190701 C8 Ronchigrams by Mike Chibnik

C8 Ronchigrams

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Mike Chibnik

Mon, Jul 1, 8:51 PM (8 days ago)

to ceztom, tomt, me, tscrawf3, TCrawf3, kometes, rberry, 54healeyguy, bahu, jerryawilsonphd, Christopherulivo, emilanes

My old C8 had enough dirt and smoke residue on the corrector plate to grow radishes, so it was time to give it a good cleaning.  So I removed it from the tube assembly and used a lot of water Dawn soap and after a bit of soaking followed by gentle use of cotton swabs the gunk was removed.  I followed that with a liberal rinse of distilled water and a spritzing of 90% alcohol and gentle use of canned air and dabbing with tissue paper.  In it went making sure i oriented to the position marks I made before removal.  I used the scope for Mercury viewing the last week and noticed optical alignment was slightly off.  I did the best alignment I could do last night with it unfocused using concentric circles with the secondary mirror.  After alignment I I wanted to do a star test but the air was too unsteady.  So I used the  Ronchi screen eyepiece I had loaned to the other Mike to see what type of figure my scope might have.  It appears my old scope has reasonable optics.  Images were taken with my old iPhone.

 

190508 Foucault test tolerance origin

You asked about the origin of the tolerance (x=2pR/r), and I didn't know, as it came from page 3 of Tom's 4-page Foucault test description.  The detailed description is also avoided in Harbour's paper.  Harbour says:  The amount of error that is allowed for the location of the focal plane to deviate from its ideal location for any given zone on a mirror has been worked out for us with the science of geometry. For our purposes it is not necessary to elucidate the entire method of determining the allowed error.  Thank you very much Mr Harbour!

My curiosity was piqued, so I did a Google search.  It comes from the Rayleigh criterion and the Airy disk first minima width (small angle sine approximation used, i.e. sin(x) = x).  See: https://www.quora.com/Where-does-this-%CE%B8-1-22-%CE%BB-D-come-from-Whats-its-derivation. 

The variable names used in the quora paper are different than in Tom's derivation, i.e., Tom uses D as the mirror focal length, while the quora reference uses D for the lens (mirror) diameter (I'll call it D' herein to avoid confusion).  They are related by f/, the focal ratio (your mirror is f/5.09). 

So, the quora paper has p=1.22λ/D', and Tom uses 1.22λf/D.  These formulas are equivalent and confusing because of the D's represent different measurements.  Note that if D' is the mirror focal length, and Tom's mirror diameter is D,  then D = D'/f, where f is the f/ ratio.

I know the above is a lot of information.  If you have any questions, I welcome them.

Enjoy,

Bruce

190502 Foucault mirror measurement data reduction spreadsheet

Foucault measurement data reduction spreadsheet

bkm	Thu, May 2, 1:45 PM (7 days ago)

to Tim, Thomas, me, Mike

TimC, TomT,

We were rushed a the end of Tuesday's workshop, and we didn't have much time to explore the Foucault data reduction spreadsheet I created for you.

In the short time we had, you picked up very quickly on what Excel can do, and asked some great questions.  The purpose of this email is to explain spreadsheet operation in more detail so you can peruse it at your leisure.

The attached pdf file has three pages of Excel screen captures for the Foucault data reduction.  The first page is the spreadsheet as normally presented, i.e., numbers in the cells.  The second page shows the actual cell contents (numbers, formulas, labels, etc.), and the third page is the resulting plot showing the average measurement results for the dummy data I created in cells M17 through Q21.

You can follow along what the Excel spreadsheet does from the Foucault hand calculations page also appended.  The hand calculations simply progress from left to right, i.e., the number calculated in a column is used in the next column to the right.  In the hand calculations, the formulas used are shown at the top of each column.

Every Excel cell has an address consisting of a letter (for the column), and a number (for a row).  The column letters and row numbers are shown in the borders at the top and left side.  An Excel cell can contain any of several things: a label, a number, an equation, or be blank.  For example, cell B17 contains the number 0.9, and cell C17 contains an equation that references cell B17.  More on this later.

Labels are simply typed in and 'enter' depressed.  Labels provide spreadsheet instructions, guidance, or information, such as in data entry, or data results output.

In a cell containing a number, the number is just typed in and 'enter' depressed.

A cell containing an equation always starts with an equal sign"=".  If you wanted to calculate 3 + 4, you would type in =3+4 (no spaces), then enter.  The number 7 would be displayed in that cell.  If that cell is highlighted with the cursor, the cell contents will be shown at the top left of the spreadsheet as "=3+4".

