HOWIE GLATTER'S LASER COLLIMATOR PAGE
In order to achieve the best possible resolution and contrast, the optical elements of a telescope must be put into near-perfect alignment. Collimation is the adjustment of the position and orientation of the optical elements to achieve best performance. Laser collimation is a relatively new way to accurately and precisely collimate a telescope.
When practiced with accurate tools and correct techniques the various methods of collimation will converge to the same result, but laser collimation has several unique advantages. The laser collimator provides its own light source, so collimation can be readily accomplished or checked after dark without additional equipment. Unlike passive collimation tools, your eye position is not constrained by a peep-hole and cross hairs, and you don’t need to scrutinize elements at different distances simultaneously.
Laser alignment and Shock Resistance
A laser module inside the collimator emits an intense, thin, parallel beam of light, which exits a front aperture and projects along the central axis of the cylindrical collimator body. The beam acts as a reference line from which alignments are made.
The most important thing about a laser collimator is that the beam be aligned with the collimator's cylindrical axis. If the beam alignment with the collimator body is off, the collimation will be off and the telescope will not achieve its best performance.
For a collimator to serve as a reliable reference tool over the long term, the internal laser alignment must withstand mechanical shock. My collimators incorporate features to make them highly shock resistant. After I align the laser to the collimator body within 15 arc seconds, I test the collimator by whacking it against a block of urethane, striking at least a dozen times on three axis. I then check the alignment, and if it hasn’t changed the collimator goes into stock. The collimators usually withstand drops from eyepiece position up a ladder without losing alignment. I believe my collimators are unique in this respect.
If the laser in a collimator is misaligned, rotation of the collimator on its axis will cause the beam impact to trace a circle. However, rotating a collimator in an eyepiece holder is not the best test ofa collimator's alignment due to the small space between an eyepiece holder and the collimator. The collimator may precess like a top as it is rotated, and then even a good collimator's spot can travel in a circle. For a valid test the beam impact location should be carefully noted, then the collimator unclamped, rotated, and re-clamped, and the beam location checked to see if it has wandered.
I produce the collimators in three different body sizes: a 1¼" only, a 2" only, and a combination 2"-1¼" size. The combination size is 2" at the back, and steps down to 1¼"at the front. The 2"- 1¼" or 2" collimator is recommended for accurate alignment in a 2" eyepiece holder, but the 1¼" collimator is o.k. in a 2" holder if used with an accurate adapter bushing. The adapter can be itself checked for accuracy with the collimator by rotating the adapter and reclamping it, and seeing if the laser spot wanders.
The red collimators are offered with a choice of either 650 nanometer or 635nm wavelength. Both lasers have the same beam power output, but because the human eye's sensitivity to the shorter wavelength is greater, the 635nm. laser appears about two or three times brighter. The 635nm laser is more expensive, but it enables Barlowed or holographic collimation in higher levels of ambient light. In darkness the 650nm laser is adequate.
I also offer a 532nm green collimator, much brighter than the red ones. In most circumstances it is overly bright for night time collimation, but it is useful for Barlowed or holographic collimation daylight or room light. It is stocked in the 2"-1¼" combination size only.
The 1mm stop Attachment
The beam produced by red lasers used in collimators is fuzzy-edged and elongated. When making collimating adjustments you will have to judge the location of the center of the spot by eye. To improve adjustment precision I supply my collimators with a detachable aperture stop accessory having a knife-edge 1mm pin hole and a white screen front. The stop is included in the basic collimator price. The stop screws into the laser aperture and restricts the beam, producing a tiny circular impact surrounded by a series of concentric rings. The edge of the pinhole diffracts some of the laser light, forming the concentric rings, which facilitate precise centering. With the stop attached to the collimator, the beam impact looks like a star diffraction pattern. The diffracted light that forms the rings is divergent, and this fact allows the stop to also be used to implement a low-contrast form of “Barlowed” collimation, explained below under the heading of Barlowed collimation.
