In tune with the recent discussion about mirrors, I'd like to pose a question about off-axis masks. During the course of reading about telescope designs and modifications, I have read about modifying Newtonians with offset circular masks of roughly 40% of the primary mirror's diameter to reduce diffraction and increase the effective focal length, and about curved spiders to reduce diffraction effects, but nowhere else have I encountered the type of mask that I will discuss below. I hope it may prove beneficial to others, and provoke some discussion. Is this a now discredited concept, or does it have merit like my own experience would suggest? I have copied at the end of this email the text from "Helpful Telescope Hints for Observers" by Frank L. Goodwin. This text came to me along with other paperwork accompanying the 1956 8" Cave f/5.84 that I inherited, but it evidently had been printed in S&T in 1954 or 1955 (see below). (BTW, old issues of S&T and Astronomy are in the stacks at the U of U.) Other correspondence between Mr. Goodwin and the telescope's original owner indicate that my adjustable Barlow was also made by Mr. Goodwin. He appears to be a credible source. Briefly, the Goodwin Mask is a black paper cutout that is taped onto the primary mirror. This cutout matches the shape of the combined secondary and spider, but is 1/4" wider all around the secondary and 1/4" wider on each side of the spider's struts. The concept is that you can improve the image by "throwing away" the light affected by the diffraction of the secondary and spider. Further description of this is in the text at the end of this email. My first attempt at this a couple of years ago appeared to be ineffective, so I removed that mask. But in a recent attempt to dismiss any temptation to buy a new scope, I decided to "hotrod" my existing Cave to make it more easily portable, precise and user-friendly while keeping it as close to original as practical. It was quite neglected and abused when I got it. My first experiment was with a single quadrant offset mask at the "top end" of the scope. This didn't allow any light to pass the secondary and spider, increased the effective focal length, reduced the light gathering by about 75%, but gave much sharper views of Saturn, Jupiter, and the limbs of the pine trees on Mt. Olympus above my house in East Mill Creek. Let's call the unmasked mirror "A". And let's label this mirror with offset masked as "B". View "B" is better than view "A". This success led me to make a new Goodwin Mask, but since installing it required removing the primary mirror and then replacing it in somewhat oversized holes in the aging fiberglass tube, it required recollimation. Let's label the mirror with the Goodwin Mask as "C". Due to the time required for this modification, it was not possible to directly compare view "A" with view "C". However, since mask "B" could be removed in less than a second, I could easily compare it with both "A" and "C". It is hard to tell much difference between view "B" and view "C", even though the focal lengths are different. This could lead me to conclude that "C" is also better than "A". Except for the fact that I also just began using a Cheshire collimating tube, and may have collimated the optics better than ever before, and this might attribute to the improved view. Nevertheless, I'm leaving the Goodwin Mask on the primary because Saturn and Jupiter have never looked sharper, it is historically accurate to the period when the scope was made, light loss is minimal, and it evidently doesn't degrade the image. (BTW, I was in the southern Nevada desert a year or two ago and was able to read a sign on Interstate 15 from 9 miles away, even though I was at about a 45 degree angle from it.) I welcome any comments about the Goodwin Mask concept, why I've never encountered it anywhere else, and from anyone else who may give this a try. Bob Grant The text from Mr. Goodwin follows: How a Patterned Strip of Paper Laid on the Mirror can Transform any Reflector into an Off-axis Telescope. An off-axis reflector is one in which the light coming from a sky object is not obstructed by the Newtonian diagonal or its struts, - thereby avoiding diffrac- tions of the latter. A Newtonian diagonal and its struts produce two types of diffraction of light reaching the mirror's surface. One diffraction is caused by obstruction of the light, - the other is the 'temperature diffraction' when the diagonal and struts are slightly colder or warmer than surrounding air inside the tube, - this temperature differential warps the incoming light passing by in close proximity to the edges of these surfaces. Because of this temperature factor it is necessary to make a metal tube of the reflector 3/4 inch between the edge of the mirror and the inner wall of the tube, or 1 1/2 inch total inside diameter of the tube wider than the diameter of the mirror. This wider diameter of the tube also helps to lessen tube wall currents from distorting finest details