Exit Pupil using Barlowes.

[Astro Imp]

As I understand it a Barlowe increases the focal length of the telescope by the factor of the Barlowe. 
Using my 1200mm focal length scope and a 6mm eyepiece I would have an exit pupil of 1mm. If I use a 2x Barlowe this makes the focal length 2400mm, using the same 6mm eyepiece will this give an exit pupil of 2mm?
 

[NGC 1502]

 

Hi Alan. It works the other way round. If you add a barlow to an eyepiece, that will produce a smaller exit pupil.

If a 6mm eyepiece gives a 1mm exit pupil with your scope, adding a 2x barlow to that eyepiece would produce a 0.5mm exit pupil, as well as doubling the magnification.

HTH, Ed.

[Astro Imp]

Thanks Ed.

[vlaiv]

Yes exactly - you can think of exit pupil as "shrunken" aperture - by factor of magnification. Add barlow - increase magnification - you "shrink" aperture more - smaller exit pupil.

200mm F/6 with 6mm will give x200. 200mm / x200 = 1mm exit pupil. Add x2 barlow you are now at x400 and if we follow thru - 200mm / x400 = 0.5mm.

[Ricochet]

I find it easiest to remember that 

Exit Pupil =  Eyepiece Focal Length / Focal Ratio

Therefore with your 6mm eyepiece and f6 telescope you have 6mm / 6 = 1mm

If you add a 2x barlow you not only double the focal length, but the focal ratio, so now you have 6mm / 12 = 0.5mm.

[Astro Imp]

3 hours ago, Ricochet said:

Exit Pupil =  Eyepiece Focal Length / Focal Ratio

Yes I knew that and had a senior moment and used Focal Ratio/EP Focal Length. 
Thanks to all who replied and pointed out my mistake.