The Holographic Attachments
Optional holographic attachments screw into the laser aperture and have a white screen front surface. They contain an optical element that diffracts most of the laser light into a diverging symmetrical pattern around the central beam. The projected pattern is useful for centering optical elements by making it symmetrical with the edge of the optic.
Three different patterns are available:
- A 10 x 10 line square grid pattern is supplied as standard unless otherwise requested because it is the widest pattern. It spreads 21 degrees which allows centering of optics as fast as f/ 2.7. This pattern is recommended for general use because it can be used with the fastest telescopes likely to be encountered.
- A nine-concentric circle pattern is available that spans 10 degrees and will reach to the edge of f/ 5.7 optics. This pattern is recommended for scopes around this focal ratio or slower. Because the laser light is spread over a smaller area it is brighter than the square grid pattern, and this makes it particularly useful with Cassegrain scopes, where the pattern impact is sometimes scrutinized on the mirror surfaces. The projected pattern is seen only by light that is scattered from dust, dirt, or optical roughness, so a brighter pattern is better, especially if the mirrors are very clean.
- A cross-hair and circle “scope” pattern is available that spans 10 degrees. It has utility for non-Barlowed , conventional Newtonian primary collimation, where the primary is adjusted to return the reflected central laser beam back into the laser aperture of the collimator. The cross-hair intersection makes it easier to see when the return beam is centered on the collimator face.
The lasers in my collimators have a maximum power of 5 milliwatts, and are quite safe if used with reasonable precaution. Direct or mirror-reflected eye exposure to the laser beam should always be avoided, so be careful when collimating to ensure that the beam doesn’t enter anyone's eye. The beam's impact on a surface can be viewed with no problem if the surface produces a diffuse reflection. The beam impact on a mirror or lens surface may be safely viewed if the reflected or transmitted beam is not directed towards your eye. A badly miscollimated Newtonian or Cassegrain may allow the beam to exit the telescope, so check first by pointing the telescope at a wall or screen to see if the beam is escaping. With unobstructed telescopes such as refractors the beam will always exit the front of the telescope, so a strip of masking tape should be run across the dew cap or lens cell as a safety beam stop.
Collimator alignment within the eyepiece holder
Inconsistent registration of the collimator in the eyepiece holder is a main cause of laser collimation difficulty. When the collimator is clamped, a small sideways displacement may occur, but that won’t cause a problem if the collimator and holder axis remain parallel. However, significant problems do occur when the eyepiece, collimator or camera adapter tip, and the two axis go out of parallelism. If the tipping is consistent and repeatable, collimation can be accomplished, but if the eyepiece holder does not provide a stable pointing direction for the collimator, consistent results cannot be expected.
The collimator, with its long light beam as a "lever arm" is a very sensitive tool for detecting focuser problems. All too oftenthe laser beam impact travels in a circle as a helical focuser is rotated, or jumps back and forth as focus direction of a rack-and-pinion focuser is reversed. Whatever focuser axis instabilities exist should be fixed or minimized, up to and including focuser replacement. However, even if the problems are not fixed, laser collimation of the telescope can be accomplished if the focuser is adjusted to a stable position and not disturbed over the course of collimation.
Newtonian collimation is done by making the optical axis of the primary mirror and the eyepiece axis coincide, through reflection in the secondary mirror. This is axial collimation.
Additionally, the flat, oblique secondary mirror should be positioned so its edges are centered within the converging image beam from the primary, and so that the converging beam it reflects is centered on the focuser axis. This insures that the focal plane will be evenly illuminated, without uneven darkening at the edges of the field. This is lateral collimation.
Ideally, the optical axis the telescope should coincide with the mechanical axis of the tube structure. This is not necessary for star-hopping or push-to operation, but for accurate operation of setting circles or go-to telescopes, it is necessary. I believe it is desirable to align the optical elements with the tube structure in any case, for then centering or off-setting of the mirrors can be done by measurements from the telescope structure.
