Baffled by a large exit pupil

[John]

I thought I understood a little bit about exit pupils but I'm in doubt now

On another forum I read a post from a person about using a Meade 56mm 2" plossl with an 8" F/4.8 newtonian for ultimate wide angle views.

I posted a polite reply that I thought the exit pupil generated would be nearly 12mm which, amongst other things, would mean that some of the light gathered by the scope would not get into the eye, assuming a maximum dilated pupil diameter (for the average eye) of around 6-7mm. That was my understanding anyway

My post was picked up by a couple of experienced forum members who reckoned that there would be no problem at all with that scope / eyepiece combination and that the only risk was seeing the shadow of the secondary occasionally. Certainly, as far as they were concerned, no issue of wasted light.

This seems to overturn completely my understanding of the issues that very large exit pupils create. I'd appreciate some other views on this - where have I managed to "get hold of the wrong end of the stick" ?.

Thanks.

[nightfisher]

Im not experienced enough to comment, maybe there is some one on here that can try a set up like this, but from what i have read i would side with your original opinion john

[Ags]

I'm pretty sure you were right the first time. If you bring the light to a focal point then let it spread out for 56mm approaching the eye lens, at approx F5 that will form a bundle of rays over 11mm wide.

Despite the lost light, you will however be looking through the maximum aperture possible for your eyes at that magnification.

[arbacia]

John,

I own dobsonian 12" f/5 and 16" f/4,5

I own (among others) Ethos 21mm, Nagler 31mm, Takahashi LE 50mm.

I aged 47, but I still have a large dilated pupil (for my age)

I don't know about the "ultimate" experience of other observers, but about mine and about what I found in my colleagues in the field.

The "ultimate" wide angle views is determined by the maximun field stops of 2" eyepieces, not for the lesser magnification. At very low power, there is a loss of contrast and no more field of view is added. The three eyepieces has almost the same field stop (at least Nagler 31 and Takahashi 50) , therefore the same field of view

I found that a lot of friends and colleagues -some of them "very experimented" ones- don't recognize vignetting (wich is very common in SCT at low power). I recognize that against the black background of the sky is not easy to perceive the darken areas or the dimming of stars created by vignetting, but it is easy as we defocus and the stars show clearly the cutted edge. Central obstruction is more difficult to perceive at night as it use to fill the field of view. In fact in day time is very annoying. The results is juts a dimmer FOV with a loss of resolution due to difraction. At very low power the loss of resolution is disguissed and sometimes difficult to dintinguish, but it's there.

Of course there are a loss of gathered light and, of course, the secondary shadow fill the central area (or all the FOV).

In my dobsonians, I never use the Takahashi LE 50mm. In fact, I use the Nagler 31 only as a finder. For observation I use 21mm or less (usually 8mm)

I hope my experience helps you in some ways.

Patricio

[Ags]

Anyway, John, what are you doing on other forums? Explain yourself! :-)

[Chris H]

If an ep produces an exit pupil of 9mm and your eye can only dilate to 6mm then it stands to reason that some light cannot enter the eye but the light that does enter will be brighter than that from an ep which produces a 6mm exit pupil so this compensates for the wasted light. As long as you can't see the secondary the ep will work just fine. Any loss of contrast is down to the local light pollution conditions as a low power ep won't darken the sky background, from a dark observing site the sky background is dark at any power and contrast is not an issue.

[Macavity]

I suppose I'd have to go with the "Al Nagler argument":

Tele Vue Optics: Telescope Formulas, Common Telescope Myths

Light MUST be wasted, but it is (somehow) of "no consequence"? <wibble>

On the other hand, a non-subjective "experiment" can never really be performed? I KNOW that I do have trouble with mirror shadows greater than ~2mm under daylight conditions. I sense (to some extent) that is alleviated in the dark, due to pupil dilation. But an 8"/F4 Newt is never going to replace a 3-4", f = 400-500mm refractor for really wide fields. Doh! The Newt's image is a "lot brighter" though...

It MAY be that the observer is correct re. his "more standard" F4.8 Newt.

At 18x, his/her mirror shadow is still below 3mm (by my calculations)?

A 2" 56mm "Plossl" is certainly well up to the internal field stop limit. <G>

[Mr Spock]

The 8" f4.8 with a 56mm eyepices will have the same light gathering power of a scope of approximately 120mm aperture, depending on how large your pupil will get. The background will also be lighter (unless you are in totally black skies) and you will see the secondary... I could see it with my 10" f4.8 and 42mm eyepiece; that and the exit pupil is why I didn't use that eyepiece with the 10", my 22mm being much more effective.

[swamp thing]

I have tried some very long (40mm) focal length eyepieces in my Dobs (which are fast f/4.5) hoping to notice the secondary shadow on bright objects.

Must admit, I am yet to see it

Regards Steve

[cs1cjc]

I think too much is made of "wasted" light. I use a 38mm eyepiece in my newts as a finder. Particularly since the eyepiece is cheap so is soft at the edges and has a very curved focal surface, I am not getting the most detail nor the most contrast and, arguably, I am wasting light. However I am getting the brightest possible image and the widest possible field of view for the 'scope, both of which are useful for locating an object.

