aimed for is

Figure 10.4. Looking exactly down the optical axis of a precisely collimated f4.8 Newtonian (secondary in line with its reflection), the offset of the secondary away from the focuser is clear.

perfect concentricity of the reflections as seen through an eye-stop (such as the film canister) placed in the drawtube. The focuser should be adjusted first, if possible, then the secondary, and then the primary. A standard laser collimator can be used to collimate the secondary, but not to collimate the primary, because the beam cannot strike the centre of the primary, since it is perforated.

To use a standard laser, first check, by measurement, that the secondary is physically centred in the tube. Place the collimator in the drawtube, make sure it is collimated in its own tube, and then square the focuser on the optical axis so that the laser beam strikes the centre of the secondary. This can be judged by looking at the reflection of the secondary in the primary, by looking in from the top of the tube (take care, as always, not to look directly into the laser). The spot will be easiest to see if the surroundings are dark and the mirrors are not too clean. Once this has been accomplished, the secondary collimation screws should be adjusted (and they are nearly always non-sprung on Cassegrains, so one always has to be tightened when another is loosened) so that the beam returns centrally to the target built into the collimator. Most collimators feature a cut-away area of the cylindrical case, allowing this target to be seen from the side. The secondary is then collimated.

The primary is normally collimated by sighting the central dark spot through the stop and centring by visual estimate, followed by star-collimation. However, a different type of laser collimator is also available, called a holographic laser collimator, which projects a grid pattern of light, rather than a simple beam, at the mirrors. This can be used, with a white screen fixed some distance in front of the telescope, at night, to project this pattern on to the screen. The primary collimation screws are then adjusted until the projected pattern is symmetrical. I have not tried this method.

Schmidt-Cassegrains and Other Catadioptrics

These telescopes generally only have one user-adjustment allowed by the manufacturer, the secondary collimation. The primary is fixed to a mechanism that moves up and down the baffle tube, to focus, and cannot be adjusted. The secondary usually uses three hex-head or Phillips-headed screws, but these can be replaced by third-party knobs (such as Bob's Knobs), which are easier to use. Rough collimation is achieved as in other Cassegrains, by using an eye-stop in the drawtube and centring the circular dark reflection of the secondary. Precise collimation is done using a star. Alternatively, the holographic collimator can be employed.

Star Collimation

This is the final stage, and only complete test, of collimation for all telescope types. In practice it is rarely carried out on refractors, but frequently on other telescopes. It is highly desirable that the telescope is accurately driven, but if not, the test star should be Polaris, in the northern hemisphere, because it doesn't move very much.1 The test star, of second magnitude, or third for large apertures (above 30 cm or 12 in.), should be centred exactly in a medium-power eyepiece (giving about 200x), using cross-hairs. If you do not have a crosshair or reticle eyepiece, one can be made by screwing an inexpensive reticle attachment into many ordinary eyepieces. When centred, the star should be slightly de-focused, observing the appearance on both sides of focus. On one side, providing seeing is not too bad, and the telescope is thermally equilibrated with its surroundings, the pattern of diffraction rings will be clearly seen. Note, this is not the "doughnut" shape with a dark centre that is observed in telescopes with secondary obstructions when far from focus. This is the stage of defocusing just before the doughnut becomes obvious.

Further from focus, a doughnut with an asymmetrical hole will reveal bad miscollimation of the primary, and a non-circular doughnut will show that the secondary is not positioned correctly, but for precise collimation, the doughnut should be not quite visible. What should be visible is a pattern of bright and dark rings as in Fig. 10.5. The primary collimation should be altered to make this pattern symmetrical. The difficulty is that altering the collimation shifts the star in the field, even with perfect tracking, and also knocks out the alignment of any finders. So if you make more than a tiny alteration at one time, you risk losing the star.

The basic rule is that you have to find the adjustment that moves the star in the direction of the thickest side of the diffraction pattern. Then you re-centre the star in the field using the telescope's slow motion controls, or slow slew rate. The asymmetry in the pattern should then have diminished (unless you have gone too far). For a telescope that is already quite well collimated, by the methods described above, the amount of turn of a primary screw required to perfect it is likely to be less than one quarter. Again, it is sensible to keep a note

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