Second 16" f5 Dobsonian

New telescope, Mark II

Inspired by Obsession Telescopes and "The Dobsonian Telescope" by David Kriege & Richard Berry.
Using the optics, groundboard and rocker (much cut down) from Mark I.

Secondary cage

The focusser board mounting is different from Kriege & Berry's. Everything is set up so the draw tube is just outside the cage inner diameter for the closest focussing eyepiece. The spider and secondary mounting are homemade but seem to work.

The distance, h, from the focal plane to the optical axis is 327mm, so the minimum secondary minor axis diameter would be 402 x (327 / 2045) = 64mm. Mine is actually 85mm, which seems to allow a reasonably sized fully illuminated field.

The secondary is glued to the mount with 3 blobs of clear silicone (Si). The separation is maintained while the glue is curing by 4mm packing (P). I found the easiest way of incorporating offset away from the focusser was with a simple strip of card, J. The diagram is confusing because it looks as though the offset is in the wrong direction.

Truss tube

The 25mm truss tubes are all the same length. There are four left-handed and four fight-handed ones.

The top fitting is straight out of K&B and seems to be used on 12.5" Obsessions. It's completely brilliant. Very simple, imparts no strain to the secondary cage, and is very solid. I haven't bothered with making the knobs captive. I haven't dropped one yet.

The bottom fitting is also from K&B, with the clamp and stop screw fixed to the channel, and the whole thing bolted to the mirror box with just one M6 bolt. It's also completely brilliant, not least because the tubes are held in almost exactly the right position as soon as you put them. Watching videos of people assembling 'scopes with other fittings, you see the poles waving about all over the place.

I think they would still work if they were only half the length, which would be useful if I ever make a low-profile mirror box.

The whole tube is extremely rigid when assembled. Just as rigid as Mk I. I can't detect any flex at all, however hard I push and pull.

Mirror box and altitude bearings

The altitude bearings are certainly massive, partly because the balance point of the telescope turned out to be 85mm above the top of the mirror box because of my light mirror. The friction and stiction (lack of it) are perfect. The teflon pads are double the size required for 15 psi. (Why?).

It would be possible to try bigger side bearings, probably without changing the friction very much. These would clear the truss tubes, making it much easier to shroud the telescope.
The rocker would have to be cut down, of course and there might be some spring in the bearings at low altitudes. I won't know until I try them.

Mirror cell

The steel tailgate (painted with beige metal primer found in the cellar), stainless steel wiffletree and sling are all exactly as described in Kriege & Berry. The welding was done by a local engineer using a jig I supplied him. The mirror box was built round the tailgate, so that it fitted with the minimum of packing and the box is absolutely square.

Rocker and groundboard

The rocker box and groundboard come from the Mk I telescope, much cut down. The groundboard pads are twice the area required for 15 psi pressure. (Why?) The friction was reduced by the tuning washers around the pivot bolt. Difficult to set up, because the telescope has to be completely dismantled and reassembled between each adjustment.

When the telescope was brought in from the garage the pivot bolt was replaced with an A2 stainless coach bolt because the original had corroded. The slightly naff locking nut arrangement was also replaced with two half nuts.
The bearings were waxed with Turtle wax. This reduced the friction a bit, without introducing stiction, so tuning washers are not required. How long this lasts remains to be seen.

There's still a tiny bit of slop as the telescope is moved in azimuth. It's barely noticeable, but removing it would require a proper bearing.

Wheelbarrow fittings and storage

The wheelbarrow fittings are rather different from Kriege & Berry's because I had to push the telescope through a narrow doorway in London, and along a narrow path in Wales. The trolley on castors is not needed in Wales, but a wheelchair ramp is needed to get it out of the French windows. The handles now reach up to a much more convenient height.

Eyepiece height

A 16" f5 Newtonian is inevitably going to require some sort of platform for viewing near the Zenith. The one I use is 7" tall, which is eminently manageable. It might be an improvement to make a triangular one with three feet.

There is a benefit to having the tube slightly higher off the ground - I can still observe comfortably at 45° standing up. The rocker box height could be reduced by a couple of inches by reducing the profile of the ground board bearing and changing the orientation of the collimation bolts, so the lower rail, which swings closest to the bearing, has two bolts rather than one central one, and recutting the bearing arc. All this is a lot of work, and I'd still need a platform.


The mirror is quite thin, c. 30mm, and made from a blank with a pattern of ridges on the back which are not concentric. I think this means it is Schott's Suprax, and was cut out of a 500mm square block.

I'm pretty sure it was made by Oldham Optical who are no longer in business. The engraving shows its focal length is f = 80.5" = 2045" mm. B.H probably stands for Beacon Hill.

Surface deterioration, June 2020

It's now at least years old, and the coating is showing signs of decay. The two areas of most significant damage are about 2cm in diameter overall and the clear holes in the centres of each patch are about 1cm in diameter. The glass they reveal doesn't look bad, but I can't really tell. I can also see lots of small holes in the coating if I hold the mirror up to the light.

Although the holes only cover a small percentage of the area of the mirror and have a small effect on the optical performance, I'm thinking about getting it re-aluminised, because it's certainly not going to get any better.

Will the mirror need refiguring? All the advice I've received is that this is unlikely. Is there any way I can tell without sending it away?

The mirror has now (July 2020) been recoated by Galvoptics

In the future I will store the telescope in the house rather than in the garage, which is damper than I would have expected for a modern house. Hopefully the new coating should see me out!