Longest FL EP's in Newt's

[Lee]

I was wondering what experience people had with really long focal length eyepieces in Newts... I remember Lee Andrews saying that after a certain FL you start to see the secondary mirror - I'm sure the limit was about 30mm??
Any experiences?? ta

[elusiver]

el :)

[ausastronomer]

Lee,

That post by Mark Hodson is very accurate, I suggest you read and then re-read it to take it all in.

A short version is :-

It depends on the your maximum dilated pupil size and you need to use an eyepiece that creates a marginally smaller exit pupil than your own pupils dilated size. You calculate the eyepiece exit pupil size by dividing the eyepiece focal length by the F-Ratio of the telescope, hence a 30mm eyepiece in an F5 telescope creates a 6mm exit pupil. The darker the sky the larger the exit pupil you can use and still have a dark sky background and good contrast. You can measure your own dilated pupil size if you wish. Things can vary a lot from individual to individual for many reasons, eg alcohol and tobacco consumption, blood pressure levels, diet etc. The numbers can also vary a little between individuals depending on their preference for high contrast and an ink black sky background. However, a reasonable guessamatic is as follows regarding maximum useable eyepiece exit pupil under reasonably dark skies:-

Under 25 yrs of age = 7mm to 7.5mm exit pupil
25yrs to 40 yrs = 6mm to 7.0mm
40 yrs to 50 yrs = 5.0mm to 6.5mm
over 50 yrs = <6mm

I am 46 yrs old and try to stick with eyepieces creating an exit pupil of 6mm or smaller regardless of sky conditions in my F5 scope.

CS-John B

[Lee]

Thanks guys..... :)