of the mirror. The effect of these two diffractions is seen on stellar photographs by large observatory mirrors, - both right angled crosses of light around bright stars, and circles of light from tube diffractions enclosing the crosses. Now have you noticed the phenomenon in a telescope, any size, when a tree branch or twig is in the path of light coming from the star into your telescope? The obstruction of the tree twig causes a star obstructed by it, to appear as a string of multiple stars on each side. The farther away the tree twig obstruction is, the greater is this multiple star diffraction effect. While usually in amateur reflectors the Newtonian diagonal and its struts are too close to the mirror to exhibit this ' interference' phenomenon, - it exists anyway with the result of detracting from maximum sharpness of detail. To prevent these diffractions of Newtonian diagonal and struts from reaching the mirror's surface to be reflected back to the diagonal and eyepiece, confusing the image by mixing in with the pure unobstructed light from the sky object, we simply do this: We place a paper pattern right on the mirror to mask off these diffractions. The paper pattern (black paper) represents the 'shadow' cast by the Newtonian diagonal and its struts on the mirror, only you cut the pattern 1/4 inch wider all around than the diagonal itself and ¼" on each side of the strut (4) 'shadows'. Care must be taken to have the paper pattern exactly where the diffraction falls on the mirror, not carelessly to one side of the occluding strut and diagonal structure. True, by this pattern masking off the diffractions right on the mirror itself, you lose a little light gathering area, but it is confused (diffraction) area you can well afford to get rid of. To properly collimate the paper pattern to the strut and diagonal 'shadow' on the mirror you should have a motor drive to follow the stars. Then point the reflector directly at the Sun, - ascertained as squarely direct by a white cardboard at the base of your telescope tube, which shows when the Sun is optically squared sufficiently to make the pattern. Or if your finder is accurate, it will do. Place a white piece of paper on the mirror, anchored to the sides of the cell by scotch tape. Note the Sun's shadow of the diagonal and struts on the white paper over the mirror, and make the pattern wide enough to occlude the slight 'blurring' on each side of the struts, and outside of the diagonal's shadow. Your telescope tube necessarily should have a hole near its base to draw an outline of the pattern, before cutting it out with black paper for permanent use. Otherwise look down the barrel of your tube, and with the pre- pattern white paper marked with suitable lines, you can note between which lines the limit of the "blur" occurs, caused by the Sun's disc shining down from all sides of its limb, - a sort of 'penumbra' effect, - which will amply suit for lesser extent of star and planetary diffraction spreading, and equals the Moon's effect, due to lesser brightness of the Moon, and near comparable visual solar disc. Anchoring the resulting black paper pattern on the mirror with scotch tape on the mirror or cell sides, you are all set. The reason for the paper right on the mirror's surface is to minimize the distance from the pattern's edge to the mirror's surface, since the paper's edge would interpose diffraction were it as far away from the mirror surface as the diagonal and struts are. The theoretical diffraction from a mirror's circular edge to the eyepiece has been grossly over- rated, - for which reason regular 'one-sided' off-axis mirrors are circular with an idea of minimizing the diffraction from mirror edge. But here with the paper pattern you need not have circular areas on four sides, since by having them as 1/4 'slices of pie' in shape, the gain in light gathering area far offsets the impractically insensible paper edge diffraction going back to the diagonal and eyepiece. There being no sensible distance between the paper pattern and the mirror's surface to multiply any diffraction effect. By this paper pattern method, known to one large observatory as the "Goodwin Mask" planetary, lunar, etc details come out so remarkably on nights of minimum atmospheric turbulence, as to make one fairly 'gasp' in astonishment. Here is an off-axis mirror, all around, instead of on one side, - in which the light coming from a celestial object does not strike an obstruction of diagonal and strut to cause a 'diffraction shadow' on the mirror. All four sides of the pattern receive the light unobstructed by a Newtonian diagonal and its struts. Space does not permit legions of other instrumental and observational techniques found in my 53 years of observing since a kid, - but trust the foregoing may prove of some interest. Frank L. Goodwin, ScB., (Vice President Chicago Burnham Astronomical Society). The 4 HELPFUL HINTS were printed in Sky & Telescope Magazine in Dec. 1954 and Jan, Feb., March 1955 issues)