jcham21
sage
Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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In this thread I will show my journey through making a 6" F/15 achromatic refractor. I am a physics/math undergrad at Louisiana State University and I decided to make my own telescope as my senior research project. I am hoping to get some great planetary views once the project is complete.
I started the initial planning of the project around January 2008. I bought the book "Making A Refractor Telescope" by Norman Remer and read it about 5 times. The book suggested that I make a 4" mirror as a preliminary project if I did not have any experience in grinding and polishing glass. So that is exactly what I did. I tried to make all of my mistakes and learn the process from the 4" mirror. Around this time I also started to get my lens making tools. The most important tools are a wedge tester, center thickness tester and spherometer. I ended up making 4 different spherometers. I will include pictures of them later. I was able to convert my wedge tester into a center thinkness tester. I will also show pictures of this later.
As for coming up with a lens design, I had a lot of help from Mike Jones and Dick Parker. I would like to thank them for all of their help thus far. I initially came up with a design myself and got a quote from GlassFab. The quote ended up being close to $1200. That was a little steep. So I ended up buying the F/15 airspaced doublet kit from Newport Glass for $550. This included the two lenses as well as a plate glass tool for each surface. I ordered the lenses in April and didn't receive them until June 23.
I decided to make a Baker type achromat. This design has the two inner radii (R2 and R3) the same. Dick Parker has done this with the Newport kit as well. There is a nice video on Stellafane where Mr. Parker goes over a lot on making an achromatic refractor.
The lenses come with pregenerated curves to a specific design chosen by Newport. I had Mike Jones design an equal internal radii achromat where the radii were close to the pregenerated curves done by Newport.
I worked most of the summer grinding with 220 carbo trying to get to the correct radii. I will give more details about my endeavour later.
I am currently at the polishing stage and hope to be done within the next month.
I will also talk about my Richard Berry refractor mount and my experience in getting the physics machine shop to machine my lens cell, tube adapter, and focuser back plate. I will include many pictures along the way.
Thanks for reading my intro.
James
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James
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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8334
Loc: Bremerton Washington
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Sounds fun James...can't wait to see your pix.
I will be making a 10"f/16 myself so watching you will be informative.
Rob
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www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
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Jeff Morgan
Carpal Tunnel
Reged: 09/28/03
Posts: 1987
Loc: Prescott, AZ
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James,
I look forward to hearing about your endeavor. I have built a slightly scaled down version of the Berry alt-az mount for a 6" f/5 refractor, it is extremely rigid.
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Jeff Morgan
Prescott, AZ
Wile E. Coyote School of Telescope Making
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mconnelley
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Reged: 03/14/06
Posts: 271
Loc: Fremont, CA
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Hello:
I'll look forward to reading of your efforts. It sounds like you're already fairly far along since you're polishing the glass and machining the lens cell. I'm finishing up a 5.7" f/14 refractor myself. The lens was already finished and in a cell, so "all" I had to do was make a push-pull collimation ring to mate the cell to the tube, get a tube, make a back plate, stick the focuser on, and mate it to my mount. I've used the scope a bit and have been quite impressed with the view. I'm at the point where next I need to paint the inside of the tube and add the baffles.
Cheers
Mike Connelley
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jcham21
sage
Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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First I will show some of my spherometers. The first spherometer I made consisted of a 4" pulley, 3 ball bearings epoxied to the bottom, and a dial indicator clamped in the center. This wasn't the greatest spherometer due to my failure of epoxying the bearings on a common radius about the center. The dial indicator wasn't exactly centered either.
So I made a better one out of aluminum.
This is a 3" spherometer shown on a 3.1" opthalmic glass blank. The ball feet are synthetic ruby balls. The hardness of synthetic ruby greatly extends the life of the feet while testing during rough/fine grinding. I epoxied the balls into the head of a machine screw. I made sure the feet were perpendicular to the base plate before tightening the nuts. The mitutoyo micrometer head was purchased from Ebay for just $25. The micrometer spindle spins as the dial is turned. I used this spherometer to make a 3" test plate and test the sphericity of my 6" lenses.
My next spherometer was a 5.5" with an electronic dial indicator.
I had to use huge ball bearings and I believe this was a mistake. Smaller bearing might have been better. The indicator is held inside its base by a small set screw. Care must be taken not to overtighten. This digital indicator reads down to .0005" which was just not good enough for my needs. So I would call this a failed spherometer.
So I proceeded to make another spherometer.
This is another 5.5" spherometer. This micrometer head is a Starrett with a spindle that does not rotate. I purchased this one from Ebay as well for $5.00. I had to take it apart though in order to mount it onto the aluminum plate. I epoxied the bottom of the micrometer to lock it in place. These ball feet are steel ball bearings. They are also epoxied onto machine screws.
When I first begin making these spherometers, I first draw a circle on the aluminum with a nice compass. I then find the three points where the feet will be using the compass again. After the points are found, I punch a small hole using the head of a finishing nail. The hole helps to keep the drill bit in place when I begin to drill. I start with a small drill bit and then progress to the size I need. The center hole is also done this same way. I cut the spherometer out of aluminum with a jig saw. So no special tools are needed.
Here is the 3" spherometer design on the aluminum before cutting:
Larger Version: http://farm3.static.flickr.com/2117/2397153317_03b3a945e9_b.jpg
You will need an optical flat or some other flat surface to zero your spherometer. I used a granite flat from Grizzly. It was only $12 and has an accuracy of plus or minus .0001".
Another good thing to have is an optical surface where the radius of curvature is known very accurately. This can be a mirror or another lens. You can use this surface to find the radius of your spherometer accurately. The more surfaces you have to check your spherometer against the better. The worst feeling is having doubt in your spherometer when you are grinding. I took apart a couple meade 4.25" mirrors at the university and used them to find the radius of my spherometer. I used my foucault tester in finding the radius of the mirrors. I also used an 8" uncoated mirror and a 10" coated mirror.
Another method is to use the concave R3 surface of the flint lens. You could maybe do 2-3 wets per abrasive grade and make a pitch lap to put a flash polish on the surface. Use a foucault tester to find the radius of curvature and use R3 as your calibration piece.
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James
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jcham21
sage
Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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I received my lenses and tools from Newport Glass on June 23, 2008. Here is what was in the box:
Larger Version: http://farm4.static.flickr.com/3134/2614428213_39d3349a92_b.jpg
The top lens is the crown and the bottom lens is the flint. The crown is BK7 and the flint is F4. Around the lenses are the 4 plate glass tools with concentric circles cut into them. Newport supplied a fairly thick flint and a relatively thin crown blank.
The above picture shows my wedge testing marks.
And here is the crown lens in my wedge tester.
The tester consists of a dial indicator on a magnetic base holder, two rods to butt the lens up against, and 3 ball bearings to hold the lens. A measurement is taken at each mark around the lens and the total wedge can be determined. My crown lens had little wedge but my flint lens had a ton.
Here is a shot of the ball bearings supporting the crown:
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James
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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8334
Loc: Bremerton Washington
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Very interesting. I just put in my RFQ to Newport for 10"f/16 blanks. Since they are special order, I expect to pay far more then you. But a lot less then other venders.
My optical expert will aid me in the wedge process on mine...but what exatly is the wedge test for and what info does it reveal?
Rob
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www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
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jcham21
sage
Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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Wedge is basically the two sides of the lens not being parallel to one another. If one side of the lens is tilted with respect to the other, the lens will act like a prism spreading white light into a spectrum of colors.
The wedge tester is used to identify which way the face of the lens is tilted. This way you can correspond a high and low point on the lens face. You then know which side you need to grind down in order to get the two lens faces parallel.
Another way to think about it is you are trying to get the same edge thickness all the way around the lens. If one side of the lens has a larger edge thickness, you must apply more pressure during grinding on that side of the lens to reduce the thickness.
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James
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Edited by jcham21 (09/24/08 04:25 PM)
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radiomaan
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Reged: 10/10/05
Posts: 44
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Just curious. How much does a lens making kit cost these days?
