Rebuilding my 5" F:15 refractor:

*
I had wanted to rebuild my 5" F:15 refractor achromat into a telescope for some time. I made the lens probably three decades ago but it had lain dormant for probably the last half of those years. It needed a new tube, objective cell, mounting rings and a new focuser. The plywood original parts had disintegrated over time. The all brass replacement parts had proved far too heavy for me to lift above my head onto the MkIV mounting. Don't use recycled, artillery shell casings except for a permanently set up instrument!

Once the decision had been made to try again I quickly found a length of 6" galvanised steel, spiral ventilation tubing locally. This proved to be quite lightweight but very stiff though it lacked the flawless beauty of an aluminium tube. Then I made some plywood baffles with dowel stringers while I looked for a pair of modern, hinged tube rings online. I doubled up on 3/4" birch plywood to attach the push-pull cell. I also did a scale drawing to work out the spacing and diaphragm aperture diameters for the baffles. These I sawed out by hand using a fretsaw to rings marked with a screw bow compass. Only light sanding was required to tidy up the cuts.


This telescope has been a bit of an uphill struggle ever since and is taking far longer than I had anticipated thanks to postal and dealer delays. The tube size is not matched by any readily available (or affordable) tube rings. I find this completely unbelievable. Why is there such a big gap in the mass-produced "Skywatcher" type ring sizes just where one would assume a whole range of tube diameters would lie?

I quickly made a crude lash-up of 3/4" plywood just to be able judge the beauty of a long tube on the MkIV. I tried moving it around and took a couple of pictures.

Then I found a pair of 182mm hinged rings for sale on the "UK Astronomy BuySell" free ads website. Lined with thick, black, foam, self-adhesive tape these rings should match my needs nicely. The thick foam liners will absorb the spiral seam of the main tube.


The 182mm hinged tube rings have arrived. Now to find some thick, black, self-adhesive foam.

After struggling for years to insert ring screws into mountings and tighten wing nuts while supporting the OTA in mid air I will never go back to them again. Hinged rings are the only sensible way to go if dovetail wedges aren't available. These old Fullerscopes have nice long (and strong) cradles to carry the tube rings. So it's just a matter of dropping the bare OTA into the permanently attached rings and tightening the screw clamps. The tube is safely resting in the well-spaced rings while the captive clamp screws are tightened. What is more the clamping screws can be loosened slightly to allow rebalancing or rotation of the OTA.

My 90mm Vixen Custom 90 has captive, non-hinged rings fixed onto a matching dovetail. I still prefer to use hinged rings rather than fiddle with long screws through the MkIII cradle. An equatorial mounting cradle with rings already attached just needs to be pointed East/West to give a nice resting place for the OTA while the ring clamping nuts are tightened. I'm really not convinced a short dovetail and declination wedge is the equivalent of a long cradle attached directly to the declination axis for stiffness. The short dovetail on the Declination axis of a modern mounting must be under considerable local stress. This must lead to flexibility limiting the mountings usefulness with long telescope tubes. So the telescope manufacturers build ever shorter instruments despite their optical "difficulties". And so it goes round and around and around.

Meanwhile back at the 5" rebuild:

Now I needed a focuser. My original, 50 year-old, RAS threaded, push pull, brass focussing mount was bought with my paper round money when I was still an impoverished young teenager. (child slave labour was still acceptable back then) The brass focuser was bought for the first telescope I ever built. That one used a 1/2 diopter (2 meter focus) 60mm spectacle lens and some battens to make an "aerial" telescope. So it was about the finished length of my intended 5" F:15 once it has its dewshield added.

Fifty years later I really hadn't a clue what to look for in a focuser. They had already been fitted to the few OTAs I had bought so I had never been in the market for a commercial focuser. I knew I wanted 2" capability and a matching 2" dielectric star diagonal. The latter could share duties on my 6" CR150HD6 Celestron refractor. A (Japanese) Vixen 2" R&P focuser was mounted on my 6" Celestron refractor when I bought it secondhand. Though I wasn't sure if I wanted to move it over to the 5". The Vixen appears below and a big, rugged lump it is too. It's 80mmm (~3.25") focus travel has the capability to be fine tuned via tiny grub screws almost hidden in equally small holes in the main casting. The 2" dielectric star diagonal shown attached was remarkably cheap at £72 + P&P. I just hope it lives up to its promise. The fat end of the Vixen, where it fits the telescope tube tailpiece casting, is ~112mm (~4 3/8") in diameter.

