University Lowbrow Astronomers

Some People Never Learn.

by Doug Scobel
Printed in Reflections: February, 2003.

Ever notice how some people never seem to learn from experience?

Set your Wayback machine to 1985, Mr. Peabody, to the time when I had been using my new, homebuilt 13.1 inch f/4.5 Dobsonian for about a year.  Although it was good for low power deep sky observing, I needed (”needed”?) a telescope specifically for high magnification planetary and double star observing.  So, I decided to build a new planetary scope, to be completed in time for the favorable opposition of Mars that was coming up in 1986.  It was to be an 8 inch f/8, with a tiny secondary to minimize diffraction, and optimized for high magnification viewing.  I made the mirror myself, and achieved a pretty good figure on it, but Mars had come and gone by the time I finished it.  Still, once the scope was finished I was quite happy with it, as it gave some darn good views of the planets and close doubles.  I remember one December night when the air was as steady as I have ever seen it, and Jupiter was simply astounding!  At over 300x I could see all the festoons and details that, prior to that, I had only seen in photographs in magazines.  It was a pretty good scope with a pretty good mirror, I thought to myself proudly.

Unfortunately, two events conspired to make that a temporary situation.

The first was that after learning how to test and figure a telescope mirror the “right” way (I had made two six inch mirrors previously), I decided to test the figure on my 13 inch Coulter mirror, because it never gave me satisfactory images.  Much to my chagrin, I discovered that it had about a one wave error at the wavefront!  Way, way more than the quarter wave standard that is considered to be the worst you should settle for.  But I had used it for a couple of years already, meaning it was too late to return it to Coulter.  So I decided to refigure it myself, thinking that I could not make it worse than it already was.  After a few months work I was reasonably successful in fixing it, and I have been very happy with the images it produces ever since.  The added benefit was that refiguring the 13 inch, which was at least triple the difficulty of figuring the 8 inch, taught me a lot about mirror figuring and more importantly mirror testing.  With newfound confidence I made the fatal mistake of going back and retesting my 8 inch, and found that it was a pretty mediocre mirror - maybe a quarter wave at best.

The second conspirator was that the coating on the 8 inch deteriorated tremendously in about a year.  It became very thin and semi-transparent.  I think that it was the fiberglass tube in which it resided, which always stunk like fiberglass resin.  I think that whatever was being outgassed from the tube attacked the mirror coating.  But regardless of why the coating went bad, the mirror would have to be recoated.

But why recoat a mediocre, quarter wave mirror?  Now that I’m such a great mirror maker (ha ha), why not refigure it to say, 1/10 wave, and then have it recoated?  Yeah!  Great idea!  NO!  BAD IDEA!!!  I’ll spare you all the details, but that mirror has never seen a figure even resembling a paraboloid ever since.  It was as if I developed mirror making amnesia.  I’d work on it for several weeks, get nowhere, or make the figure worse, get frustrated, and put it away for a year.  After a few cycles of this, I put it away “for good”, and now have a polished chunk of Pyrex that is totally useless for any astronomical purpose.  More than once I thought of having it coated and using it as a magnifying shaving mirror.

Fast forward to today.  I’ve got the bug to give it one more try.  So why would I want to resurrect this project again, especially with all the difficulty I’ve had in the past?  Two reasons - a most favorable Mars opposition is fast approaching late August this year, and I’ve never totally given up on building a really good planetary scope.  In the past few years Sky and Telescope magazine has run numerous articles on improving the basic Newtonian reflector, and I’d like to incorporate many of these ideas into one new scope.  Of primary (pun intended) importance will be to create a very good primary mirror.  If I had my druthers, I’d make a flex cell for it.  This is where you use a relatively thin, spherical primary, and the cell is designed to pull the center of the mirror back, flexing it into a paraboloid.  Mirror making amnesia notwithstanding, I know that I can create a sphere, and probably in short order.  But, given the time (gotta get those honeydo’s done first so that my wife doesn’t make me sleep on the couch) and money that I have to work on it, I’m not sure that I can make a new, thin mirror, and create a working (emphasis on “working”) flex cell, and the rest of the scope, all before August.  I really don’t want to miss another favorable Mars opposition, so, I’ll probably take one more stab at getting a decent paraboloid on what I have*.  Once I have a good primary, then I can “throw” the rest of the scope together in short order, hopefully before Mars is at its best.  All the other fancy features, including a flex cell, I can add later.

So, can I pull it off?  I don’t know, but I’ll keep you posted.

[*Footnote:  I have since discovered what my difficulty was while attempting to refigure the 8 inch.  I now know that my basement has extremely low humidity in the winter, which it always was when I was doing my refiguring work.  Low humidity can cause all kinds of figuring difficulties due to evaporative cooling effects on the exposed portions of the mirror and lap, especially during long, overhanging parabolizing strokes.  All I should have to do is deal with the low humidity and things will go much better, but please don’t quote me on that!  Stay tuned.]

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This page originally appeared in Reflections of the University Lowbrow Astronomers (the club newsletter).
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