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]

Originally Posted by mhodson

To work out what is the lowest power eyepiece you can use on a reflector multiply your eyes dark adapted pupil size by the focal ratio (f5 for example) this gives the EP size in mm as the lowest power for an obstructed scope.

In this case it would be 35mm eyepiece if your pupil size is 7mm. >>> 7mm x F5 =35mm

The average dark adapted pupil is 7mm but becomes smaller with age and could be as small as 5mm for example at age 40+.
My own pupil size as measured by my optomitrist is 6.5mm and I am over 40.
So this depends on the individual.


OK so what does this mean.

If the exit pupil is to large as in the 42mm in an f5 scope it will be possible to see the central obstruction as a dark blob in the center of the FOV at night.


On my f6 Newtonian I don't have a problem with this at night but during the day it is quite obvious when using the 42mm.

In a **refractor** this does not happen as there is no central obstruction.

If you have ever used a **reflector **during the day you may have seen this with even moderatly power EP's as the exit pupil could be well under 5mm in bright light.

I would stick with the 30mm in the F5 but don't expect to much with these EP's in an F5 scopes.
The outer edge will show stars which don't appear as small points of light as in the center of the field of view.

This is a result of both coma from the mirror and the simpler 5 element design of the eyepiece not working well with fast ratio scopes under f6.

If your new to scopes these abberations may not bother you to much but as you gain experience you will want better .
This will come at a high price.

Below quoted from Televue web site about exit pupil.

Myth #1: A 7-mm exit pupil gives the lowest useful magnification.
Not so! With a refractor there is no limit on the size of the useful exit pupil. Use whatever is necessary to get the field you need to frame the subject. A reflector's low-power limit is reached when the black spot in the exit pupil (caused by the secondary obstruction) becomes obtrusive.
While a 7-mm exit pupil, by matching that of the eye, does give the brightest views of deep-sky objects, it does not necessarily give the best ones. Higher magnifications, despite their smaller exit pupils, will reveal more details, maintain contrast, show fainter stars, and help bypass defects in the eye itself.



Myth #2: Exit pupils larger than 7 mm waste light and resolution.
With refractors larger pupils do waste aperture. But the magnification is so low that the wasted aperture is of little concern: both image brightness and resolution are as great as possible at that magnification. With reflectors, however, larger pupils do waste light, but primarily because the black spot in the pupil caused by the secondary obstruction becomes larger. Both light loss and field shadowing occur with reflectors, but as with refractors there is no resolution loss because of the low power.

Hope this helps.

P.S. I use my 42mm GSO SV in my f7.5 refractor as a finder eyepiece and it works exreamly well at this F ratio.

el

[ausastronomer (John Bambury)]

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.....