150320 why Ronchi lines are straight...

>     On 3/20/2015 12:46 PM, Jerry W wrote:
>>     Hello Tom
>>     At last Tuesdays workshop you asked why Ronchi lines are straight for a perfect parabolic mirror when viewing a star but also straight for a perfect sphere on the test bench.
>>     The explanation is this. The primary mirror in our telescopes is a paraboloid because that shape has the property of bringing incident light from a point source at infinity to a single point in the image plane of the telescope. Recall the image plane for an object at infinity is located at a distance in front of the mirror equal to the focal length or one half the radius of curvature. A sphere, on the other hand, has the property of bringing incident light from a point source at the radius of curvature to a single point in the image plane of the optical setup we use in Broder. In this case the image plane is located at the radius of curvature.
>>     This ability to produce a point image from a point object in these two different setups is due to the geometry of the two different surfaces. You may have studied this in geometry or algebra, wherever conic sections were covered.
>>     In terms of wave optics the two different setups involve different geometry wave fronts.  The light waves from a star at infinity has the geometry of a plane wave while that from the point source in our Ronchi setup constitutes a diverging spherical wave front. A paraboloid brings a plane wave to a point while a sphere does it for a spherical wave. After the light bounces off the mirror, if it's heading toward a single point in the image plane, it will constitute a converging spherical wave. This is the case for a plane wave bouncing off a parabola or a spherical wave bouncing off a sphere.
>>     Now when you put a Ronchi reticle in a converging spherical wave the reticle will be the same distance inside or outside of the image plane and what you see will there for be straight lines.
>>     Hope this helps.  Jerry

> On Fri, Mar 20, 2015 at 4:28 PM, President SBAU wrote:
>     Jerry
>     Thanks for the explanation.  Add this to your book of "101 optical explanations for telescope mirror makers"?
>     However, I am not totally clear on why a star is a planar wavefront, that is, why is it different from the test point source which is a diverging spherical wavefront?  Probably because the part of the sphere we see from the star is hardly different from a flat surface by the time we see it in our little telescope?  A star test (coma?) must be the ultimate check on a telescope mirror compared to Ronchi or Foucault tests and guesses.
      
On 3/20/2015 5:23 PM, Jerry W wrote:
> Hi Tom
> Your supposition of why a star presents a planar wave is exactly correct. Close to the star it is also a sphere, but by the time it gets to us it is a plane for all practical and intensive purposes. The presence of spherical aberration, coma, astigmatism etc are all aberrations on the simple wave front explanation I gave. They are of a secondary nature compared to the primary one of producing a true point. We can get away with ignoring them because when we are squinting through the Ronchi grating with our naked eye we can't see them.  If we were to greatly magnify the image then they would show up.
> Jerry

140708

 

 

140702 TimC: From the Workshop...#7

From the Workshop...#7 (from SBAU July newsletter):
by Tim C

http://hubblesite.org/the_telescope/hubble_essentials/

    I believe we are at a point where we talk about polishing and testing. Before we do, I am going to take a brief aside and tell a good story - the story of the Hubble Space Telescope and what happened to it. I will not go into specifics of how the testing
was done. Instead, I'm going to tell you what happened and how a "shower" solved a giant problem. The Hubble telescope became a political football for a time as the huge cost was viewed as wasted money because the main mirror was flawed, and seemingly beyond correction. 
    The Hubble mirror was finished and figured on a ground floor room. Above this was a small, rather awkwardly situated room. This small upper room housed the testing equipment. This design made it possible to test the mirror in place, so there was no need to pick up the heavy mirror and put it on a testing rack. Instead, they only needed to open access to the test apparatus. The “Null Corrector" was an arrangement of mirrors and lenses held to each other with specific-length rods. The only problem was the "field caps" covering the ends of the rods. Reflections from these field caps introduced a 1.3 mm error into the optics. This error was overlooked due to pressures to finish the mirror. The damage was done. The outer edges of the mirror were over-corrected or too flat. 
    So, when the Hubble Space Telescope saw “first light” there was much consternation as to what happened. The views were terrible. How do you go about fixing something like this? Replace the main objective, or replace the corrector mirror, or use a series of corrective mirrors with the instruments on board? Any of these techniques should work but there was no way to get it done. Think of the spacesuits of the astronauts with their big bulky gloves. In principle there was no way to delicately replace the mirrors.
    In Europe a Strategy Panel met to address the different ways of fixing the problem. Jim Crocker, an engineer by training, thought if they could fix the cameras it would solve the problem. Maybe they could place corrective mirrors in front of the camera? But there were several instruments on board. One mirror would not fit all of the instruments’ needs. He went back to his room one night thinking hard on the subject and went to take a shower. The European shower system was unlike those found in America. A rod holds a showerhead in place. The showerhead can slide up or down to adjust to the height of the person in the shower.  During the day, the maids slid the showerhead all the way to the bottom of the rod. As Crocker slid the shower head up and pivoted it outward to be over his head, he had a vision of several different
“showerheads” swinging into position with their corrective mirrors in the light path between Hubble’s secondary and the science instruments. If they sacrificed one of the first generation instruments in place and plugged in this corrective mirror unit, it would fit the simple kind of maintenance the Hubble team had planned for decades. It would work. In fact, and as you may recall, it worked perfectly. Moral of this story: you just never know when or where a solution to a problem will present itself!
see also...http://history.nasa.gov/SP-4219/Chapter16.html

140701 TimC's dilemmas

140701 SBAU Telescope Workshop
Attendees: Angela, Paul, Tim, Jerry, Chris, TomT, Mike

> On Tue, Jul 1, 2014 at 10:40 PM, Tim C wrote:
>     Okay guys,   

 

 

bulge plus TDE insideR?

