Hoods, Grazing Reflections and the Fuzzy End
The removal, or should I say suppression, of stray skylight from the image plane in a Cassegrain type optical system is a major challenge for any telescope designer. The singular Cassegrain problem is that one can look at the sky directly from the eyepiece tube. This is a serious dilemma and there is no one solution that suits all applications. It can, however, be adequately controlled within specific applications. The two extreme applications of the Cassegrain type are the moderately wide field astrographic Ritchey-Chretien and the narrow field Dall-Kirkham planetary type. The current popular approach for both types is to use a hood, a conically or funnel shaped extension that surrounds the secondary and is set so as to block off-axis ambient light at the expense of the smallest possible secondary size. Where significant off-axis areas of the sky are desired the importance of a hood can be appreciated but in the end it can significantly increase the size of the secondary, and the greater the off-axis field the greater the increase in secondary size.
For narrow field, high-power, high resolution work, and where small, cleanly defined obstructions are preferred, a hood is really not required if a properly designed baffle tube is employed. This will allow for smaller obstructions and suppress any noticeable ambient sky-light for almost any observing situation. But much is demanded of this baffle tube and it will be a little different.
The illustrations above give a basic idea of how ambient skylight light can enter a Cassegrain type system. The green rectangles represent the primary and secondary mirrors. The black rectangles near the central hole is the area shadowed by the secondary. The black dashed line angling in at the left represents the contracting focusing light as it's reflected from the primary and the line angling in from the left side of the central hole represents the secondary shadow as it emerges from the face of the primary. Both lines merge at the focus.
The left-hand picture shows how light will enter the image plane without a baffle tube of any kind. The red lines represent light that directly enters by simply looking around the secondary. The hood (solid red lines) can stop some of the off-axis light but to stop all of it would require a very large hood. The orange lines represent light that enters but is stopped by the mirror itself and reflected harmlessly away. The blue line represents light that is stopped from entering by the secondary itself.
The middle picture shows the impact of a baffle tube in stopping some of the red line light that might otherwise enter the image circle. However, the orange line light is now capable of entering the image plane by low angle grazing reflection from the interior of the baffle if the walls of the baffle can not completely absorb it.
At this point it is interesting to look at the cross-sectional shape of the hood. A subtle but important feature is its conical shape. It should follow the angle of the contracting rays of the primary mirror. This has nothing to do with suppressing incoming off-axis ambient light but rather to reduce the shadow from the hood on the opposite side.
The right hand picture shows that if the baffle tube is somewhat constricted or extended in length even more light can be suppressed. If the baffle tube is extended until the edge of the secondary is just touched or covered when observed at the image plane, no light can enter. Practically, I bring the baffle tube up until it just approaches the edge of the secondary.
Note that it is very important that the baffle tube is not so large in diameter, or so long, as to obtrude out of the contracting secondary shadow. This will cause an effective increase in the secondary size by effectively expanding the shadow.
It can be seen that a properly designed baffle tube must not only be long enough and of the correct diameter but it must completely suppress any grazing reflection from light that strikes its internal walls. When a hood is not used all of the stray light suppression is accomplished with and within the baffle tube. Optically, the baffle tube of a Cassegrain is the only real functioning tube. The outer tube (if there is one) is really superfluous. Assuming now that there is no hood, or tube, a lot of light that otherwise would be blocked out is allowed to enter the baffle tube and if not stopped at the wall would enter the eyepiece or CCD by means of internal grazing reflection. Traditionally, the grazing reflection problem has been attacked by the use of disk stops within the baffle tube. While this is to a certain extent effective it does not completely nor adequately address the problem so I began searching for something better.
At first I tried making a traditional baffle tube with several internal disk stops and looked for grazing reflection light by simply pointing the tube off to one side of a bright light and looking through the bottom end. This pretty much simulates the condition in the telescope and shows the effectiveness of any light suppression system. One should not see any reflected light from the walls of the tube. Using conventional stops, reflected light from the edges of the stops was clearly visible as glowing rings. I then removed the stops and began looking into various light suppressing cloths stuck to the sides, concentrating on fuzzy light-trapping velvets and velours. All of these, even those sold as telescope tube liners, gave off light from grazing reflection. Remember, the incidence of reflection here is quite low and low incidence reflection is the most difficult to suppress.
After some experimentation I became very interested in black velour cloth, even though it gave off a sheen of grazing reflection. Velour struck me as a very promising material because its surface is made up of millions of little short hairs standing on end - light traps! Velour is that fuzzy cloth your grandmother's furniture used to be covered with. I looked at the surface under a microscope and saw that the ends of the hairs were machine cut flat and functioned as a myriad of tiny specular surfaces. In a moment of wild inspiration I grabbed a can of flat black paint, sprayed the cloth and re-inserted it into the tube - once dry, the reflection disappeared - completely. Microscopic examination showed that the hair ends were now covered with individual, hemispherical blobs of paint, each a poor reflective surface but also round and acting to disperse the light, thus weakening it into invisibility. And that's the stuff I put on the interior walls of my baffle tubes. This is the blackest surface I have ever seen, no light gets out, when held up to bright light sources no reflection can be seen - none.
There is one remaining problem, the edge of the opening of the tube. Ordinary manufacturing aesthetics dictate that this should be clean and smooth in appearance. The problem is that a clean edge will act just like the edges of those stops and give off a ring-like reflection. The answer is to leave the edge a little rough and fuzzy, thereby creating maximum dispersion of light. It may not look smooth and machine-like but unfortunately finished here is not good. I've considered machined rings and collars, even semi-fuzzy rings and collars; but none improve the performance, so, I try and make it look as good as I can but it's still a little fuzzy.
The fuzzy end