Excel provides algebraic and trigonometric functions.  For example, the relative percent stick position is the square root of the percent Zone area; hence, the numbers displayed in column C are the square root of the numbers in column B, i.e., 0.95 shown in cell C17 is the square root of the number (0.9) shown in cell B17.  "SQRT" is the mnemonic Excel uses for square root, thus cell C17 contains "=SQRT(B17)".  The argument is always enclosed in parenthesis and no spaces.  You can have multiple nested parentheses for complicated expressions.  The actual cell contents (numbers, equations, or labels) can be seen on the second page of the attached pdf file.

Excel aids in copying cell contents by automatically indexing cell callouts.  For example, When I created the spreadsheet, I typed into cell C17 "=SQRT(B17)".  I then highlighted that cell and selected 'copy' (ctrl-C, or copy from the edit menu).  I then pasted this expression into cells C18 through C21 (ctrl-V, or paste from the edit menu).  Excel automatically indexed the SQRT argument (cell reference) to B18 through B21.

Dollar signs indicate cell references that are not supposed to be automatically indexed, such as constants that are calculated and stored in other cells, such as f/ ratio, as calculated in cell G5, or p as calculated in cell G6, or numbers that are input, such as the mirror diameter in cell C5, or the radius of curvature in cell  C6.

The radius of curvature is used in the calculation of tolerance as shown in column E.  In the second pdf page (with the actual equations), C5 is referenced through dollar signs, i.e., $C$5, which tells Excel to not automatically index this reference.  You can limit automatic referencing to just cells or columns, i.e., $C5 would not allow automatic referencing on columns, but would allow automatic referencing on rows.  Likewise for the converse: C$5.

A cell can have any of 65,000 different background colors, sixteen of which are standard Windows colors, such as turquoise.  I have colored in turquoise the backgrounds of data entry and data output cells to make them stand out.  The background color has no effect on any operations.  A cell can have one of many outlines in the same or different color (or none).  I have outlined the data entry and output cells in dark blue.

Text can also be many colors.  I made the pin location output numbers dark blue, otherwise, the text is black.  These pin location numbers are what you can easily calculate as I detailed in the hand calculation page previously appended, and again appended here for convenience.

I may be approaching information overload, or maybe have exceeded it.  If you want to explore Excel and the Foucault data reduction spreadsheet, I'll bring my laptop to the next workshop.  Let me know.


Bruce

On 7/8/2018 10:31 PM, Tim Crawford wrote:

We will get together this Tuesday, July 10 for a workshop. We meet in the Broder Building from 7:30-9 pm. Come join in if you get the chance. Jacques, may we please get the east gate open? Thanks.

I’m not sure where we are with projects, so let’s check in and assess where we are.

I have a new addition to the Dobsonian design- altitude bearings. Now, these are the genius design of Tom Whittemore. Here’s a look:

These altitude bearings slip over the Sonotube ( telescope tube of a Newtonian scope). What is brilliant is they tighten to the sides of the scope when you tighten the knobs you see in the image above. I truly believe in my heart this design could replace all the modern designs we see in magazines and articles.

We need a few tweaks to get there though. Come and speak to these issues.
1). Tube must be smooth
2). The tolerances have to be trued.
3). There is a “ rocking motion” when the bearing assembly is in place. Strategic placement of pads or Teflon pads may take up the “ raw fit” of this assembly.

Also, we may get a little figuring done to get ever closer to finishing an 8” mirror.

See you there.
Tim
——————————
July 10th was my father’s birthday. He was a ship captain most of his life. He used to tell me he had been at sea since Christ was a deckhand.

Without going too far, I just  want to tell you of one incident. ( I had the extreme pleasure of going to to sea with him for a couple of years).

We were in the middle of the Atlantic Ocean. I was a young man just out of City College. I had just finished introduction courses in Physics. My dad came out to the boat deck and asked me if I could tell him how fast we were moving through the water. He gave me a couple of factors and I plugged these into newly studied motion laws.

I gave him my results and he shook his head and said, “ No Timmy. Wrong. You are missing another factor.” I asked him what and he simply said we are on a great circle.

He was using a sextant. That’s all he used to navigate the globe. It was just before GPS became the standard. And yes, he always got us there and almost to the minute. Try to imagine that. It wasn’t too long before that the long sought after “ Longitude” was used at sea.

It may not sound like much but, I got to witness it “live” right in front of my eyes.....

170201 Latest Telescope Workshop photos

Find the latest photos I have taken at the Telescope Workshop and other Santa Barbara Astronomical Unit (SBAU or AU) events at Flickr:
https://www.flickr.com/photos/sbaushots/albums
You can also search Flickr for keyworks like sbau, workshop, month, year, mirror, Ronchi, etc. which might help you find what you need more quickly.

You might want to look at the SBAU Forum for Telescope Workshop and other astrophotography or telescope related comments:
http://www.sbau.org/sbauforum/

Hope to see you on most Tuesday nights!
 Tom Totton

150809

TimC looking for scratches...found one!  Time for a new lap!

