Bob, This is an interesting idea, but I don't see that it would make the view any netter. The mask still acts like the spider, and in fact is worse because it is wider. It MAY get rid of the temperature effects mentioned in the article. I would be scheptical about it working, and would have to conduct several careful tests to see if it made an improvement. Brent --- Marilyn Smith <bob-marilyn@worldnet.att.net> wrote:
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,
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