Am I missing something here?

[parallaxerr]

When weighing up new scope options, inevitably there is a choice to be made over focal ratio. I have read in numerous posts over the years how faster scopes are more beneficial for deep sky observation and I've always assumed this is simply down to the brighter image presented in any given EP vs that when used in a slower scope.

However, we can of course control image brightness by adjusting the exit pupil with different focal length eyepieces. I know for example that from my home observing location, an exit pupil of 3.2mm is perfect for the majority of DSO objects (quite small some may think, but it works well for me given the sky background brightness). I discovered this using a ES16mm 68° in my ST120 and have chosen EPs for subsequent scopes based on this value, give or take a little. The 24mm 68° in the 4" F7 served up a similar image with comparable contrast at 3.4mm exit pupil, albeit with slightly less magnification.

Take the following for example though - two 6" scopes, one at F6 the other at F8.  By using different eyepieces (with same aFoV) the exit pupil, magnification and FoV can be matched as near as makes no difference.

So what exactly is the faster scope offering over and above the slower scope? 

 

 

[andrew s]

Lighter weight and  more compact. 

On the down side stresses the eyepiece more as it presents more oblique rays to it.

Regards Andrew 

[John]

I'll just give one example.

I have two 4 inch refractors - a Vixen ED102 with a focal ratio of F/6.5 and a Tak FC100-DL with a focal ratio of F/9.

With my Nagler 31mm eyepiece the Vixen will show a true field of view of 3.8 degrees. With the F/9 the true field with the same eyepiece is a degree smaller.

When I want to see the whole of the Veil Nebula in a single field of view, the F/6.5 will do that comfortably but the F/9 won't get near it.

 

 

[andrew s]

5 minutes ago, John said:

I'll just give one example.

I have two 4 inch refractors - a Vixen ED102 with a focal ratio of F/6.5 and a Tak FC100-DL with a focal ratio of F/9.

With my Nagler 31mm eyepiece the Vixen will show a true field of view of 3.8 degrees. With the F/9 the true field with the same eyepiece is a degree smaller.

When I want to see the whole of the Veil Nebula in a single field of view, the F/6.5 will do that comfortably but the F/9 won't get near it.

 

 

But, what if you used a 43mm eyepiece (assuming I did the sums right) with the F/9 to match the magnification? 

Regards Andrew 

[vlaiv]

I would say that illuminated field has something to do with it as well.

Most fast scopes are built to illuminate larger field? They can for this reason display wider true field of view than slower counterpart.

Faster scopes are therefore - "richer field" telescopes - able to put more stars into single field of view and also able to put really large objects into FOV with enough context to make them stand out better.

There is another factor - faster scopes can produce max exit pupil with reasonable eyepieces. Take F/10 scope, in order to have 7mm exit pupil - you would need 70mm eyepiece. You'll have hard time trying to get one. With F/5 scope - that is 35mm EP - easy (but not cheap if you want good correction all the way to the edge).

 

[andrew s]

16 minutes ago, vlaiv said:

I would say that illuminated field has something to do with it as well.

Most fast scopes are built to illuminate larger field? They can for this reason display wider true field of view than slower counterpart.

Faster scopes are therefore - "richer field" telescopes - able to put more stars into single field of view and also able to put really large objects into FOV with enough context to make them stand out better.

There is another factor - faster scopes can produce max exit pupil with reasonable eyepieces. Take F/10 scope, in order to have 7mm exit pupil - you would need 70mm eyepiece. You'll have hard time trying to get one. With F/5 scope - that is 35mm EP - easy (but not cheap if you want good correction all the way to the edge).

 

It's swings and roundabouts as a faster scope has greater field curvature, needs higher quality with shorter length eyepiece for high magnification. Unless you are going to extremes I don't see much in it optically.

Regards Andrew 

 

[parallaxerr]

42 minutes ago, andrew s said:

Lighter weight and  more compact. 

On the down side stresses the eyepiece more as it presents more oblique rays to it.

Regards Andrew 

That's a good practical point, though my question is more about the view in the EP I suppose.

37 minutes ago, John said:

I'll just give one example.

I have two 4 inch refractors - a Vixen ED102 with a focal ratio of F/6.5 and a Tak FC100-DL with a focal ratio of F/9.

