Building an 8 inch F3.2

[Rod]

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/1SPFSfqrpjP18MpOIAflINUFPQs4Uik2D/view?usp=drivesdk

[Rod]

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.

[Rod]

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/13LGrKySIBXFrOm2g3ZbfKrHvNwpC38nH/view?usp=drivesdk

Here is a short video showing the assembly:

https://drive.google.com/file/d/1WGMoYyX_6JNNOIyblcNHNK9qOAOptHpR/view?usp=drivesdk

Below is the completed tube next to an 8 inch F6.

[multiweb]

Impressive gear. Love that table. Very professional. :eyepop:

[awbroady]

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.

[Rod]

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

[multiweb]

I assume you still have a cut out to get a round aperture at the front?

[Stefan Buda]

Nice to see some challenging optics being made locally.

Can you tell us what size figuring lap/s you used?

[Rod]

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

[Stefan Buda]

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.:bowdown:

[Rod]

Hi Marc

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

Rod

[Rod]

Thanks Stefan

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

Rod.

[Sunfish]

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.

[Rod]

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

[Sunfish]

Thanks Rod. Much appreciated.

[Rod]

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.

[Rod]

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?

[Tinderboxsky]

Small flat magnet buried inside the broad wooden base that the handle is attached to?

[Rod]

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.

[Tinderboxsky]

A nice and elegant solution..

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

[WattleHill]

Rod, fantastic work mate - from the mirror to the beautiful craftsmanship of the tubes Well done!

Reminds me of the only 6" mirror I made back in the 1980s - plaster tool with tiles stuck on it with pitch! - A real labour of love so I appreciate the effort that goes into mirror making.

[Rod]

Thanks for the kind comments Stephen.

[ausastronomer]

Hi Rod,

Nice looking scope and grinding machine. The mirror also looks like its coming along nicely.

I just wanted to point a couple of things out re your material choices that you may or may not be aware of.

On the basis that a telescope tube is used outdoors only and can get quite wet on the exterior due to dewing, the bunnings red oak plywood, isn't a good choice. This is an interior only grade plywood and it does not use exterior grade, water resistant glue to bond the laminations together. It also uses timber species on the inner plies that are less water resistant. That's why it's "cheap". The Baltic Birch plywood uses "A-Bond" glue which is highly water resistant and it also uses Baltic Birch on all plies, internal and the faces, which is a more water resistant timber species. Baltic Birch is one of the best plywoods for telescope construction because of these properties and is the reason its used by professional telescope builders like Peter Read at SDM and Dave Kreige at Obsession. You could use a few coats of marine varnish, or polyurethane on your tube inside and out and this may help its water resistance and longevity, but one ding that damages the integrity of the varnish and the water will get in and eventually the ply laminations will bow and separate.

The other thing is that while Titebond 2 is an excellent glue and specified for external use, it is classed as "Water Resistant". Titebond 3 is classed as "Waterproof". I have both Titebond 2 and Titebond 3 on hand and use both for different purposes. They are both excellent glues, but if I was choosing one to glue a telescope tube together, I would use Titebond 3. That having been said Titebond 2 should work fine in your case. If someone is starting from scratch and planning to buy some glue for this purpose, I would be buying Titebond 3, not Titebond 2.

Cheers
John B

[Rod]

Hi John

Thanks for the feedback. I take your point re the red oak. Most of my scopes are birch ply so I’m familiar with it. I think for the smaller scopes the red oak might be ok but I get that you would need to monitor the integrity of the finish.

I have Titebond 2 and 3 too. I don’t like using Titebond 3 on light coloured woods as I’ve seen examples of it leaving a dark line at the seam - I’m probably overly obsessed with how the final product looks! As you say Titebond 2 is good enough.

Rod

[multiweb]

It is a very fast scope. Are you visual solely or will you image with it? If you do what will you use as a coma corrector? Off the shelf or make a corrector?

[Rod]

I’ll use it visually. I have one of these:

https://explorescientificusa.com/products/coma-corrector

Rod

[mura_gadi]

Nice scope... can't start to think how you would make such a fast small mirror. Great little desk top unit!