On 8/9/2015 7:04 AM, Tim Crawford wrote:
 Greetings all,
 We are planning to meet Tuesday, August 11th from 7:30-9pm at Broder .

 I went over to my good friend Tom Whittemore's home this weekend. Our intentions were to pour a new pitch lap for my 8" mirror project. The early afternoon sun poured over the vegetable gardens and humming birds swooped to the many feeders in the yard. Tom and his wife Maureen along with their dogs Patch and Flanagan made my stay most welcome.

 I became mesmerized by Tom's expertise of many things. One is baking bread! Sampling some of his work with some home grown tomatoes was a true treat. Many of our members are multi talented people. Tom is no exception. I hope Tom and Maureen don't mind my sharing this with you.

I will tell you we poured a  truly fine pitch lap. This is a thing of beauty. No flaws at all. Great work Tom! We used a combination of pitch. Medium tempered pitch and hard pitch. The process is fairly simple and straight forward, especially using these new pitch molds. I am thinking someday we may want to film this process and share it with the club at one of the members' nights?

 We will probably need a tester brought to Tuesday's meeting as Tom will not be there. I will bring mine, but it is not the best. Aside from testing there will be the opportunity to work on projects. I hope you get a chance to come by and participate. It's a blast!
> T

150329 TimC on isolating problem causing scratches

Tim C  8:17 AM (2 hours ago) to Jerry, Mike, Richard, Tim, me, Tom:    Of course I am very sensitive to this. Let's check Tuesday. I am so sorry to hear of these scratches. I am very confused by all of these scratches. It seems to me we have (as a group) never had so many show up in finishing stages. Coincidence? I am not sure. It only takes one grain of grit to do the damage. The question is, " where is the likely source of this contamination?" How does one try to isolate the source? As I have told many of you, in the dental lab, when we have a problem ( not necessarily this kind of problem, but similar) we try to list all the components in the production to the point of the problem, then one at a time change a component till the problem ceases. With the problem at hand, if it cannot be fixed, the problem occurred late in the polishing/ figuring stage. To me, it calls for looking at : A. The environment the problem occurred B. The cerium oxide and container it is in C. The surface you are working on D. The pitch / pitch lap E. Cleaning and storage materials and containers Are there more I have not listed? If it were me, and boy it has been, I think I would group the pitch and cerium into one group. Otherwise changing just one perpetuates the problem mercilessly, while replacing both and fixing the problem still isolates the cause greatly. I do have a question here though. That is, is it really necessary to change these out now? Can Mike go back to 500 then come forward with the same materials, especially the cerium, as it did no damage for a long time? Is the pitch now contaminated or is it just a matter of cleaning the lap carefully? If it is likely that this was a random piece of grit that has now been expelled from the working environment, it is just a matter of being careful with work surfaces and all materials. Again Mike, I am so sorry to hear this and I'm obviously sympathetic. As you know, I am going to go back a sixth time to regrind the donated mirror. I'm going to complete another 8" I have simultaneously. I want to know more of if one piece of Pyrex can be different from another. I still want to know if the cause that persists with this one mirror can come from the blank and not all the other standard components. I did change out all of the others and the scratches persisted. If my methods hold up it seems the problem point to the Pyrex. As Jerry suggests this may be highly unlikely. So that leaves me. Because I work in a very abrasive environment everyday, I may be the source. I may be bringing in contaminants under my fingernails, on my clothes ( which I change before each workshop) or hair. One last component is possible. Randy, from Astrosystems told me he had to put a plastic sheeting above the work area he was in. The acoustic ceiling tiles were a source of contamination for them. As many in the workshop have commented in the past, "it's too bad we do not have a dedicated place we can keep our projects and even a dedicated clean room to polish." T

150328 MikeC mirror status -- scratches?

---------- Forwarded message ----------
From: Mike C
Date: Sat, Mar 28, 2015 at 5:32 PM
Subject: Re: Am I close enough or should I tempt fate making my mirror more perfect
To: Richard B
Cc: Tim C, Jerry, TomT, Tom W

Latest update:

I managed to get the mirror back to a sphere due to the over correction. We were about to think about parabolizing.   However, at the last meeting we seemed to have a contamination problem and as a result I got one big scratch and a pit on my mirror.  After that I tried to polish out the scratch I brought it over to Tom's house to pick up more cerium oxide.  However, we noticed many more scratches. This Tuesday Tom and Jerry will determine if I have to go back to grinding out the scratches.  This might set me back six months depending on how much work I need to do.  At the very least I'll have to chuck the old pitch lap and polish, but I'm tending to think that I need to go back to 500 grade or worse.  That will mean I'm back to square one which is where I was over a year ago.