With my Nagler 31mm eyepiece the Vixen will show a true field of view of 3.8 degrees. With the F/9 the true field with the same eyepiece is a degree smaller.

When I want to see the whole of the Veil Nebula in a single field of view, the F/6.5 will do that comfortably but the F/9 won't get near it.

 

 

I suppose this is similar to my example John and as Andrew says, by using a Nagler of a longer focal length you could achieve the same view.

28 minutes ago, andrew s said:

But, what if you used a 43mm eyepiece (assuming I did the sums right) with the F/9 to match the magnification? 

Regards Andrew 

Agreed!

15 minutes ago, vlaiv said:

I would say that illuminated field has something to do with it as well.

Most fast scopes are built to illuminate larger field? They can for this reason display wider true field of view than slower counterpart.

Faster scopes are therefore - "richer field" telescopes - able to put more stars into single field of view and also able to put really large objects into FOV with enough context to make them stand out better.

There is another factor - faster scopes can produce max exit pupil with reasonable eyepieces. Take F/10 scope, in order to have 7mm exit pupil - you would need 70mm eyepiece. You'll have hard time trying to get one. With F/5 scope - that is 35mm EP - easy (but not cheap if you want good correction all the way to the edge).

 

Another practical consideration with respect to the EP's there Vlaiv.

[vlaiv]

3 minutes ago, parallaxerr said:

 

38 minutes ago, andrew s said:

But, what if you used a 43mm eyepiece (assuming I did the sums right) with the F/9 to match the magnification? 

Regards Andrew 

Agreed!

43mm 82 degrees eyepiece will be 3" - max 2" can offer is something like 40mm 70degrees (or maybe closer to 38mm).

[parallaxerr]

It seems that my thinking was logical then. Using different FL EP's, one can match the FoV, magnification and image brightness between scopes, up to the point where practical issues such as the slower scopes requirement for very long FL EP's may come into play.

So, as far as the view at the EP is concerned, the faster scope offers the ability for larger TFoV, but all other considerations are more "practical".

 

[parallaxerr]

1 minute ago, vlaiv said:

43mm 82 degrees eyepiece will be 3" - max 2" can offer is something like 40mm 70degrees (or maybe closer to 38mm).

Haha good point, so again it all comes down to the practicalities of reaching maximum TFoV. 

So only really a deciding factor if chasing the wide fields.

[John]

44 minutes ago, andrew s said:

But, what if you used a 43mm eyepiece (assuming I did the sums right) with the F/9 to match the magnification? 

Regards Andrew 

The background sky would not be as dark with the longer focal length eyepiece. I have some light pollution here and that really makes a difference.

I do have a 40mm eyepiece with a 70 degree AFoV (max field stop for the 2 inch barrel) so I can see the above very clearly when I compare the views of the Veil Nebula with the 31mm Nagler, the 40mm SWA and indeed the 21mm Ethos which shows a little less sky but darkens the background sky even more.

Also certain exit pupils are more effective when using O-III and UHC filters.

I think I would need to move to the 3 inch format to get an AFoV that is large enough to show 3.8 degrees with the F/9.

 

 

[parallaxerr]

1 minute ago, John said:

The background sky would not be as dark with the longer focal length eyepiece. I have some light pollution here and that really makes a difference.

I do have a 40mm eyepiece with a 70 degree AFoV so I can see the above very clearly when I compare the views of the Veil Nebula with the 31mm Nagler, the 40mm SWA and indeed the 21mm Ethos which shows a little less sky but darkens the background sky even more.

Also certain exit pupils are more effective when using O-III and UHC filters.

I think I would need to move to the 3 inch format to get an AFoV that is large enough to show 3.8 degrees with the F/9.

 

 

But I guess this comparison is in the same scope, correct? - The 40mm in the Tak would produce a smaller exit pupil, thus I assume darker sky background than the 31mm Nagler in the Vixen.

[John]

1 minute ago, parallaxerr said:

But I guess this comparison is in the same scope, correct? - The 40mm in the Tak would produce a smaller exit pupil, thus I assume darker sky background than the 31mm Nagler in the Vixen.

Well it does not seem to work like that in practice

 

[parallaxerr]

7 minutes ago, John said:

Well it does not seem to work like that in practice

 

Maybe that's the bit I'm missing then John - that regardless of exit pupil, the sky background brightness is a function of the eyepiece focal length only?

