Switching prisms.

[Merlin]

If I switched the PST's pentaprism for a standard 1.25" right-angled prism, can I insert the latter into the pentaprism holder, so as to continue using the black-box's focuser? Also, how long would the eyepiece extension tube need to be to accomodate the change in the focal position?

[fwm891]

That would depend on the light paths between the two types of prism. I would guess the right angled prism will give a longer back focus than the pentaprism. How much would depend on the prism sizes. You could get a rough idea by simply sketching the side views of the two prisms and doing a quick pencil line trace and measure the lengths of the internal reflections

[Peter Drew]

The RA prism will give a longer back focus as mentioned. You probably can't use it in conjunction with the original PST focusing system as the pentaprism has optical properties different to that of a RA prism. I have replaced a couple of PST pentaprisms with external RA prisms during PST mods as these are incorporated into the larger Coronado blocking filter diagonals. There is a general opinion that a good RA prism is better than the stock PST pentaprism.

[Merlin]

Thanks. Does this mean that the extra back-focus would allow for attaching a shortish focuser to the external back face of the black box?

I have a small rack-and-pinion ( all-metal ) focuser for 24.5mm eyepieces. I'd like to continue with the original BF5 for the time being, if possible.

[Peter Drew]

Probably not enough extra back focus to suit a rack and pinion. During some preliminary experiments I found that smooth side eyepieces push pulled to a focus without problem, the eyepice locking screws are nylon so it doesn't mark the barrell.

[Merlin]

Thanks Peter. So, what sort of arrangement should I have for focusing, if I replace the pentaprism with an RA one, while still retaining the BF5?

What's the simplest set-up?

[Peter Drew]

The simplest would be to just push-pull the eyepiece and then lock it at best focus. It might be possible to get a low profile Borg helical focuser for a controlled action.

[Merlin]

Would the Borg focuser need to be on top or at the back of the black-box? You mentioned sliding the eyepiece back and forth to get focus. it occured to me that, if I used a zoom eyepiece ( which I do with the PST ), I would be able to reach critical focus by raising or lowering the whole eyepiece to the required height, then, turning the zoom ring to reach exact focus.

[Merlin]

I don't want to botch the job. I've already done a stage 1 mod on the PST in question but, as you know, a good R.A. prism will produce a brighter and cleaner image. All I wish to do now is replace the pentaprism with an R.A. prism.

If I viewed from the top of the box, as mentioned above with the zoom, would I only need to insert an extension tube ( approximately 100mm long ) in the existing eyepiece holder? I have several extension tubes of differing length that I can try.