140422 Earth Day NASA #GlobalSelfie SBAU Telescope Workshop

Santa Barbara AU ‏@SantaBarbaraAU   #GlobalSelfie 2014 Earth Day NASA by SBAU Telescope Workshop mirror SBMNH Science Center Jupiter Magic Planet globe pic.twitter.com/GlA46lrKrk

On 4/23/2014 8:57 AM, Tim C wrote:    Greetings all,

Thanks for coming on by last night. We had to move to classroom 1 because of a mixer that had been scheduled for Broder. This is an example of what I have alluded to in the past. From time to time we have to give way to events that are scheduled ahead of time in Broder. Much thanks to Bill C who tried to give me advanced notice. Sorry Bill- I did not give you my new number and that is now corrected. Bill made it so easy for us to shift over to the classroom. I want to let you all know, the classroom is the fallback room we use in case Broder is busy. I try to leave a note on the door of Broder ( the door nearest the footbridge) telling where we will be or if (as in some rare cases) the workshop will be cancelled.

    Once we carefully moved a couple of items from 2 tables, we set up a Ronchi test for Christopher's 10" mirror. (Used to be Chuck's "Mr. Peanut".) Wow! Chuck had really come a long way since I last saw this mirror! Part of my reason for the follow up email is to inform 2 head pros in our workshop, Tom W. and Jerry W. of the results we looked at. Christopher is using a sub diameter lap to polish this mirror out and some of the correction strokes, I am not qualified to answer to. So, Tom and Jerry ( I always like saying that), we saw an almost perfect sphere that has a turned down edge way out at the 97-100% zone.

For you in the workshop unfamiliar with what I am talking about, the dreaded "turned down edge" is a typical result of grinding and polishing your own mirror. Imagine your polishing your mirror and testing it with a glass slide that has etched lines that are parallel. If your mirror is perfectly spherical, you will see parallel vertical lines extending from edge to edge. But, if those lines turn at the edge, you have a turned down edge (TDE). Tom or Jerry have always told me that you must not think of the edge as low but instead think of the adjacent zone as high. Okay, now confused? Just think of this. What Chris will want to do to correct this is work on the adjacent zone to the inside of this turned edge. Work it all the way around the mirror, just as you would polish by using strokes that are predictably random. Now if you think about it, this will lower the adjacent zone to the level of the edge and now you have corrected the edge. I can hear you all say, "but the middle has not been worked on and now you have a high middle". Correct! But it is much easier to correct the middle than a TDE. What type of stroke to use and for what duration, I will leave for our experts. Tom Totton took some images of this last night and if he would be so kind he may send these to Tom and Jerry to look at.

    Last night, speaking of TomT, we also took a photo of members of our group in the Planetarium. It was really fun watching Tom set up a make shift platform for his timed shot and scramble over to get in the picture. He had to fiddle a bit to get the big planet Jupiter to show in the photo. This added some seconds to the shot. Tom kept leaving the shot early and we really enjoyed his ghost like figure leave the photo. One thing I thought of only now Tom is that this was in honor of Earth Day and you used Jupiter in the photo. Now we'll have to wait another year to set it up again!

    Anyway, a good time was had by all. Paul W asked me a while back to get permission to use images and diagrams from the Stellafane website. I told him last night they just responded and told me it was okay as long as it is not in a commercial form. I want to finish though by thanking Paul for all his hard effort on the club website. He told me last night he just completely re-did the whole site (40hours +!!!!!) thank you so much Paul from all of us. You members out there, spread the news. And use the forum! In fact, Tom W. told me members are using the newsletter to sell equipment. Please know you can use the Forum and reach a whole lot more people!

    See you all soon. Remember, May 10th is Astronomy Day. We will be out at Calle Real  Market Place. I have a permanent shoulder injury and it would be great to see a few of you spell me with mirror grinding.
    TimC