Does the Askar FRA300 have back focus requirements

[RugbyRene (Rene)]

Hi all,

Looking at pulling the trigger on an Askar FRA300 but am confused by what I’ve read regarding back focus.

I’ve read that because it’s a Petzval design back focus isn’t an issue and you don’t have to worry about it.

On another site it lists the back focus at 71mm, implying it is something you have to cater for.

Any one have any idea which it is?

Rene

[joshman (Josh)]

It has a set focus plane, like a camera lens. There is *always* a set distance from the last glass element where all the light rays converge, however because the last glass element is inside the tube, the focuser will get your imaging equipment out to the focal plane.


This of course changes if you use the Reducer, then you will need to fiddle with spacers to get the exact back focus from the rear shoulder of the reducer.


Hopefully the picture below shows this in an easier fashion.

https://cdn.shopify.com/s/files/1/00...g?v=1650512913

[xyren (Jimmy)]

You would still need some spacers but the precise amount does not matter. Just slap on some spacer and start focusing. If you're in focus, you're all set.

[Oldtimer (Alex)]

Short answer is don’t worry about It Rene. It only becomes an issue if you fit a focal reducer.
Cheers,
Alex

[Nikolas (Nik)]

it's a petzval design just add spacers and focus till it comes into focus, For my setup I usethe 294mc pro and the spacers that came with it backfocus isnt critical at all