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jcham21
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Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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I bought the 2 lenses and 4 glass tools from Newport for $550. This did not include the grit or pitch.
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James
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jcham21
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Reged: 12/02/07
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Loc: Tucson, Arizona
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Next I had to make a grinding stand. I knew I would be spending a lot of time going around this thing.
larger version: http://farm4.static.flickr.com/3007/2906609580_6e30ea8038_b.jpg
Here is the grinding stand in use at my optics shop at the university.
My optics shop is actually a storage area for telescopes and other astronomy equipment but it is very nice.
My grinding stand was based on Dick Parker's design. The stand has been really stable thus far. I have a few cement tiles in the bottom as a ballast.
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James
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jcham21
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Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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And sometimes when I couldn't make it to the university, I turned my apartment into an optics shop.
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James
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jcham21
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Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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In order to test and figure R3, I would need a foucault tester. I made mine from the Stellafane plans.
larger version: http://farm3.static.flickr.com/2006/2532304120_886cddcaf5_b.jpg
larger version: http://farm3.static.flickr.com/2243/2531488655_f138696d4d_b.jpg
larger version: http://farm4.static.flickr.com/3191/2531488613_f33588b5a0_b.jpg
And of course a lens holder in my case
larger version: http://farm3.static.flickr.com/2189/2532304158_c3a6b04fdf_b.jpg
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James
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plyscope
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Reged: 11/23/06
Posts: 327
Loc: Perth, West Australia
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This is a very interesting project James, thanks for sharing it. I have a copy of Norman Remers book, it is very informative. I have a 6" f15 D&G lens which I have mounted in a home made tube and use with an alt-az mount. It is a lot of fun and gives great views. I admire your patience and hard work to fabricate your own lens.
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Andy
6" f15 refractor
90mm f16.7 refractor
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Mason Dixon
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Reged: 06/29/07
Posts: 43
Loc: Huntsville, Alabama
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Can't wait to see this progress, nice job!
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jcham21
sage
Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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Thanks for all the comments! I have been really busy trying to finish up the scope and I am currently taking midterms. But on with my progress.
While waiting for my lens blanks to come in, I decided to make a test plate to test R1. I had bought some 3" opthmalic glass blanks from ebay last year and thought I would use that as the test plate. Grinding this small optic would also give me a bit more experience in the glass pushing process and polishings.
Here is a shot I took after I started polishing:
Larger version: http://farm4.static.flickr.com/3270/2890531772_4dab96a303_b.jpg
I probably could have had less channels in that pitch lap.
http://farm4.static.flickr.com/3231/2889696273_8b0c7ece1e_b.jpg
The pitch is acculap very firm (too hard) and I used a soldering iron to make the channels.
Here is the lap and test plate in an overnight coldpress. I used aluminum foil to prevent drying:
http://farm4.static.flickr.com/3116/2889696177_12f466737a_b.jpg
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James
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jcham21
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Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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In order to get nice readings using the test plate against R1, I went ahead and made a Newton Interferometer. I got the idea from Gary Fuchs in this thread:
link
Here is my version:
http://farm4.static.flickr.com/3005/2946440998_1fe0e3caa1_b.jpg
It is a rather crude one I will admit but it works great. I made it from some scrap wood my dad had in his work shop. I believe it is 1/4" thick wood. I used small finishing nails and glue to hold it together.
Here is the top part that holds the lamps. I got the bulbs from ebay and the fixtures from Lowes. I have two lamps in the top
http://farm4.static.flickr.com/3239/2946441198_01401f9043_b.jpg
I also used some green cellophane to help filter the light:
And just like Gary did, my beamsplitter is a two-way mirror:
http://farm4.static.flickr.com/3052/2946442094_017a0febb7_b.jpg
Edited by jrcrilly (11/15/08 10:36 AM)
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jcham21
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Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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And here is what the fringes look like when viewed through the two way mirror:
http://farm4.static.flickr.com/3002/2945662337_1474010452_b.jpg
This is the crown lens on top of the flint, R2 against R3. I can count the fringes all the way to the edge. I counted 59 in this case. From the number of fringes and the known radius of R3, I can determine the radius of R2.
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James
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jcham21
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Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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As for my 3" test plate, here is how it turned out:
http://farm4.static.flickr.com/3246/2946552100_17d38022b8_b.jpg
I decided I could use the central 2" as my test plate and didn't care for the outer zone. I just used a mask to cover the outer zone when I was counting fringes.
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James
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1572
Loc: Fort Worth TX
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Hi James,
Enjoying the pictures! Can't you just use shorter strokes on the test plate and get a good sphere across the full aperture? The more of that test plate you can use, the better.
Mike
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56 mirrors, lenses, 16" f/6 Newt, 6" f/10 refractor, TOA-130S, Tinsley 5" f/15 Mak, 6" f/4 RFT, Coronado PST. Still to build: 24" f/10 Modified Dall-Kirkham, 10" f/26 Mak, 8" f/12 apo, spectrohelioscope, Herrig, Schupmann, and others.
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jcham21
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Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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Hello Mike,
Yep I probably could do that. I was polishing the test plate before my lenses came in. Once they arrived, I put the test plate aside and focused on starting the grinding of the lenses. I am now to the point where I am using the test plate. So it might be a good idea to figure the test plate a little better now. I will see what I can do.
Thanks
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James
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jcham21
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Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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Now I will talk about my mount.
While I was working on getting my lenses to shape and bringing them to the correct radius, I started to work on the mount. I knew it wouldn't be any fun if I finished the telescope and there was no mount to put it on.
I basically made an exact replica of the classic Richard Berry refractor mount. I made everything the same dimensions as used in the book "Build Your Own Telescope." I incorporated pine boards and oak plywood together in the mount. It took me about two weekends to cut, glue, screw, and assemble everything.
Here are some boards that will make up the legs:
http://farm4.static.flickr.com/3213/2730245172_797df93379_b.jpg
The first leg put together:
And here are the three legs after assembly:
Edited by jcham21 (10/17/08 03:19 PM)
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jcham21
sage
Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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Next comes the hub:
Stabilizer bar screwed into flange in the bottom of the hub:
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James
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jcham21
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Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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Now the tricky part, getting the legs to fit and sit properly onto the hub:
I had to chisel wood away on the hub in order for the legs to close all the way.
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James
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jcham21
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Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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Here is my stabilizer and spreader bar setup, a little different than Berry's:
http://farm4.static.flickr.com/3047/2949294973_e1c6edc48a_b.jpg
Notice I haven't screwed the stabilizer boards to the circular plywood yet.
And here is the height of the tripod in relation to me. I would say it is around 6':
http://farm4.static.flickr.com/3048/2950146700_45230e20be_b.jpg
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James
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jcham21
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Reged: 12/02/07
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Loc: Tucson, Arizona
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And now with the cradle on top of the tripod:
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James
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jcham21
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Reged: 12/02/07
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Loc: Tucson, Arizona
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Sorry for the huge delay in posts, I'm trying to wrap up my final classes as an undergraduate.
When my lens blanks and tools first arrived back in June, the first order of business was putting a bevel on all the surfaces. I was lucky enough to have to bevel 4 lens edges and all 4 tool edges. That amounts to about 25 feet of bevel! I spent nearly a whole day grinding the edges. I was really cautious at first since it was the first time I was working with optical glass. I used a SiC sharpening stone to do the job.
Next up was establishing uniform edge thickness or removing wedge from each lens. I used my wedge tester to find the thickest edge of each lens. The flint lens had a lot of wedge in the beginning (0.0108"). My goal was to get it down to 0.0005" and it took me two days to do so. I would use accented pressure over the thick edge for a prolonged wet and then another wet with regular center-over-center strokes. This was repeated until my wedge was within tolerance.
The crown lens was nearly wedge free from the beginning and I was able to remove it during the mating of tool and lens for each surface.
