Sunday, June 8, 2014

140608 TimC next workshop invite

On 6/8/2014 7:40 AM, Tim C wrote:
We are scheduling a workshop this coming Tuesday, June 10th. Hope you can join us. ... We are continuing projects.

I will be testing a new surface I put on my work board. I drilled new dowel holes on my board that will handle 6,8and 10" mirrors and tools. I then coated it with a water based polyurethane. These holes are located so you can wedge a mirror or tool on top the board so it will not move. I have been using Jerry's trick till now of putting plain newspaper on the work table. Spray the newspaper and it becomes the perfect surface to work on. At the end of the session you simply roll up the newspaper and the mess is clean. Now I am polishing there is a lot more lateral force on the work surface. It may need a little more staying power, hence the dowels. But have I blown it? Will the slicker surface create an "ice rink" for the disks? Let's find out Tuesday. 

Also, we may have a discussion on building testers. I am toying with updating a tester I've been using for years. I'd like to build one like Tom is using. The light source is what I'd like to improve on. With mine I cannot pot it down in intensity. Tom has told me I need to use a different set of resistors to bring the ability of the potentiometer up to our standard. You want the light source to go from bright to all the way down to just lit up. As well as this I'd like to create an ability to go from fully open light source to a pin point or slit. I think we can do this easily. So, if you can join in please do so.

Wednesday, June 4, 2014

140603 From the Workshop #6...Tim C

From the Workshop #6...Tim C from SBAU June, 2014 newsletter:

Let's review briefly. We have a tool and a blank that will ultimately become a mirror for our reflecting telescope. We would like our blank to end up an 8", f/6, mirror. This translates to a focal length of 48” and a radius of curvature of 96". At this point we are using 60 or 80 grit silicon carbide to grind our blank to the desired focal length. Our mental state at this stage is optimistic: (1) this blank will become a fine mirror that we will place in a great telescope, and (2) any errors at this point can be redone. How do we know when we have reached focal length? Recall in workshop #4 we used a basic equation to find the depth of our curve of our mirror:  s = r2 / 2R.  Using this equation our 8" mirror will have a Sagitta at its center of .083".  Conveniently, this depth at the mirror’s center may be approximated by the thickness of a drill bit resting under a straight edge. As an example, a 5/64” drill bit has a thickness of .078". Although this drill bit is a “bit” undersized, it can still be used as a crude guide to tell us when to stop grinding since we are near the desired focal length of our mirror. When this depth is reached, a straight edge will easily slide over the bit without rocking. This is a good mechanical test. But we need a test that is a little more precise. We need some kind of optical test. Consider the following.

Wet the surface of the mirror, and go outside when the sun is reasonably high in the sky. Reflect the sunlight onto a surface and watch the image that your “mirror” makes. As you move your mirror toward and away from the surface you will see a spot that becomes larger or smaller. Stop when you see the smallest spot. Measure the distance from your mirror to the surface. Since the sun is essentially an infinite distance away, you have just determined your mirror’s focal length to a precision of an inch or two from the desired target of 48”.  Until we bench test your polished mirror, this is one of the best ways to estimate your mirror’s focal length!

Once you achieve your desired focal length, reverse the roles of the tool and the mirror in order to maintain your mirror’s focal length. With the mirror on top (MOT) you affect the center depth of the mirror. Working with this positioning will tend
to preferentially deepen the center of the mirror, and thus shorten the focal length of the mirror. When the tool is on top (TOT), the mirror’s focal length is increased. Alternating the two positions allows us to (1) maintain the desired focal length and (2) smooth out any aberrations on the mirror’s surface that do not conform to a sphere.  We will use this technique all the way through our list of finer and finer abrasives. Our mirror will become smoother and smoother as it becomes more and more spherical. Ultimately we will move to Aluminum Oxide, a lapping powder, in order to
smooth our mirror even further. But, even after this fine course of material, our mirror will still not reflect light when it is dry. In future issues I will discuss polishing and figuring your mirror and the tests we use in the workshop to determine the quality of your mirror.

Sunday, June 1, 2014

140601 TimC

On 6/1/2014 10:57 PM, Tim C wrote:
Greetings all,
We are scheduling a workshop for this week, Tuesday, June 3rd. Bill, would you please have the east gate open. If any of you need to drop off bigger items to the workshop, you may bring them down and then park back In the Museum parking lot till the workshop ends. We will try to help bring your projects back to your cars at the end of the class.
As usual, we meet at the Broder Building across the creek at the Museum of Natural History. We normally meet from 7:30-9 pm each Tuesday.
Way back last year Javier received an 8 inch mirror as a donation. This person only requested we show him our finished scope. I am delighted to say I am now in the final polishing stages of this mirror. I changed the focal length of this mirror from a much longer length to a much shorter one. In fact, it is shorter than we originally thought. It is now an f/3.92. We targeted an f/5.5 but found it to be shorter. As such you would think we are disappointed but, no we are pleased. This will be a fine scope. We have received some parts members have donated to us for our project. After a few set backs, I am finally polishing and looking now to the business of designing and building a Dobsonian type reflector for our mirror. As John Dobson passed away just a few short weeks ago, this will serve as a fine testament to his memory. I hope you will join us by participating and putting this together. 
In the meantime, there are many projects coming to fruition. What is fascinating is that all of our projects are in a finishing stage of polishing and figuring. It is an exciting time for you all to see the many nuances polishing and figuring has to offer the mirror making process. Coinciding with this, some of our articles in the newsletters will indeed deal with the testing of these optics. This is where the rubber meets the road- where the theories meet reality. We will take these pieces of borosilicate glass, polish and coat them with aluminum, to reflect and magnify starlight received from eons past and deem to understand them. Not interested in that? Well, maybe you may find answers to questions you have in astrophotography. Let's say you have an urge to understand how to image stars in the night sky, or current events taking place in the night sky. For instance, in early July asteroids Vesta and Ceres will appear to come close in space. Want to image them? We have experts to tell you how.
 Please feel free to come join us.