I can still vividly remember seeing M81 ans M82 in it for the first time in it - wonderful!

[Merlin66]

I've been designing telescopes (and more recently spectroscopes) for almost 50 years.

If I were to design a system where the exit pupil far exceeded the receptor ( ie a Mk1 eyeball v's a 30mm exit pupil) I'd be laughed out of Optical School!!!

As Mr Sprock mentioned in his reply, you are effectively reducing the aperture by artificially introducing a field stop, thereby reducing the Entendue (throughput).

Sure, you can do anything with your scope and eyepieces.....but somethings are smarter than others.....

[catburglar]

Sure, you can do anything with your scope and eyepieces.....but somethings are smarter than others.....

Very nicely put.

I used a 40mm plossl in an F4.7 dob, no problems with secondary shadow, but a bright washed out view that was only good for a finder.

[nightfisher]

I thought I understood a little bit about exit pupils but I'm in doubt now

On another forum I read a post from a person about using a Meade 56mm 2" plossl with an 8" F/4.8 newtonian for ultimate wide angle views.

I posted a polite reply that I thought the exit pupil generated would be nearly 12mm which, amongst other things, would mean that some of the light gathered by the scope would not get into the eye, assuming a maximum dilated pupil diameter (for the average eye) of around 6-7mm. That was my understanding anyway

My post was picked up by a couple of experienced forum members who reckoned that there would be no problem at all with that scope / eyepiece combination and that the only risk was seeing the shadow of the secondary occasionally. Certainly, as far as they were concerned, no issue of wasted light.

This seems to overturn completely my understanding of the issues that very large exit pupils create. I'd appreciate some other views on this - where have I managed to "get hold of the wrong end of the stick" ?.

Thanks.

On the "other" forum i read and posted on this issue, just to stir them up, looks like they prefer to "tell posters" how to post!, looks to be a place i wont spend much of my time!

[John]

Thanks for all the feedback folks, and especially Jules for entering the fray on the other forum (where I usually browse but don't post much for this very reason !).

Since my post and the critical responses there seems to have been a general admission that a 12mm exit pupil is not going to be optimum and that it does have the effect of reducing the effective aperture of the scope as Mr Spock says.

It seems I committed the "sin" of using the term "wasted light" which some of the forum seniors thought was sloppy terminology

I have a 31mm Nagler which generates an exit pupil of 6.45mm when used with my 10" F/4.8 newtonian. It's a nice combination for scanning but I find the 20mm Nagler (E.Pupil 4.16mm) is the one I turn to for getting the most contrasty views of extended DSO's against the blackest background sky

[nightfisher]

the "said" forum is hard work, and they seem to have an attitude on there, that i find offensive, hence my post count on that forum is up to.............6

[arbacia]

I just read the "other forum"... some scholastic gurus there. Time to time, I also found that not acceptable attitude.

Lower focal eyepieces give bigger apparent field of view and therefore more inmersive experience with better illumination. Why to use a large aperture tube to stop it at the eye? Ok, I understand that if we have such a large focal EP in the case we can play with it, but nothing more.

Patricio

[catburglar]

For me, with modest light pollution, I find the optimum exit pupil for most DSO's is between 2 and 4mm. I sometimes go a bit wider with a UHC or OIII filter, and a bit smaller for globular clusters and planetary nebulae.

[Ags]

I like to get the exit pupil down to 1-2mm, mostly due to strong LP.

[arbacia]

I use to keep from about 1mm to 3mm. In dark or suburban areas, in both of them.

[Anweniel]

I wonder what the members of the 'other' forum would say about your use of the word 'contrasty'.. ... just messing ofc.

I think I would have thought the same as you to be honest doing the maths but as always the proof is in the pudding the best way to find if something fits is to try it or at least get some feedback of experience, to me it sounds like you recieved a backlash for giving your own opinion, personally find this kind of snobbery unpalettable and I'm glad I chose to join this community instead as on the whole I haven't come across this much here.

Difference of opinion is inevitable on all walks but there are ways and means of expressing this.. my twopenneth

[John]

Having reviewed the thread again this evening it's turned out to be quite an interesting one, albeit with the usual squabbling between forum seniors

I agree though, if possible it's good to find things out for yourself first hand - what works for one person does not necessarily work for another

[Moonshane]

it just occured to me how appropriate the homophonic word, 'baffled' is to the thread header! maybe you intended this and it just took a long time to sink in with me.

[John]

it just occured to me how appropriate the homophonic word, 'baffled' is to the thread header! maybe you intended this and it just took a long time to sink in with me.

I can't claim that Shane - I've only just "got it" now you have pointed it out

[Anweniel]

Need some time to 'reflect' on this point... or is it 'refract'?....

[John]

Need some time to 'reflect' on this point... or is it 'refract'?....

Ho Ho Ho