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James
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jcham21
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Reged: 12/02/07
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Loc: Tucson, Arizona
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When I received my lens blanks, each surface had been curve generated to a certain radius according to a lens design supplied with the lens kit. The design was a Fraunhofer with a small airspace. A better design is the Baker achromat where the two internal radii, R2 and R3, are equal and with a larger airgap between the two lenses. With R2 and R3 nearly the same, this would allow the two surfaces to be tested by interference. The larger airgap also helps to reduce ghost images and allows for the correction of a small amount of spherical aberration during figuring if need be.
The initial radius of each surface was measured with a spherometer. The measurements are given below in a table. Based upon these initial values, I had the help of Mike Jones in designing a new optical system. The new design values are also given in the table below.
T1 = center thickness of the crown lens
T2 = airspace between the two lenses
T3 = center thickness of the flint lens
Crown glass type is Schott N-BK7
Flint glass type is Schott F4
As you can see I was given a very thick flint lens and a fairly thin crown lens. Compared to a 6" pyrex blank, the flint lens is a lead brick. With the crown lens being so thin, I had to be careful to avoid flexure of the lens during grinding and polishing.
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James
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Edited by jcham21 (11/07/08 01:50 PM)
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jcham21
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Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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Hello all,
The first surface I started my work on was the concave R3 element. I ground it to shape using 220 SiC. Standard mirror making techniques could be employed on this surface and once polished, the figure could be accurately determined with my Foucault tester. I wanted to refine my technique on this surface first before moving onto the convex surfaces as well.
In order to bring the surface of R3 to the correct shape, I needed to slightly reduce its radius of curvature. Standard mirror making techniques call for working a concave surface on top of a convex surface in order to decrease the radius of the concave. Therefore I worked with the lens in my hand and the grinding tool on the stand. A center-over-center stroke mixed with a w-stroke was used with the 220 SiC abrasive. The target radius was reached after about 1 hour of grinding.
After achieving good contact between lens and tool, I could switch to the next abrasive grade and begin the smoothing process of R3. Smoothing consists of removing pits from the previous abrasive grade with a finer abrasive. The stroke used is the center-over-center stroke alternated with a w-stroke. These strokes keep the surface spherical. In order to keep the radius at the current value, the first wet is spent grinding with the lens on top followed by the next wet with the grinding tool on top. Any shortening of the radius with the lens on top is cancelled out by grinding with the tool on top the next wet. When ever working with the lens on the bottom, some type of holder for the lens needs to be made due to the convex bottom. I used the other grinding tool for this. I first put the tool for the convex surface on the stand, then put a circle of rubber mesh on the tool, the lens is then placed over the mesh, and finally the lens is lightly taped into place. A picture will explain it better:
Larger Version: http://farm4.static.flickr.com/3071/3030188671_cede6f1eee_b.jpg
The tool for R3 is on the left and the flint lens is taped onto the tool of R4. I got this idea from Dick Parker.
The abrasive sequence that I followed was 220 SiC, 25 micron white aluminum oxide, 15 WAO, and then 9 WAO. The total time spent grinding from 220 micron silicon carbide to 9 micron WAO was 11 days for R3. Actual time spent at the grinding stand was approx. 11 hours (88 Wets total)
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James
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jcham21
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I put a quick polish on the lens to see how accurate my spherometer was. To my surprise I measured the radius to be 31.97", which was very close to what I was shooting for. Here is how I measured the radius of R3:
My measuring rod is a retractable antenna purchased from Radio Shack. I would usually use books to hold it up in the middle but then I found a cool little jack. A screw helped to keep the antenna level. After finding the null position with the Foucault tester, I would place one end on the lens and then lengthen the rod until it touched the knife edge. The foucault tester was then moved out of the way and I measured the rod with a tape measure.
I used the found radius value to get a better calibration for my spherometer as well.
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James
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dan chaffee
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Loc: Kansas City
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One thing you might want to consider is the tendency for
concave radii to shorten very slightly after the real polish
has been achieved, just as convex radii tend to lengthen.
When I made my 7" f/18 achromat, I deliberately overshot my
concave radii by a couple of mm or so, and undershot my
convex ones at the fine grinding stage and it polished out pretty close to the target.
Don't be surprised if you have to "chi-chi" the radii back anfd fourth to get it on the mark.
Dan
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jcham21
sage
Reged: 12/02/07
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Dan,
You are correct, the radius of R3 shortened a little bit by the end of polishing.
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James
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jcham21
sage
Reged: 12/02/07
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Loc: Tucson, Arizona
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After smoothing R3, I then started work on convex surface R1. This surface required the most glass removal and turned out to be the most difficult surface in controlling its surface radius. Since I had to shorten its radius, I worked with the lens on the grinding stand and the grinding tool in my hands. I used an overhang stroke to remove glass quickly. I ended up removing too much glass and overshot my target radii. The shortest I brought R1 was to 51.70" according to my spherometer. My design radius was 52.23" and thus I had to lengthen the radius.
I found out that lengthening a radius is a lot harder than shortening one. At this time I also started to doubt my spherometer readings and constructed another one to test side by side. Achieving good contact between R1 and its tool was also very difficult. With time running out and two other surfaces needing to be ground, I ended the smoothing of R1 with a radius of 51.77" The total time spent grinding from 220 micron SiC to 9 micron WAO was 43 days. Actual time spent at the grinding stand was ~ 27 hours (255 wets). I had the intention of lengthening the radius some more during polishing or redesigning the lens system after the smoothing of all surfaces was complete.
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James
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Edited by jcham21 (11/25/08 12:59 AM)
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jcham21
sage
Reged: 12/02/07
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Loc: Tucson, Arizona
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I then turned to work on convex surface R4. This surface has a very long radius with a very shallow convex curve. This means that changing its radius is quick and doesn’t require as much glass removal as a shorter radius. I had to shorten its radius and thus I worked with the lens on the stand, concave tool on top. An overhang stoke was used to remove glass from the edge of the lens. I also overshot my target radius for this surface. After smoothing was complete, my spherometer indicated I had a surface radius of 144.42" . I also decided to lengthen this radius during polishing. The total time spent grinding from 220 micron SiC to 9 micron WAO was 18 days. Actual time spent at the grinding stand was ~ 15 hours (134 wets).
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James
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Edited by jcham21 (11/25/08 01:00 AM)
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jcham21
sage
Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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The last surface that needed to be worked was the convex R2 surface. This surface radius needed to be lengthened and this also proved to be very difficult for some reason. I was able to bring the radius to 31.77" . My target radius was 31.96". I also had the intentions to lengthen this radius during polishing. The total time spent grinding from 220 micron SiC to 9 micron WAO was 31 days. Actual time spent at the grinding stand was ~ 12 hours (103 wets).
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James
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jcham21
sage
Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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The smoothing operation was a very tedious and time consuming task. Between the repetitive motions of the grinding strokes and walking around the grinding stand, I would have to take measurements of the lenses. After about five wets, I would take a radius measurement with my spherometer. Once I was done with an abrasive grade, I would check center thickness and wedge. My daily tolerance for working at the grinding stand was 2-3 hours. Taking all of the wets into account and the number of days I worked, my average time at the grinding stand per day was 1.5 hours.
I started work on the lenses June 24 and finished fine grinding on September 11. During this time I also had to focus on summer independent study work (spanish), cope with a couple ear infections, and suffer 11 days at my apartment with no electricity due to Hurricane Gustav. So it was an interesting journey.
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James
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Edited by jcham21 (11/25/08 01:12 AM)
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jcham21
sage
Reged: 12/02/07
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By this time I had already ordered my tubing from Hastings, 7" OD and 8 feet long. Total cost with shipment was $163.69. (Shipping was $98)
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James
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John Jarosz
Astro Gearhead
Reged: 04/25/04
Posts: 2287
Loc: Chicago area, IL
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Really nice tripod! 
