Focusing on Focusers.

How to adapt the Skywatcher 200P focuser to attain better collimation, as well as easier inter-change between 1.25″ and 2″ eyepieces.


Newtonians are tinkerers’ telescopes; they appeal to amateurs who want to learn about optics and above all other designs, they respond well to a bit of TLC. Such is the case with my modified Skywatcher 8″ f/6 Newtonian, which I have steadily improved over the last few years to transform it into my dream telescope; a modest instrument that can do all things well, from ultra-high power double star observing, through lunar and planetary studies and onwards into wide-field, deep sky observing. If I had to own only one telescope from all the varieties that now exist, I’d choose this one in a heartbeat.

The best way to improve a telescope is to spend as much time with it in the field. How else can one even begin to understand whether or not an alteration is warranted?  In this capacity, I have clocked up hundreds of hours under the starry heaven inter-changing two inch, long focal length eyepieces for their shorter focal length counterparts, which are invariably in the 1.25″ format. But doing this in the usual way is frankly a bit of a chore. On my telescope, one has to remove the 1.25″ focuser housing, insert a two inch tube adaptor and then plonk in the big 2″ eyepiece. As you can imagine, this can get a bit fiddly on a dark night and cause needless interruptions to one’s observing schedule. Can this interchange be speeded up? Thankfully, the answer is yes!

The standard 1.25″ eyepiece adaptor that comes with every Skywatcher 200P Dob.


The first thing I did was to remove the standard 1.25″ eyepiece adaptor that comes with the telescope and replace it with an inexpensive 35mm extension tube, such as the one featured in the first image of this blog.

Next, I inserted an old Orion precision centring adapter into the 35mm extension tube, as illustrated below:

A very useful adapter: the Orion precision Centering Adapter seen inserted onto the top of the 35mm extension tube.


The precision centring adapter features a helical tightening mechanism that precisely centres any 1.25″ eyepiece in the focuser draw tube. This has immediate benefits to attaining ultra-precise collimation, as it removes the remaining wiggle room that would normally attend the insertion of a laser collimator in the standard 1.25″ eyepiece adapter.

Using the precision centring adapter improves the accuracy of collimation using a laser collimator or Cheshire eyepiece.


Once collimated, one can insert a 1.25″ eyepiece as shown below:

The centring adapter keeps 1.25″ oculars snug and precisely centred in the focuser drawtube.


To use 2″ eyepieces all one needs do is remove the centring adaptor, and insert the eyepiece into the 35mm extension tube as shown below:

Removing the centring tool is child’s play and allows for rapid interchnage with a 2″ ocular.


Testing this new system out during the day, I can report that it allows all my eyepieces; orthoscopics, Plossls, super-wide, ultra-wide angles and even Kellners to reach sharp and precise focus at infinity, as well as targets located as close as approximately 50 yards in the distance. It greatly facilitates the inter-change between 1.25″ oculars and their 2″ counterparts in the field because it always has the 2″ eyepiece adapter in the focuser drawtube. No more fumbling about in the dark.

So, if you’ve got these adapters lying about in your ole box of tricks, I’d encourage you to give it a go. It can only serve to increase your enjoyment of a very fine telescope. You know it’s worth it!

Octavius: working more efficiently for its master.


Note added in proof (August 2 2018):  I’ve managed to get out on a few nights before the weather turned and test the new configuration on both my 8″ f/6 and 12″ f/5 instruments (both of which are endowed with 2-inch focusers) under rapidly darkening skies. I can report that the alteration was definitely worthwhile!. The centring adapter is very easy to remove or insert at will, allowing me to go from wide-field scanning at low power, using heavy, 2-inch eyepieces covering a couple of angular degrees of sky or more, and then zoom in at inceasingly high powers, sometimes much beyond 50x per inch of aperture, to look at high resolution stuff.

All in a jiffy!

One downside noted: some(but not all) Barlowed oculars can’t come to focus with this modus operandi.

No’ bad ken.

Neil English is author of Choosing and Using a Dobsonian Telescope.


De Fideli.


5 thoughts on “Focusing on Focusers.

  1. Hi Neil,have to say you have done a fantastic detailed review of this scope!…and now i have one too,its an excellent viewing experience looking through it. i seem to have the same problem as you wrote about in achieving focus with a 2inch eyepiece. I have followed your advice in the description you wrote …but i am finding it a minefield to track down an extension tube…some people have mentioned is it an M45 thread??…i can`t locate a 35mm extension tube…would a 40mm tube be ok in `our`scope?if you could advise me i would be so so grateful as i want to unlock the scope`s potential with a 2 inch eyepiece.thanks ever so much.

    • Hey Mark,

      Good to hear from you again.

      I’ve got a couple of these adapters in me ole box of tricks, so if you let me know your address, I’ll pop it in the post to you!

      No charge, gratis!

      A while back, I removed all my Newtonian blogs from public viewing but I’ve decided to take a hiatus from my writing, so they will all be available for public viewing again.

      Best wishes,


      • Hi Neil,
        Thank you so much for the reply and thank you for being so generous with your! very much appreciated indeed! Would my email be on show here??


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