Fred’s ATM Dobsonian Webpage

"How to overcome those first time worries…"

The purpose of this page is to encourage others who have once considered grinding their own optics to take the plunge by sharing my own rewarding 1st time ATM experience.  After reading several inspiring "grind your own" articles in our club newsletter I took the plunge and bought myself a 10" mirror kit. The purpose of this page is not to give you a 1,2,3 recipe for grinding your own mirror, but to share my own enjoyable ATM experiences and hopefully to convince many of you to "Just Do It!" There is no better way to gain an appreciation for optics than to make your own.

General consensus states that you should start with a 6" or smaller mirror as your first.  Being my usual unconventional self I opted to ignore consensus and go with a 10" blank.   After all was said and done I believe that it was easier for me to grind a 10" because the blank was easier for my large hands to handle.  I believe that a 6" or smaller blank would have been awkward to handle.

I did a great deal of up front reading. As such I also advise you to know what you're doing before you do it. As the familiar flying saying goes: "Plan your flight and fly your plan". Have a goal in mind for each step of the mirror production process and understand the process.

Like they say: "One picture is worth a thousand words". As such I purchased John Dobson's 90-minute videotape (available at the S&T website http://www.skypub.com/). This videotape takes the mystery out of mirror grinding. Dobson brings mirror grinding down to Earth, literally.  In one part of the tape when he finishes rough grinding he tosses a 16" mirror blank (mirror face up of course) on the ground.  Dobson, after many years is obviously not intimidated by a piece of glass.   I also believe that intimidation is probably the biggest obstacle to overcome in making your own mirror. Don’t be intimidated. It’s only a piece of glass. Short of dropping and breaking your mirror blank there’s little that you can do that will destroy your mirror. In the long run a mistake, such as a scratch, will only cost you a little extra grit and time.

As far as ATM books are concerned I found the Texereau book "How to Make a Telescope"  to be confusing and intimidating.  The Texereau book in my opinion is a good 2nd reference for the experienced ATM'er.  Only after having "been there and done that" am I starting to understand various parts of Texereau's book. The book basically seems to lose something in its translation from French to English. On the other hand Richard Berry's "Build Your Own Telescope" was much more practical.  For a first timer I believe that the Berry book and Dobson tape will answer most 1st timer questions about mirror grinding and mount construction.   There is also a very good and helpful email list which most times will get you specific answers from other experienced ATM'ers in minutes or less.  To subscribe to the list or search the ATM archives visit:  http://www.system.missouri.edu/atm/

As far as mirror size and rumors that I’ve heard are concerned I was also told that making a 10" mirror as a first mirror will take you twice as long as a 6" and because of this you should make a practice 6" mirror, throw it away and then start your 10". From what I experienced this wasn’t the case. I was shooting for a f6 mirror, which gives you a manageable focal length of 60". I ended up with a f6.7.  No big deal! The tube simply ended up a few inches longer. Speaking of length…. I found that it’s difficult to find the focal length of the rough ground mirror within +/- several inches.

During my course of information gathering I heard several horror stories about 6" mirrors that took 5 years or longer to finish. If you're dedicated and determined I don’t see why you should spend more than a few weeks of evenings completing the entire mirror and mount. I only spent 27 logged hours hogging, fine grinding, polishing and figuring the mirror.  These 27 hours took me from two pieces of flat glass (mirror blank and plate glass tool) to the "ready to coat" finished mirror.  The wood working part of the mount took about 2 days from start to finish. Now maybe I was just lucky, but I’d rather think that luck didn’t really play that big a part in the production of the mirror and mount. I feel that "preparation" played a much larger part. If you have a goal and understand how to reach your goal it’s basically only a matter of time, a little sweat and much enjoyment.

Being impatient and unable to come up with reliable quantitative Foucoult test data I opted to rely on the qualitative Foucoult image, qualitative Ronchi images and most importantly star testing the polished and figured blank using sun glints reflected from a street light (PLEASE NOTE:  I USED REFLECTED SUN GLINTS.  I DID NOT POINT THE UNCOATED POLISHED MIRROR BLANK AT THE SUN.  NEVER POINT ANY MIRROR,  OR MIRROR BLANK DIRECTLY AT THE SUN! ) After viewing the Dobson tape one thing that really struck me was that John Dobson never mentions the words "Foucoult or Ronchi". Could this be possible? No engineering lingo in a technically difficult area such as optics manufacturing? Well the conclusion that I came to was "If a simple star test is good enough for Dobson, it’s good enough for me." Dobson seems to be an avid believer in the "Keep it Simple" principle.

After I collimated the Dob, allowed the glass to cool down to ambient temperatures and performed the star test I saw classic text book diffraction rings. Needless to say a real thrill! This stuff about grits between two pieces of glass really works! Obviously the quality of optics can be very subjective, but from what I've seen so far this seemingly crudely self made mirror using techniques hundreds of years old shows more detail than the commercial optics I currently own.

Although I was a bit skeptical at times, these old time tested grinding techniques really work.  Short of dropping the mirror blank it's a forgiving process. I had a great time making the mirror and encourage all of you to give it a shot.  Most of the work is "cave man" type work.  It really helps if you grunt a little during "hogging".  Grunting really helps during polishing!

It's hard to find a more rewarding experience.  If you've ever thought about "grinding your own" go for it.  You'll love it, learn from it, and with a little luck produce a mirror as good or better than most commercial mirrors.

Clear skies,

Fred

 

 

Polishing and figuring is done with a lap.  Since this was my first mirror and I didn’t have any other tool lying around I used my plate glass grinding tool as the lap base.  I was lucky in terms of not having to go back and fine grind a scratch out. But then again, you also need to be practical. As a first mirror ATM'er you're undergoing a learning experience. Learning experiences involve mistakes and we all know you learn from your mistakes. If you do have one or two small scratches, don’t worry about it. For all practical purposes that large spider in the following image is much, much larger than any scratch you’ll ever make with even the coarsest grit. As of yet I have never heard anyone complain about those large spider shadows cast upon the primary mirror! I also think that most ATM’ers and professional opticians will attest to the fact that there has never been a perfect mirror and most likely there will never be a perfect mirror. Just look at the history behind the Hubble primary mirror!

 

 

This business end image shows the primary mirror, spider, focuser, telrad, finder and secondary mirror.

 

A side view of the finished Dob. Time to party!