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There are numerous tools available and many procedures documented for collimating newtonian reflector telescopes. The tools include sight tubes, cheshire eyepieces, autocollimators, single beam laser collimators, holographic laser collimators and barlowed laser collimators. The tools are available in 1.25" and 2" sizes to fit the commonly available amateur newtonian telescope focusers. One of the most commonly used tools is a combination sight tube and cheshire eyepiece. The qualities and prices of the tools vary widely. As for the procedures, some are quite simple and some are very complex - some are effective and some are not.
The need for accuracy and precision in Newtonian collimation depends partly on the focal ratio of the telescope. Faster focal ratios suffer more severe effects from poor collimation. The need for accuracy and precision also depends on your use of the telescope and on your tolerance for the errors that mis-collimation causes. For example, collimation errors may be more apparent in long exposure photographs made through the telescope than they are visually through an eyepiece.
My newtonian reflector is a 203 mm f/4.9, and I use it mostly for long exposure DSO photography with a digital SLR. When used for this purpose accurate collimation is mandatory. Occasionally I use it for planetary photography, with a barlow lens to increase the effective focal length and a small CCD imager that captures a very small field. While the small field of view and long effective focal length in this mode are more forgiving of collimation errors, the sharpness and fine detail desired for planetary imaging makes accurate collimation of prime importance.
At first I used a 1.25" combination sight tube/cheshire eyepiece to collimate this telescope, and I feel that I achieved the best collimation I could achieve with the tool. The biggest problem was that this telescope has an oversized secondary mirror, and the outline of the secondary mirror is not visible through the 1.25" combination tool even when the focuser is cranked all the way out. It is necessary to also retract the tool in the eyepiece holder considerably and clamp it in that position, in which position it wobbles significantly. So I never felt really confident that my alignment of the secondary mirror position, an important element in collimation, was as good as it needed to be.
Recently I purchased a 2" Howie Glatter Laser Collimator, with a 635nm laser, self-barlow attachment and holographic attachment. Included now with all of the Howie Glatter Laser Collimators is a 1 millimeter aperture attachment with a matte white face that can be used when the optional attachments are not in use.
While learning to use the collimator, at first I did not understand the instructions for using the holographic attachment to position the secondary mirror, and when I finally understood the instructions I found the procedure to be a lot of trouble with my solid tube newtonian. As a result I continued using the (too narrow) sight tube for that step in the collimation procedure, while using the laser collimator for the other steps. Ultimately I realized that I was still not achieving the desired collimation accuracy, because of the poor fit of the retracted sight tube in the focuser and because of the use of the 1.25" adapter during only one part of the collimation procedure.
I resolved to develop a repeatable process for collimating with the laser collimator alone, and to practice it until it became second nature. I have achieved that goal now and I have documented the procedure here.
The focuser should be "square" in relation to the OTA. In other words, it should be perpendicular to the OTA from all angles. Some focusers are collimatable, meaning that the tilt of the focuser can be adjusted, in a manner similar to the way the mirror tilts are adjusted, in order to square the focuser.
If your focuser is collimatable, it is a good idea to set it to the flat, un-tilted position before starting this procedure. If your focuser is not collimatable and is not square in relation to the OTA, you may shim it in order to make it square, but the penalty for not squaring the focuser is small, so you may choose to skip this procedure and proceed to the second procedure, adjusting the position of the secondary mirror. If you choose to shim your focuser, One way of doing it is to remove one or more of the screws that fasten the focuser to the OTA and reinstall them with one or more small flat washers between the focuser and the OTA. This procedure assumes that your focuser is collimatable, but lets you know when you need to shim the focuser if it is not collimatable.
This procedure requires removing the spider and secondary mirror from the OTA. Note that if your spider hub contains three collimation set screws (I believe this is the most common configuration), one (and only one) of those set screws should be positioned in line with the focuser axis when this procedure and the next procedure are performed. This requirement helps to assure that the secondary mirror holder ends up correctly aligned with the OTA axis.
Use the following procedure to test and adjust the squareness of the focuser:
The secondary mirror needs to be centered optically under the focuser. This adjustment is often made with a sight tube, but it can be made using the Howie Glatter Laser Collimator with the optional holographic attachment.
