Is there a useful size limit in f5 scopes?

[Smithfire]

Please forgive the naivety of the question....but i get the feeling that 35mm is top end of what is credible in wa eyepiece for use with a 12in f5 dob.

I may bd way off thd mark if course!

Mant thanks

Clive

[John]

There is no hard and fast rule but as eyepiece focal lengths get longer and are used in scopes with "faster" focal ratios, the exit pupil that is developed (thats the bundle of light that exits the eyepiece and enters your eye) gets rather large. Once it is larger than the maximum dilated pupil diameter of your eye some of the light gathered by your scope is just not contributing to what your are seeing.

In my case, I'm 55 years old so my pupil probably does not exceed 6mm in maximum dilation, probably a bit less. So I try and limit the size of the exit pupil to 6mm at max. With an F/5 scope this means that a 30mm eyepiece is probably about the longest that will be effective. My fastest scope is actually F/5.3 so I limit my longest focal length eyepiece to 31mm for a 5.8mm exit pupil.

To find what size exit pupil a particular eyepiece will generate with a particular scope, divide the focal length of the eyepiece by the focal ratio of the scope. So a 35mm eyepiece with an F/5 scope generates an exit pupil of 7mm which might be OK if you are young but a little on the large side if you are my age.

So you are correct in thinking that there is an upper limit when you are considering making the most of the scopes aperture.

Another factor is that low magnifications (which are the other thing that long focal lengths give of course) show more background skyglow if there is light pollution present. Increasing the magnfication a bit can darken the background sky which allows faint deep sky objects to stand out just a little better.

Hope that makes a little sense !

PS: I ought to add that there are no hard and fast rules about the above, it's just guidance really.

[Knighty2112]

To be honest if you want some wider views than a 32mm eyepiece with a scope you may as just well use a good pair of 15 X 70 binoculars. I see M31 much easier and quicker through my binoculars than I ever do thru my scope. I've just got a new 120mm f5 refractor and waiting to see what the view is thru it with my 32mm Celestron Omni 1.25" EP when we get a clear night, but I still use my binoculars for some nebula, galaxies, globular clusters etc, as the wider view thru the binoculars make the objects much easier to locate sometime if the skies are good enough.

[Moonshane]

I agree with John. If you have no or very little light pollution you'd get away with it at 35mm but a 7mm exit pupil would leave skies very washed out if you have (like most of us) to battle LP.

I have f4 scopes (f4.5 with paracorr) and I find that even with a 26mm eyepiece, the skies are OK but could do with being a little darker. At darker sites though it's an awesome sight. As with all things it's a compromise. Personally, I'd not go above a 6mm exit pupil unless I cannot help it.

[Moonshane]

Good point above. I actually bought a short refractor for wide field views when I want them. Cheaper than a good quality 35mm eyepiece.

[Smithfire]

John. That is possibly the most useful answer i have ever received. I now "get it"

Thanks very much.

Just got to sort a way of measuring my dilated pupil!

Clive

[Hobbes]

Have a look at this page http://jimshomeplanet.com/EP/ep.html 

Near the bottom of the page is a really useful way of measuring your dilated exit pupil at a dark site

A fellow local astronomer was telling me about this method at our society meeting last night so I looked it up today and will be have a bash myself.

[Smithfire]

Got a 404 error on that page hobbes?

Clive

[John]

Once you have a handle on exit pupil in relation to fast scopes, you start to realise why ultra wide angle but highly corrected eyepieces are popular with the owners of such scopes. You want to keep the exit pupil an effective size while seeing as much sky as possible and also having the view sharp across that view. It's a tough trick to pull off for the optical designer which is why there is quite a price attached to the units that deliver in all 3 areas.

[kev]

Fwiw. My skies here are reasonably good, i aim for a 5mm exit pupil max. Im 56. Dunno if that helps.

Sent from my iPhone so excuse the typos!

[Charic]

Please forgive the naivety of the question....but i get the feeling that 35mm is top end of what is credible in wa eyepiece for use with a 12in f5 dob.

I may bd way off thd mark if course!

Mant thanks

Clive

Clive, You could have a 21mm eyepiece with an affective field of view of  100°, that's wide angle.

But you have understood the answers from above, its not just the focal length that matters.

I'll be same age as John   ( by end of Year! ) and the rules don't always apply? because according to a recent eye-scan, I have 4mm pupils (avg) and could possibly only achieve 4.5 - 5mm  at most?

Using the basics of focal ratio multiplied by entry pupil (eye) I went on recommendations to buy the 32mm Panaview. Now working with an f/6 scope and  7mm pupil (42mm EP ) and a 6mm pupil size (24mm eyepiece ) [7mm is/was ' the ' standard if there is one, and 6mm was closer to what I thought I had?) 42+24=66/2= average 33. So a 32mm eyepiece suited and suits my present needs admirably , not having tried anything else larger.

I'm not really sad reaching Johns age, my new age will bring more financial rewards    

[pipnina]

If you go over the size your pupils can dilate to in a newt, won't it create a shadow of the secondary in the middle of the image?

I tried holding my webcam up to my 25mm EP to show some friends a view through my scope... Big black dot in the middle of the image. Exit pupil was 3.6mm... Camera lens much smaller. Perhaps less than 1mm?

[rockystar]

Got a 404 error on that page hobbes?

Clive

There's a funny character at the end, just delete it in your browser's address bar

[John]

If you go over the size your pupils can dilate to in a newt, won't it create a shadow of the secondary in the middle of the image?

I tried holding my webcam up to my 25mm EP to show some friends a view through my scope... Big black dot in the middle of the image. Exit pupil was 3.6mm... Camera lens much smaller. Perhaps less than 1mm?

Thats right. When the exit pupil gets really large you can start to see the shadow of the secondary against a light background such as the sky or the moons surface.

I'm not sure at what point that happens though in terms of max eyepiece focal length v's telescope focal ratio

[estwing]

40mm plossl shows it big time in mine john

[Dave In Vermont]

Ditto on 404. And no "funny character" to remove.

Server Error in Application "57998752-JIMSHOMEPLANET.COM"

Here's one to get you through the door:

http://jimshomeplanet.com/

Dave

[rockystar]

The character gets converted in the browser address bar to %C2%A0 if you delete these the address is fine http://jimshomeplanet.com/EP/ep.html

Very interesting read though, suggesting that too long an eyepiece reduces the effective size of the mirror on fast scopes.

[John]

The character gets converted in the browser address bar to %C2%A0 if you delete these the address is fine http://jimshomeplanet.com/EP/ep.html

Very interesting read though, suggesting that too long an eyepiece reduces the effective size of the mirror on fast scopes.

Which is the point I was making here:

http://stargazerslounge.com/topic/256353-is-there-a-useful-size-limit-in-f5-scopes/?p=2800231

[Hobbes]

Apologies for the duff url all

[rockystar]

Which is the point I was making here:

http://stargazerslounge.com/topic/256353-is-there-a-useful-size-limit-in-f5-scopes/?p=2800231

Sorry John, agreed, it sometimes just needs explaining in a couple of ways to fully understand.

I got that you lost some of the light  but didn't fully appreciate the reason.