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COLLIMATION OF THE TELESCOPE

  Scope City's Optics Crash Course
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PART VIII. COLLIMATION OF THE TELESCOPE

Collimation is the alignment of the optical parts of a telescope. It's intuitively obvious that these must be lined up to produce high-quality images, but the necessity of doing so becomes more severe the shorter the focal ratio of the scope. On the other hand, the same techniques for collimating the short focal ratio can be equally well-used on a longer focal ratio, so the techniques you learn for one will be used on the other.
Of the telescope types we've discussed, the one most likely to need collimation is the Newtonian reflector (which includes Dobsonians). The Schmidt-Cassegrain, though easy to collimate, is relatively unlikely to need it, so instructions for SCT collimation will not be found here.

The most common tools that can be used to collimate the Newtonian reflector are:

--laser collimator
--Sight Tube
--Cheshire
--Autocollimator
--Combination Sight Tube/Cheshire

What follows is a step-by-step method of how to collimate the reflector, and which tool to use for each task. What is described is the Modern Partial-Offset Collimation Protocol, which is described many places in the literature. In this protocol, all optical surfaces are aligned to the focuser axis, so we will start out assuming that is OK. If the focuser is tight on the tube, its alignment will be good enough for excellent collimation. The entire procedure can be done in daylight, and it helps to point the telescope at a bright sky for this, BUT DON'T POINT CLOSE TO THE SUN.

COLLIMATION OF THE NEWTONIAN TELESCOPE

1. Aligning the Secondary mirror in the tube. The tool used here is the Sight Tube, a longish cylinder with a set of crosshair wires at one end and a peep-hole on the other. The Sight Tube is inserted in the focuser until the outline of the inside diameter of the sight tube appears to surround the outside diameter of the secondary mirror, and then is fastened tightly with the setscrew. If you can't tell where the edge of the secondary mirror is, hold a piece of white paper against the inside of the tube opposite to the focuser. This will surround the secondary mirror with a white background, making it easy to see where the edge of the mirror is. What we're trying to do here is to make the outside edge of the secondary mirror appear concentric with the inside diameter of the end of the sight tube, as seen through the peep hole. If the secondary appears too far up the tube, away from the primary, its center bolt will need to be loosened and the secondary lowered in the tube toward the primary. Do this with the tube nearly horizontal to avoid the possibility of dropping a tool on the primary mirror.

Once the mirror is in the center in the up-down tube direction, it will need to be rotated until it is centered in the side-to-side direction. Start out by rotating the secondary on the center bolt until it appears round to the eye. Then, carefully use the screws on the secondary to make the round image of the secondary appear concentric with the inside diameter of the sight tube.
This will only have to be done once, so take your time to make it right.

2. Aligning the Secondary mirror to the primary. The tool used here is the Sight tube or the point-source red laser collimator. The laser must itself be collimated or using it will misalign the scope. Many low-cost lasers come out of the box miscollimated. It is for this reason we recommend the sight tube instead. Insert the sight tube and fasten the setscrew tight. Look at the crosshairs through the peep-hole and note the distant center dot on the primary mirror*

[*if the primary mirror does not have a center dot, you will have to remove it and put one on the mirror. 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.]

You will carefully move the collimation screws on the secondary until the center dot is lined up exactly behind the crosshairs intersection. What you're doing here is to tilt the secondary mirror to point directly at the center of the primary mirror. That's why we're adjusting the secondary mirror. This may seem to move the round image of the outline of the secondary out of concentricity with the inside diameter of the sight tube. If so, repeat step one and step two again. Each iteration brings the secondary closer to exact alignment. If you have to choose between the two, pick the dot line up with the crosshairs. The centering of the secondary mirror in the focuser is only to provide even illumination of the image all the way around the edge of the field of view-less important that correct pointing of the secondary mirror.

3. Aligning the Primary mirror. The tool used here is the Cheshire eyepiece, or the combination sight tube + Cheshire tool. A Cheshire eyepiece is 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. Be careful when you insert this tool that you do not cover this hole with the bill of a cap.

