From SE8 to 10" f4.7 dob...what ep's

[Bigwings]

My f10 SE8 SCT was forgiving with its EPs. My new dob is f4.7 and I need over time to replace the stock EPs for something better.

I read somewhere that over 36mm EP's  presented problems with exit pupils to large for normal pupil sizes....

I would like a good wide field EP but I need to understand the above problem.

??Thinking  

Aero ED SWA 2"

but  35mm or 40mm that is one question.

Sorry guys no green on black advice needed...not on a pension lol.

Thanks

[cloudsweeper]

At low power, the exit pupil can be quite large.  This means the contrast with faint objects is not great.  Also, some of the available light might be wasted - not really a problem with bright objects.  But you will still be able to enjoy wide angle views of starfields.  

I'm quite happy to use 42 and 50mm EPs in my f/5 frac for that very reason.

Doug.

[Stu]

A frac can cope with larger exit pupils as there is no risk of seeing the shadow of the secondary.

A 40mm eyepiece in an f4.7 scope will give you an 8.5mm exit pupil, a 35mm will give 7.4mm.

Your maximum pupil size tends to reduce as you age, I can't remember the basic formula but it is not unlikely that you will have a maximum of 6 or even 5mm. Plenty of light will be wasted, plus large exit pupils tend to result in washed out images if you live somewhere with any degree of light pollution.

I might be tempted to stick to a 28mm 82 degree which will give you a 5.9mm exit pupil, x42 mag and 1.95 degree fov.

[cloudsweeper]

Stu's point:

A frac can cope with larger exit pupils as there is no risk of seeing the shadow of the secondary.

........is of course highly relevant to Bigwings' situation with a Dob, compared to mine, with a frac!

Thanks for pointing it out!

Doug.

 

[Stu]

3 minutes ago, cloudsweeper said:

Stu's point:

A frac can cope with larger exit pupils as there is no risk of seeing the shadow of the secondary.

........is of course highly relevant to Bigwings' situation with a Dob, compared to mine, with a frac!

Thanks for pointing it out!

Doug.

 

Thanks Doug.

The vital piece of info I don't have is when you do actually start to see the secondary shadow in a Newt. I think up to 9 or 10mm is probably ok, but perhaps someone else can comment.

What is clearer is the fact that large exit pupils do not mix well with any form of LP. Under a very dark sky they can be great, but result in a washed out view with a brighter sky background.

[Bigwings]

On 10/09/2016 at 13:25, Stu said:

A frac can cope with larger exit pupils as there is no risk of seeing the shadow of the secondary.

A 40mm eyepiece in an f4.7 scope will give you an 8.5mm exit pupil, a 35mm will give 7.4mm.

Your maximum pupil size tends to reduce as you age, I can't remember the basic formula but it is not unlikely that you will have a maximum of 6 or even 5mm. Plenty of light will be wasted, plus large exit pupils tend to result in washed out images if you live somewhere with any degree of light pollution.

I might be tempted to stick to a 28mm 82 degree which will give you a 5.9mm exit pupil, x42 mag and 1.95 degree fov.

Thanks guys, (and Steve at FLO)

The first EP is ordered, I have gone for a Panavision 26mm given that I'm very mature and probably have pupils of tiny proportions . I should manage an exit pupil of 5.5 and a x46 magnification. I do like M42 and it fits in just nice.  I guess the next on the shopping list will be a Barlow.