Complete collimation of a Newtonian telescope requires at least seven separate alignments to be made, but once the initial alignments have been made most of them remain stable. Usually only two, the angular alignment of the primary and secondary mirrors, need to be checked and corrected as necessary with each set-up. Sometimes telescopes arrive from the manufacturer with most of the alignments correct, but checking all alignments it is the only way to be sure you are getting best performance.
On the other hand, if you are collimating a telescope that is known to have been properly collimated previously, you may want to just skip to the angular adjustments of the secondary and primary mirrors.
Focuser alignment with the telescope tube
The focuser axis should intersect the main tube axis. They usually intersect at right angles in order to have the shortest possible distance between the secondary and the focal plane, so that the smallest possible secondary mirror can be utilized. However, it is not necessary that the intersect angle be 90 degrees for the Newtonian system to work. There are telescopes known a “lowriders” where the angle is about 30 degrees, in order to lower the eyepiece position.
The focuser can be aligned using the collimator in single beam mode, preferably with the aperture stop attached. The secondary mirror and its holdermust be temporarily removed, but the spider can be left in place. First the focuser axis is adjusted to intersect the main tube axis. A measuring tape or ruler is held inside the tube at right angles to the laser beam, and a measurement is made from the tube wall to where the beam grazes the ruler. This measurement is repeated from the opposite wall of the tube, and the focuser base is adjusted with screws, shims or washers until the measurement is the same from the opposite sides if the tube.
Next, the focuser can be aligned to intersect the main tube axis at a right angle. If the telescope tube front opening or the upper cage top ring is square with the tube it can be used it as a reference for the adjustment. This can be checked with a carpenter’s or framer’s square. Then, measure back from the tube’s front edge to the laser beam, first at the focuser side of the tube and then at the tube wall opposite the focuser. Adjust the focuser base by tipping ittowards the front or back of the scope using shims or washers so that thetube front edge-to-laser beam distance is the same on the focuser side of the tube andon the side opposite the focuser.
The secondary should now be replaced. In telescopes with fast focal ratios, where the distance to the mirror focus is a relatively small multiple of the mirror diameter (say, 5 or less) the light cone converges steeply to the focus. Where the converging light cone first touches the oblique secondary mirror, it has a larger diameter than where it last touches the secondary mirror, closer to the top of the telescope tube. For this reason, if the secondary has been made the minimum size to just capture all of the light cone, its geometric center should be offset a little from the primary mirror axis, away from the focuser, so that the secondary edge which first meets the light cone just captures the larger diameter, and the edge which last meets the light cone just captures the smaller diameter. If you know the amount of offset for your telescope, you may adjust the secondarymirrorcell orspider to move the secondary away from the focuser by the required amount. This can be done with a ruler or caliper, measuring from the secondary edgeto tube wall. In f/6 or slower telescopes the offset is so small that the secondarymirror can be centered within the tube.
Next, move the secondary parallel to the tube axis so that the converging light cone reflected to the focuser will be centered on the eyepiece axis. When the secondary is correctly positioned, the center of the face of the secondary is offset or displaced towards the primary, so that the secondary edges are centered in the light cone converging on the focuser axis. You can look through the empty eyepiece holder or use a sight tube to adjust the secondary so that its edges appear concentric. If you have a holographic attachment you can adjust the secondary by making the projected reticle pattern symmetrical on the face of the secondary. Here is a link to a page by Nils Olof Carlin the has good information on this : Collimating with a holographic laser.
Both methods produce the proper offset towards the primary.
Next, rotate the secondary on the main tube axis so that the mirror faces the focuser squarely. This is usually done by looking through the open focuser or a sight tube and rotating the secondary on the main tube axis so that its edge appears circular.
This adjustment can also be done with using a single beam collimator. Temporarily misadjust the secondary by tipping it eitherdirectly towards ordirectly away from the focuser using its angular adjustment screws. Then, rotate the secondary holder on the tube axis; the reflected laser spot will trace an arc on the primary at the bottom of the tube. Look at this spot from the front of the telescope with your eye gazing through a line between the secondary and the focuser.Rotate the secondary so that the laser spot reaches a point in line with both the secondary and focuser. Lock the secondary rotation adjustment at this point.