John
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6" F4.6(w/Paracorr) GEM reflector, 8" F11 Dall Relay Scope
6" F5 RFT Refractor, Garrett Gemini 20x80 LW
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jcham21
sage
Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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The middle of the tube got a couple dings during shipment:
And I could even fit the entire tube in my Honda Accord:
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James
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jcham21
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Reged: 12/02/07
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Loc: Tucson, Arizona
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Quote:
Really nice tripod!
John
Thanks John. Wait till you see it with the wood stained and moldings added to the legs.
James
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James
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mikey cee
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Posts: 3531
Loc: bellevue ne.
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James.....The dents at least look shallow. A pipe expander could fix those from an auto exhaust place if they got one that big. It looks like they're near the declination bearing so you could hide them under the scope maybe. Otherwise it does look good and the images will be tops!  Mike
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7x35 and 10x50 sears tower binocs, 3" f/10 edmunds reflector, 2.4" f/11.7 manon refractor, 6" f/8 jaegers refractor, "The 8 Ball" 8" f/13.3 brandt refractor, 3" f/15.8 sans&streiffe refractor, 3.1" f/15 selsi refractor(towa 339), 2.4" f/15 sears refractor, selsi 30x30mm spyglass, criterion 5-draw 25x45x75x spyglass(1957), 4.25" f/14.8 tasco 20te.
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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1990
Loc: Hockessin, De
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Very nice work. You can remove the dents by making a wood disk with it's OD matching that of the ID of the tube. Put a slight taper to the disk so the one ID is slightly smaller. Put this end into the tube first and push it through the tube. In college I repaired pipe organs and we had a set of disks from Delrin to removed dents from the pipes.
You might consider polishing the surface out using polishing pads. They will get you to an optical "black" polish in about 1/2 the time as pitch. Also with the pads stuck to the glass tools the radius doesn't change as much. Once you achieve a full polish you can switch to pitch for the final figuring. The pads also allow you to but a quick polish on the glass to check the radius, then pull them off the glass tool and keep grinding if need be.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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scout72
sage
Reged: 05/12/08
Posts: 407
Loc: SF Bay Area
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Looking forward to hearing about your experience with the lens cell, adapter place and back plate!
Very cool thread- thanks for the updates so far.
Did you get the standard wall thickness for your 7" Hastings tube?
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Scout72 is my 1972 International Scout 4x4
Some Scopes, Some Binocs, and Some Mounts,
and a much shorter signature
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Anil
member
Reged: 11/21/07
Posts: 128
Loc: Bangalore, India
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Great work James!!!!.
I love DIY work.
BTW are you planning to install slow motion controles to the mount?
Anil
My 4" DIY refractor
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DIY 100mm CARTON @ f/13
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jcham21
sage
Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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Quote:
James.....The dents at least look shallow. A pipe expander could fix those from an auto exhaust place if they got one that big. It looks like they're near the declination bearing so you could hide them under the scope maybe. Otherwise it does look good and the images will be tops! Mike
Yep they are near where the two tube rings holding the bearings will be. I went ahead and put bondo in the dents and smoothed them out. It was my first experience with bondo and it went fairly well. Lots of sanding was needed to smooth the bondo out. After the tube was painted, I could not see any dents.
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James
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jcham21
sage
Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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Quote:
Very nice work. You can remove the dents by making a wood disk with it's OD matching that of the ID of the tube. Put a slight taper to the disk so the one ID is slightly smaller. Put this end into the tube first and push it through the tube. In college I repaired pipe organs and we had a set of disks from Delrin to removed dents from the pipes.
You might consider polishing the surface out using polishing pads. They will get you to an optical "black" polish in about 1/2 the time as pitch. Also with the pads stuck to the glass tools the radius doesn't change as much. Once you achieve a full polish you can switch to pitch for the final figuring. The pads also allow you to but a quick polish on the glass to check the radius, then pull them off the glass tool and keep grinding if need be.
- Dave
I will have to try the wood disk dent removal method on some of my next projects.
I wanted to use pads to do the initial polish but I went ahead and polished out using standard Acculap. More about my polishing later.
Thanks for the info and comments!
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James
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jcham21
sage
Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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Quote:
Looking forward to hearing about your experience with the lens cell, adapter place and back plate!
Very cool thread- thanks for the updates so far.
Did you get the standard wall thickness for your 7" Hastings tube?
The only thickness offered for a 7" tube was 0.06" which could be thought of as standard thickness I think.
Here are some teaser images of my lens cell, retainer ring, tube adapter, and focuser adapter plate made in Google SketchUp:
Focuser adapter plate:
With focuser in place:
Initial plan of completed refractor on mount:
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James
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jcham21
sage
Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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Quote:
Great work James!!!!.
I love DIY work.
BTW are you planning to install slow motion controles to the mount?
Anil
My 4" DIY refractor
Nope I was not planning on installing slow motion control, I'm not sure how to do that with an alt-az mount. Hand control will be good enough for me!
Your 4" refractor is amazing, very nice DIY project!
Thanks for the comments.
James
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James
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jcham21
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Reged: 12/02/07
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Loc: Tucson, Arizona
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During fine grinding, I started to design the lens cell and other parts that would be attached to the tube. I used the free 2D CAD software Draft IT : http://www.cadlogic.com/products/draftit/
It was easy to learn and had enough features to get my design across to the machine shop folks. And man did they do a good job with the machine work and mounting everything to the egged tube.
First up is the lens cell and lens cell tube adapter:
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James
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jcham21
sage
Reged: 12/02/07
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Here is the tube adapter without the lens cell mounted to it.
And the lens cell with lens retainer ring alongside:
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James
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jcham21
sage
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Loc: Tucson, Arizona
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Focuser end with a 2" Crayford focuser:
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James
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scout72
sage
Reged: 05/12/08
Posts: 407
Loc: SF Bay Area
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beautiful! slobbering over here- I have my lens and can't wait to get the tube assembly going.
How is your tube adapter attached to the tube itself? I don't see threads or holes?
Do you know if the machine shop that made your adapters would be willing to make another set? The only thing I would need to do different would be the spacing of the holes from tube adapter to cell....
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Scout72 is my 1972 International Scout 4x4
Some Scopes, Some Binocs, and Some Mounts,
and a much shorter signature
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scout72
sage
Reged: 05/12/08
Posts: 407
Loc: SF Bay Area
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doh- looking back at the picture I do see the one screw at the bottom of the tube adapter- must be 3 equal spaced holes and I just can see the other two- anyway, looks great.
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Scout72 is my 1972 International Scout 4x4
Some Scopes, Some Binocs, and Some Mounts,
and a much shorter signature
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jcham21
sage
Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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I left my tube with the machine shop guys so they could fit all the parts to it. They were also kind enough to paint my tube for me. I was planning on painting it myself. I walked in one day to check on the parts and the tube was midnight black. I asked them if that was primer and they said nope. I told them I kinda wanted it to be white. Then he recommended a metallic paint. So I chose silver and he painted the tube that color.
Well after handling the tube a couple times, the flakes were all over my hands. So I ended up sanding the tube for a couple of hours, getting rid of all the flakes and stuff. Then I put bondo in the dents and smoothed that out.
I put primer on the entire tube and then sprayed 3 coats of white gloss enamel. The end product was this:
I like the classic white a lot better!
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James
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jcham21
sage
Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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Quote:
How is your tube adapter attached to the tube itself? I don't see threads or holes?
Do you know if the machine shop that made your adapters would be willing to make another set? The only thing I would need to do different would be the spacing of the holes from tube adapter to cell....
I originally wanted the tube adapter to be threaded on but the tube was too thin. So the shop guys press fitted the adapter onto the tube first and then used three screws to anchor it in place.
They might be willing to make another set but they are usually very busy and it might take awhile. The total machine shop charges were $819 for labor and $128 for the 8" diameter, 7" long solid piece of aluminum everything was machined from. So the parts cost way more than the lenses which were just $570.
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James
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Anil
member
Reged: 11/21/07
Posts: 128
Loc: Bangalore, India
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Hi James, What is the material used for lens & focusser adapter? looks very good. Are you planning to powdercoat the machined items?
