16" Dob remake

[Sfarndell]

The thought of making a new cell....nooooooo. I struggle enough with wood. Metal is currently too big a leap so I'm trying to keep as much of the 'machined' parts from the 1st scope as possible. The 18 point cell works ok and my PLOP analysis shows it should be adequate for the job. The weight is also relatively low as it's aluminium.

[Sfarndell]

The truss poles are now at the correct length and I've stripped the OTA components down to the wood to start varnishing/ painting later this week.

The bearings came out nicely. They're 3x9mm pieces of plywood glued together and are slightly wider than the rocker box to accommodate tracks/teeth for the goto gears. There was a moment of panic after a little accident whereby the bearing on my pattern router broke while trimming the main edge of the OTA bearing, but fortunately it was minor and could be corrected fairly easily. The router is now my favourite tool! Wish I'd learned to use it properly for cutting straight edges while doing the mirror box.

The rocker is incomplete and will be trimmed and cutouts in the sidewalls made once the bearings are mounted and I can measure where the supports, placed 70 deg apart, are meant to go.

[swamp thing]

Looks nice Scott

What Goto system are you going for?

[Sfarndell]

Thanks Steve.

A few years ago, I picked up a good condition Meade DS 90mm scope with dual motors (see pic) and an autostar 497 controller at a garage sale for about GBP100 - too good a bargain to pass up! I took the motors & controllers off that and modified them for my ultralight, which worked well. I'm not sure how they'll cope with the additional mass of the current scope, but it should be ok if the scope is well balanced and I can reduce the friction on the bearings.

The old scope used roller ball bearings on azimuth and rollerblade wheels on the altitude and had a smooth movement. I'm keeping the roller-balls on the azimuth, but will need to cook up something else for the altitude supports.

[Sfarndell]

For the rocker box supports, I came across Mel Bartels' 20" scope which illustrates what I want to achieve. Mel used 1mm thick aluminium strips to make a border on his mirror-box bearings, preventing the metal bearing from cutting into the mirror box bearing. I intend to not use a lining on the mirror-box bearing.

For my components, the metal bearing dimensions are 20mm diameter and 9mm thickness. The rocker box side walls (as measured) are 23mm.

Using 2 metal bearings side by side doesn't give much room to work with for countersinking the axle bolts (M10) so I'm wondering if 1 would do? or would this place too much force on the mirror box-bearings and eventually wear a groove in them?

Any input welcome.

Scott

[Sfarndell]

I made some progress this weekend on the rocker box, as well as on woodworking skills and I'm happy with the outcome. The screws were replaced by dowels when the glue dried, improving the overall look . The camo-style finish on the ply is a bit Marmitey - love it or hate it - but the flash distorts the appearance a bit and it looks nice enough after varnishing the top cage that I'll use varnish for the rest of the components. For now .

The cutouts for the metal bearings at the top of the rocker box went ok too, but the thinness of the sides combined with the difficulty in cutting on the curvature of the support made the decision to use two metal bearings per side - i.e. a single bearing in width rather than 2.

Additional neatening up will be done prior to varnishing, but this is the rocker box while the glue was drying.

[8kids]

Looks neat and tidy. Replacing the screws with dowels is a nice idea. I like the beer mug, I haven't seen one of those for a long time......Mmmmm beer.

[Sfarndell]

Things are finally coming together with 1st light expected tomorrow evening! Can't wait!

Final coat of varnish was applied last night so things should be dry enough by tomorrow. A few small to do like shortening/trimming the primary mirror collimation bolts and the bearing bolts with a grinder and assemble. Here are some pictures from last night after the varnishing. No flash used so the pics approximate reality.

[swamp thing]

Looks nice Scott.

Look forward to seeing it all together

[Moonshane]

Looks neat and tidy. Replacing the screws with dowels is a nice idea.

I agree, this had never even occured to me!

[Sfarndell]

It's DONE!!

Well, almost...

The varnish was dry by Friday so put all the bits together and tested it on Saturn - WOW! I've never seen it that bright. I used a cheshire to collimate, but didn't star-test, but even so I could see the faint inner C-ring in moments of clear seeing with a 2" 15mm eyepiece (clearly this wasn't a target in the ultralight ).

Mechanical performance was ok. Not brilliant, but ok primarliy due to stiction on the mount so tracking wasnt smooth.

Altitude

Issues were that the wood MB bearings riding on steel rollers was not perfect - small irregularities in the circular cuts, as well as too much friction against the varnished wood let to unevenness in the movement. This was corrected by fixing some 25mmx 2mm aluminium strip to the edge of the MB bearing and voila...problem eliminated - the altitude movement is melted-butter smooth and the 'scope balance is good.

A smaller issue is a misalignment in one of the roller bearings which makes the MB drift about 5mm off centre from vertical to horizontal. This is within tolerances and has no impact on visual observing but will impact on the GOTO accuracy when the motors are eventually added. I have another design for mounting the rollers using steel brackets,but for the 1st attempt I was aiming for a more natural look to the scope ...part of the trial and error fun .

Azimuth

The roller bearings were not properly aligned in the pre-cut grooves, which was a simple fix. There was some stiction issues at first until, after a few 360 deg spins, the bearings bedded down somewhat. Movement is even and smooth, with less friction than on my 10' intelliscope, so I'm happy with this. The smaller radius than the altitude bearings is compensated by a higher friction which makes the differential in 'feel' of torque required to move the scope in az and alt reasonably simillar. In future, I *might* need to add an aluminium/formica ring to the underside of the rocker to reduce friction further depending on how the motors cope with the current friction.

Other tweaks

I need to extend the baffle opposite the eyepiece by +- 10cm

Add the red-dot finder

Make a dolly/means of transporting the scope more easily around the garden.

So, while there are some tweaks to improve functionality, the scope is very useable. Here is the pic just after I added the aluminium strips, prior attempt #5000 at sweeping the garage clean. I'll post more detailed pics of the individual components soon.

[swamp thing]

Very nice Scott.

Looking forward to hearing how ya first test drive goes

[Moonshane]

superb effort! how did you coat the trusses? were they painted? any special tips on painting ali tubing? I suppose it needs an etch primer first?

[faulksy]

very nice scott you must be so pleased with the out come, and you said you had no wood working skills. well done mate

[Sfarndell]

Thanks everyone! The woodworking skills certainly developed as I went along - clearly the bearings/trunions and rocker were last !

I cheated on painting the trusses and wrapped them in the mattest insulation tape I could find. Works well and took only 2x 20m rolls. The new pva surface is also apparently a lot easier to paint - possibly a future mod??

However, for now, I'm happy to have a beer or two and focus (haha) on using it a bit.

[JRM]

Looking good Scott, I might suggest wheel barrel handles for easy moving but think that might upset the beautiful wood work you

have done.

Rick

[Sfarndell]

Thanks Rick

The broom handle lying next to the scope is part of version 1.0 of a moving mechanism using 2xpoles and some wheels to slide under the rocker & move the scope around. sourcing appropriate sized wheels is proving problematic, but the old rollerblade wheels from my last build should do the job on a hard surface. Otherwise, the scope is light enough to move around in 2 trips (disassembled), but if I need to take it anywhere e.g darksite, then something more robust will be required.

[JRM]

I see a dark site in your future then Scott, you will want to pull all the photons possible with that beauty and only a dark site

will do the trick.