2″ replacement focuser for 80mm f/5 refractor

Unlike the larger aperture refractors in the Sky-Watcher range, the StarTravel 80 only has a 1¼” focuser. At my age I probably can’t benefit from an exit pupil wider than 5mm, so there’s little point in me using an eyepiece with a focal length greater than 25mm with an f/5 scope. Any 25mm eyepiece whose apparent field of view is limited by a 1¼” barrel is outside my price range, so why change the focuser?

The StarTravel 80’s focuser draw tube is quite long and includes two baffles which may cause some vignetting of wide field views. It is quite easy to calculate what size of baffles would allow an unvignetted view, but less easy to compute just how much light is lost by an undersized baffle. For example, a baffle close to the objective lens is effectively an aperture mask, reducing brightness across the entire image. A baffle close to the eyepiece acts as a field stop, creating a sharp reduction of brightness at the edge of the image, but leaving the centre undisturbed.

Having measured, as accurately as I could, the diameters and positions of each baffle and other obstruction, I created a simple spreadsheet to allow me to see which baffles cause problems, and just how large the unvignetted field is. Note that the distance of the focuser baffles from the objective (when the scope is focused at infinity) will depend on the optical path length of the diagonal. With wide field eyepieces the nosepiece of the 1¼” diagonal itself will produce some vignetting.

Assuming a path length of 65mm for the diagonal, and aiming for a 27mm fully illuminated field, the most restricting element is the end of the focuser draw tube and the front baffle in the draw tube. It is quite common for owners of “short tube 80″ refractors to cut off the end of the draw tube to reduce this, but a better solution is to fit a 2″ focuser, which has a larger draw tube and allows use of a 2” diagonal.

These pictures show a 2″ replacement focuser I bought from Owl Astronomy, but similar focusers are available from other sources such as Telescope Warehouse, ScopeStuff and Teleskop-Service, among others.

As with every Chinese made rack & pinion focuser I’ve ever encountered, this focuser arrived with some nasty, sticky “grease” on its rack and pinion gear. So before attempting to fit it to my scope, I dismantled and cleaned it as follows.

First, undo the four screws that hold the focuser gear cover plate and remove the plate. Note that this focuser doesn’t have the springy piece of metal pushing the focuser pinion against the rack that I have found in the focusers of all my Synta scopes. Instead it has some cardboard shims to stiffen up the focuser action, as shown in the pictures below.

Having removed the focuser pinion gear and shaft, the draw tube can be pulled out easily. After unscrewing the eyepiece holder from the end of the draw tube, I cleaned the rack, pinion and focuser body with some degreaser to remove all traces of sticky grease.

The pictures above show the focuser body and draw tube from the eyepiece end and from the objective end. The focuser body holds three PTFE strips that the draw tube slides against. Note that the draw tube has two baffles, which are clearly visible. The one at the objective end is about 45mm diameter, that at the eyepiece end is about 40mm. The maximum field of a 2″ eyepiece is about 46mm, so this 2″ focuser is designed to vignette some 2″ eyepieces!

Although I only intend to use the scope with 1¼” eyepieces, my spreadsheet shows that the new focuser would still vignette my widest views. The draw tube as supplied is longer than needed, so I decided to cut 50mm off the end. Before doing this I drilled a new fixing hole in the rack and through into the tube while the rack was in place, helping to get correct alignment later on.

After removing the rack from the draw tube I shortened both parts by 50mm with a mitre saw. The picture above left shows the components after cleaning up with a file, but before blackening. The other picture above shows the eyepiece end of the draw tube after reattaching the rack. One other task before reassembly was to file away some metal from the finder dovetail slot, as it was too small for any of my finders.

When reassembling the focuser I lightly greased the rack and pinion. I didn’t fit the cardboard packing pieces, but fitted the pinion shaft retaining spring from the scope’s original focuser. After tightening the tiny adjusting screws either side of the focus lock knob the focuser is very smooth running, with minimal wobble.

The final pictures show the reassembled focuser before and after fitting it to the scope tube, and fitting some nicer looking knobs.

Looking through the eyepiece aperture of the assembled telescope showed that the main baffle in the scope body was now the most obtrusive vignetting element. To reduce vignetting a little I pushed this baffle away from the objective lens by about 10mm, then blackening the exposed unpainted metal. I also noticed that the new focuser draw tube is quite shiny at low angles of incidence, so I lined it with flocking paper.

Having fitted the new focuser, when I tried the scope with a 2″ diagonal I found that I could not focus at infinity with some of my eyepieces. However, my 1¼” diagonal has a shorter optical path length and can be used (with a 1¼” to 2″ adapter) without a problem. The new focuser is not much longer than the original one. The ideal solution would be to shorten the scope body, but that’s just a bit too much surgery for my liking.