Vixen 2" refractor focuser. (at full extension)

Now I had to look for a modern, commercial focuser and preferably an upgrade without any real idea what to look for. I began to search the many online astronomy equipment dealer's websites and was mazed how many 2" Chinese made Crayfords were available. The prices varied enormously but they all looked much the same with a few expensive exceptions. There were also a number of high end American eyepiece focusing mounts with mouthwatering finishes and claims but they would have run to many hundreds of £s by the time VAT and freight and customs had all been added to the inevitable range of upgrade options.

So I read the glowing reviews of the more affordable options on the popular amateur astronomy forums and was completely taken in. How could something so simple as a Crayford focuser go wrong? A tube, an axle and four bearings? I placed my order for one of the least expensive, 2" Crayford refractor focusers on the online market. Eventually, this is what turned up:

Inexpensive Crayford focuser sans skirt.

The finish was very impressive indeed for the price but the large, fine, ~3" thread on the bottom had me completely stumped. I had been told by the dealer that this large thread fitted a number of commercial refractors and popular SCT rear ends. It was years since I last saw an SCT and almost as long since I looked at an astronomy magazine. The actual focus travel of 56mm wasn't even close to the claimed 68mm either so I wasn't very happy. The GSO refractor focuser had a claimed 98mm of travel which is rather more useful for refractor use. Particularly when Barlow lenses and star diagonals are constantly being added or removed. This compact Crayford focuser certainly seemed smooth enough and the knobs did what they should. Though it would hardly carry loads which the 2" R&P Vixen would simply shrug off.

I spent the whole of the next two days searching online for a matching 3" female threaded adapter to fit the thread on the bottom of this focuser. How was I to know such a thing didn't exist? I emailed various companies without success and trawled endless websites. The images on the commercial websites were always too poor to recognise anything and the descriptions even worse.

Finally I found a well illustrated web page by somebody doing a lube and rebuild of a 2", slow motion GSO Crayford which looked just like mine. Except that it had a removable base to fit a telescope tube. That was the only clue I needed after two days of fruitless searching on the computer. My base was simply missing. That big, fine thread fitted nothing but the unknown manufacturer's own bases!

GSO Focuser

I almost placed an order for a suitably sized base from eBay but I had just emailed the owner of the website to thank him for his very useful article. Fortunately he was a knowledgeable enthusiast and immediately asked me for an image of my new focuser. He recognised my focuser was not a GSO but a cheaper Chinese copy. And what is more no GSO tube adaptor would ever fit it! The GSO has a plain cylindrical stub base which fits into the many different bases and adapters which match all the various commercial telescope tube sizes. I had been sold a "refractor focuser" with a rather short focus travel but without its refractor base. Luckily the dealer had a SCT cylindrical base which I will plug into a turned aluminium adapter flange/backplate of my own making to match my main telescope tube. Now I'm waiting on the post yet again for the SCT skirt. I shall use a 2" fitting drawtube to make up for the lack of focus travel.

The SCT skirt has arrived. The matching threads are shown above. Beautiful CNC work and very nicely finished. Sorry about my fingerprints!

The Crayford focuser now respectably dressed with an SCT skirt.


The 50mm (~2") SCT threaded end of the useful skirt is 82mm Ø. (OD) It offers a perfect flange to attach it to a turned backplate via small screws.

The finish is slightly shinier than the focuser body, but who cares? All I have to do now is turn an adapter ring to match the external diameter of the SCT skirt to the inside ø of my spiral steel main tube. Luckily I found a 24" length of solid 7" diameter aluminium bar in a scrap metal bin a couple of years ago. Equally fortunately I can get this huge bar neatly sawn into useful slices on a band saw by a local engineering firm. I'd hate to have to hacksaw a slice off the bar by hand!

A comparison of scale. 2" Crayford with 1.25" star diagonal & Vixen 2" R&P with 2" star diagonal.

This chapter will be updated regularly as the build progresses. In the meantime I have placed wanted ads online for another 2" Japanese Vixen refractor focuser. Until I find one this little Crayford focuser will probably do fine.

I have turned the adaptor ring in my lathe to a perfect fit in the main tube. (rubber mallet tight!) I was careful to take just enough material from the central stopped hole to obtain a wring tight fit on the SCT skirt. Both ring and focuser would probably stay in place indefinitely without fixing screws. Though I shall drill and thread some holes for fixing screws just to ensure they don't suddenly fall out on a very cold night! Differential expansion of the different metals involved might have this unexpected and undesirable effect.

