What Dia hole should i cut !!

[gfletcher]

Hi Guys

I have a spare end cap for my 10" Skywatcher newt.

I was thinking of cutting a hole in the end cap to use when observing planets (giving me a longer f ratio)

My first question is , what diameter do you guys suggest that i cut in the end cap and secondly, should i cut it centrally or off axis ?

If i were to cut it:

127mm ( 5" dia ) gives f 9.45

102mm ( 4" dia ) gives f 11.76

76mm ( 3" dia ) gives f 15.8

thanks

Graham

[steelfixer]

I am intrigued.

I may have read this wrong but how does reducing the apature increase focal length?

Thanks

Graham

[gfletcher]

Hi

By reducing the aperture of the mirror (effectively), this alters the f ratio as the focal length stays the same .

i.e

a 127mm aperture in the end cap makes the mirror work at 127mm dia in lieu of its full 10" (254mm dia)

Hence, the focal length of the mirror is 1200m, therefore

1200mm/127mm = f 9.45 ratio.

HTH

Graham