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Old 12-09-2021, 10:14 AM
Rod
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Building an 8 inch F3.2

Over the last 18 months I have been working on a fast 8 inch scope. I thought I’d share my progress. I’ve made several mirrors between 4 and 12 inch. They were all in modest focal ratios between F5 and F10. The tolerances at at F3 are tight so the mirror was a challenge. I used a conventional Foucault tester and a ronchi screen for all the testing. When I had a good result there I tested the mirror in autocollimation.

I bought the blank here on iceinspace. It had the fast curve already ground into it. So I made a tile and plaster tool to complete grinding. Melbourne was in lockdown when I began so I used my at home time to build a new grinding / polishing machine shown below. If you are interested in the machine, I prepared a short video about it for my club last year:

https://drive.google.com/file/d/1SPF...w?usp=drivesdk
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Old 12-09-2021, 10:31 AM
Rod
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Testing the mirror

Testing the mirror involved using a Foucault tester and an 8 zone mask. I had to observe pairs of zones darken at the same time and not their positions with the tester’s micrometer. This is easy at modest focal ratios but with a mirror this fast it’s difficult to see the zone pairs at the same time, particularly at the edge which is the most important part of the mirror. With practice I became better at testing this mirror. But I needed a way to verify the test results.

I had purchased a 12 inch optical flat from a friend a few years ago. It was uncoated and it was made by someone I trust. I had it aluminised and used it in the configuration shown below. Essentially it is a roughly made telescope set up in front of the flat mirror. A bright LED is mounted behind a ronchi screen in the eyepiece holder. As you can see the test showed straight lines, indicating a good parabola. Autocollimation is a double pass test so any errors shown are doubled in size, so it’s an excellent null test for a completed telescope.

The only error it does not fully test for is astigmatism. I hope to test for this with a star test once the tube is complete. If Melbourne’s restrictions ease up, a friend has offered to test the mirror with his newly completed Bath interferometer so I may also do that.
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  #3  
Old 12-09-2021, 10:59 AM
Rod
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Making the tube

For smaller scopes, I like to make octagonal tubes. They remove the need for a tube box and locate the eyepiece at the right angle and have a flat area at the top for a handle and a finder. I’ve made several of these tubes. I cut the pieces from 6 to 7 mm plywood on my table saw then add a 22.5 degree bevel on each of the long edges. You can add the bevel with a router and a 22.5 degree bevel bit if you prefer.

Here is PDF version of a presentation I did showing how I assemble the tube:

https://drive.google.com/file/d/13LG...w?usp=drivesdk

Here is a short video showing the assembly:

https://drive.google.com/file/d/1WGM...w?usp=drivesdk

Below is the completed tube next to an 8 inch F6.
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  #4  
Old 12-09-2021, 11:34 AM
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multiweb (Marc)
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Quote:
Originally Posted by Rod View Post
Over the last 18 months I have been working on a fast 8 inch scope. I thought I’d share my progress. I’ve made several mirrors between 4 and 12 inch. They were all in modest focal ratios between F5 and F10. The tolerances at at F3 are tight so the mirror was a challenge. I used a conventional Foucault tester and a ronchi screen for all the testing. When I had a good result there I tested the mirror in autocollimation.

I bought the blank here on iceinspace. It had the fast curve already ground into it. So I made a tile and plaster tool to complete grinding. Melbourne was in lockdown when I began so I used my at home time to build a new grinding / polishing machine shown below. If you are interested in the machine, I prepared a short video about it for my club last year:

https://drive.google.com/file/d/1SPF...w?usp=drivesdk
Impressive gear. Love that table. Very professional.
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  #5  
Old 13-09-2021, 06:29 AM
awbroady (Andrew)
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Looking good! The octagonal design has a lot of practical advantages. Am about to start my own 8" f6 dob and will now be considering this option. Thanks for posting.
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  #6  
Old 13-09-2021, 09:48 AM
Rod
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Thanks for the nice comments. If you are looking for a good cheap ply for an octagonal tube, I’ve found Bunnings’ red oak 7 mm thick works well. The tube comes out a bit stronger than the birch ply I normally use. I think it may be because the individual plies are thicker. You need to check the sheets carefully. The veneer quality can vary as can sheet flatness.

I use to use polyurethane glue for these tubes but now I find Titebond 2 is just as strong and much cleaner to work with.

Rod

Last edited by Rod; 13-09-2021 at 11:07 AM.
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  #7  
Old 13-09-2021, 01:22 PM
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multiweb (Marc)
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Quote:
Originally Posted by Rod View Post
For smaller scopes, I like to make octagonal tubes. They remove the need for a tube box and locate the eyepiece at the right angle and have a flat area at the top for a handle and a finder. I’ve made several of these tubes. I cut the pieces from 6 to 7 mm plywood on my table saw then add a 22.5 degree bevel on each of the long edges. You can add the bevel with a router and a 22.5 degree bevel bit if you prefer.

Here is PDF version of a presentation I did showing how I assemble the tube:

https://drive.google.com/file/d/13LG...w?usp=drivesdk

Here is a short video showing the assembly:

https://drive.google.com/file/d/1WGM...w?usp=drivesdk

Below is the completed tube next to an 8 inch F6.
I assume you still have a cut out to get a round aperture at the front?
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  #8  
Old 13-09-2021, 01:24 PM
Stefan Buda
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Nice to see some challenging optics being made locally.

Can you tell us what size figuring lap/s you used?
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  #9  
Old 13-09-2021, 03:44 PM
Rod
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Hi Stefan

Thanks for your interest.

