Friday, January 31, 2014

TimC From The Workshop #2...(Draft)

On 1/29/2014 8:41 AM, Tim Crawford wrote:
From The Workshop #2...(Draft)
    Crafting a telescope mirror can be a rewarding endeavor. Once, in my earlier years I went downtown and bought a telescope mirror kit from Douglas Telescopics. I bought a 55gallon drum to use as a work top . I set it all up and opened Allyn Thompson's Making Your Own Telescope.... I was stunned ! It made no sense. I put everything away-for years! The reason for this short aside is to let you know it is not uncommon to be lost in the beginning when you try to do it a cappella. I strongly advise you to find someone to guide you, like you get in a workshop like ours. There are many in the group with hundreds of hours crafting mirrors. They have vast experience and can guide you through all aspects of crafting your own mirror.
    So you are handed a round blank and another tool of let's say the same diameter. As in the previous letter, you will use a grinding medium between the two to grind a telescope mirror. Before you begin though, there is one important step to do. You must bevel the outer edge of your blank. You may use a carborundum sharpening stone to do this. Soak it in water and using a downward 45 degree angle, grind slowly the edge of your mirror. For purposes of these essays, I will use Pyrex as the material we use as a mirror. This bevel will prevent chipping of the edge of your Pyrex blank while grinding. The bevel should be at least 1/8th inch wide. Choose a workplace that has somewhat constant temperatures and a work surface that has strong support.
    How does it all happen then? How do two flat surfaces become a perfect sphere and a perfect convex compliment of this sphere? The answer is an elegant one and a little obvious on hindsight. Let's not think so much about the abrasives here but instead, think of the two discs passing over one another. The disc on top just by its own weight has downward pressure on the lower disc. The upper disc is worn in the center and the lower disc is worn on the edges. But, let's think of it another way to further illustrate the process. If the upper disc is passed over the lower, using chordal strokes let's say, first a little to the right and then a little to the left, then the middle of the disc on top is getting worked full time, while the edges get a break when the disc is either right or left of center. The center has to wear more than the edges. Now, add one more element- controlled randomness. Sounds kind of contrarian but it is the most important. By walking around a worktop in one direction while turning the mirror the opposite direction while grinding, the discs become worked evenly, in fact perfectly. But, how deep do we make them? How do we stop from going deeper and deeper. Next edition we will cover this as well as terms you will likely use if you decide to grind your own optics.


140128 Telescope Workshop Notes
TimC, DmitriiZ, TomT

Tim continued working 500grit on the donated 8" mirror, while being interrupted with questions on dental technician techniques.

Dmitrii, after weighting his pitched tool to take the shape of the mirror, put in about 1 hour of polishing on his 12.5" mirror and kept the conversations rolling.

Larger pictures/video here:

Thursday, January 30, 2014

Tim's notes From the Workshop #1+

On 1/19/2014 9:35 PM, Tim Crawford wrote:
> Greetings once again,
> There will be a workshop this week, January 21, at the Broder,from 7:30pm till 9. This week I will continue to grind the donated mirror. It is coming along famously. I will start to use 500 grit this week. It will not be to long now. It is now time to start the design of the new scope. Thanks to Dr. Tom and Dr. Jerry I am able to easily see these projects come to fruition.  If you are able, come I'm by and  see us. Bring your ideas, projects or questions.
> T

On 1/13/2014 8:06 AM, Tim Crawford wrote:
> Greetings all,
> I am scheduling a workshop this Tuesday, January 14th, at the Broder Building  starting at 7:30pm. Please feel free to drop in if you can make it. I really don't have a specific topic for the evening but I would like to continue the fine grinding on the donated 8". I do have a question for you all. What would it take to create a permanent dedicated set up to do, say, asteroid occultation studies? Make this guesstimating with the precondition that a location is already available. It can also be remote setup with remote access. Do not consider cost, but instead what hardware/software would it take?
> T