With the A-Bonded plywood I think you also get no gaps or holes when cut. The others will require filling. Flexiply looks really versatile for round tubes and has a high grade surface and A-bond glue. Also make sure it is hard wood core, not a softwood fill if your taking weight on it. A light colour for the wood is better over dark for dewing.

There are companies in Oz that will make plywood to spec, for glue finish, wood types and surface panel finish.

[Rod]

Hi Steve

Just to clarify, this scope is not made of red oak. I bought the plywood from plyco. It’s exterior grade and there are no voids. Most of the ply I use in scopes is like this:

https://plyco.com.au/pages/birch-plywood-collection

I’ve made two round plywood tubes as described here:

https://www.iceinspace.com.au/forum/showthread.php?t=174149

I like round tubes for larger scopes and octagons for 8 inch and under.

Rod.

[mura_gadi]

Sorry, was just trying to point out the hard core and abond glue feature on the plywood options as well.

Who's mirror cell design are you using on the scopes?


Thanks

[Rod]

No worries. For this scope I’m trying Gary Seronik’s design:

https://garyseronik.com/a-simple-double-plate-mirror-cell-for-your-reflector/

Rod.

[Rod]

I have done a bit more work on this scope so here is an update. I added a wire spider, installed the altitude bearings and my friend Mark 3D printed a nice secondary holder for me. I’ve also lined the tube with black velvet. Temporarily I’ve mounted it on one of my other scope mounts.

Here is the wire spider being installed with a jig and the diagonal holder.

[Rod]

Here is the OTA temporarily placed on one of my other scope mounts. My son is standing next to it for scale. Notice that the altitude bearing is located high up on the tube. The weight of the eyepiece and coma corrector on a short tube make vertically balancing the tube much more important than on a more conventional sized dob. This also means a normal dob mount won’t allow access to the zenith as you can see when the scope is fully elevated.

[Rod]

I thought I would finish off this discussion by showing the recently completed mount. Here are the parts.

[Rod]

Here is the finished scope. Again I’ve placed a magnetic strip just underneath the rim of the base ring. This allows some adjustment for the pointer above the degree circle and eliminates the need for a metal strip on the surface. The rocker is a little wider than normal to accommodate the altitude bearings sitting higher than normal to compensate for the heavy eyepiece and coma corrector.

Rod.

[Tinderboxsky]

Your finished scope looks fantastic, Rod. I like the detailing highlights.

Have you had a successful first light observing session?

[Rod]

Thanks Steve

Yes I have had it at the ASVs dark sky site once although there was only 2 hours of truly dark sky and I’ve used it at home for tests. That was all on the temporary mount shown in earlier posts. After some testing I removed the moonlite focuser shown earlier. It was a little redundant as all the focusing is done by the coma corrector. It’s been replaced with a kineoptics helical focuser I already had. I used it to get the position of the corrector right and then left it alone. All the focusing is done with the corrector’s threaded top.

The scope works well optically. Nice wide fields as expected. The only issue I’ve had is with the Explore Scientific coma corrector. The threads on the focuser section of the corrector are to me looser than they should be. This causes my heavy eyepieces to lean a little and this showed a single small spike on stars. A friend had a look and found if we adjusted the three locking screws we could minimise it’s size. I’ve added a little grease to the threads which has also helped. This issue is only noticeable when using a 20mm 100 degree ES eyepiece.

I’ve been surprised at the quality of the views at higher magnification with a 5 mm APM eyepiece. The Moon is very detailed and just fits inside the field.

I’m going to Heathcote this Saturday so will hopefully get a few hours dark skies to test everything out further.

Rod

[Rod]

I thought I would give an update on a recent change to this scope. Overall I have been very happy with it. The only nagging issue has been the spike in star images that would show up if the locking knob on the focuser was too tight. I also found that the knob that adjusted the helical focuser friction was pushing the corrector slightly off centre.

I think I have a good solution now. Since the focuser just holds the corrector in place and the coma corrector does the focusing, I drew up the contraption below in Tinkercad. The three adjustment knobs keep the coma corrector centred and the star images are now nice and round and much easier to keep that way.

Rod.