Larger Exit Pupil

[cloudsweeper]

8 hours ago, John said:

Don Pensack has recently posted this on exit pupils on the Cloudynights forum which I think is an interesting contribution to this topic:

https://www.cloudynights.com/topic/592949-no-exit-pupil-limitation-for-refractors/?p=8122944

Is that Starman 1's post, John?

Very interesting indeed.  He asserts that with excessive exit pupil, light misses the eye, but because of reduced mag, the brightness / mm^2 increases, and this results in constant brightness as the exit pupil continues to go up.

The brightness / mm^2 is crucial I think, and accepting its constancy means that as the mag drops, the integrated brightness (or perceived brightness if you prefer) must still fall.

More wastage of light - reduction of effective aperture - less light flux - so total/integrated brightness would be expected to fall.

So what Stu and I believe to be so still holds, I reckon.  It's all down to the distinction between brightness per unit area and integrated brightness across the area.  I contacted Randy Culp (rocketmine), who concurs with this.

It is indeed an interesting and challenging topic!

Doug.

[Littleguy80]

Assuming I'm understanding being posed, it strikes me that this should be testable. Light is well understood and we can measure brightness so getting a definitive answer should be quite achievable. What isn't testable is how we perceive what we're seeing. The image our brain presents to us is not the image that is formed on our retina but rather a composite of what our brain expected to see and any differences it found in the actual image. How many times have you looked at your keys and not seen them because your brain was telling you that you'd lost your keys? I tell my kids all the time that your brain is a liar and not to be trusted  Our own preconceptions will always affect what we see. 

It wouldn't surprise if many of the eyepiece manufactures have test data that would answer the question from the perspective of a brightness measurements across the exit pupil. 

[YKSE]

Don's comments are very interesting, I would pay as much attention to Ernest's and Sixela's comments too, Ernest as PhD in Optics, and Sixela has much experience in observing and optical theories.

[Louis D]

But nothing I've read here addresses the issue of looking through a nebula because it basically fills the field of view and is therefore unidentifiable as such.  By reducing the magnification, even if it means not making it any brighter per unit area, the nebula's image is compressed enough to make it fill the fovea which might now be able to pick up on it, especially if a nebula filter is used to knock down the sky background glow and the scope is knocked about a bit to engage our moving target recognition centers of the brain.  At least this has been my experience when trying to discern large nebula.  Don't be afraid to waste a few photons experimenting with large exit pupils.

[Mark at Beaufort]

I have found this thread very interesting. I have a pair of Russian 7x50 quality binos which clearly has an exit pupil of over 7. I have never measured my exit pupil but my age suggests 5 or 5.5.

I am going to attach 2 H.Beta filters to these binos and see what the California Neb looks like - assuming I can see it in the first place.

[jetstream]

From an observational stand point I find that the 200mm f3.8 obstructed 37.5% (minimum, no vanes included) gives a "brighter" (wrong use of word) view than the SW120ED f7.5. With the 200mm a 21mm Ethos or Lunt 20mm is used for an exit pupil of 5.5mm and with the 120ED a 42mm LVW gives 5.6mm exit pupil.

The difference is the mag at which the objects are viewed- 36x for the 200mm and 21x for the 120ED.

The 200mm operates at about 127mm aperture vs 120mm for the frac.

When viewing large objects that need no extra mag to enhance ie the NAN- why does the 200mm give "brighter" views to the eye?

[Littleguy80]

19 minutes ago, jetstream said:

From an observational stand point I find that the 200mm f3.8 obstructed 37.5% (minimum, no vanes included) gives a "brighter" (wrong use of word) view than the SW120ED f7.5. With the 200mm a 21mm Ethos or Lunt 20mm is used for an exit pupil of 5.5mm and with the 120ED a 42mm LVW gives 5.6mm exit pupil.

The difference is the mag at which the objects are viewed- 36x for the 200mm and 21x for the 120ED.

The 200mm operates at about 127mm aperture vs 120mm for the frac.

When viewing large objects that need no extra mag to enhance ie the NAN- why does the 200mm give "brighter" views to the eye?

Disclaimer: This is nothing more than my best guess  I wonder if this is question of light density. I googled "photon density" and it appears to be a valid concept. The exit pupil is a measurement of area not density. So with the 200mm there's more light coming in leading to a greater density of photons being funneled into the same area (exit pupil) making it brighter. 

[Stu]

1 hour ago, jetstream said:

From an observational stand point I find that the 200mm f3.8 obstructed 37.5% (minimum, no vanes included) gives a "brighter" (wrong use of word) view than the SW120ED f7.5. With the 200mm a 21mm Ethos or Lunt 20mm is used for an exit pupil of 5.5mm and with the 120ED a 42mm LVW gives 5.6mm exit pup

How are you defining brightness Gerry, surface brightness or 'total brightness'? If the latter it makes sense surely as you have same SB over a larger area?

