[astro744]

Originally Posted by OneCosmos

Thanks Peter. Yes I reckoned using prime focus would need a significant Barlow and shutter speeds would be very slow. That’s what made me think of EP projection instead. The image always seems so bright with an eyepiece that sometimes you even need a filter so I thought magnification plus brightness was the winnimg combination.

I did a lot of eyepiece ptojection in the days of film using my 6” f5.5 (840mm f.l.) Telescope and a Vixen 5mm (0.965”) ortho. I later switched to using Clave 10mm and 6mm eyepieces.

Using the Vixen 5mm ortho and the particular eyepiece projection adapter that I had, I got 10920mm effective focal length working at F72. Exposures of planets varied from 1/30s to 1 sec and were done manually by first putting a black card box (or black velvet) over the aperture, locking the shutter open then slowly taking the box off and covering again counting the exposure in my head all the time making sure the telescope was not bumped. Exposures of 1sec and 1/2 sec were easy to judge and common with film speeds between 100 to 400 ISO. 1/4s and less was more difficult to accurately state the exposure but the end result was what counted and if it meant a very brief ‘unknown length’ exposure so be it. It was all trial end error and to a few of rolls of film to get right.

Using a Barlow was not an option as e.f.l. would be nowhere near long enough. Technically using a Barlow or Powermate is not called prime focus since prime focus is primary lens/mirror focal length only.

Note at f72 the image soon becomes very dim hence the longer exposures. All captures were single shots and could really only be called good on nights of exceptional seeing. Most of the time detail captured was nothing like that seen at the eyepiece visually. (Completely opposite to what processed images can show now).

I had a fixed distance eyepiece projection adapter and changed the eyepiece if I wanted different magnification but only for the Moon I would use less than maximum sometimes. Variable distance ones allow changing of the e.f.l. without changing eyepieces.