In refractors, the eyepiece faces the upper end of the telescope tube where the light is collected. While a star diagonal is normally used the effect is to make the eyepiece face indirectly toward the upper end of the tube so the effect is the same.
Some light reaches the eyepiece that is not useful because it is outside the useful Field of view. Suppose our eyepiece gives a 1/2 degree field of view. At the same time light to make up the 1/2 degree field enters your telescope there is scattered glare light entering from the field of 5 to 15 degrees in diameter. In observing you see illuminated objects such as stars viewed against a dark background. The scattered light makes the nearly black background brighter to be dark gray or even light gray. The contrast becomes poor or detail can not be seen. Painting the inside of the tube flat black will adsorb most of the light reflected from the inside tube wall. However some of the light is still reflected and some scattered glare light reaches the eyepiece with no reflection.To improve optical performance of refractors a series of or glare stops are placed at regular intervals inside the refractor tube. The glare stops are nothing more than discs with holes in them. The holes become smaller as the light beam comes to a focus. This traps stray light that would normally be reflected to the eyepiece off of the sides of the tube and would reduce contrast.
In a Cassegrain the stray light problem is the same but the physical situation is different. A Cassegrain has a tube length of only about 1/3rd that of a refractor of the same aperture and f-ratio. This is accomplished by folding the light path. Between the primary and secondary mirrors there are three light paths folded together. One of the light paths is the full aperture of the telescope for the full length of the tube. Glare stops inside the telescope tube must be at least the full aperture to let light into the telescope. But these can remove little of the most troublesome glare.
The most troublesome glare comes from the 1/4th or 1/3rd of the light path just before the focal plane. This means that the light baffles to exclude stray light in a Cass must be between the secondary mirror and the focal plane. A baffle tube is placed in front of the primary mirror which works the same as a glare stop but does not interfere with other two light paths. For optimum performance of Cass type systems most experienced telescope builders recommend a series of light baffles mounted on both the primary and secondary. The effect of stray light is most apparent when Cass telescopes are used in daylight. Scattered light is very high so the contrast is very poor to give a white glow to everything observed. The best test is looking at a black object. The whiter it looks thru the telescope the greater the scattered light and the poorer the contrast. However even at night faint objects and planetary detail can be more difficult to observe. Scattered light from city lights and the moon adds to the problem. The effect on observations at night depends on the objects being observed and other factors. If you are undecided whether to use baffles for a Cass telescope now under construction or being planned it is suggested that the telescope be assembled and used without baffles. If you find a glare problem in your observations the baffles can be added.
Commercial baffle sets are available but they can be homemade of brass or aluminum tubing. For small telescopes the baffle tube can be attached to the focusing tube of the focusing mount. The baffle tube can also be attached to a metal disc which is in turn attached to the mirror cell. On nine point flotation systems the disc should not interfere with any of the flotation pads but should be attached to a metal disc which is in frame. In either case the disc should be made with a number of large holes to increase air circulation. It should be mounted on the cell between the end cap and the cell rather than between the mirror and the cell frame. Most often the baffle is attached to the center of perforation of the primary mirror.
Bob's Note: The baffle tube may also be used to locate the primary mirror laterally as this is a good low stress point to do this. The stresses that are formed tend not to affect the operation of the scope as they are applied to only a small part of the mirror and are applied in the direction perpindicular to the mirror's surface, causing little deformation of the mirror.
For 1 1/4" and 1 1/2" perforations common bathroom drain fittings can be adapted by using machine collars on either side of the mirror to hold the tube in lace. To keep the metal from straining the mirror place rubber washers between the mirror and the collars. Rubber washers to fit the tubing are available in the plumbing supplies. For other sizes you can use "O" rings. Aluminum tubing can also be used with collars. It is possible to thread the tubing and use nuts on both sides of the mirror. However the most common aluminum tubing is made of an alloy that is not easy to cut threads in.
