A question of focul length

[Davey-T]

I thought I new the answer to this but doing some planetary imaging for the first time using various combinations of bits I got a headache especially when Peter asked why I was using a 6.3 f/r and a 2X Powermate.

So using 2500mm fl at f10 20mm ep at prime focus so f10

Same thing above with 2X powermate making 20mm ep into 10mm ep is this  f20 at 5000mm f/l ?

Same as above but introducing 6.3 f/reducer, now I'm totally confused and going to take a tablet.

Dave

[Laurie61]

Barlows and focal reducers effectively increase or decrease the scopes native focal length, dependant on what you are trying to do you would pick one or the other. A  2x barlow and 6.3 focal reducer would almost cancel each other out whilst putting a huge amount of extra glass into the light path, this would create optical aberrations and reduce light through put from the scope. So although you can do it there's no benefit and a measurable reduction in optical performance. Your examples above are correct when you say " Same thing above with 2X powermate making 20mm ep into 10mm ep is this  f20 at 5000mm f/l"

Using a 2x barlow doubles the scopes focal length to 5000 mm, f20, and a 20 mm lens produces 5000 / 20 = 250 x if you then fitted the focal reducer you get 2500/ 6.3 = 1575 mm x 2 = 3150 mm fl, if used with a 20 mm lens this gives 157 x. If you used the scope at the native focal length, 2500 mm, a 20 mm lens gives 125 x .   

 

[Davey-T]

Thanks Laurie, the strange thing was that I seemed to get better images using the 2X Powermate and 6.3 f/r than with either one on their own, but as trying to video planets the seeing changes continuously it's a job to do meaningful comparisons.

Dave