[ATM] 25" mirror possibilities (Chapter 2)

[dmikk2@netscape.net]

Hi everyone,

 Late last year I asked the list for some advice on how to best put to use a 25" full thickness mirror I purchased on Ebay.  The mirror is from a huge "Wright Schmidt" collimator made by Tinsley Laboratories for Boeing. I greatly appreciated the comments and offers of help.   At that time, I was not even able to safely set up the 180 lb mirror to test it.  I've been reading, thinking, lurking on the list, and progressing VERY SLOWLY, within the constraints of a "day job", family, etc.  Since everyone was so helpful, I thought I should at least give a progress report.

 I've finally gotten a combination storage/shipping crate and test stand made and just did a very basic Foucault test on the mirror.  The results are something like:

Focal Length: 160 inches, f6.4

Figure: oblate ellipsoid, SC about 1.8

 I say "something like", since I really didn't have a suitable location to test the mirror.  I did a basic test with the mirror in one corner of my garage, facing diagonally across the garage, to a point about 5 feet outside of the garage door.   The thermal characteristics of the mirror, garage floor, garage air and outdoor air over a 26 ft. 8 inch. path tend to make the image look more like flames in a fire place than a Foucault test.  After trying the test at various times of the day and night, I finally hit a time one morning when the inside and outside air temperature rose to the point where they approximately equaled the mirror temperature, and I got a few usable views.   Comparing what I saw at different knife edge positions with the images from Michael Lindner's Foucault Simulator program lead to my "guesstimate" of SC = 1.8.  I'm at least reasonably confident the value is between 1.6 and 2, so it really is an oblate ellipsoid.

 Based on the responses to my original post, it appears that the most practical way to put this to use is to have the mirror refigured to a paraboloid.  However, with a focal length of 160 inches, a conventional Newtonian would still have the diagonal about 12 feet above the face of the mirror.   In a Dobsonian, the front of the mirror would probably have to be  at least a foot above the ground.  At the zenith that puts the eyepiece at least 13 ft above the ground.  Even a very tall person would have to stand on the top of a 7 ft ladder to view at the zenith.   This does not seem to be safe or practical.

 So...what about some sort of folded Newtonian?  Some very rough numbers, that allow reasonable illumination of a 35mm size film or CCD are:

Case 1:
  6" flat secondary mirror, tilted 15 degrees from the optical axis, 32 inches from the focal plane.  This reflects the light path back toward the side of the "tube" at a 30 degree angle from the optical axis.  The focal point is then outside of the tube, 16 inches from the optical axis of the primary, and about 28 inches closer to the primary, than the secondary is.   This reduces the height of the focal plane above the primary from 144 inches to 100 inches.  Also, the ladder height decreases from about 7 ft to about 3 ft, with a central obstruction of 24%.

Case 2:
  8" flat secondary mirror, tilted 10.5 degrees from the optical axis, 45 inches from the focal plane.  This places the focal plane 42 inches closer to the primary than the secondary is, and reduces the height of the focal plane above the primary to 73 inches.   The ladder height needed by a tall person at the zenith would now just be about 1 foot, however, the central obstruction is 32%.

Are there any major problems with such folded Newtonians, such as problems with baffling?  Also, both of these possibilities have fairly large central obstructions.  How "bad" would a central obstruction of 32% be?

Case 3:
 The alternative to a folded Newtonian seems to be to actually regrind the mirror to a shorter focal length, say f4 at a much higher cost and more coma.  For an f4, a conventional 5" diagonal 16" from the focal plane would give a similar size illuminated field with a 20% central obstruction, and a required ladder height of about 2 feet.

 Any comments or suggestions would be appreciated.

Thanks,
Dennis


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