Sigh....

Mike

On Sat, Mar 14, 2015 at 5:14 PM, Mike C wrote:

Thanks for getting back guys.

I'll be making the Ronchi eyepiece and masks but won't be able to use them until I get back to Santa Barbara as the cloud cover is solid and might not be gone until a day or so.  I might go far as to make another offset circle with a 3.5 inch circle.

In the ;meantime I need to get serious with a more permanent way of holding the optics.  My setup is a handful to keep pointed when the angles are too high and too low.  That's because the board used for elevation needs to be adjustable.

Mike

On Sat, Mar 14, 2015 at 11:22 AM, Richard B wrote:

Hi Mike--

Have you ever done a star test on a "perfect" mirror? If you haven't, an easy way to do so is to make a circular off-axis stop about 2 inches diameter for your 10 inch. Use a magnification 1/5 as great, and run a star test. The errors over the small aperture will be small compared to the full miror, and you'll see what a "perfect" image looks like.

The effect you describe -- hard edge one side of focus with soft edge the other side -- is very common. However, the description is qualitative, and that makes it hard to know whether you can ignore what may be a 1/20-wave error or be worried about a 1/2-wave error. I used to take a Ronchi eyepiece (an eyepiece housing with a 100-line Ronchi inside) to star parties, and I looked a lot of mirrors. In general, it's best best to slip in the Ronchi, do your test, and not say anything to the guy who owns the telescope!

I suggest you make a ring-shaped stop with a diameter of 9 inches. Make sure the edge is nice and clean, such as an Xacto-blade cut edge in file-folder cardboard. Just for fun, you could make a mask that cover only 180 degrees of the mirror's rim. If you have a turned-down edge, it probably won't be as much as 1/2 wide, so you see what the star test shows with a non-turned edge.

Another useful diagnostic is to place a Ronchi screen at the focus, and view the mirror surface in the light of a star. With four or five bands, they should be perfectly straight. If the bands bow in or out, the mirror is over- or under-corrected.

The purpose of testing on stars is that "you get what you see." There is no math to interpret or figures to misinterpret. The fact that you see errors in the formation of your star images says your mirror will always perform that well and no better. The Ronchi test on a star is essentially a (not very sensitive) null test; straight bands mean good. You can also use a knife edge at focus, but you also see all the air currents.

Testing at center of curvature is good because you do it indoors under controlled conditions, but you're measuring the first derivative of the figure, not the figure itself.

--Richard


On 3/14/2015 8:57 AM, Tim Cwrote:

Hi Mike,

Tom or Jerry are the experts here, but, I want to give you my take- what I would generally interpret first. Then, you can check with Tom, Jerry and Richard for more advice and most likely a clearer explanation.

Before I get there though I would like to say I use a program written by Reifke to interpret my Foucault results. I do not know what algorithm he uses in his program but I like the way it shows results. The pin stick I gave you does not use the same settings for pin placements. You will have to manually measure and change these within the program. The output of this program gives you a picture of the wavefront and where to make changes. It looks like this:

The lower horizontal line is a perfect mirror. The output is the line traced from left to right representing the mirror surface. The vertical lines represent the zones. You can visually see where the correction is needed. Both Tom and Jerry ( actually several others as well) tell me you really can't trust this because the algorithm is unknown.

In Richard's book on page 212, you see the pattern in general you describe showing an over correction. To me, that means the curve is too deep. Where I get all fouled up is visualizing if this means you have too long or too short a radius of curvature. I always seem to get it backward. But, I think it means the radiuses are shorter in an overcorrected mirror. For me, I just visually see a deeper curve or dish if you will. It means we need to flatten the curve. 

What I would do, depending on where the errors are on the wavefront is to use TOT and a spherizing stroke in general. 

The hard part here is you sent some images that do not represent this exact picture. I am going to search some pictures in Suiter's book a bit and see if I can't find something a little more definitive . In the meantime, yes some Ronchi pictures might really help here. Try for 4-6 lines inside or out. The other thing is, you can wait and we'll all look at this next Tuesday. 

T

PS- just for fun see if the pin stick placement measurements on my pin stick are the " effective radius" you see in the picture. I believe these are the measurements you would need to manually use in Reifke's program. ( if you choose to check it out)

Sent from my iPad

On Mar 13, 2015, at 9:56 PM, Mike C wrote:

Hi Tim:

I did center over center last night as per Tom, followed by large w's to blend and got similar results. I'm not sure why things are looking exactly the same in star testing. I see a bright outer ring out of focus and then a diffused circle with bright spot inside focus.  That either means that I've got over correction or turned edge.  I;ve got to figure out again how to get some decent Ronchi grams to show.  Again what does over correction mean with respect to the curve and what part of the curve needs working on?

Mike

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