Anil
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DIY 100mm CARTON @ f/13
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jcham21
sage
Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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Quote:
Hi James, What is the material used for lens & focusser adapter? looks very good. Are you planning to powdercoat the machined items?
Anil
The material is 6061-T6 round aluminum stock. I wanted to powder coat the parts black but I think I like the polished aluminum look better.
James
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James
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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8334
Loc: Bremerton Washington
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Quote:
Quote:
How is your tube adapter attached to the tube itself? I don't see threads or holes?
Do you know if the machine shop that made your adapters would be willing to make another set? The only thing I would need to do different would be the spacing of the holes from tube adapter to cell....
I originally wanted the tube adapter to be threaded on but the tube was too thin. So the shop guys press fitted the adapter onto the tube first and then used three screws to anchor it in place.
They might be willing to make another set but they are usually very busy and it might take awhile. The total machine shop charges were $819 for labor and $128 for the 8" diameter, 7" long solid piece of aluminum everything was machined from. So the parts cost way more than the lenses which were just $570.
Scopes coming along..... 
Looks like you've spent roughly $1800 thus far on this scope for materials and labor costs, not including your own time spent and what ever you braught to the project.
It can be clearly seen why these large scopes cost so much.
My 10" objective is about $2000 for the glass..like you I am polishing it myself with help from a professional shop.
I'm making the adapter rings/cell and back plate myself.
And I made the 5" electric helical forcuser myself as well.
That dog gone Hastings tube is costing an arm and a leg to buy and ship. A 12"x13' tube with shipping is roughly $600.
Since I tend to make all the parts either from scratch or from scrounged parts that were something else....coughing up that much money for a tube is tough.
I'm impressed with your progress. I will be painting my scope *Celery* green.....
Great thread......
Rob
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www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
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jcham21
sage
Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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Good luck with your scope Rob!
Now I will talk about my experience with polishing. I decided to use acculap and I initially got the hard grade. Here are the laps after pouring, shaping, and channeling:
My method of lap making was fairly simple. I used the grinding tools as the lap substrates. Masking tape dams were fixed around the tool and molten pitch was poured within the dam. The thickness of the pitch layer was around 1/4 inch thick. After waiting a minute or two, the masking tape dam was ripped off and then the lens was pressed onto the pitch. Cerium oxide was also added before this pressing. Both the tool and lens were also soaked in warm water for a good while. Once the lens surface was impressed into the lap, the lens was taken off and the lap was put under cool water to set it. Pitch usually flowed past the edge of the substrate and needed to be trimmed off.
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James
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jcham21
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I chose to use masking tape and a soldering iron to make my channels. I found this to be the cleanest and fastest way to get the job done.
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James
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jcham21
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Many air bubbles came to the surface of a few laps and can be seen as holes in this lap. I don't believe this will hurt the polishing though:
Once the laps were made I pressed microfacets into the pitch with fiberglass window screening. Here is how I accomplished cold and hot pressing in the beginning:
And here is what the microfacets looked like after pressing:
Here is the edge after trimming:
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James
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jcham21
sage
Reged: 12/02/07
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Well after polishing for a couple hours, I found out that the hard grade was too hard. I had never worked with pitch before so this was a learning experience. Quoting Mike Jones, hard acculap is like granite! I then switched over to the standard grade acculap and remade all 4 pitch laps. I can say I have lots of experience in pitch lap making!
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James
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jcham21
sage
Reged: 12/02/07
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Loc: Tucson, Arizona
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Polishing with the standard grade acculap was a lot better. The pitch was able to be pressed easier with less heat and less weight. The polishing motions were also a lot smoother.
With the hard grade, the channels never closed. With the standard grade, the channels started to close after 2-3 hours of polishing.
Here is a pic of the crown element on a pitch lap showing the channels beginning to close. The BK7 glass polished very fast as well.
And here is some lens on top work on R3, the flint lens:
--------------------
James
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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 867
Loc: Easton, PA, USA
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Hi James,
I'm not sure how I missed your thread for so long but have just spent some time with it.
Definitely one of the best documented projects I've seen. At least for me there can't be too many good clear pictures or too much detail explained.
Just to pick one that thing that stood out after a not thorough enough reading: the antenna for measuring radius seems like a fine economical solution.
Looking forward to more...
Gary
(Hello Mods: I'd like to suggest this be made "Sticky")...
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jcham21
sage
Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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Thanks for the comments Gary.
Happy New Year everyone! Tigers steam rolled GT last night woo!
Now for testing and figuring the concave R3 flint surface.
Once R3 has been fully polished with all pits removed, the surface may be figured to a sphere. The first step is to
use the Foucault test to determine the initial figure. The picture below shows how the figure looked after 5.15
hours of polishing was complete. (All Foucault pictures show the knife edge cutting into the light cone from
the left with the picture series starting near the lens and moving away.)
The initial Foucault test shows that I have two different radius zones. The outer zone is focusing closer
to R3 than the center zone. This is also known as an oblate spheroid. The surface can also be thought
to have a central hill. I would like to keep the radius of the central zone because it was measured to be
closer to my design radius than the edge zone. Therefore I need to polish away glass from the edge. I
do this by working the pitch lap on top with a W-stroke.
I also have a turned edge but I will not worry too much about it. The lens cell will cover a good portion of a bad edge.
--------------------
James
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jcham21
sage
Reged: 12/02/07
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After a total of 8.2 hours of polishing on R3, I proceeded to do another Foucault test as well as a Ronchi test.
Foucault:
Ronchi:
The figure is still an oblate spheroid. I need to continue polishing glass away at the edge if I want to preserve the central radius. If this were a mirror and the radius of curvature was not critical, I would try to polish glass away from the center to bring the mirror to a sphere.
The Ronchigrams for the surface figure have a slight curvature inside and outside the radius of curvature.
I would like for the dark bands to be straight.
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James
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jcham21
sage
Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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After 10.3 hours of polishing R3, another Foucault and Ronchi test was taken:
The figure is starting to get smoother but I am not making any progress in lengthening the radius of the outer zone. I have decided I will try to reduce the central hill. I will do this by polishing with the lens on top of the lap. This will concentrate the polishing strokes over the center
and help to bring down the central hill.
The turned edge is still there but I believe it is small enough to be covered by the lens cell.
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James
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jcham21
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In order to bring down the central hill, I applied pressure over the center of the lens as I was polishing.
After the accented polishing strokes, another Foucault and Ronchi test was taken. There are multiple
zones in the Foucault test but the Ronchigrams are showing fairly straight lines. I will start
regular polishing strokes to smooth out the zones.
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jcham21
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After 11.65 total hours of polishing R3, I was successful at reaching a sphere. The radius of curvature at the
end of polishing was measured to be (31.96 ± 0.01)"
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James
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DAVIDG
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Reged: 12/02/04
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You have a turned edge and a residue of a hill with a slight depressed ring around the hill still visible in the last image of the Foucault test but they are not showing in the Ronchi test. Your other Ronchi images are also not showing the defects that are visible in the Foucault images. What are the number of lines per inch on your Ronchi screen and how are you mounting the screen ? The low contrast and lack of sharpness in the ronchi pictures indicates the slit width is larger then the spacing between the lines and it is reducing the senstivity of the test.
I would go at a little longer and get it as perfect as you can. Since your going to use this surface as a test plate for the rear of the crown you need to get the edge perfect so you can tell if there is edge problem on the crown.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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jcham21
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The last ronchigram was made with monofilament fishing line. I dont remember the diameter of the line. I believe it was around 4 pound test line. The other ronchigrams were made with transparency film using the stellafane link, I believe it is 100 lpi. The actual lpi on the transparency is probably smaller. I will have to get a better ronchi grating later.
The ronchi gratings that I made were mounted in a 35mm slide holder. The gratings were placed over the LED and the return beam was allowed to hit the grating right above the LED.
