[pvelez (Pete)]

I have a question regarding focal reducers and impact on back focus.

If I have backfocus of 170mm in my optical train and I introduce a reducer, is there a simple way to determine how much it reduce my back focus?

Here's the background. I have a LISA spectroscope that I used with my RC8. An ST10-XME is the imaging camera. The LISA has a standard 2 inch nosepiece into which I previously screwed an Astrop-Physics Telecompressor as the LISA prefers to operate at about f/5 - The RC8 is f/8 hence the reducer.

I'm looking to work out what size spacer I need to use the LISA on my new PW CDK 12.5. Without the reducer, its reasonably easy (I think). The LISA adds 41mm to the imaging train and I can then use the PW adapter charts to set the image to come to focus on the slit of the spectroscope (the spacing from slit to imaging camera is preset and needs little adjustment).

If I add the reducer, the focal plane will shift forward so I need a shorter adapter. But by how much?

I've found a few websites with formulae (and funky spreadsheets) dealing with this but they are focussed (excuse the pun) on the optimal distance between the reducer and imaging plane - I'm more interested in determining the distance from the back of the focuser to the focal plane with the reducer in place.

Any thoughts?

Pete