by Donald E. Pensack
Collimation is the alignment of the optical parts of a telescope. Though lining up the secondary under the focuser is essential for uniform illumination of the field of view, there are only two critical alignments in Newtonian collimation: the Focuser Axis (aligned by adjusting the secondary mirror), and the Primary Axis (aligned by adjusting the primary
What happens if the scope is not collimated? At the very least, the position of best focus will not be located in the center of the field of view. At worst, every star in the field will display a flaring away from a poorly focused image that resembles a small comet. Extended objects, like the Moon or planets, will have details “smeared” out and focus will be nearly impossible to achieve. A very small miscollimation can introduce visual effects exceeding ¼ wavelength of aberrations, the maximum allowable error before the image deteriorates to less than the aperture should display.
What follows is a description of how to collimate accurately to a level where collimation errors no longer influence the image quality.
Collimation is the alignment of the optical axis of the primary mirror to the focuser axis (utilizing tilt adjustments on both mirrors), so we start out assuming the focuser is tight on the tube and relatively square to the tube axis. The entire procedure can be and is easier done in daylight. A bright sky is useful to allow us to see all the necessary reflections.
Then follow the steps described below:
THESE STEPS ARE DONE IN A-B ORDER:
STEP A Aligning the focuser axis by adjusting the tilt of the secondary mirror (which is what we are really doing when we adjust secondary mirror tilt): The tool used is the Combination Sight Tube/Cheshire tool (the tool above is a sight tube, not a combination tool. It has just a peep hole and crosshairs only). Insert the sight tube and fasten the setscrew tight. Look at the crosshairs through the peep-hole and note the position of the distant center marker on the primary mirror*
[*if the primary mirror does not have a center marker, you will have to remove the mirror and put one on it. If you don’t know how to do this, have a local shop or astronomer friend help you out. You cannot successfully collimate without one. A transparent mirror center-spotting template such as the ones available from Catseye (for all mirror sizes) and FarPoint (for mirrors under 15″) allows a home user to perfectly position the center dot to less than 0.02” (0.5mm), which is accurate enough for good collimation. If the mirror already has a center dot, it’s position should be checked—some come misapplied and off-center as much as ¼” (6mm).]
Carefully adjust the collimation screws on the secondary until the Primary’s center marker appears lined up exactly behind the crosshairs’ intersection. What you’re doing in this step is to tilt the secondary mirror to point the reflected focuser axis directly at the center of the primary mirror. That’s why we’re adjusting the secondary mirror. [If you have a lot of light in the tube, or the sky light illuminates the bottom of the Sight Tube’s crosshairs, a distant reflection of the underside of the crosshairs will be visible, but will appear a lot smaller than the near-to-the-eye crosshairs in the tool (which appear dark because they are not lighted on the side nearest the eye). When the telescope’s primary is collimated, this distant reflection of the crosshairs will be hidden behind the near-field crosshairs. This distant reflection of the underside of the crosshairs is removed for simplicity in the BEFORE image that follows.]
Some people have trouble focusing on the center mark and the crosshairs at the same time. It helps to use glasses in that case, or back the eye up far enough to allow both to be in focus. Even if the crosshairs are slightly out of focus, the diffraction put up by the crossing of the hairs creates a “dot” in the vision that can be lined up with a center mark that has a hole in it. If you back away from the sight tube, hold on to the scope or focuser with one hand—this steadies the body and the eye so you won’t be bobbing back and forth trying to see the image through the peephole.
BEFORE ALIGNMENT OF CENTER MARK WITH CROSSHAIRS
AFTER ALIGNMENT OF CENTER MARK WITH CROSSHAIRS, WITH PRIMARY NOT YET COLLIMATED
AFTER ALIGNMENT OF CENTER MARK WITH CROSSHAIRS, WITH PRIMARY COLLIMATED (STEP B)
CHESHIRE PART OF COMBO TOOL
STEP B Aligning the Primary mirror: The tool used is the Cheshire eyepiece (The Catseye Black Cat is pictured above), or the combination sight tube/Cheshire tool. A Cheshire eyepiece is either a cylindrical tool with a hole in the side of it and an internal 45-degree mirror to reflect light from the sky down onto the primary mirror and back or an even-simpler tool with a bright ring on the bottom and a simple peep hole on the other end (pictured).
Be careful when you insert a 45-degree Cheshire like the Tectron or most combination tools on the market (the type with an open window on the side) that you do not cover the side hole with the bill of a cap so the 45 degree surface can reflect the bright light of the ceiling or sky; for the simpler bright ring type (pictured), the tool should be fully inserted. What this tool provides, when viewed through, is a reflected bright ring with a dark center. Use the collimation screws on the Primary mirror to move the reflected image of the center marker into the dark center of the bright ring of reflected light. It may be necessary, if a lot of movement is required, to repeat Step A for the Secondary mirror, and then repeat Step B (this is usually only necessary when a scope is assembled from components or a major shift in primary alignment follows disassembly of a mirror cell for the replacement of springs). In a properly collimated telescope both secondary and primary alignment will agree at the same time. If a combination tool is used, the crosshairs, center dot, and dark center and reflected crosshairs will all line up at the same time.
BEFORE PRIMARY MIRROR ALIGNMENT THROUGH THE CHESHIRE
AFTER PRIMARY MIRROR ALIGNMENT THROUGH THE CHESHIRE
NOTE: In the dark, a bright red LED flashlight aimed at the primary mirror’s center mark will make collimation possible and easy with the combination sight tube/cheshire tool described above.
Donald E. Pensack
Los Angeles, CA