As for the turned down edge, I placed the lens in the cell and tested R3 with its edge covered by the mounting rim of the lens cell. The turned edge was completely covered doing this test.
When I was figuring R3, the last images above were the best figure I was able to attain after many hours of polishing. I had a deadline to meet and I had lots of work left to do, so I left the figure as it is. Since I was not able to bring R2 to its correct radius, I will not be using R3 to figure R2. Instead I just used R3 to get a radius measurement of R2. Here are what the fringes looked like when testing R2 on top of R3:
I believe I would need 2 or 3 fringes in order to determine the figure of R2? And I have 60+ above.
If I was making a mirror out of R3, I would have definitely continued the figuring. But since this is a refractive surface, the magnitude of the effect on the wavefront from the surface error is only about half that of the surface error. Also, the Foucault test is a reflection test and any surface error will be doubled. So the errors in the previous foucault image are four times greater than what they are when the surface is used under refraction. (A mirror must be figured four times more accurately than a lens.)
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jcham21
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Now for polishing R2.
The design radius of R2 was not achieved during fine grinding. In order to lengthen its radius, I did all polishing work with the convex lens on top. I mainly used a wide W stroke hoping to lengthen the radius. The lens polished out fairly quickly. Total time spent polishing R2 was 6 hours. The final radius was measured by using R3 as a test plate. The convex R2 surface was placed onto the concave R3 surface in the Newton interferometer. Fringes were counted from the center to the edge and a radius difference was calculated. The final radius achieved on R2 was 31.82". This value was 0.14" too short. I decided to redesign the lens at this point. The new design will not have R2=R3.
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James
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jcham21
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Polishing R1.
The design radius of R1 was also not achieved during fine grinding. Therefore I needed to lengthen the radius during polishing. In order to lengthen the radius I used an overhang stroke with the lens on top of the pitch lap. This stroke concentrated the polishing over the center of the lens and thus lengthened the radius. After the overhang strokes were complete, many hours were spent polishing with a W-stroke and center-over-center stroke in order to attain a spherical figure. Since there was no way of easily monitoring the figure of the convex surface, the best practice was to stick to these known sphere making strokes. Total time spent polishing R1, including strokes used to lengthen its radius, was 13.75 hours. The final radius achieved was 51.94" . This radius was determined by using a test plate in contact with R1 in a Newton interferometer. My target radius for R1 was 52.23" and I was short by 0.29" . At this point I worked on a new design.
I used OSLO design software and experimented with different values of R4 and the airspace until an acceptable design was found. I was able to find a very good design with low spherical aberration and coma. Here are the new parameters and spots:
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James
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jcham21
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And here is the longitudinal spherical aberration curves:
The lens is corrected for d-light.
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James
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jcham21
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Polishing R4.
The new design called for lengthening the radius of R4 by nearly a half inch. Similar to what was done with R1, I used an overhang stroke to quickly polish away the glass in the center. This was done by working with the convex surface on top of the pitch lap. A W-stroke and center-over-center stroke was used afterward to bring the surface spherical. Since the radius of R4 is very long, the uncertainty in its measurement is greater than the other shorter radii. I used two methods to measure its radius. The first was using a spherometer. The measured radius was (144.80 ± 0.5)" . The other method was by interference with an uncoated mirror my grandpa gave me. The radius (136.7") of this mirror happened to be fairly close to the radius of R4 and therefore fringes could be counted. There were a large number of fringes when the two were in contact so I only counted fringes in a three inch circle.
The radius of R4 was determined to be (145.1± 0.1)" The R4 surface has a large tolerance and I believe I am within it with these radius measurements. Total time spent polishing R4 was 9.1 hours.
--------------------
James
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jcham21
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Now that I had all the lenses polished out and at the correct radii, I was ready to conduct the first star test of the complete system.
Here are the two lenses polished out and ready for testing:
First up is how I placed the lenses in the lens cell and what I used for an airspace. Before placing the crown into the cell, a plastic shim stock ring was cut for the lens to rest on. Glass should not be allowed to contact metal. Shim stock was also placed around the edges of the crown lens. Here is the crown lens after being lowered into the cell:
A large roll of duct tape was covered with a soft cloth and the lens cell was placed over this so that the covered roll of tape protruded high enough to place the lens on. The lens cell was then lifted up to catch the lens.
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James
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jcham21
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The airspace ring is now placed on top of the crown lens. I also used plastic shim stock rings to create the airspace. Rings of different thickness were combined until they their total thickness was at 0.17" when miked.
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James
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jcham21
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The flint lens is then placed onto the spacer ring with R3 facing down. I will have to grind a flat edge around R3 later on so it sits flat on the rings but for now I believe it does not need it. A roll of duct tape was also used to lift up R1 and the spacer rings to allow the flint lens to be put into place.
Here is a close up of the shim stock placed around the edge of the flint lens:
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James
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jcham21
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The lens retainer ring has a recessed edge to hold the lenses in place. This extra space also allows for changing the airspace. I have to fill up the extra space with plastic shim stock before the retainer ring is placed over the lenses.
Retainer ring:
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James
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jcham21
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Now the retainer ring can be placed over the lenses and spacer rings:
And screwed down to secure the lens assembly:
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James
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jcham21
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Lenses installed for the first time and ready to receive its first starlight!
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James
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dan chaffee
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Reged: 03/22/06
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Nice progress, James.
The thing I would be most concerned about for the initial
star testing would not be the figure, but looking for astigmatism. If you detect it and can't diminish it by rotating the lenses wrt each other or by tilting them with collimating screws, you'll have to go back to grinding. It happened to me. ...and I was doing all by hand.
If all is in order, figuring the SA will be very easy.
Looking at your design perfomance evaluation, the color correction and off -axis performance looks nearly identical
to my 7" f/18 fraunhofer variant , so I have a very good idead of what you can expect and you won't be disappointed
as long as you don't mind a touch of purple around the brightest objects. Expect to handle 400x with that thing for the Moon or Saturn if the seeing is good:-)
good luck,
Dan
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Dick Parker
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Reged: 08/17/07
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James -
Very impressive. your professors should give you an A+ for this project. My senior engineering project was making an 8 inch mirror and reporting on the testing. You have done so much more.
Good luck. Let us know how your star testing comes out and what you got for a grade from your professors.
Dick Parker
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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1990
Loc: Hockessin, De
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Very nice job. For future reference if you want to let OLSO optimise the design, place a V for variable in the box next to what you want to change. If you click on the VARIABLE box on the tool bar in data spread sheet and it will show you variables and the plus/minus range. You can reduce the range of variablity so you don't get a lens thickness or radius that is not practial. Now go up to main tool bar and click on OPTIMISE. Click GERNERATE ERROR FUNCTION and click on GENII Ray Abberations. CLick on that and tell it OK. That is OSLO "canned" function. Now go back to OPTIMISE and you'll see that INTERATE is not grayed out, click on it and tell it OK. Now all your variables will have updated with a new design.
If you want R2 = R3 make R2 a variable and make R3 a PICKUP of R2. Now what ever value R2 as, R3 will be the same.
Do you have a optical flat to test the completed lens by autocollimation ? If not you can use a distance mercury vapour streetlight and green eyepiece filter to isolate the line 546nm.
Again very nice job !
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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jcham21
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Reged: 12/02/07
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Loc: Tucson, Arizona
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Here is the first photograph taken through the refractor before any figuring was done:
Larger: http://farm4.static.flickr.com/3491/3183352820_fba5716aee_b.jpg
This is a prime focus shot. I had my Canon Xti on a tripod with no lens and brought it next to the focuser draw tube. I also did not have any baffles in the tube. Light that reflected from the walls would enter the eyepiece when viewing the moon.
Although the moon shot looks fairly nice, star images were not that great. The first star test showed the figure of the lens system to be overcorrected; the central zone of the lens system was focusing light closer to the lens than the outer zone. Instead of having sharp stars, there was a small halo of light around each star at best focus.