I actually only used two laps. For most of the work I used a slightly oversized lap. It’s about 6 mm wider than the mirror. I followed Mel Bartels advice a bit on this project. I’ve used slightly oversized laps before and found they do help to prevent turned edge.

To deepen the centre I used a chordal stroke. To extend the correction out I used a long centre over centre stroke. This left the intermediate zones lagging. I used a 4 inch lap with a tangential stroke to address these zones. The end result led to a slightly over corrected mirror. Fortunately some gently W strokes tool on top brought the correction down again and smoothed it out a bit too. It sounds straightforward but it wasn’t! I had to return to spherical twice after over correcting the outer zones. It took me several months free time to find a combination of strokes that worked. I kept a log which helped a lot. I did try a 2 inch lap at one point and found it created far too much roughness.

Virtually all the figuring was done by hand with the machine acting as a motorised turntable.

Rod
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  #10  
Old 13-09-2021, 06:31 PM
Stefan Buda
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Thanks Rod for the detailed reply. It all makes sense and I think you did very well. Fast mirrors like this one are very difficult to figure.
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  #11  
Old 13-09-2021, 07:14 PM
Rod
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Quote:
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I assume you still have a cut out to get a round aperture at the front?
Hi Marc

Yes I do. Not sure it’s necessary though. I haven’t bothered before.

Rod
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  #12  
Old 13-09-2021, 07:16 PM
Rod
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Quote:
Originally Posted by Stefan Buda View Post
Thanks Rod for the detailed reply. It all makes sense and I think you did very well. Fast mirrors like this one are very difficult to figure.
Thanks Stefan

You’re mirrors are larger and faster though! I’m very envious of your lens making and engineering skills.

Rod.
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  #13  
Old 14-09-2021, 07:19 AM
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Looking at the auto collimation test I can see a number of offset images.

I have tried a very rough telescope auto collimation set-up to see if it would work and so I can also star test the mirror when finished. I tried it on a small f4 in progress and a finished f7 mirror.

Can you explain what the different parts of the image are showing. I have have trouble lining up the centre of the parts of the return image but perhaps that is not essential as your image shows. I assume the crucial part is the overall straight test lines in the outer part of the image.



Quote:
Originally Posted by Rod View Post
Testing the mirror involved using a Foucault tester and an 8 zone mask. I had to observe pairs of zones darken at the same time and not their positions with the tester’s micrometer. This is easy at modest focal ratios but with a mirror this fast it’s difficult to see the zone pairs at the same time, particularly at the edge which is the most important part of the mirror. With practice I became better at testing this mirror. But I needed a way to verify the test results.

I had purchased a 12 inch optical flat from a friend a few years ago. It was uncoated and it was made by someone I trust. I had it aluminised and used it in the configuration shown below. Essentially it is a roughly made telescope set up in front of the flat mirror. A bright LED is mounted behind a ronchi screen in the eyepiece holder. As you can see the test showed straight lines, indicating a good parabola. Autocollimation is a double pass test so any errors shown are doubled in size, so it’s an excellent null test for a completed telescope.

The only error it does not fully test for is astigmatism. I hope to test for this with a star test once the tube is complete. If Melbourne’s restrictions ease up, a friend has offered to test the mirror with his newly completed Bath interferometer so I may also do that.
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Old 14-09-2021, 08:08 AM
Rod
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Hi Ray

Part of the problem is that the image is from my handheld phone so it doesn’t grab the whole picture. Getting it all aligned was a bit tricky. Yes straightness of lines is what you are looking for. I’ve also been advised to look for any ‘clocking’ of the image as you move from outside to inside of focus or vice verse. If the orientation of the lines change at all that probably indicates astigmatism.

The different parts of the image are the secondary which had a reflection of the ronchi grating and light source. You can also see the spider gains. I think it would be more centered if I used a beamsplitter. My impression is that the light source and grating need to be extremely close. This is the first time I’ve used this test so I’m learning too. However I’ve had very experienced people confirm the test shows a finished mirror.

Rod
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Old 14-09-2021, 01:36 PM
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Thanks Rod. Much appreciated.
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Old 17-09-2021, 05:22 PM
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First assembly

A quick update. I have added the focuser, telrad, a handle and the main mirror cell. I have ordered a secondary mirror mount from astrosystems. Once that arrives, I’ll make a wire spider and the OTA will be done. There are a few parts that need more polyurethane coats, so a bit of touching up still to do.
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Old 17-09-2021, 05:26 PM
Rod
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Magnetic wood?

I couldn’t resist having a bit of fun with this. I like to use a wixey gauge and a degree circle as setting circles. I didn’t want an ugly bit of metal on the woodwork. Can you guess how the inclinometer is being held in place?
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  #18  
Old 18-09-2021, 08:02 AM
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Tinderboxsky (Steve)
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Small flat magnet buried inside the broad wooden base that the handle is attached to?


Quote:
Originally Posted by Rod View Post
I couldn’t resist having a bit of fun with this. I like to use a wixey gauge and a degree circle as setting circles. I didn’t want an ugly bit of metal on the woodwork. Can you guess how the inclinometer is being held in place?
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Old 18-09-2021, 10:53 AM
Rod
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Hi Steve

Yes very similar to that. It’s 6 x 20mm circular rare earth magnets. I bored 3 holes (2 magnets per hole) with a Forstner bit so there was less than 1mm wood between the magnets and the surface. I saw something similar on this woodworking video:

https://youtu.be/Szm-pnU3raM

The inclinometer seems to hold quite strongly.

Rod.
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  #20  
Old 18-09-2021, 12:21 PM
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A nice and elegant solution..

The scope looks terrific. I enjoy working in wood too; it is a much under rated material.
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