[parallaxerr]

An interesting excerpt from an article I saved to disk some time ago...

"The brightness of extended objects (galaxies and nebulas) is proportional to the square of the exit pupil. Therefore, a low power 4mm exit pupil (4 squared = 16) is four times as bright on galaxies and nebulas as a medium power 2mm exit pupil (2 squared = 4). To put it another way, twice the power results in one-fourth the brightness on the faint fuzzies outside our solar system.

On the other hand, the brightness of a point of light (a star) is a function of the aperture of your scope – not the exit pupil. The bigger the aperture, the fainter the star you can see. Stars do not get dimmer as a scope’s power increases and the exit pupil gets smaller. Extended objects do, however, and the sky (the most extended object you’ll ever see through your scope) becomes progressively darker as the power goes up. The result is that faint stars are usually more visible at higher powers, as the contrast between the unchanging star brightness and the progressively darker sky background increases."

It would suggest that equal magnification and exit pupil should provide much the same image between two scopes of equal objective diameter but differing focal length as sky brightness is a function of power, which by default is a function of EP focal length AND telescope focal length...

I think this calls for a multi scope, exit pupil matched shoot out.

[vlaiv]

8 minutes ago, parallaxerr said:

An interesting excerpt from an article I saved to disk some time ago...

"The brightness of extended objects (galaxies and nebulas) is proportional to the square of the exit pupil. Therefore, a low power 4mm exit pupil (4 squared = 16) is four times as bright on galaxies and nebulas as a medium power 2mm exit pupil (2 squared = 4). To put it another way, twice the power results in one-fourth the brightness on the faint fuzzies outside our solar system.

On the other hand, the brightness of a point of light (a star) is a function of the aperture of your scope – not the exit pupil. The bigger the aperture, the fainter the star you can see. Stars do not get dimmer as a scope’s power increases and the exit pupil gets smaller. Extended objects do, however, and the sky (the most extended object you’ll ever see through your scope) becomes progressively darker as the power goes up. The result is that faint stars are usually more visible at higher powers, as the contrast between the unchanging star brightness and the progressively darker sky background increases."

It would suggest that equal magnification and exit pupil should provide much the same image between two scopes of equal objective diameter but differing focal length as sky brightness is a function of power, which by default is a function of EP focal length AND telescope focal length...

I think this calls for a multi scope, exit pupil matched shoot out.

Here we need to distinguish couple of concepts:

- light intensity

- visual magnitude

- actual perception of brightness

Light intensity is easy - it is number of photons or energy - in any case, physical - measurable quantity, a flux of sorts. Above is true for light intensity.

People noticed long time ago that doubling the intensity of light does not produce twice as bright light in the eye of observer. We sense in logarithmic scale - hence magnitude system that is based on logarithm. What we see as one step increase in brightness is in fact multiple / power of intensity.

This works well in regime where there is enough light, but when we step into threshold regime, things get even weirder - our brain starts doing some funky stuff to the image we see. We never see shot noise although on that levels of light - we should. Noise filter kicks in. We form image via "long integration" - we remember what we saw a few seconds ago and once we see it - it is easier to see it again because our brain combines signals with our expectations and memory.

No light / absence of all light is not black. When we have no light and also have no reference to compare with - we see gray rather than pitch black. This effect is called EigenGrau - see https://en.wikipedia.org/wiki/Eigengrau

In order to see sky background as gray - we need field stop and it needs to provide baseline value of being black. If there is too small difference between the two - we will see both as eigengrau - with no contrast difference (too high mag at too dark skies).

Similarly we need enough contrast between target and background sky.

There is that, but there is also real difference in scopes / glass used. Not all scopes are equal and not all eyepieces are equal.

Not all fields are equally illuminated - there might be some vignetting which is higher at the edge of the field - where field stop / our reference black should be. Some eyepieces have lower transmission, but also some eyepieces impart certain tint to the image - like slightly yellowish / warm tone or slightly bluish / cold tone.

Change of tone can change our brightness perception as not all colors are perceived equally bright (green is the brightest, red is dimmer and blue is dimmer still, but combinations can be brighter - like yellow, it is perceived brighter than green - this is for same intensity / flux).