I mounted a razor blade on the end of the drawtube and did a knife edge test on Vega. The shadow pattern showed that the center zone was focusing closer to the lens.
Since I did not have an optical flat to perfrom an autocollimation test, I used the modified Hartmann test to monitor my figuring. I will explain this process later.
--------------------
James
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jcham21
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So how did I mount the tube assembly to the Berry mount?
Here's how:
The tube rings were made by Parallax and are the 7" OD standard tube rings. These rings are mounted to plywood bearings with ebony star formica around their circumference. The ebony star rides on teflon strips mounted on the cradle. The motions are very smooth.
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James
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mikey cee
Post Laureate
Reged: 01/18/07
Posts: 3531
Loc: bellevue ne.
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James.....It's nice to have that feeling of self accomplishment isn't it?? Very well done and you should feel great. I share your mechanical abilities and the feel for the finished result. However I could never see myself tackling a lens job....no way Jose'! Mike
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7x35 and 10x50 sears tower binocs, 3" f/10 edmunds reflector, 2.4" f/11.7 manon refractor, 6" f/8 jaegers refractor, "The 8 Ball" 8" f/13.3 brandt refractor, 3" f/15.8 sans&streiffe refractor, 3.1" f/15 selsi refractor(towa 339), 2.4" f/15 sears refractor, selsi 30x30mm spyglass, criterion 5-draw 25x45x75x spyglass(1957), 4.25" f/14.8 tasco 20te.
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jcham21
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Reged: 12/02/07
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Loc: Tucson, Arizona
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And here is what the refractor looked like on it first light night:
The scope is missing bits and pieces so this is not the final result.
James
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James
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plyscope
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You are doing a great job James. Thanks again for sharing this with us. I love seeing threads like this. I am inspired to try a dob mount too.
--------------------
Andy
6" f15 refractor
90mm f16.7 refractor
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jcham21
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Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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Sorry for the delay in posts, I hope to finish telling you about my project within the next couple days.
Quote:
The thing I would be most concerned about for the initial star testing would not be the figure, but looking for astigmatism. If you detect it and can't diminish it by rotating the lenses wrt each other or by tilting them with collimating screws, you'll have to go back to grinding. It happened to me. ...and I was doing all by hand.
Dan
During the first star test, I saw a slight elliptical out of focus star image. I believe this was due to not collimating the telescope (I need a cheshire eyepiece). Also I have a very bad diagonal and whenever I put a laser collimator in it, the laser spot hits the lenses about 3 inches away from the center of the lenses. My focuser might not be square to the tube axis as well. How much astigmatism would be seen with a refractor out of collimation? I know achromatic refractors are not corrected for astigmatism off axis and this could be what I was seeing.
Quote:
James -
Very impressive. your professors should give you an A+ for this project. My senior engineering project was making an 8 inch mirror and reporting on the testing. You have done so much more.
Good luck. Let us know how your star testing comes out and what you got for a grade from your professors.
Dick Parker
All my professors really liked the telescope and the project received an A. It was my last requirement in order to graduate. I graduated from LSU on December 19, 2008 with a B.S. in physics and minor in math.
Quote:
Very nice job. For future reference if you want to let OLSO optimise the design, place a V for variable in the box next to what you want to change. If you click on the VARIABLE box on the tool bar in data spread sheet and it will show you variables and the plus/minus range. You can reduce the range of variablity so you don't get a lens thickness or radius that is not practial. Now go up to main tool bar and click on OPTIMISE. Click GERNERATE ERROR FUNCTION and click on GENII Ray Abberations. CLick on that and tell it OK. That is OSLO "canned" function. Now go back to OPTIMISE and you'll see that INTERATE is not grayed out, click on it and tell it OK. Now all your variables will have updated with a new design.
If you want R2 = R3 make R2 a variable and make R3 a PICKUP of R2. Now what ever value R2 as, R3 will be the same.
Do you have a optical flat to test the completed lens by autocollimation ? If not you can use a distance mercury vapour streetlight and green eyepiece filter to isolate the line 546nm.
Again very nice job !
- Dave
Thanks for the OSLO tutorial and kind words. It will come in handy when I work on other projects. Also I do not have an optical flat (wish I did!). I tried to test the lens in autocollimation using a spherical telescope mirror and altering the lens airspace. But the mirror I was using still induced some spherical aberration and the mirror was astigmatic. I could not find any other mirrors that could have worked by adjusting the lens airspace as well. So I used the modified Hartmann test to monitor my figuring.
Quote:
James.....It's nice to have that feeling of self accomplishment isn't it?? Very well done and you should feel great. I share your mechanical abilities and the feel for the finished result. However I could never see myself tackling a lens job....no way Jose'! Mike
Yes it felt great to have finished the telescope on time. I started work in June and finshed at the beginning of November in 2008. I will have to go through my logbook and count all the hours spent grinding, polishing, building, testing, etc.
Quote:
You are doing a great job James. Thanks again for sharing this with us. I love seeing threads like this. I am inspired to try a dob mount too. Andy
Thanks Andy for the kind words. I find the Berry mount is very stable and was a perfect choice for this refractor.
--------------------
James
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Mike I. Jones
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James,
Well done, guy. Thank you for including me in the design phase, I enjoyed working with you. You have a great telescope that you will enjoy all your life, and your grandkids will love! Hopefully it won't be the last one you do. If you ever need help again, just shoot me an email and I'm there.
Mike
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56 mirrors, lenses, 16" f/6 Newt, 6" f/10 refractor, TOA-130S, Tinsley 5" f/15 Mak, 6" f/4 RFT, Coronado PST. Still to build: 24" f/10 Modified Dall-Kirkham, 10" f/26 Mak, 8" f/12 apo, spectrohelioscope, Herrig, Schupmann, and others.
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jcham21
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Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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After R3 was made spherical, the next step was to test the entire lens system. At this point I had already made the optical tube assembly and mount. This made the Hartmann and star tests a lot easier to conduct.
The Hartmann test is a screen test. It is used to evaluate the longitudinal spherical aberration of the finished lens system. A screen is placed over the aperture of the telescope that has slits along a diameter of the lens at equal distances from the center. The sun is used as a light source at infinity. The test requires two slits to be open on either side of center at one time. All other slits are masked while this zone is being measured. Here is my mask over the objective:
Larger: http://farm4.static.flickr.com/3329/3230417844_2fa31f279b_b.jpg
The two slits form two images of the sun on a cardboard screen at the back of the telescope. When the screen is placed past the lens focal point, the solar limb separation of the two images can be measured.
Here is the image of the sun with no mask. Very bright!
Larger: http://farm4.static.flickr.com/3095/3230447874_da7738ffcc_b.jpg
I was seeing spots after this test.
Larger: http://farm4.static.flickr.com/3398/3229597215_9c34c8265c_b.jpg
And here is what the solar image looked like when the mask was in place.
Larger: http://farm4.static.flickr.com/3478/3229597351_4cd9b5252e_b.jpg
Measuring the solor limb separation.
Larger: http://farm4.static.flickr.com/3442/3230448002_40de829ec2_b.jpg
The solar limb separation is then measured for the rest of the slit pairs. The screen at the back of the telescope is then moved in front of the lens focal point. The solar limb separation is then measured for all slit pairs at this plane. Since the distance to each of these measuring planes from the back of the lens is known, the distance between the two planes can be calculated.
The measured solar limb separations are A and B in the above diagram. The distance from the back of the lens to plane A is known. We add this distance to d1 in order to determine the back focal length (BFL) of a zone. From the geometry, d1 is found to be:
The BFL of each slit pair is calculated and a plot of zone radius versus BFL will show the longitudinal spherical aberration of the lens system.
--------------------
James
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jcham21
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Quote:
James,
Well done, guy. Thank you for including me in the design phase, I enjoyed working with you. You have a great telescope that you will enjoy all your life, and your grandkids will love! Hopefully it won't be the last one you do. If you ever need help again, just shoot me an email and I'm there.
