ED-100 Focuser Swap pt 1

By Antony McEwan

If you refer back to my review of the Skywatcher ED-100 Pro 4” apochromatic refractor, you’ll see that I was generally very well pleased with the telescope, and intended to keep and use it. The colour correction and quality of the views had impressed me, especially considering the asking price. The only small concern I had was the focuser, or more precisely, the lack of any real tension on the drawtube in the focuser. This resulted in the drawtube slipping out at inopportune moments, and meant that it was difficult to aim the telescope near the zenith when using heavy eyepieces, as I could not depend on the eyepiece staying in the same place!

I was not alone with this concern, and there are now several third party focusers available that will fit the ED-100 tube with no modifications needed and no special skills required, other than the ability to hold a screwdriver without dropping it. I had a look around on the Internet and decided to go for the ‘2” Crayford Focuser for Synta Refractors’ made by Baader Planetarium. I spotted it on the web page of Telescope-Service in Germany, and it appeared to be a well-made and professional looking unit. The price was good too, working out at about 166 Euros (about £108 at the time) including postage and packaging. After some discussion with the company about its suitability, I sent the order.

Arrival

Several days later a box arrived. Inside that box was another box. Inside that box was the new Baader 2” Crayford focuser. With tentative fingers I parted the bubble wrap and plastic wrapping and looked at the nice silky black assembly in front of me.

It certainly looked to be in a completely different league from the standard Synta Crayford. Where the Synta was clunky and covered in crinkle finish white (yes, white…) paint, this was super smooth satin finish black. The focuser wheels were actually a little smaller in diameter than the Synta’s, but they turned very smoothly and were quite sturdy to the touch. The Synta focuser used two setscrews to hold diagonals or eyepieces in place in the drawtube, while the Baader focuser uses a compression ring for a much more secure grip. It also came complete with a 1.25” adapter, also with compression ring fitting. So I sat there for a while playing with this new focuser, admiring the sleek blackness of it, and turning the focus wheels just for the fun of it. But then I realised I couldn’t do this forever – I had to actually fit the thing to my ED-100!

Removal and Installation

The standard focuser on the ED-100 was held in place by three crosshead screws, spaced 120 degrees apart around the tube assembly. Baader had designed their focuser to use the very same holes for attaching their replacement unit, so in essence the whole replacement process should consist of undoing three screws, sliding the old focuser out, sliding the new focuser in, and tightening the three screws again.

Now, if you’re looking for an exciting read, with graphic descriptions of trials and tribulations overcome in heroic manner, I’m afraid this isn’t the place! The whole process was exactly as described above! Sometimes things really are as simple as they’re supposed to be…

I mounted the telescope on my Giro mount so that it wouldn’t roll around or drop off the table while I was messing about with it. The three screws came out with no problems at all. I placed them carefully to the side, as they would be needed later. The Synta focuser was quite a tight fit in the tube, but came out after a sharp tug. I then took the Baader focuser and checked the fit inside the tube. It was not as tight a fit as the Synta focuser, and left perhaps a millimetre to spare around the circumference of the part of the focuser that went inside the telescope tube. I didn’t worry about that, but simply lined it up so the three screw holes were in the correct place, and then inserted and tightened the screws. Looking back now, I should really have wrapped a piece of electrical tape around the focuser to provide a more snug fit, but I was caught up in the heat of the moment and didn’t think. With the screws tightened there was no sign of any buckling or creasing of the tube, so I was quite happy with the installation.

Instantly the telescope looked a lot better! A satin finish black high quality focuser certainly looks the part, and a lot more so than the clunky white-painted Synta model it replaced. The new focuser also came with a pair of rings to house an 8x50 size finder-scope, and there is a slot on the focuser body that accepts the small bar that these rings attach to. The screws that tighten against the finder-scope are nylon-tipped so that they do not mark the surface of the finder- another nice touch.

So the new focuser was in place, but that was not the end of the worry. When replacing a focuser you have to get it in such a position that it is aligned correctly with the objective lenses at the front of the telescope. If the focuser is misaligned the images shown at the eyepiece will suffer from distortions and it will not be possible to get a sharp satisfactory image. The best way to check that the focuser is aligned well is to take it out under the stars and do some observing, preferably with a high power star-test as well.

I waited for a clear night.

Click here to go to part 2