advice about focusing an eyepiece in a finderscope

[DancingStar]

Hi everyone, can I get some advice about focusing an eyepiece in a finderscope?
I have a William Optics 50mm f4 finderscope/guidescope; it is supplied with 2 eyepiece holders: a diagonal prism and a straight-through attachment. The diagonal is for an eyepiece and the straight-through part is for a guide camera [which I don’t have]. I am able to get an 18mm Orthoscopic eyepiece to focus in the diagonal, but I want to use an illuminated reticle because I just cannot successfully judge when a star is correctly centred in the 18mm.
I have a 12mm illuminated reticle eyepiece but it is too uncomfortable to use in the finderscope because it doesn’t come to focus with the diagonal prism; it only focuses using the straight-through attachment, which means that I must kneel on the ground and squint through the reticle [I’m 60 years old], plus the FOV of the 12mm isn’t suitable for using in a finderscope.
Using the diagonal prism, the 12mm must be moved inward towards the finderscope to approach focus [the out-of-focus star gets smaller and smaller], but it stops before the star comes into focus. Someone suggested that I need an extension tube, but I thought that this is needed when the eyepiece can’t be moved far enough in the other direction – away from the scope to achieve focus.
Can anyone tell me if an extension tube to move the eyepiece away from the prism might help or just make it worse?
Thanks
Tony

[Wavytone]

Will make it worse, and a 12mm eyepiece is rather short for a finder - the field of view will be small and the magnification too high - you really want one around 20-26mm.

There are some cheap ones around e.g..
https://www.bhphotovideo.com/c/product/917605-REG/celestron_93781_illuminated_raci_fi nder_scope.html

http://www.apm-telescopes.de/en/optical-accessories/finder-pointer/apm-finderscope-50-mm-90-erect-image.html

APM have them with the right matching eyepiece and illuminated reticle, which is what I'd buy to make sure it all fits correctly.

You could also replace it with something like the little skywatcher "heritage" 130mm newtonian scopes - pull the OTA off and put that on your scope, and toss the mount.

[traveller]

Hi Tony,
If your 1.25" diagonal has a screw off top with a M42 thread, then this adapter/extension may be useful.
http://www.ebay.com/itm/New-M42-to-1-25-034-1-25-inch-Adaptor-anodized-aluminum-Telescope-Adapter-/321757913944
Basically, screw off the top (not the "nose" of the diagonal, just the tip to expose the M42 thread), thread the adapter in place and the eyepiece can be clamped in.
Bo

[DancingStar]

Hi Bo thanks for that. It has a helical focuser eyepiece holder, so I can put an eyepiece into it but my 12mm illuminated reticle one won't focus. It can't be removed.
Cheers
Tony