Mike
Thanks Mike, I really enjoyed the design process. You were a tremendous help in getting this telescope started. This will definitely not be my last telescope project. Perhaps I should make a Newtonian now Or a Rosin astrograph.
--------------------
James
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jcham21
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In order to conclude that the figure was overcorrected, I performed the modified Hartmann test as described above to measure the longitudinal spherical aberration. The results are plotted below:
The data from the Hartmann test confirmed that the lens system was overcorrected; light was focusing in front of the average BFL from the center of the lens to the 70% zone.
The Hartmann test gave an average BFL of 88.48" and is plotted as a pink vertical line. Using this value of
the BFL, I was also able to use OSLO to find a better value for R4. The radius of R4 that gave the measured BFL was determined to be 144.9".
--------------------
James
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dan chaffee
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Reged: 03/22/06
Posts: 286
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Quote:
During the first star test, I saw a slight elliptical out of focus star image. I believe this was due to not collimating the telescope (I need a cheshire eyepiece). Also I have a very bad diagonal and whenever I put a laser collimator in it, the laser spot hits the lenses about 3 inches away from the center of the lenses. My focuser might not be square to the tube axis as well. How much astigmatism would be seen with a refractor out of collimation? I know achromatic refractors are not corrected for astigmatism off axis and this could be what I was seeing.
James,
It depends on how many rings you are defocusing to. If it's
eliptical with more than 5 fresnel rings, that's pretty strong astigmatism. The fewer the rings, the more sensitive the test. 3 inches off center would be enough to detect
some astigmatism, althought without my software (changed OS and haven't reloaded) I can't remember how much. I suggest
testing for astig without the diagonal; rotate the lens assembly and see if elipse axis follows lens's orientation.
I haven't had time to go over the details of your cell, but you want to ensure that the lens is loose in the cell. If it doesn't shift ever so slightly when moved, it's too tight and aluminum is notorious for its behavior regarding thermal expansion.
Dan
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jcham21
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In order to fix the overcorrection, I needed to choose a lens surface to apply the needed corrections. I chose R1. In order to increase the BFL of the central zone, I needed to remove glass from this area. I did this with a three inch sub-diameter pitch lap.
This small lap was used to concentrate polishing strokes over the center of R1. A W-stroke was used with the small polisher. After a couple sessions with the small lap, an hour was spent smoothing out the surface with the full sized polishing lap. This procedure was repeated two more times.
After the figuring sessions, the lens system was again tested with the Hartmann screen. The results are plotted below.
The test gave an average BFL of 88.34" which agreed well with the expected value of 88.29" given by OSLO with the new value of R4. There is not a major amount of overcorrection anymore but each zone radius alternates from being over- and under-corrected. A star test would have to be done in order to evaluate the new correction. The table below sums up the longitudinal aberrations determined from the Hartmann test.
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James
Canon XTi
Canon EF 200mm f/2.8L II Camera Lens
Celestar 8
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jcham21
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Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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Star test after corrections.
The initial star test gave star images with light halos at best focus indicating an unacceptable amount of spherical aberration. These light halos have disappeared after the above corrections were made. The star images are now white pinpoints. Brighter stars such as Vega have a blue halo surrounding the white star which is due to the inherent chromatic aberration of the system. At this point the figuring of the lens system was complete.
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James
Canon XTi
Canon EF 200mm f/2.8L II Camera Lens
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jcham21
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Reged: 12/02/07
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The finished crown lens:
Larger: http://farm4.static.flickr.com/3518/3237032619_cf543a33ee_b.jpg
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James
Canon XTi
Canon EF 200mm f/2.8L II Camera Lens
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jcham21
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Reged: 12/02/07
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Loc: Tucson, Arizona
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The finished flint lens:
Larger: http://farm4.static.flickr.com/3505/3237032699_569c458860_b.jpg
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James
Canon XTi
Canon EF 200mm f/2.8L II Camera Lens
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jcham21
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Reged: 12/02/07
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So how close did I come to the original design? Here is a table comparing the final design parameters to the initial:
Larger: http://farm4.static.flickr.com/3132/3236988375_f398d5703a_o.jpg
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James
Canon XTi
Canon EF 200mm f/2.8L II Camera Lens
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How To Make a 6" F/15 Refractor
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jcham21
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Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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Now on to baffles. I made two baffles out of aluminum flashing and used a piece of the refractor tube to make retainer rings. A section was cut out of each ring to allow it to be put inside the tube. The baffles were cut out of the flashing using scissors and a razor blade.
One baffle and two retainer rings ready to be installed:
The other baffle installed within the tube with retainer rings:
And placement of each baffle was done according to this equation and diagram made by Mike Jones:
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James
Canon XTi
Canon EF 200mm f/2.8L II Camera Lens
Celestar 8
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How To Make a 6" F/15 Refractor
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jcham21
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Reged: 12/02/07
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Loc: Tucson, Arizona
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And man do baffles make a world of difference on views!!
Before baffles:
After baffles:
View down the cannon looking at the focuser end:
I did look at the moon without baffles and the tube walls act like mirrors dumping loads of stray light into the eyepiece. And nope I did not paint the inside of the tube, I don't think I need to. Perhaps some flocking would help between the lens and first baffle.
With just two baffles, the views were greatly enhanced. Lots of contrast when viewing the moon.
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James
Canon XTi
Canon EF 200mm f/2.8L II Camera Lens
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How To Make a 6" F/15 Refractor
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jcham21
sage
Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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I went ahead and spray painted the front of the focuser adapter plate flat black as well.
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James
Canon XTi
Canon EF 200mm f/2.8L II Camera Lens
Celestar 8
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How To Make a 6" F/15 Refractor
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jcham21
sage
Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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The focuser adapter plate fits on the back of the tube nicely. Here is the crayford focuser installed onto the adapter plate showcasing its range of motion:
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James
Canon XTi
Canon EF 200mm f/2.8L II Camera Lens
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How To Make a 6" F/15 Refractor
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jcham21
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Reged: 12/02/07
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Loc: Tucson, Arizona
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I decided to make my own finder scope rings. The "finder scope" will be a 60mm f/15 refractor. The rings were water jet cut out of a piece of scrap aluminum my dad had. They came out nicely:
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James
Canon XTi
Canon EF 200mm f/2.8L II Camera Lens
Celestar 8
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How To Make a 6" F/15 Refractor
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jcham21
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Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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Flat black paint party, wooo...
Notice the white overspray past where the newspaper was put down when painting the OTA... Sorry Dad!
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James
Canon XTi
Canon EF 200mm f/2.8L II Camera Lens
Celestar 8
Main Astrophotography Gallery
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How To Make a 6" F/15 Refractor
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jcham21
sage
Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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Did I mention this refractor is portable? Everything is able to fit into my 1993 Honda Accord, of course with the back seat folded down. I even had room to fit a celestar C8:
Larger: http://farm4.static.flickr.com/3505/3237994348_ff4450dc09_b.jpg
Larger: http://farm4.static.flickr.com/3462/3237153563_eab3700449_b.jpg
Larger: http://farm4.static.flickr.com/3087/3237994512_38aa2f837e_b.jpg
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James
Canon XTi
Canon EF 200mm f/2.8L II Camera Lens
Celestar 8
Main Astrophotography Gallery
Comet Lulin Gallery
How To Make a 6" F/15 Refractor
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Albert92
member
Reged: 06/27/08
Posts: 10
Loc: Europe
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Good work!
You should be an example for many ATMs.
I will be waiting for other photographs (planetary or even terrestrial) taken through the refractor.
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7wt
member
Reged: 09/04/06
Posts: 15
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Wow, that is one of the coolest projects I have ever seen. Nice work all the way around.
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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8334
Loc: Bremerton Washington
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Great job James.....Great job.
Not too many young folks are into building their own scopes...let alone a refractor.
Nice work. Maybe you should have thought outside the box and painted it green...instead of that *avarage* white color..  
Rob(my hats off to you)
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www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
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