What this tool provides is a reflected bright ring with a dark center. We will use the collimation screws on the Primary mirror to move the reflected image of the center dot into the dark center of our bright ring of reflected light. It may be necessary, if a lot of movement is required, to repeat step two, and then return to step three. In a properly collimated telescope both steps will agree at the same time. If a combination tool is used, the crosshairs, center dot, and dark center will all line up at the same time.

[If you have a lot of light in the tube, a distant reflection of the underside of the crosshairs will be visible, but a lot smaller than the near-to-the-eye crosshairs in the tool. When the telescope is collimated, this distant reflection of the crosshairs will be hidden behind the near-field crosshairs.]

If your reflector is f/6 or longer, you can stop here. Indeed, if you have been careful, any Newtonian will be well collimated by now. But, for the users of f/5 scopes and shorter, some improvement can yet be had:

4. Eliminating all residual errors. The tool used here is the Autocollimator. No, it doesn't do the work for you, but it allows you to see tiny misalignments of the mirrors and correct them to a high degree. On short f/ratio scopes, where the allowable tolerances on miscollimation are in the thousandths of an inch, such a tool comes in quite handy. This tool is so sensitive to misalignment that simple mechanical flexure will be visible. If your secondary spider vanes are too loose, the telescope will not hold collimation to a few thousandths through all the altitude changes the telescope goes through, so the first thing to do here is to tighten the screws that hold the secondary spider to the tube. Be careful! Though they should be quite tight, most of them are small screws that can be stripped if applying too much force. Most commercial scopes come with these screws quite loose, however, and tightening is almost always called for.

The Autocollimator is now inserted and tightened. This is a tool with an internal mirror that faces the secondary mirror. As you can guess, when alignment is achieved, there is no light in the peep-hole and the whole field goes dark. Because 4 reflections of the mirror's center dot will be visible, you are essentially collimating the telescope exactly on a point 8 times the focal length of your telescope away. If done carefully, collimation will be accurate to a small number of thousandths of an inch. This accuracy rewards the observer with the best possible images that can be had with the telescope.

When looking in, there will be 4 images of the primary's center mark. If your previous steps were carefully done, these 4 images will overlap or nearly "stack" on top of one another. But, if all 4 are seen, better alignment can be achieved. By carefully moving the secondary collimation screws, the four reflected images of the center mark will stack tightly on top of one another. It is desirable to return to the Cheshire at some point and check alignment of the primary. If it's off, correct it in the Cheshire (turning the primary collimation screws), and return to the autocollimator. After a couple times back and forth, all four images of the center mark will "stack" tightly on top of one another, and your telescope is perfectly collimated.

The next time the telescope is taken out, step one will not need to be done. It's probably not a bad idea to check the alignment of the secondary with the sight tube, but it is unlikely that more than a minor tweak of one screw will be necessary. Then, proceed to steps two and three. If you use an autocollimator, skip step one and start with step two. The autocollimator will perfectly align the secondary.

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TELESCOPE COLLIMATION TOOLS

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HOWIE GLATTER LASER 1MM APERTURE STOP
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Antares CHESHIRE TELESCOPE COLLIMATION EYEPIECE
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Antares CHESHIRE TELESCOPE COLLIMATION EYEPIECE
Item No. 717-02100
Manuf. No. Antares CCOLLI

Antares Deluxe Cheshire/Sight Tube Combo Telescope Collimator, CColli, is great for complete alignment of telescope componets and mirrors. Fit into standard 1 1/4 inch eyepiece sockets.
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Officina Stellare Green Laser 532nm w/ Mounting Bracket
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Officina Stellare Green Laser 532nm w/ Mounting Bracket
Item No. 784-20280
Manuf. No. Officina Stellare LASFIN