It happened again.  I put the mirror up on the stand tonight.  It tested out that it has a slight dome overall but it looks like I am also introducing a very small turned edge.  That's okay. 

major scratch...what caused it?

But afterward I decided to check my new flashlight on the mirror surface.  I found the mother of all scratches on the mirror.  This time I can feel it with my nail.  Wow, at first I was bummed out (for some time).  Now I'm just frustrated.  I have never had a mirror come I with so many problems.  I have gone backwards 3 times so far and I was at first thinking I would be unwilling to go back again.  If all you guys think it's okay, I'd like to continue polishing for a while.  Hey, what can it hurt.  I was trying to get this done so the donor could see the results.  Now, I'm kind of thinking I'd like to see the results.  Could be many reasons for tonight's debacle.  Without boring you all with the rundown, I'd like to see if a particle has lodged in my pitch.  That will make it easy. 

indent on pitch lap

Dump the pitch, go back to 320 and bring it on back one more time.  If this is the course I take,  I will definitely keep this sucker.  I'd deserve it.  It owes me.  (Only kidding).  It is a learning experience and as such I am delighted to take on this challenge.  I hope you all will join me to try to figure out what keeps going on.  It is a mystery right now.  Like I said earlier, this has never happened before.  Are there sub-surface bubbles that chip up onto the surface?  Am I dislodging grit from my workplace from under my fingernails?  Am I just not taking care to keep a "clean room" atmosphere around my workplace?  All good questions.  I am really sorry you guys for screwing this up over and over.  I am trying to be patient.  I hope you will be patient with me.
>     T
On 7/1/2014 10:52 PM, Dmitrii Z wrote:
> Tim, I am sorry to hear that! How frustrating! Hang in there, though. We must not let each other give up.

TT: see more photos/vids, full size, at Flickr link.

JerryW analyzing TimC iris adapter...let's see what happens if we take it apart!

peg holes for tool mirror holding on work board

 

Chris asking Jerry for star hopping help

Paul showing Starry Night Pro Plus

PaulW Seahorse nebula astrophotography

140608 TimC next workshop invite

On 6/8/2014 7:40 AM, Tim C wrote:

Greetings,

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.

T

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.

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.
T

140527

140527 SBAU Telescope Workshop
Attendees:  TimC, TomW, JerryW, PaulW, MikeC, EdK, TomT

TomW made Ronchi pattern printout based on
his mirror's f5.9? final shape using macRonchi program.

More pics/vids at:
https://www.flickr.com/photos/27241501@N03/sets/72157644849872496/

MikeC's pitch lap divot; white coating is embedded cerium
oxide on the pitch lap

MikeC w/ HP waveguide carriage
as a base for a possible mirror test setup

MikeC 10inch Ronchi pattern...center not polished due to
pitch lap center divot?

EdK chordal polishing starting to smooth out outer ridge?

TimC donated 8 inch mirror 1st polish Ronchi result

PaulW showing TomT use of Registax5 program for bringing
out Jupiter moon shadows from Celestron NexImage5 frames

TomW getting TimC to use glycerine and detergent in polishing
compound to avoid stickage (worked for Sam grinding 10in)

140520 TimC comments

On 5/20/2014 11:21 PM, Tim Crawford wrote:

 Well, was that cool or not! We had 12 people including three students from UCSB tonight. I ground a little with my newly trimmed tool and freshly beveled Pyrex disc. The scratches and gouges are disappearing slowly. Paul mentioned one thing tonight very promising. Some of the lighter scratches and lesser gouges will disappear in the polishing stages. At that point we are smearing glass around at a molecular level. I think these were great comments that we can test in time. Thanks Paul!

Tom poured not one but TWO laps tonight. Thanks Tom!!!!!! We set up two different mirrors on the test stands. Both are on track to becoming great mirrors.

 Remember, we are working with wavelengths at this point- although the Ronchi patterns look sometimes different from ideal, they are very close to the end. It is just tweaking from here. There is a tendency to get overwhelmed by what you see through the tester. Do not be discouraged. You are very close! Stay in there. Don't rush- enjoy it. In the end, the absolute joy in looking through optics you have crafted is nothing short of spectacular.

At the near end of polishing, the laps Tom is pouring for us are just fabulous. In the past we poured pitch into dams created by using masking tape on the tools we were grinding with. Now, Tom and others are using molds they have purchased. But I'm the hands of a craftsman, these new laps can look flawless and clean! A true artist preserving an art form that would be easily lost in time if there were not people like you Tom.

My new disc that Christopher gave me, complete with a "king tooth" drawn on it is just great. I am pleased to say the Cerium Christopher purchased looks like it is going to be just fine. I was getting nervous with the deep red color of the Cerium. I thought it might be less than optical grade, but with just a little touch tonight, I realized it is really fine material and dispersed in water it looks a lot less red. It's good stuff. What a pleasure this class has become. Thank you all for your participation. We missed Jerry tonight. That would have made it an 11 on a scale of 1-10. Hope you are okay Jerry. We miss you. T

 [ChrisU small pitch lap tool not set to match his mirror, so contact surface shows spots of polishing compound in center of each square of pitch...possibly heat caused each square to sag?  tt]

[see more photos/vids: https://www.flickr.com/photos/27241501@N03/sets/72157644761492162 ]