Next, check with a ruler that the primary mirror is centered within the tube, so that the optical axis will coincide with the tube axis, and adjust if necessary.
Next, the angular alignment of the secondary mirror is adjusted so that the beam strikes the center of the primary mirror. With a single beam laser collimator and an unmarked primary it is impossible to judge this accurately by eye, so a collimation mark, ring or triangle placed on the center of the primary is necessary. Most commercial telescope primary mirrors come with a collimation mark, but often they are not placed accurately, so the mark position should be checked with a ruler.
Don't worry about having a mark on the primary. The center is not in use because it's in the shadow of the secondary. I supply self-adhesive collimation "donuts" with the collimators, and instructions for their safe placement.
If a primary mirror has no center mark, thesecondary can still be adjusted using a holographic attachment, by centering the grid pattern on the edges of the mirror.
Next, the angular alignment of the primary is adjusted so that the beamfolds back on itself, retraces its path, and returns to the laser opening in the frontof the collimator. The returned beam can be seen striking the face of the collimator, and the primary is adjusted to move thebeam impacttowards the center of the collimator face until it disappears in the brightness of the laser aperture. With a solid tube, the best way to view the beam impact on the collimator face is from the front of the telescope, looking down the tube, by double reflection in the primary and secondary. You may need someone to help you by turning the primary adjustment screws while you look down the tube.
With an open truss tube, the collimator face may be seen by reflection in the secondary.
The primary adjustment can also be done by making the up and down going laser beam impacts on the secondary coincide.
BARLOWED LASER PRIMARY ADJUSTMENT
Primary mirror collimation is the most critical adjustment in a Newtonian, and it’s been realized that with modern fast focal ratios of f/4.5 and faster, the errors which can occur in laser collimating the primary using the conventional method of folding the beam back on itself can exceed good collimation tolerances. The Barlowed laser procedure, invented by Nils Olof Carlin, is a more accurate laser-based method of collimating Newtonian primary mirrors. It was originally done by inserting a laser collimator into a Barlow lens in the focuser, with a paper screen attached to the front of the Barlow. The combination projectsa silhouette shadow of the primary center mark back to the screen on the Barlow device. The primary is adjusted by centering theshadow on the screen. Unlike conventional laser primary collimation, the adjustment is relatively insensitive to inaccuracy or "slop" in thefit of the collimator in the focuser, or small errors in secondary alignment.
How it works
Normally, parallel rays of light from a distant startravel down a telescope tube, and upon reflection by the paraboloidal primary mirror, are converted into a converging beam in the form of a cone. The cone forms a point image of the star at the mirror’s focal plane. Barlowed laser collimation takes advantage of the fact that a telescope will work in reverse. When parallel rays of light from the laser pass through the negative Barlow lens they are refracted into a diverging beam, also in the form of a cone. If the rays are traced back through the Barlow lens, they would appear to emanate from a virtual point source in the center of the focal plane, just like a star image. Thediverging rays are reflected by the secondary mirror onto the primary mirror, where they form a blotchy patch, similar to a magnified image of the original irregular laser beam.
When the primary mirror reflects the beam back up the telescope tube the parabola works in reverse, converting the diverging rays into parallel rays. Where the primary is covered at its center by a collimation mark or ring, it is blocked from reflecting, so the upward travelling beam contains within it the superimposed dark shadow of thecollimation target. The shadow remains sharp because the rays are parallel. The beam is reflected by the secondary back to the Barlow screen, where the shadow of the collimation mark surrounded by laser light isseen. The primary is adjusted using its collimation screws to center the target shadow around the hole in the screen.
Since the virtual point source is located at the focal point of the primary, the position of the shadow on the screen is a true indication of the primary mirror optical axis (if the collimation mark is accurately placed), and is effected very little by variations in the aim of the point source beam. It is startling at first, as the collimator and Barlow are wiggled around in the focuser, to see the shadowof the collimation mark remain almost stationary.