[Stu]

3 hours ago, Louis D said:

But nothing I've read here addresses the issue of looking through a nebula because it basically fills the field of view and is therefore unidentifiable as such.  By reducing the magnification, even if it means not making it any brighter per unit area, the nebula's image is compressed enough to make it fill the fovea which might now be able to pick up on it, especially if a nebula filter is used to knock down the sky background glow and the scope is knocked about a bit to engage our moving target recognition centers of the brain.  At least this has been my experience when trying to discern large nebula.  Don't be afraid to waste a few photons experimenting with large exit pupils.

That's a case of it all being about contrast I guess. You don't see a nebula filling the fov because there is no contrast. Get an edge in view, enhance the contrast between Nebula and sky with a filter and jiggle your scope and you have much more chance of seeing it.

[John]

I've looked right through large nebulae a few times by using too much magnification I reckon

 

[Alien 13]

The hard part is trying this out, even my ZS 71 with a Celestron 32 mm plossi only gets me an exit pupil of 5.4 mm with a quite uncomfortable eye relief, I would need a 60 mm eyepiece to get the exit pupil into double figures and I imagine it would be difficult to look through.

Alan

[jetstream]

38 minutes ago, Stu said:

How are you defining brightness Gerry, surface brightness or 'total brightness'? If the latter it makes sense surely as you have same SB over a larger area?

In this case it is brightness in the eyepiece-what I look at.

Object surface brightness is fixed I believe and contrast is the difference between the objects surface brightness and the sky "darkness". The TFOV/exit pupil of the 200mm/21E and 120ED/42 LVW are very close, with the nod to the 120ED. They engage very similar amounts of receptors in the eye. The apertures are extremely close as well (127mm effective aperture/120mm). If I included the effect of obstruction with the 200mm the exit pupil shrinks a bit giving the 120ED even more of a lead.

The little newt also shows fainter and more nebulosity than my refractors- and I've tested them straight through, with a mirror diag and the prism diag (worse). The 42mm LVW is an extremely good nebula eyepiece as are the others mentioned.

I have obstructed the little newt a bit more -below 120mm, no difference to my eye.

[Stu]

25 minutes ago, jetstream said:

The apertures are extremely close as well (127mm effective aperture/120mm).

How are calculating effective aperture, by objective diameter less secondary diameter? If that's the case, would it not be more accurate to consider total area less secondary area which would, I think, still leave the 200mm a long way ahead?

All interesting stuff!

[Piero]

I would also agree with Stu that the difference between the two areas is more accurate. 

Said this, I believe it is more complicated than that. Gerry and I discussed about this some time ago without reaching a conclusion! 

It may sound a bit weird to think in this way, but at that time an analogy with a solar eclipse came into my mind. The analogy works like this. The light coming in the tube is a bit like the Sun. The secondary mirror is like the Moon between the Sun and Earth. The primary mirror is like the Earth. We know that the Moon during an eclipse would project cones of shadow of different darkness. Does this happen with a Newton telescope too? I'd be tempted to think that point sources are not affected much by these cones because the light in this case either enters or it is blocked by the obstruction and the energy is all focused on a point. However, I'd expect that there is some form of shadow caused by the CO affecting the background sky and extended sources.

If this thought is correct, (1) the subtraction between the two areas (aperture area - CO area) is not sufficient. My first thought would be something like this: aperture_area - CO_area - k1*CO_umbra_area - k2*CO_penumbra_area, where k1 and k2 are scaling factors between in [0,1] regulating how much these cones of shadow cut off in aperture. Said in another way, the amount they dim aperture. (2) do these cones of shadow affect the background sky and extended sources differently? If the background sky is more affected, this would be darker, and therefore there should be a gain in contrast between extended source and background sky when some CO is present. 

Feel free to ignore this post if felt too awkward! 

[YKSE]

For those who're interested in the effect of CO, I'd recommend a good read of this thread, you should find who knows what he's talking about:

https://www.cloudynights.com/topic/590382-size-obstruction-of-the-secondary-mirror/

Also some harder read of this one:

http://www.telescope-optics.net/obstruction.htm

My understanding is: For all the usually scopes with CO less than 50%, the effect is very minor, compare to a perfect none-obstructed scope, a 33% CO give resolution a 1/4 wave (diffraction limited)  none-obstructed scope of the same size for dim low-contrast detail, but give slightly better resolution than the perfect none-obstructed scope for bright low-contrast detail. In short, an 8" is an 8", resolution is about the same, 33% CO or an apo refractor. Lens and mirror quality has more impact than CO size.

[John]

I tend to feel that you can get away with a relatively large central obstruction (eg: SCT) as long as the scope is in good collimation. With larger obstructions contrast and resolution seem to decline quite quickly if the collimation is off.

 

[YKSE]

Yes, I'd rate collimation as important as mirror coating quality, for SCT, maybe even vital.

[jetstream]

8 hours ago, Stu said:

How are calculating effective aperture, by objective diameter less secondary diameter? If that's the case, would it not be more accurate to consider total area less secondary area which would, I think, still leave the 200mm a long way ahead?

All interesting stuff!

Yes, I 've figured it out both ways and I'm not sure it matters really. All I really know is that the little newt shows things my other scopes don't in the faint nebula dept, including the VX10.