Passive Collimation Tool Tutorial

"COMPLETE", "PRECISION" Newtonian collimation can be achieved with a collection of 3 "Passive" collimation tools WITHOUT the need for a "laser":

  1. The CROSS-HAIR SIGHT TUBE - The classic 1.25" tool is typically a 4" to 6" long metal tube with a peephole on one end. Two thin wire "cross-hairs" at the other end span the tube opening and cross at 90 degree angles to one another.  This tool is used primarily for initial positioning of the diagonal along the tube axis and also to aid in squaring the focuser to the tube.  It is also essential in assuring a "face-on" presentation of the diagonal to the eyepiece by comparing its degree of circular projection relative to the sight tube opening. It is primarily used only at initial scope setup but may be required later on if the diagonal is "bumped" or removed for maintenance. 

    I offer two versions of a 2" size sight tube" called the TELETUBE XLTM and the TELECAT XLTM combination tool. These sight tubes are unique in that the length is adjustable to match the telescope's f/ratio for optimum Secondary positioning accuracy. If you only have a 1.25 " focuser, I recommend the AstroSystems Brand that can be purchased separately from AstroSystems (They also sell 1.25" autocollimators).

  2. A CHESHIRE - typically has a 1.25" diameter barrel and is 3" to 4" long. It is characterized by a reflective surface with a hole in the center and a peephole on one end.  In the classic Cheshire, the elliptical reflective surface is positioned on a 45 degree angle to the line of sight inside the tube and is illuminated by light entering an adjacent cutout in the tube side.

    With CATSEYETM brand Cheshire tools, the annular reflective ring surface is on the END of the tool opposite the peephole and is perpendicular to the line of sight.  Illumination is accomplished by light directed at the primary mirror which is reflected back to the reflective ring via the diagonal. 

    A Cheshire device is used in conjunction with a "reference" center spot on the primary mirror to adjust the primary tilt.  The CATSEYETM System utilizes a special highly reflective triangle center spot sized to just fit inside the image of the reflective ring; the points of the triangle are aligned with the primary adjustment screws.

    The advantage of CATSEYETM brand Cheshires is that the image of its reflective ring is very distinct and easy to see especially at NIGHT when illuminated with a red LED flashlight. The BLACKCAT XLTM is a 2" diameter version of the Standard CATSEYETM designed for larger, longer-focal-length scopes.

  3. An AUTOCOLLIMATOR - This is typically a 1" long, 1.25" diameter tube open at one end with a peephole at the other end.  There is a round, flat mirror on the backside of the peephole inside the tube with the mirrored surface facing toward the open end of the tool.  A small amount of the mirrored surface has been removed at the center of the mirror to allow sight through the peephole.  This tool operates on the principal that 2 mirrors (the primary and the autocollimator) faced opposite each other will reflect an image between one another, and if the angle of incidence of both mirrors is "exactly" 90 degrees (i.e. collimated), the reflected "ghost" images will merge together. 

    This tool is used to adjust the tilt and rotation of the diagonal mirror.  In the CATSEYETM System, the triangular center spot is the reference image.  Each successive reflection is reversed and when the multiple images merge through correct diagonal positioning, they approach the combined shape of the "Star of David" and actually should "disappear" at the point of complete convergence.

    I offer a 2" autocollimator called the INFINITY XLTM or the next-generation 2-pupil XLKPTM. I highly recommend this high-axial-error-resolution tool for ultimate collimation precision. If you only have a 1.25 " focuser, I recommend the AstroSystems Brand that can be purchased separately from AstroSystems (They also sell 1.25" sight tubes).


Collimation Application Notes:

Indirect sky light or a brightly lit wall can be used to implement the CATSEYETM System, but it's strength and real advantage is at NIGHT with the scope in position for observing where a bright red LED light directed at the primary clearly and distinctly illuminates the reflective surfaces of the triangular center spot and reflective ring.

Iterative, alternating use of a CATSEYE/BLACKCAT XLTM Cheshire and the INFINITY XL/XLKPTM autocollimator will very quickly and progressively converge both Primary and Focuser optical axes to a level of precision unsurpassed by any other collimation tool combination.