Keep in mind that "centered optically" in the case of the secondary mirror is not the same as "centered geometrically". Due to parallax, the optical center of the secondary mirror when it is tilted 45 degrees and seen as circular through the focuser drawtube is not at the geometric center of the mirror, so it is not possible to center the secondary mirror by marking its geometric center and using a single beam laser to adjust the position of the mirror until the laser beam hits the center mark.
In order to be centered optically, the secondary mirror needs to be offset towards the primary mirror, and ideally it should be offset away from the focuser by the same amount. This arrangement results in "fully offset collimation", and guarantees that the optical axis of the collimated telescope is exactly centered in the OTA. Setting the correct offset towards the primary mirror is easy - when offset correctly, the secondary mirror as seen through the focuser drawtube, from a point near the focal plane, will appear to be centered in the drawtube in the direction of the OTA's long axis, due to parallax. Setting the offset in the direction away from the focuser, however, is more difficult. Most modern collimation procedures forego this adjustment, resulting in "partially offset collimation". The effect of partially offset collimation is that the optical axis of the collimated telescope is not exactly coincident with the long axis of the OTA. The error, however, is small, and the consequences of the error are negligible for most purposes.
A word here about the importance of the distance from the collimator to the secondary mirror is appropriate. Due to parallax, the position of the secondary mirror's optical center as seen through the focuser depends on the distance of the observer's eye (or in this case the distance of the laser's holographic aperture) from the secondary mirror. The goal of collimation is to center the optical axis in the focuser axis in order to center the fully illuminated field in the observer's eye, or on the film or digital sensor for prime focus photography. Therefore the laser aperture should ideally be positioned near the viewing position, or near the focal plane for prime focus photography. It may be difficult to achieve this ideal position with a laser collimator without extending the focuser drawtube, but the penalty for not doing so is small. A slightly de-centered optical axis will likely not be perceptible through an eyepiece, and though it may be noticeable in long exposure prime focus photographs, the effect of uneven illumination in photographic images is easily corrected by the use of flat field frames. And with a focuser that is not perfectly square to the OTA, or one that shifts in alignment as the drawtube is extended, the misalignment caused by extending the drawtube far past the normal position or by adding an extension to the drawtube may introduce more severe collimation errors. You can determine how well your focuser's axis remains aligned when the drawtube height changes by noting the position of the laser spot on the secondary mirror, or on the inside wall of the OTA with the secondary mirror removed, when the focuser is extended and retracted. If it is reasonably stationary, perform the following procedure with the drawtube fully extended, or if a drawtube extension is used, position the drawtube so that the laser aperture is near the normal eye position, or near the focal plane for photography. If extending the drawtube causes the focuser axis to shift noticeably, perform the following procedure with the drawtube at approximately the height you will use for observing or imaging.
On my newtonian reflector the position of the spider hub can be adjusted by adjusting knurled nuts that attach the spider vanes to the OTA, loosening one knurled nut and tightening the opposite knurled nut, thereby moving the hub in relation to the center of the OTA opening. Other newtonian reflectors have different mechanisms for adjusting the position of the spider hub. If your spider hub position is not adjustable and the secondary mirror is not centered under the focuser in the direction perpendicular to the focuser axis, you will need to tilt the focuser to center it. This procedure assumes that your spider hub is adjustable, but points out when you will need to tilt the focuser if it is not adjustable.
Use the following procedure to adjust the position of the secondary mirror:
If you have followed the previous procedures correctly, the secondary mirror is now optically centered under the focuser and rotated so that it faces the focuser. Now the tilt of the secondary mirror needs to be adjusted so that the originating laser beam strikes the primary mirror precisely at its center. This adjustment can be made with a sight tube or with a correctly aligned laser collimator. The Howie Glatter Laser Collimator with the 1mm aperture installed provides a means of making this adjustment very precisely.
If a large adjustment is needed in this procedure, the position of the secondary mirror relative to the focuser axis, established in the previous procedure, may change. After this procedure it is advisable to return to the previous procedure and check the position of the secondary mirror using the laser collimator with the holographic attachment. If re-adjustment of the secondary mirror position is needed, repeat this procedure after making that adjustment.
Use the following procedure to adjust the tilt of the secondary mirror:
The tilt of the primary mirror needs to be adjusted so that its optical axis is aimed precisely at the optical center of the secondary mirror. Performing this adjustment may result in enough movement of the primary mirror so that the reflected laser beam no longer strikes the primary mirror at its center mark. Therefore, it is advisable to return to the previous procedure, adjusting the tilt of the secondary mirror, upon finishing this procedure, and then to repeat this procedure. In worst cases several iterations of these last two adjustments may be necessary.