The new tube rings temporarily fixed in place on the 24" long cradle are show above. It is amazing how the scale of the MkIV shrinks with a long OTA attached. Moving the tube around is very easy, with so much extra leverage, compared with the stumpy 6" f:8.


The black, ring-lining sponge I cut from an old camping mattress was a little too thin to get a really good grip on the telescope tube. So the tube tends to slide through them when it is pointed high in the sky! I didn't much like the look of white, double-sided tape I used either. I'm just going to have to look for some thicker, black, high quality, self-adhesive, sponge strip at the builders merchants. Closed cell foam would be best if I can get it.

Well, finding suitable self-adhesive foam strip proved impossible. So a search began for raw materials from which to cut suitable strips. Eventually I found a cheap garden kneeling mat in dense black foam at a supermarket.

Trials proved that 10mm (~1/2") strip was a suitable thickness to get a snug fit on the tube. After sanding my knife-cut strips, to ensure uniformity all round, I used a high quality, water based, contact adhesive on the foam and rings. I had already removed the original fibrous ring lining material with oil-based, house paint thinners. Which proved an easy and safe solvent. The idea was to expose clean, painted metal to ensure a good surface for the glue to adhere to. Trying to glue to the fibrous ring liner would have greatly increased the chance of the foam liners separating from the ring. The thickness of the new foam inserts easily absorbs the spiral seam of the ventilation tubing.

Now I need to turn an adjustable, push-pull cell for my 5" objective lens. I had better get a slice or two cut from the 7" diameter aluminium bar so I have something to work with. Brass is nice but would make the OTA as nose heavy as a modern Chinese refractor. I'm still not sure whether I want to add a larger diameter dewshield. A proper dewshield looks better than an extended tube of the same diameter. The difference between lens size and tube ID makes the choice optional. I had better make a decision before I start turning an inch or more off the outside of the 7" bar. In the meantime I shall use a plywood cell to speed things up. I have had to shorten the tube by another 10" (250mm) as the dewshield overhang was over 2 feet long once I had achieved focus with a star diagonal in place.

A shot of the 5" F:15 (now with 12" dewshield overhang) alongside the 6" F:8. The extra length of the 5" makes it much more unwieldy to carry around and getting it safely into storage. Conversely, the 6" seems to weigh twice as much as the 5"! You either accept or hate the spiral wound tubing. I prefer it to square sided box, plywood tubes. You make your own choice. I liked the low price compared with buying new, aluminium alloy tubing for this humble rebuild experiment.

Here are a couple of snaps taken with a digital still camera hand held to the 15mm Meade 4000 eyepiece. (no drives running on the MkIV so I couldn't lock the clutches)

The colour correction is excellent and the visual image far sharper than these images would suggest. The Moon was sailing along just above the ridge of the roof which has been heated by hot sunshine all day. The objective has been quickly aligned as has the focuser. Neither are fixed with screws yet. Both could do with more tweaking as there are obvious comatic/astigmatic images on bright stars. The spiral steel tube is slightly oval just where I want the objective cell and this is making alignment unnecessarily difficult. The Cheshire eyepiece showed multiple images no matter how I tried to push the plywood cell around at arm's length inside the tube! I didn't trust myself to try and align with the cell's push pull screws working blind. So I was nudging the cell with my fingertips against the tightness of the oval tube.

This alignment problem will be sorted out in daylight once the parts are safely fixed with screws. A shiny new aluminium tube is beginning to look very attractive right now! The popular irrigation tubing used in the USA isn't available in Denmark. So a new tube will have to be bought from a metal stockholder.

The inside of the tube, the adapter rings and baffles have not even been painted matt black yet. The rings of Saturn, despite the planet being quite low in the sky, were nicely sharp. With the rings in front of the planet a thin, hard black line. Needless to say I am very pleased with the results so far. Except for the focuser. Which fails to move no matter how much I adjust the screws. So it has to be pushed in and out by hand! You get what you pay for. Cheap crap in this case!

I did paint the baffles black but the new, aluminium, objective cell is still clamped in the lathe chuck.The drive belt keeps slipping so I can't get enough torque for a plunge cut.

I haven't done much practical astronomy this winter. It has been the coldest and longest, with the most snow, I can remember. Snow has been lying from before Christmas when we had 16".  It lay for 3 months in all! Permanently overcast day and night seems to be the norm. It has hardly risen above freezing for over two months! I can't even reach my telescope stand and couldn't move it through the deep, hard-frozen snow even if I could reach it. My workshop has plummeted to a steady -6C so there's not much going on in there either! 


Click on any image for an enlargement. Back click to return to the blog.




*