8" GS Dob mirror cell upgrade - DIY

[janoskiss (Steve H)]

The standard mirror cell that comes with GS 200mm f6 Dobsonians is little more than a ring, with the mirror supported at its very edge by three cork pads (3 on the base and 3 on the sides).

http://members.optusnet.com.au/~jano...ll-mod-001.jpg
Note: All images in this post are thumbnail links to larger versions.

This kind of support is said to be insufficient for mirrors larger than 6" or 150mm in diameter, because the mirror will distort excessively under its own weight, with significant degradation in performance.

The following is a step-by-step account of my fix, thanks to my great little dodgy-digi-camera.

The main difficulty was to keep this mod a mod, and not let it become an entirely new mirror cell. With the stock mirror cell, there is little room to put anything either behind it without losing reasonable adjustment range for collimation screws, or in front of it, between the mirror and the cell, without losing side support and needing longer mirror clamp screws. The use of flat L-brackets with 9mm plywood provided a solution: little bit in front, little bit in back, and still enough room for everything (only just for side support!).

Step 1: Cut 180mm diameter circle from 9mm plywood to fit interior of mirror cell. A cheap jigsaw makes this easy. The plywood I had lying around, but originally it came from Home Timber & Hardware.

http://members.optusnet.com.au/~jano...ll-mod-002.jpg http://members.optusnet.com.au/~jano...ll-mod-003.jpg

Step 2: Lay out L-brackets and mark plywood for drilling and sawing:

http://members.optusnet.com.au/~jano...ll-mod-004.jpg http://members.optusnet.com.au/~jano...ll-mod-005.jpg

The L-brackets are from Tony's in Oakleigh, the greatest little hardware store in Melbourne. At Tony's you can scavenge and see what will work and what won't. Every screw, nut, nail, bolt, etc is sold by the kilo and things are not in little sealed plastic packages. I took my mirror cell with me and found the L-brackets and screws, nuts & washers to suit. $2.50 for the lot.

Step 3: Drill holes in plywood for L-brackets, and cut holes for ventilation.

My creation with drill+jigsaw ended up pretty rough because plywood splinters very easily. It needed a fair bit of file + sandpaper work to get rid of the worst of it. For a nice clean job, one of those thin & skinny blade bench saw setups would be needed (I forget what they are called; the thing for crafty woodwork, e.g., making model planes out of ply & balsa wood).

http://members.optusnet.com.au/~jano...ll-mod-006.jpg

Step 4: Bolt brackets onto the plywood. This picture shows the mirror side:

http://members.optusnet.com.au/~jano...ll-mod-007.jpg

Step 5: Stick cork pads to the tips of the bottoms of the brackets. The new support assembly will sit on the cork pads, which will rest on the rim of the original mirror cell:

http://members.optusnet.com.au/~jano...ll-mod-009.jpg

The sticky-backed cork pads were $1.50 for a pack of assorted sizes from Bunnings.

Step 6: Check assembly for fit, and fine tune as necessary.

For me this meant:

- filing back a tiny bit of the ends of two of the brackets, so the whole assembly could be placed in any orientation without jamming.

- placing a strip of rubber between the stock mirror cell frame and one tip of one of the L brackets. This eliminated any noticable sideways movement.

http://members.optusnet.com.au/~jano...ll-mod-010.jpg http://members.optusnet.com.au/~jano...ll-mod-011.jpg

The rubber is from a roll I got from Clark Rubber for something else some time ago (around $5/metre; ask for a free sample).

Step 7: Affix mirror support pads. These are three cork pads placed on the metal brackets.

Old school says that in a 3-point mirror cell the support points should be on a circle with a radius 71% that of the mirror. This number comes from the "equal weight" principle: weight inside support circle = weight outside. There is no reason why this should work best, but it sounds good.

