Saturday, February 12, 2022

Collimating a Newtonian

 My physical therapist has a telescope given to his kids by a grandparent, and like most gifts, it has been a disappointment.  There are three areas where the Christmas gift reflectors disappoint and since I was going to write this up for him, you get the benefit as well.


Newtonian reflectors need the mirrors and eyepiece tube lined up pretty accurately.  (So do refractors, but unless abused, this is seldom a problem.)  You will get imprecise fuzzballs for Jupiter, barely discernable rings around Saturn, and uncrisp focus on the Moon.  Properly collimated, all but the worst or smallest reflectors will show cloud bands on Jupiter.

These steps assume a reflector made with some modicum of care and not dropped or stomped on by gorillas.  Collimation can be done with a keen eye and a Chesire eyepiece, but most people use a laser collimator.  It is easier and faster.  A laser collimator goes in the eyepiece tube and projects a laser beam that reflects off the diagonal mirror, hits the primary mirror, then bounces back and hits a projection screen.  This is best done in shade or at twilight.

1. Put collimator in eyepiece tube.  Turn the locking screw as you would on an eyepiece, no tighter or looser.  Make sure the centering screen is pointing straight back at the rear of the telescope.

2. Turn on the laser.  

3. Aim telescope at a wall.  If you see the beam hitting that wall, you are way out of collimation.  While the power of the laser is so low that it will not injure your eye, it will dazzle you.  Besides, for all you know, the manufacturer might have got a deal on laser LEDs rejected by the Navy's antimissile laser project.

4. There are typically three adjustment screws on the back of the primary mirror mount (the piece of aluminum that holds the mirror).  In the best telescopes, the mirror mount may have locking screws that lock it in position.  These screws will not be spring loaded, unlike the adjustment screws.  By turning the adjustment screws you are moving where that laser beam is aimed.  Adjust until the beam no longer exits the telescope. 

5. Now look down the tube.  You want the beam exactly in the center of the primary.  Nearly all commercial reflectors will have either a dot or white paper reinforcement ring centered on the primary.  How do you get the beam centered?  On the back of the assembly that holds the diagonal mirror you will find either three adjustment screws or (ideally) three thumbscrews.  Turn these one at a time until the beam is in the center of the primary.   This may take a couple minutes.

6. Go to the back of the telescope.  Look at the centering screen on the collimator.  You should see the beam somewhere on the screen.  Now adjust the three screws on the back of the primary mirror mount.  Turn them, one at a time until the beam is in the center.  Some have a bullseye.  Some, like the Orion Lasermate, have a tiny hole.  When the light splays into something that looks like a paintball hit a target, you are centered.

7. If your mirror mount has locking screws that lock down the mirror mount's position, turning these until they contact the back of the mount.  This will hopefully reducing the need for future collimation.  

Your image should be dramatically improved.  If you are gentle in moving it, it should stay close to collimation.  At least it will be closer the next time.

Finder Alignment

The finder is a small telescope that you use to get your telescope aimed at your target.  This alignment is best done in daylight or with a Moon.

1. Make sure the finder seems aimed the same direction as the telescope.  Is it parallel to the telescope tube?

2. Find some objective a few miles away that is distinctive.  A mountain range is miles away, but at which peak are you looking?

3. Put in your lowest power eyepiece and try to find the object.  (If you start on a target a few hundred feet away, this will not be accurate enough.)  One you have it centered the target in the eyepiece, look in the finder and fiddle with the adjustment screws until the crosshairs are on the target.  The Moon is especially good because the cross hairs will give you a nice set of black lines.

4. Put in a higher power eyepiece and repeat steps 2-3.  Repeat steps 2-4 until you run out of higher power eyepieces or patience.


This is usually the biggest cost-saving measure on cheap scopes.  The modified achromat (or if lucky Kellner design) eyepiece have a narrow field of view and are seldom crisp focus.  Orthoscopics or Plossl designs will almost always be better.  If you want to spend more on eyepieces than your telescope cost, buy those made by Televue.  You can also put Z-rated tires (good to 149 mph and above) on your Toyota Corolla, but it is not sensible.

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