Barlowed collimation with a regular Barlow lens works, but it forms a long, somewhat bulky cantilevered assembly at the focuser. I’ve streamlined the procedure a little with three different devices.
My self-Barlow attachment consists of a disc with a white screen front and a small Barlow lens at its center. It screws to the laser aperture, and offers a more compact option for Barlow-collimating. The shadow is projected to the screen at the front of the collimator, so in some cases visibility of the screen may be difficult from mirror adjustment position at the back of the scope.
For easy visibility from the back of the telescope, I offer two other devices that fit to the focuser drawtube, used in conjunction with the laser collimator to perform Barlowed collimation : the Blug and the tuBlug. They allow the adjustment to be made more conveniently, with good visibility from the back of the telescope. See: Blug page, tuBlug page
Barlowed Collimation & the 1mm Aperture Stop
The primary purpose of the 1mm stop was to increase the precision of single beam adjustments, but the small amount of light that is diffracted by the pinhole to form the rings around the central beam, being divergent, is reflected back from the center of the primary mirror, all parallel, carrying a weak shadow of the collimation mark to the screen on the stop – a new mode of Barlowed collimation! As far as I know, Vic Menard was the first to notice and employ this. It works well in the dark, where the low brightness shadow can easily be seen. The amazing thing about this procedure is you are simultaneously seeing the adjustment of the secondary by the central beam on the primary, and the adjustment of the primary by both the Barlowed shadow, and the folded-back central beam (hopefully they’ll agree)
The laser beam will exit the front of a refractor, so run a strip of masking tape across the dewcap or lens cell to act as a safety beam stop. The drawtube should first be aligned so that the laser beam passes through the center of the objective lens. The collimator is placed in the draw tube without using a diagonal, and the drawtube alignment is checked by seeing if the beam passes through the center of the objective. Check it by holding a ruleracross the front cell opening, with the beam grazing the ruler's edge. Check centering both horizontally and vertically. Using a holographic attachment, the centering of the beam can be checked directly by symmetry of the pattern at the edges of the objective. If the drawtube alignment is off, it should be corrected before proceeding with collimation. Few refractors have adjustments for this, so usually drawtube misalignment must be fixed by telescope mechanic work on the drawtube, focuser, or tailpiece. Shimming, filing, or machining may be required. Sometime the tailpiece mounting screws can be loosened, the tailpiece shifted to align the drawtube axis, and the screws retightened.
Theoretically, the angular alignment of the objective could be adjusted by folding the beam, reflected from the center of the rear surface of the objective, back on itself. (enough light is reflected even from an anti-reflection coated lens surface to do this).
In practice, objective centering on the tube axis is almost never perfect, and there is typically enough decentering to make this method untrustworthy. The possible pitfalls are exactly analogous to those of non-Barlowed Newtonian primary collimation. For angular collimation of the objective lens I would recommend using a chesire eyepiece, bringing the bright spots to the center of the objective as viewed through the chesire, or star testing.
In principle, a laser collimator with a holographic attachment can be used to fully collimate a Cassegrain telescope. All of the optical elements of the scope should be centered and made square with the optical axis, which ideally should coincide with the mechanical axis of the telescope tube. Checking, and if necessary correcting element alignment and positioning proceeds sequentially element by element, from the draw tube or back axis, to the secondary, to the primary, and then out the front of the scope to a screen in front. In practice the procedure may be complicated by restriction of the beam by baffles, and the refractive elements in some designs. However, baffles can also be checked and aligned using the projected pattern.
Since the projected reticle pattern from the collimator is perfectly symmetrical around the central beam, it can be used for checking centering. The position of the reticle pattern or central beam projected from one element onto another can be used to align the elements with each other.
Here is a general procedure:
- Check drawtube or back centering with a ruler or caliper
- Check drawtube or back squareness by projecting the single beam down the tube and checking beam centration at the front by measurement from the tube walls.