Now, it has been independently shown by at least two credible people that if refocusing is taken into account, the ideal place for the three support points is at about 40% from the centre to the edge of the mirror. See Mark Holm's website for more info. I've read all the articles referenced there and some others (the main ones being Toshimi Taki's articles in S&T), and I'm convinced that r_support = 40% x r_mirror is ideal. But with my L-brackets + plywood setup, screw heads were in the way at r = 40%, so instead of removing three screws, I placed the pads at r=50%:

http://members.optusnet.com.au/~jano...ll-mod-012.jpg

Step 8: One more thing to fix: Set up mirror clamps to provide extra sideways support.

I expected that even though the mirror is raised about 3-4mm, the mirror cell would still have enough side support to stop the mirror sliding around. This proved not to be the case.

http://members.optusnet.com.au/~jano...ll-mod-014.jpg

I decided that the easiest fix is to let the mirror clamps provide some of the side support. For this, I could no longer leave them loose, so I made up spacers out of 6mm plywood:

http://members.optusnet.com.au/~jano...ll-mod-017.jpg

I ended up using the spacers with one washer per screw, so that the screws could be tightened without any pressure on the mirror.

Here is the reassembled augmented mirror cell + mirror:

http://members.optusnet.com.au/~jano...ll-mod-018.jpg

... and here it is back in its home, awaiting collimation:

http://members.optusnet.com.au/~jano...ll-mod-019.jpg

T.B.C. with subjective performance evaluation.

[ballaratdragons (Ken)]

Coping Saw! or for even finer work- a Fret Saw!

Steve, that is a great Mod report.

My only concern would be 'Damp' entering the plywood on cold dewy nights and making the wood swell, hence putting everything under tension and forcing the mirror out of collimation.

A solution to the problem, if it occurs, is to use Perspex, Acrylic or Nylon Sheet for the plywood parts.

Well done, and instructions are written clearly.

[janoskiss (Steve H)]

I share your concern Ken, and I was going to apply at least a coat of paint to the plywood but I have no room (kitchen + dining room are the workshop, no garage or shed). Perspex would be too flexible, or much more expensive for a thick enough sheet; I'd also expect harder to cut with an off the shelf el cheapo jigsaw? Acrylic Sheet?: I have no idea what that is. Please tell me, because the plywood was pretty bad to work with.

Anyway, my scope spends most of its life indoors in a heated dining room. As far as I know, dampness does not easily enter "exterior" plywood, but moisture can build up gradually. This is why I used plywood & not hardwood or god forbid pine. But those who store their scopes in the garage and consider doing the same mod I did should rethink materials and finish.

[ballaratdragons (Ken)]

Sorry for the confusion Steve. Acrylic is just the name for thick perspex. Nylon is better.

Nylon sheet comes in all thicknesses up to many inches thick and is very easy to cut and is not all that flexible. It is available at 'Industrial Plastics' suppliers.

If someone was concerned about damp in a shed or garage but dont want to attempt cutting thick Acrylic or Nylon they can take their nylon sheet and an exact drawing to an engineering workshop and they can laser cut it to perfection.

If they want to cut costs on materials they can buy a Nylon 'Food' cutting board for only a few dollars but they are generally softer, more flexible nylon.

If the ply is well sealed I don't see much problem. Marine ply would be perfect but it is VERY expensive!

[RAJAH235]

A really concise & well executed 'fix', Steve. Looks good too.
In answer to the question re; plywood. Provided you used an exterior type which uses a waterproof glue, as you said & it's sealed, I can see no moisture prob. IMO...
As Ken has said, a kitchen polypropolene? cutting board would be suitable as well.
They are easy to work. ie; drill/cut etc... + it's cheap. L.