January, 2014 SBAU newsletter:
From the Workshop #1...
Tim Crawford
    Greetings from the workshop! Our mirror class, originally created by our own Tom Whittemore, is now in its eleventh year! It seems like just yesterday we were at the Broder Building gathering for our first encounter with grinding a working mirror to be integrated into a hand-crafted Newtonian-type telescope. For many of us these are Dobsonian scopes utilizing John Dobson's famous, simple and elegantly-designed scopes.
    Our goal is to keep you abreast of the most current techniques which Amateur Telescope Makers(ATMs) can offer. Now, most of you are very proficient with telescopes. Still, I hope to give you a first-hand example of what may happen when you are encouraged to make your own telescope mirror and, when this is done, to design and build a telescope.
    First you are handed an 8” disc of Pyrex. Next you are given a ceramic grinding tool made essentially from bathroom tile. Since your grinding tool has the same hardness as does your mirror, the tool will wear down the glass when silicon carbide is used as an abrasive between the two. This is how you start to make a truly fine mirror! Eleven years ago, along with some patient instruction from Tom, this is all we were given. But, in the meantime, many of us went on to complete our mirror-and-telescope making projects.
    In future editions of the “joys of mirror making and telescope building,” I will get into some of the finer details of making a research-grade telescope mirror. Please stay tuned! Happy New Year!

Thursday, January 16, 2014

140115 John Dobson on Wiki


140114 Telescope Workshop notes 730-9p Tuesday evening
Attendees:  TomT, JerryW, TimC, EdK, JoeD, ChrisU, DavidH

JoeD brought in 6” AstroTech f/4 that he is modifying for portability: .  He moved the focuser position farther out since he will not be using a camera.  He has made a perpendicular plywood mount with three hand holds and swivel bottom on a heavy duty photographic tripod (Manfrotto).  The mount can also stand flat on a table with its slippery feet to allow horizontal movement without a tripod. First light with the new mount was tonight looking at the Moon and Jupiter and its moons.  Some diffraction on objects, possibly from large secondary mirror, that Joe says was used to make the telescope more useful for astrophotography.  Also, Joe will try to reduce some wobble of the tripod mount and the bottom plate attachments. 

TomT mentioned that summer meetings pushed by JerryW will be attempted, hopefully members will do them, which was also suggested by Jerry.  We can have outsiders come in as well, and the backup is to use videos taken off YouTube or astronomy DVD programs.

TimC did a little more 320 Al2O3 grit grinding on the 8” donated mirror.  JerryW checked the surface with the loupe and called it good.  Tim will try to find some 500 grit, Aluminum Oxide.

EdK  brought in mirror, but no testing setup tonight.  Ed will be modifying his polishing stroke to chordal to remove the turned down edge.

DavidH’s old Celestron German Equatorial Mount that fell off table last week and bent a wormgear shaft, already has been fixed by Art Harris.
TimC-will be writing another article for newsletter, talking about steps of understanding telescope functions, but wants help from JerryW and TomW final editing. Tim wants to get around to describing the earliest telescopes and history of telescopes.

JerryW-says think of Power as diameter increase, 100X means Jupiter would look 100X bigger than what the human eye can see alone.  He discussed apparent field of view (field stop edge in the back of the eyepiece) vs. field of view thru the telescope, which depends on focal length of telescope divided by eyepiece focal length.

More photos of tonight's session:

Friday, January 10, 2014


140107 Telescope Workshop notes 730-9p Tuesday evening
Attendees:  TomT, JerryW, TimC, EdK, ChrisU, DavidH

TimC talking again about bringing in samples of Telescope Workshop stuff to monthly meetings, but tonight’s suggestions were to setup for a full meeting, during the long light evenings of the summer, discussing mirror and telescope creation and astrophotography.   Gathering questions that members want to have answered ahead of time would determine meeting coverage. 

TimC did a little more 320 grit grinding on the 8” donated mirror.

EdK  brought in mirror, but no testing setup tonight.  Ed has put in another 16 hours of polishing trying to widen spherical surface.  See his email.

DavidH brought in the old Celestron German Equatorial Mount.  David says too much grease and also that one piece was not screwed in far enough that caused the concern about too much slop in the wormgear.  Now Jerry and David looking at drive system, Jerry brought in his drive controller, cables, drives from a Vixen GPD (great Polaris) mount, with a 12V battery cigarette lighter connection (Kendrick wheelchair type battery).  Unfortunately, gear drive fell off table and bent a wormgear shaft.  Art Harris was suggested as the man to consult with for repair.

 ChrisU brought in his “Flaming Tomato” Dobsonian telescope and mount.   He is putting on a home textured formica strip on the Declination circles for friction or stiction (static+friction?).