COLLIMATION FOR MODERATE APERTURE RATIO

COLLIMATION FOR MODERATE APERTURE RATIO NEWTONIANS

A defocused star in a well collimated reflecting telescope.

This discussion is strictly for Newtonian reflectors having aperture ratio of roughly 5.5 and larger and instruments that have all of their optics concentrically aligned. For those individuals with small aperture ratio Newtonian's that require the secondary mirror to be offset from the optical axis, the following does not apply.

One of the reasons I'm so fond of moderate to large aperture ratio Newtonians is that the collimation of the optical system is not critical. This is what I call a "relaxed" system, and for astronomy, where high resolution is required and great demands are placed upon the optical system, a requirement for critical alignment because of small focal ratio is simply another cause for potentially less than perfect performance. The owner of a Newtonian having an aperture ratio of 6 and above really needs nothing more to collimate their telescope properly than a plastic film can, some heavy paper or Bristol board, a drawing compass and a number 2 pencil. Bristol board is a form of heavy paper that approaches cardboard in character. It is very smooth and durable - for a paper product. One really need not invest in laser collimators or other expensive and sophisticated optical devices. I am aware of several clever optical devices that an amateur can easily and inexpensively construct that will collimate a Newtonian very precisely. One such device was invented by a fellow I know named John Shelly and an article on its construction appeared in a past volume of Amateur Telescope Making Journal and another is the Cheshire collimator. Both work very well. However, I find that I can collimate a Newtonian to high accuracy and use none of these devices whatsoever.

Collimating a Newtonian involves two steps: first one has to properly adjust the diagonal, and secondly one adjusts the primary mirror. In order to prepare the telescope for collimation one needs to construct a couple of simple items. First of all, get a plastic film container that fits nicely into the barrel of your 1.25" eyepiece holder. If you have a 2" eyepiece holder use the adapter to reduce it to 1.25". Carefully make a center spot on the bottom of the film can and on the top of the cap of the film can. On the bottom of the film can drill a hole approximately 1/4" to 3/8" in diameter. In the film can cap drill the whole approximately 1/16" to 1/8" in diameter. This device will be inserted into the eyepiece holder and will be utilized to center the secondary mirror (the diagonal) to the optical axis of the focusing tube. The importance of the film can is twofold; it forces the observer to place his eye exactly at the center of the optical axis within the focuser, and the hole at the bottom of the film can forms an aperture through which the diagonal can be accurately centered.

The next step is to make a Bristol board mask that will fit over the front end of the telescope. This mask will be used to optically align the primary mirror to the secondary mirror. This mask should be made to carefully and snuggly fit over the open end of the telescope and the exact center in relation to the tube should be marked on this mask. Once the center has been accurately determined the pattern shown in the accompanying picture should be cut into Bristol board. The exact dimensions of the pattern will vary as to the size of the telescope it is made for. The only critical thing here is that the central hole of the mask should be about 1/2" in diameter larger than the diagonal holder. The object of this mask is to create a small target area in which the secondary is to be accurately centered when looking through the focusing tube.

Lastly, draw on a piece of paper something resembling the picture below at the left. This will indicate how the diagonal will appear to move when viewed through the focusing tube when one of the adjusting screws on the mirror cell is turned either clockwise or counterclockwise. The adjusting screws at the back of the telescope should be numbered (at least in your mind) to correspond to the picture. A simple diagram like this will save a lot of running back and forth.

Now that the necessary components have been constructed the process of actual collimation can be outlined.

Step 1: The diagonal is carefully adjusted within the telescope so that it occupies the exact geometric center of the tube. This establishes the exact center line of the collimated optical system and insures that the optical axis is parallel to the side of the tube.

Step 2: The film can is inserted into the focuser. When looking through the hole in the top of the film can the diagonal should appear exactly centered within the hole in the bottom of the film can. If the diagonal is not centered longitudinally in relation to the optical axis, it should be adjusted by turning the threaded rod provided as a means of holding most common diagonal cells and continuing the rotation until the diagonal appears to be centered. If is not centered laterally to the optical axis (inasmuch as we have already centered the diagonal precisely within the tube) the focuser itself will have to be shimmed up on one side or the other until the diagonal becomes centered.

Step 3: The secondary mirror is now adjusted until the primary mirror is in the apparent center of the diagonal. The primary mirror should be entirely visible within the secondary mirror with some room to spare. If the mirror can not entirely be seen you are not going to have a fully illuminated field. You will be losing aperture and effectively increasing the size of the secondary. The secondary mirror is usually adjusted by three pull screws at the back of the holder. Also, rotating the secondary about it's axis by the mounting rod can be an important adjustment tool.

Step 4: The sheet of paper with the drawing on it shows in which direction the diagonal moves when the adjustment screws are turned. The mask is now placed over the front of the tube. Adjust the three screws on the back of the primary mirror, carefully taking note of the motion of the diagonal as small adjustments are made, until the diagonal mirror appears perfectly centered within the central ring hole of the mask.

The telescope is now collimated and the film can and mask can be removed. To give a final test and to insure that exact collimation has been achieved, focus the telescope on a second or third magnitude star and insert the eyepiece combination that will give you approximately 100 to 150 power. Defocus the star outward so as to produce the sharpest diffraction rings. The presence of the secondary will result in a black spot in the center of the rings. The rings should be perfectly concentric with the black central spot. This is not a star test in the conventional sense to determine optical quality, but rather a star test to determine the collimation of the optical system. If the dark center is not concentric but off to one side and the rings non-concentric and egg shaped, then the collimation process has not been done precisely enough. But, I have never collimated an f/6 reflector or longer and found that the collimation required further tweaking. The star collimation test can be made more sensitive by bringing the rings closer into focus until a perfect Airy disk with its surrounding first order ring (seeing conditions permitting) is achieved. If you can get this, you've got everything you can get.