Sometimes a mounting stud is made to fit the perforation of the mirror of a bar of machinable aluminum. It is attached with a nut or the back of the mirror. A length or tapered baffles can be made of sheet brass or tooling copper available from hobby craft stores. Shim brass is available from machinist suppliers. The baffle tube is formed over a wooden form and seams sweat soldered together. This in turn is attached to the mounting stud. Since Cass perforations are usually cut from both front and back of the mirror there may be a consideration in the perforation where the cuts meet. If this construction interferes with mounting of the baffle it may be necessary to grind it out. Note that this grinding requires the same protection for the optical face is required in perforating.
Formula (4) below is for a straight baffle length for primary mirrors. If (L) is negative a baffle is not be used because:
| Where: | L = baffle length from the front of the mirror | b = back focus |
| B = separation between primary and secondary | V = baffle tube outside diameter | |
| C = secondary mirror clear aperture | K = secondary holder outside diameter | |
| W = baffle tube inside diameter | I = image size at the focal plane |
The primary mirror baffle length is selected above by considering the light path between the secondary mirror and the focal plane. But there is a chance the baffle tube is long enough to interfere with the optical path passing from the primary mirror to the secondary mirror. To take an extreme example, if the baffle inside dimeter is the same as the secondary size, the baffle length by Formula (4) and (5) would be the total distance between mirrors! There would be no stray light but no light could pass thru the telescope. Formula (6) is a safety check to give an approximate baffle length without obstructing large image forming rays passing from the primary to the secondary. It is a useful guide to prevent a loss in image brightness. The baffle length by Formula (6) is a maximum baffle length. If formula (6) gives a maximum length less than by Formula (4) then only the shorter of the two would apply. If the baffle length by Formula (6) is less than the minimum baffle length by Formula (5) then use a smaller baffle tube. For an existing baffle the inside diameter of the baffle tube can be reduced with a layer of cork or flock paper.
| where | K =secondary holder outside diameter |
| F = focal length of the primary | |
| V = baffle tube outside diameter |
for commercial baffles the tubing is about 1/16" thick so V = W + 1/8"
A proper baffle length will prevent light from reaching the eyepiece directly from the sky but light can still enter the baffle tube and be reflected down the tube toward the eyepiece. Light striking a flat black at a 90 degree angle will be almost entirely absorbed. However light striking at a very oblique angle will be almost entirely reflected. The long slender ID of refractor tubes and baffle tubes will reflect most of the light that falls at the very oblique angles in most common refractors and Cass baffle tubes. Some of this reflection can be absorbed using flock paper (Edmund No. 70621) placed inside the baffle tube at the upper and lower ends;. it is difficult to put flock paper the full length of the baffle tube. If used at the open end of the tube it will change the inside diameter so should be considered in the calculations for baffle length. It may be an advantage to reduce the baffle tube inside dimeter with flock paper if the baffle tube is too short or too large in diameter. Flock paper can be put in the tube for a distance of about twice the diameter without too much difficulty.A better solution to blocking reflections down the baffle tube is to use a paper or metal stop glued to the bottom and on the baffle tube. The inside of the baffle tube is a situation almost exactly like a regular refractor telescope so the glare stop is the same. Formula (7) gives the diameter of the hole in the stop where (Z) is measured from the mirror face toward the focusing mount.
| where | L = image size at focal plane | B = separation |
| S = glare stop aperture size | C = secondary clear aperture | |
| Z = distance from mirror face to glare stop | b = back focus |
Baffles for the secondary holder can be made of paper or thin metal glued to the outside of the secondary holder. They are less important than other baffles but are simple to add. They are easy to add if you feel the need for assurance for them. Formula (8) gives the secondary baffle length (1) measured from the face of the secondary.
| where | B = separation | l = secondary baffle length |
| C = secondary clear aperture | K = secondary holder outside diameter | |
| M = primary mirror diameter |
Like the refractor, a Cass telescope works best with a series of glare stops. But because the Cass is more complicated the glare stops are of a number of types. However they do work very well together.
Telescopes - How to Make Them and Use Them, Macmillian 1966, pages 4-35. Briefly gives the importance of a baffle tube with one formula.