The tilt of the primary mirror can be adjusted using a single beam laser collimator, a cheshire eyepiece or a barlowed laser collimator. The single beam laser collimator, while accurate enough for many purposes, lacks the precision of the other two methods and relies upon the laser collimator being precisely collimated and exactly centered in the focuser. The cheshire eyepiece is less sensitive to errors resulting from imprecise alignment of the tool in the focuser. The barlowed laser method is quite insensitive to errors resulting from imprecise alignment of the tool in the focuser. The Howie Glatter Laser Collimator with the optional self-barlow attachment provides an excellent means of making this adjustment very precisely.
As mentioned previously, all new Howie Glatter Laser Collimators now ship with a 1mm aperture attachment having a matte white face. The primary benefit of the 1mm aperture is that it produces a tighter, circular laser spot that is easier to center than the oval spot normally produced by laser collimators. A secondary benefit is that the aperture diffracts the laser beam and produces a diffraction pattern, similar to the Airy Disk pattern seen in a focused star image at high magnification. In low light, this diffraction pattern acts like the diffuse pattern produced with the self-barlow attachment. When the beam hits the primary mirror center mark, it causes a diffuse beam containing the doughnut shaped shadow of the center mark to be reflected back to the secondary mirror and to the face of the collimator. This shadow can be used to achieve a parallax-free alignment of the reflected laser beam with the focuser axis, in the same way that the self-barlow attachment is used. The diffuse pattern produced by the 1mm aperture is, however, much fainter than the pattern produced by the self-barlow attachment, but in low light it is possible to use that pattern instead of removing the 1mm aperture and replacing it with the self-barlow attachment in step b below.
Use the following procedure to adjust the tilt of the primary mirror:
The star test is the ultimate test of the collimation of a newtonian reflector. It must be performed on a night with good seeing, and after allowing ample time for equalization of the primary mirror temperature with the air temperature.
Focus the telescope on a fairly bright star (approximately magnitude 2 or brighter), using relatively high magnification (approximately 1 to 2 times the telescope's aperture in millimeters). Aim the telescope so that the star is exactly in the center of the field of view - if you have a reticle eyepiece, use it to assure that the star is centered. Verify that the diffraction rings associated with the Airy Disk are visible - this assures that the magnification is high enough and that the atmospheric conditions are stable enough for a meaningful star test. Defocus the telescope until a pattern of fuzzy, concentric rings surrounding a dark disk becomes apparent. Observe the symmetry of the pattern. The rings should be concentric with each other and with the dark disk, and all should appear perfectly circular.
Note that even when perfectly collimated the newtonian optical system still exhibits coma for off-axis stars, and depending on the telescope's focal ratio, the coma can be easily seen in the eyepiece at high magnification when the star is not precisely centered on the optical axis. Coma is evident in the star test as a non-symmetrical, comet-shaped ring pattern, with the tail of the comet pointing away from the optical center of the mirror. If your telescope is correctly collimated, you will not observe coma in the star test unless one of the following conditions is true:
To prevent the first condition, perform the star test using an eyepiece with a cross-hair reticle to center the star.
The second condition could occur if you collimate your telescope with a 1-1/4" collimator and perform the star test with a 2" eyepiece, or vice versa. Unfortunately, using an adapter to hold your eyepiece or your collimator can significantly change the collimation. The solution to this problem is to obtain a premium quality focuser or to collimate and perform the star test with the same size eyepiece adapter.
The second condition could also occur if the weight of your eyepiece or your collimator causes the tilt of their optical axes to be different in the focuser. If your focuser can be adjusted to prevent this condition, adjust it. If not, consider obtaining a higher quality focuser.
The third condition may be more difficult to overcome. Start by verifying that the primary mirror center mark is exactly in the geometric center of the mirror. If it is not, replace it and re-collimate. If the center mark is in the exact geometric center of the mirror, your mirror's optical center does not coincide with its geometric center. In this event, it may be possible to identify the optical center of the mirror, replace the center mark at that point, and re-collimate. I will not attempt to explain that procedure in this document, however, and replacing the mirror is probably a better option.