DIY telescope

PhilPassmore · December 23, 2021

I have said in other threads that I am new to astronomy, so if the direction I take in the search for great viewing takes a somewhat contrary turn that conflicts with general wisdom, please forgive me, it's just in my nature to get an idea stuck in my head and pursue it to my conclusion.

This posting is about what is usually at the heart of all of our quests, a telescope that meets our needs. For some it is a good pair of binoculars, for others it's the biggest Maksutov that they can persuade their other halves to allow them to buy (or be single, or very good at keeping secrets).

Having done a bit of exploring using a 250mm Dob with very questionable silvering (another project), I decided that there was little in the telescope world that couldn't be enhanced by increasing the aperture. There is after all, nothing to take the place of photons. The lure of a big refractor was strong, and there seemed a toss up between 'big' with very nice glass (and probably three slices of it), or 'very big' with less exotic silicon, and only two bits. The problem for me, is that in reality, both would be stretching my budget. So, in a fashion that those who know me would understand, I decided to build one myself.

I know that cheap glass is probably frowned on, but I decided to give it a go. I bought a 154mm f 7.8 doublet from China, together with an aluminium cell to mount it in.

This cell mounts onto a sectikn of common brown underground drainage pipe of almost the perfect diameter. A snug fit of the cell was ensured by using the 'soft' side of seld adhesive velcro strips. The cell is secured with three countersunk set screws.

I could have taken the easy route and kept the tube the same diameter for its whole length, but it would have been extremely heavy and unwieldy, and finding mounting rings would have been more difficult and expensive. So originally, I stepped down a size in drainage pipe, which was ok, but still very heavy, and the thick walls of the pipe presented problems when mounting the focuser at the other end. So I made the switch to carbon fibre tube for the aft section. The two sections are linked with two very snug fitting delrin (a plastic) 'doughnuts', which with a good bearing surface area, and sufficient spacing ensure the tubes remain perfectly parallel. Again machine screws are used to secure both tubes to tbe delrin.

The focuser mount is machined from aluminium, and internally threaded to accept the focusser screwing directly into it. A little silicon grease is used on the threads to prevent galling and binding that can sometimes happen on aluminium. The mount is secured in the carbon tube with yet another set of machine screws.

Both tubes are lacking baffles (not got round to it), but they are both fully flocked with adhesive and flocking powder. It's a messy operation, but personally I think that the result is extremely good, to the extent I may not bother baffling.

As you can see in the pictures, I have secured a handle onto the mounting rings, which are an off the shelf item with about half the thickness of the soft padding removed to get the fit onto the tube just right. The handle is designed for a camera carrying rig, but works well with helping get the scope onto its mount.

On the opposite side of the rings to the dovetail rail I have fitted a bit of carbon fibre box section tube just to aid maintain straightness and parallelism. It has the added bonus of acting as a very coarse 'finder scope' by sighting through its length.

I haven't got a lit to compare it to, but I am happy with it. It was great fun to build and incredibly satisfying to use. There is of course a significant blue fringe as its 'only' an achromat, but the addition of a Baader 'Fringe Killer' has made a huge reduction without cutting out too much light.

Things to improve (aren't there always?) ; Perhaps doing the baffling as mentioned above, and coming up with a collimation system for the lens cell.

As ever, feedback, questions and constructive criticism are welcome.