- Check centering of secondary mirror by measurement from tube walls
- Check that reticle pattern is centered on secondary.
- Check centering of primary by measurement to tube structure.
- Adjust secondary to reflect reticle pattern symmetrically onto primary. This should also fold the central beam back on itself.
- The pattern reflected by the primary is projected in a parallel beam out the front of the telescope. The angular adjustment of the primary is done by projecting the pattern onto a surface or screen in front of the telescope and adjusting the primary to center the patternaround the shadow of thesecondary or secondary baffle (If a baffle, its centering should be checked).
Some collimation adjustments will require observing the position of the reticle pattern on the primary and secondary surface. Viewing is done from far enough off axis so that the reflected reticle pattern is not aimed towards your eyes. The reticle beam impact on a mirror (or lens) is only seen by light that is scattered from dirt, dust, or optical roughness on the surfaces. Visibility of the pattern impact can easily wash out in daylight, room light or even low ambient light, especially if the mirrors are clean. Because of this I recommend the brighter 635nm or 532nm collimator for Cassegrain collimation in conditions other than darkness.
The biggest problem in collimating commercial SCTs is that the corrector plate, primary mirror, and back axis are not adjustable, and the secondary has no centering adjustment. Only secondary mirror angular alignment is user-adjustable.
Additionally, in some of these telescopes there is excessive clearance between the bearing surface on the outside of the baffle tube and the sliding bushing that the primary is mounted on. This allows the angular alignment of the primary to shift when the direction of focus adjustment is changed, and when the scope tube crosses the meridian when tracking the sky. Newer versions incorporate a primary mirror lock, but if you don't have this feature you should always finish your focusing adjustments and perform collimation by rotating the focus screw counterclockwise, so that the mirror will be pushed upwards, and won’t settle backward against screw backlash.
If all the non-user-adjustable alignments were correctly and permanently set at the factory, and there was no primary mirror shift, the telescope could be reasonably well collimated by adjusting the secondary so that the beam from a laser collimator folds back on itself and retraces its path back to the laser aperture, which would set the secondary square with the optical axis. However, because of possible or probable misalignments in the non-adjustable elements, the best setting of the secondary may actually be tipped with respect to the optical axis. Tipping the secondary can partially (but never perfectly) compensate aberrations induced by misalignments in the non-adjustable elements.
The best use of a laser collimator with SCTs is not to perform an initial collimation, but to measure and record the optimal secondary adjustment, once it has been obtained using the star test. Once recorded, the optimal adjustment can be reproduced with the laser collimator alone whenever required.
The star test adjustment, done under good seeing conditions, is the best method of finding the optimal secondary adjustment. It need be done only once. The secondary is adjusted to produce the most compact image of a star at high power in the center of the field. The star test (and subsequent re-collimation with the laser collimator) should be done without using a diagonal, unless the diagonal is known to be accurately collimated itself. As soon as the secondary adjustment is judged as good as possible, the eyepiece is removed and the single beam collimator inserted in the back.
It is important that the collimator be mounted and clamped in the scope identically each time, so that the registration in the holder will be the same. The laser beam reflects from the secondary and returns back to impact the face of the collimator. The location of the beam impact is a measurement of the setting of the secondary at that moment. Take careful note of the beam impact position so that you can re-collimate the secondary any time that it becomes necessary by adjusting the secondary to move the reflected beam impact to the same spot. You can place a mark on the face of the collimator at the exact location, but then the collimator must be inserted in the telescope back in the exact same rotational orientation each time.
The beam impact can be seen from the front of the telescope, looking through the corrector plate, by double reflection in the primary and secondary. It may be difficult to see the collimator face because the view is restricted by the baffles. If so, I produce a side cut-out adapter tube that makes it easy to see the collimator face at the back of the scope
Cloudy Nights Telescope Reviews has an interesting thread outlining a users' experience with the Howie Glatter laser collimator, TuBlug, and several attachments.
Here is the link:
My Howie Glatter Laser Collimator Experience