Non-optimal image plane distance and focal reduction

[SteveL]

Hi all

Got a quick question I need to clear up...

On objects near the NCP, I am unable to use my 6.3 Focal Reducer at its optimum length because the camera collides with the fork base (on a wedge). As such, I tend to use shorter T-Thread tubes so it does collide. This bring up the question of exactly how much reduction is it actually causing at this point.

Say on an f/10 OTA, the optimum distance between a 6.3 focal reducer and the imaging plane is 90mm, and my shorter tube gives me 60mm, would that make it 2/3rds of the reduction, making it f/7.78 (or thereabouts)... or am I wildly off the mark?

[centroid]

Steve, do you really mean that you want to image objects near to 'National Car Parks (NCP)' or did you mean the 'North Celestial Pole (NCP)'

Have a look at this, and see if its answers your question:

http://66.102.9.104/search?q=cache:akLj2tiFYh4J:timosastro.1g.fi/tools/focalreducer.html+focal+reducer+calculator&hl=en&ct=clnk&cd=1

Dave

[SteveL]

North Celestial Pole... but you knew what I meant

That links a great help (I had it already, just forgot I had it).

[centroid]

North Celestial Pole... but you knew what I meant

That links a great help (I had it already, just forgot I had it).

You're right of course, but I'm just in a facetious mood today, having just been 'dragged around' Tescos for the weekly 'shop'

Dave

[MartinB]

Great link Dave!