High quality green lasers for using on Officina Stellar Telescopes
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Astrozap Artificial Star for collimation
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Item No. 793-31600
Manuf. No. Astrozap AZ-1600

The artificial star is designed to provide a steady, stable reference for star collimation to be used in lieu of an actual star. This device will allow collimation without the worry of star drift or seeing conditions
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Farpoint Lightbridge Primary Mirror Spring ONLY Kit for 16 inch
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Farpoint Lightbridge Primary Mirror Spring ONLY Kit for 16"
Item No. 781-10024
Manuf. No. Farpoint FP324

All 16" models and mid 2008 come stock with plastic collimation knobs. Spring only upgrade kit.
 
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Farpoint Lightbridge Primary Mirror Spring ONLY Kit for 12 inch
Farpoint Lightbridge Primary Mirror Spring ONLY Kit for 12"
Item No. 781-10023
Manuf. No. Farpoint FP323

Spring only upgrade kit for users who do not want to replace thier stock plastic knobs with the Farpoint machined
 
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Farpoint Lightbridge Knob and Spring Kit for 12 inch Primary mirror adjustment-NEW style
Farpoint Lightbridge Knob and Spring Kit for 12" Primary mirror adjustment-NEW style
Item No. 781-10022
Manuf. No. Farpoint FP322

All newer 12" models come stock with plastic collimation knobs. 2009 AND LATER MODELS
 
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Farpoint Lightbridge Knob and Spring Kit for 16 inch Primary mirror adjustment
Farpoint Lightbridge Knob and Spring Kit for 16" Primary mirror adjustment
Item No. 781-10021
Manuf. No. Farpoint FP321

Upgrades the stock primary mirror springs and collimation screws / knobs. This is essential for models earlier than 2008
 
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BOB'S KNOBS FOR VIXEN VMC 260L/200L CATADIOPTRIC TELESCOPES
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It fits Vixen VMC 200L, VMC 260L Catadioptric secondary
 
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BOB'S KNOBS FOR MEADE 14 inch (36 cm) f/10 SCT 6-SCREW SECONDARY WITH EXPOSED FACTORY COLLIMATION SCREWS
BOB'S KNOBS FOR MEADE 14 inch (36 cm) f/10 SCT 6-SCREW SECONDARY WITH EXPOSED FACTORY COLLIMATION SCREWS
Item No. 788-20203
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BOB'S KNOBS FOR MEADE 10 inch (25 cm) f/10 SCT WITH 6-SCREW SECONDARY
BOB'S KNOBS FOR MEADE 10 inch (25 cm) f/10 SCT WITH 6-SCREW SECONDARY
Item No. 788-20201
Manuf. No. Bob`s Knobs m10-6

It fits all Meade 10 inch (25 cm) f/10 SCT versions with six screws on the secondary housing (LX90, LX200, LX200R, others). The inner screws are for collimation.
 
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BOB'S KNOBS FOR MEADE 8 inch (20 cm) f/10 SCT WITH 6-SCREW SECONDARY
BOB'S KNOBS FOR MEADE 8 inch (20 cm) f/10 SCT WITH 6-SCREW SECONDARY
Item No. 788-20200
Manuf. No. Bob`s Knobs m8-6

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BOB'S KNOBS FOR MEADE 10 inch (25 cm) f/6.3 SCT
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BOB'S KNOBS FOR MEADE 8 inch (20 cm) f/6.3 SCT
BOB'S KNOBS FOR MEADE 8 inch (20 cm) f/6.3 SCT
Item No. 788-20170
Manuf. No. Bob`s Knobs m8wf

It fits all Meade 8 inch (20 cm) f/6.3 SCT versions (2080, LX10, LX50, LX100, LX200)
 
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BOB'S KNOBS FOR MEADE 16 inch (20 cm) f/10 SCT WITH 6-SCREW SECONDARY
BOB'S KNOBS FOR MEADE 16 inch (20 cm) f/10 SCT WITH 6-SCREW SECONDARY
Item No. 788-20164
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