Collimating an TeleVue NP-101
This is a detailed description of the procedure I used to collimate my NP-101. In order to accurately collimate my scope, I decided to build a simple jig that slid over the objective cell and allowed me to accurately adjust the tilt of the objective cell. One should be handy with tools but nothing more. A screw driver, an allen wrench, a drill, a saw and a 1/4-20 tap are the only tools required.
Since you are reading this, I am assuming you have determined your scope needs collimation. This is the first step, testing the collimation by performing a star test. When I tested my scope I found that at 300x the diffraction rings were not centered on the airy disk and they were not full circles. I was not certain this was a collimation issue, it could have been a diagonal or thermal issue so I tried the tube in a number of positions to identify that the rotating the tube caused the off-centered diffraction rings to rotate as well. Once I established this, I considered it for a while, fooling with the collimation of a Newtonian is one thing, with an apochromatic refractor is quite another. I discussed it with John Biretta whom I know from Astromart and had advised me in the purchase of this scope, we talked over the various ways to build a jig and I decided to go ahead with collimating the scope.
The basic problem is that the objective in front and the Petzval elements in the rear must be accurately aligned. If they are not, the telltale signs of miscollimation are visible. My first step was to try collimation without a jig but it soon became clear this was hit and miss, the cell has no adjustment screws, only locking screws. I was not able to accurately position the objective so that I was happy with the collimation, it required a series of nudges and while I was able to get it close, invariably the attempt to make it perfect resulted in poorer collimation. You may want to try this, I don't recommend it, I liken it to collimating a fast Newtonian that lacks adjustment screws...
So, this is what I did, I can make no guarantees except to say it worked for me and I believe that if you have identified that your scope requires collimation and if you read this carefully so that you understand it, you too can achieve a well collimated NP-101 with little more than an hour's time and some care.
Materials: Acrylic Tube 6 inches long x 4.25" x 4.50" (0.125"wall)
4 1/4-20 Nylon Screws 1" long (Nylon helps to prevent marring the scope finish)
Hose Clamp 5 inch diameter
McMaster-Carr has the tubing in 12 inch lengths for $19.
Building the Jig:
- Drill and tap 4 equally spaced 1/4-20 threaded holes approximately 1/2" from one end of the tube. The Nylon screws threaded here will serve as the adjustment screws. This need not be precision, eyeballing it is fine.
- Using a saw, split the tube end to end so that it can be spread open and slipped over the front of the objective cell. Make sure you the cut is located so it will not intersect the drilled holes
- Remove the snap ring from the dew shield, it is wire and can be done without tools. The scope should be securely mounted. Slide the Dew Shield down the tube towards the focuser as far as it will go. Make sure there is sufficient room for the jig to fit, you may have to slide the scope in the clampshell.
- Spread the acrylic tube, slip it over the step in objective cell, the end with the screws should be towards the focuser.
- Identify the 3 collimation adjustment screws. Slide the tube towards the focuser until the distance from the non-threaded end to the screws is about 2.5 inches. Using the hose clamp, clamp the acrylic tube to the objective cell. Using a magic marker, mark the position of the three collimation screws, they are visible through the clear acrylic.
Remove the acrylic tube, drill three holes 3/8" diameter or larger centered on the marks. These provide access so you can loosen and tighten the collimation locking screws with the jig in place. Once this is done, the jig is finished and you are nearly ready to being the actual collimation.
First though, you must prepare the scope. The collimation locking screw heads are filled or covered and this must be removed so the allen heads are accessible. Mine had already been removed, someone had previously played with the collimation.
The actual collimation is simple. One will need a clear night with reasonably good seeing to perform the star test and an eyepiece combination capble of about 300x, I used a 3.5 mm Nagler.
- Slide the acrylic tube over the tube once again and position the 3 access holes over the collimation locking screws. Clamp the tube to the objective cell.
- Install the four 1/4-20 Nylon Screws and snug them up against the the OTA, they need not be tight, they just need to be tight enough to keep the Objective Cell from moving.
- Loosen the 3 collimation locking screws. These need to be loose enough to allow the cell to move easily but no looser.
- Perforn the star test, you will need to be able to see the diffraction rings so the star should be reasonable bright but not too bright.
- Using the 4 nylon screws, make the adjustments so that the infocus diffraction rings are centered about the star.
- Once you are satisfied, tighten the collimation lock screws. It is reasonable to do it in several steps as one would torque an engine head or other precision instrument.
- Once these are tight, loosen the nylon screws adjustment screws on the jig and repeat the star test to make sure the collimation has not shifted.
- Back out or remove the 1/4-20 Nylon screws so they allow the acrylic tube to slide freely against the objective cell, loosen and remove the clamp, slide the tube over cell to remove it.
- Slide the dewshield into position, replace the snap ring, making sure it is securely in place.
You are done.... Store the acrylic adjustment jig and enjoy your scope.
This is how I did it. Please feel free to ask questions or make comments. If it seems confusing or too much, I would advice against it. It is a simple procedure, the jig just provides positive adjustment screws. There are a number of different possible ways to do this, yours might work better for you.
Jon Isaacs