[elusiver]

AWESOME MOD STEVE!... this ATM & DIY forums working out just great isn't it?

definately on the to do list. Is tony's hardware the one on huntingale rd near the shops there? He always gives a discount.. always.. got heaps of stuff there.

el

[janoskiss (Steve H)]

No, but that's not a bad little shop either. Tony's is in Oakleigh, next to the bakery that is next to the fruiterer on Chester St. It is a real DIY-er's store, like I said, perfect for scavenging with most things loose, not in pre-measured quantities in sealed packets, and real cheap; no rip-off prices like at the big stores that never have exactly what you need anyway. Plus the place has that great Oakleigh wog atmosphere with people coming and going all the time, hanging out, sometimes talking loudly and exchanging fixing tips.

[elusiver]

Originally Posted by janoskiss

. Plus the place has that great Oakleigh wog atmosphere

when i lived in oakliegh a few years ago there werent too many places that didn't have this!!! nice place to live


el

[Orion]

Well done Steve great mod! You could also go to a metal engineering firm and get the same thing cut out of 5mm or 6mm stainless with holes drilled and tapped for the collimating screws. Get several made up in different sizes and start a little business.

[iceman (Mike)]

Very nice Steve, if you'd like, please write up the mod in a word doc with pictures after your firstlight report, and it can go as a "how-to" on the iceinspace site.

[janoskiss (Steve H)]

The weather gods must have seen what I've been up to from the look of the persistent thick cloud cover over Melbourne. Mike, once I get a chance to see if the mod makes any difference at the EP, I'll send you something you can publish on the site.

[ving (David)]

great work steve!

[elusiver]

Originally Posted by Orion

Well done Steve great mod! You could also go to a metal engineering firm and get the same thing cut out of 5mm or 6mm stainless

was thinking the same thing.. but i was thinking more 3mm-4mm. If it was metal i'd assume it'd be rigid enough to use at that thickness. Was thinking to to excatly as steve's done.. but with using metal instead of ply could lower the thickness so I wouldn't have issues with side support.

el

[janoskiss (Steve H)]

Orion & El, what would something like that cost? Add threaded holes for collimation screws and you'd be getting a new custom made mirror cell.

[rmcpb (Rob)]

Great thread. One thought, after setting it up with cork spacers and getting it working why not just seal the ply with some heavy duty polyurethane then gluing the mirror to the new cell with aquarium silicone glue. You would then not have to worry about the side clamps ever causing stress on the mirror.

[janoskiss (Steve H)]

Rob, I have thought about ditching the clamps and using glue, but wasn't sure what sort of silicone I'd need. I had this image of the mirror falling out crashing through the spider. Long drying time, added cost, and possible future mods were other factors that made me decide against using silicone this time.

[janoskiss (Steve H)]

Finally I got to check out the results tonight. In 3 words: no noticeable improvement.

Actually there was some improvement, but that is probably only because I'm getting good at collimation. But the 'fuzziness' of focused stars essentially remained the same.

I'm also getting the hang of star testing and slowly learning what to look for. I noticed that the diffraction ring patterns look quite different inside and outside focus. Although it did not really look like any idealised star test pictures I've seen, the closest thing to what I've seen at the EP corresponds to spherical aberration. So next mod will be a 7" mask over the 8" mirror.

[Starkler (Geoff)]

Originally Posted by janoskiss

I'm also getting the hang of star testing and slowly learning what to look for. I noticed that the diffraction ring patterns look quite different inside and outside focus. Although it did not really look like any idealised star test pictures I've seen, the closest thing to what I've seen at the EP corresponds to spherical aberration. So next mod will be a 7" mask over the 8" mirror.

A mask is for masking off a turned edge, it wont do anything about spherical aberration.

[janoskiss (Steve H)]

I believe spherical aberration for a supposedly parabolic mirror is the same thing as undercorrection..? For a spherical mirror the deviation from parabolic shape is greatest at its edge, so a mask should help if that is the problem.

[Sausageman (Mike Boggan)]

If you use plywood, you could make your own varnish to seal the timber.
It is easily made from Enamel paint thinners and polystyrene.
Pour some thinners into a jar with a lid and add chunks of polystyrene until the solution gets fairly thick.
Be warned though, this is a form of Napalm, so keep the quantity small and away from heat.

Mike