And there you go - why two seemingly same setups provide viewer with different experience.

[jetstream]

2 hours ago, parallaxerr said:

So what exactly is the faster scope offering over and above the slower scope? 

More mag at the same eye illumination in general.

Mag matters ie Blackwell and Clark and eye illumination matters as we all know.

This is pretty much it IMHO and its hugely important again IMHO.

[pete_l]

ISTM that "fast"-ness is purely an imaging term. Since the size of the sensor is fixed. With optical use, as you say, the two  elements: objective and eyepiece interact.

When eyeballing, greater magnification gives more detail (and darker backgrounds)  but at the cost of reduced intensity. Wide field / low FR gives brighter extended objects, but at the price of smaller size.

Edited June 24, 2020 by pete_l

[parallaxerr]

5 minutes ago, vlaiv said:

And there you go - why two seemingly same setups provide viewer with different experience.

....OK...A little beyond the scope of my initial enquiry, but interesting information nonetheless.

I guess the takeaway for me is that there are many factors to consider including variations in glass quality etc.....but most importantly my rationale behind same mag/exit pupil/FoV is sound and I'm not missing or misunderstanding something fundamental.

I can now consider the practicalities of different focal length scopes of around the same aperture, without worrying that one will be more or less suited to the type of observing I do!

[vlaiv]

Just now, parallaxerr said:

.but most importantly my rationale behind same mag/exit pupil/FoV is sound and I'm not missing or misunderstanding something fundamental.

Only when comparing two scopes with same aperture - same light gathering ability.

Not the same if aperture size is different.

[vlaiv]

2 minutes ago, pete_l said:

ISTM that "fast"-ness is purely an imaging term. Since the size of the sensor is fixed. With optics use, as you say, the two optical elements: objective and eyepiece interact.

When eyeballing, greater magnification gives more detail (and darker backgrounds)  but at the cost of reduced intensity. Wide field / low FR gives brighter extended objects, but at the price of smaller size.

Might be that origins of term fast/slow/speed have something to do with imaging - but even there, they are "wrong" .

I tend to relate them to angle of the light beam at focus.

[parallaxerr]

2 minutes ago, jetstream said:

More mag at the same eye illumination in general.

Mag matters ie Blackwell and Clark and eye illumination matters as we all know.

This is pretty much it IMHO and its hugely important again IMHO.

Well that's what the FoV calculator screenshot in my original post kind of disagrees with. Same exit pupil, same mag....perhaps those two examples are too closely matched? Anyway, they are of the rough sizes I am considering so the difference appear to be negligible.

1 minute ago, pete_l said:

ISTM that "fast"-ness is purely an imaging term. Since the size of the sensor is fixed. With optics use, as you say, the two optical elements: objective and eyepiece interact.

When eyeballing, greater magnification gives more detail (and darker backgrounds)  but at the cost of reduced intensity. Wide field / low FR gives brighter extended objects, but at the price of smaller size.

Yes, fast-ness is definitely an imaging hot topic and I did wonder if I'd confused myself with that at first.

[parallaxerr]

1 minute ago, vlaiv said:

Only when comparing two scopes with same aperture - same light gathering ability.

Not the same if aperture size is different.

Agreed, which is what I am comparing as per OP

[jetstream]

2 minutes ago, parallaxerr said:

FoV calculator screenshot in my original post kind of disagrees with

FOV?

Going from 40x to 50x at the same (appropriate) exit pupil is huge regardless of the FOV IMHO.

[andrew s]

1 hour ago, John said:

The background sky would not be as dark with the longer focal length eyepiece. I have some light pollution here and that really makes a difference.

I do have a 40mm eyepiece with a 70 degree AFoV (max field stop for the 2 inch barrel) so I can see the above very clearly when I compare the views of the Veil Nebula with the 31mm Nagler, the 40mm SWA and indeed the 21mm Ethos which shows a little less sky but darkens the background sky even more.

Also certain exit pupils are more effective when using O-III and UHC filters.

I think I would need to move to the 3 inch format to get an AFoV that is large enough to show 3.8 degrees with the F/9.

 

 

Happy to accept the your filters prefer different exit pupil sizes thought I can't  think of a reason why off hand.

Also happy to accept the wider the field you want then for a given aperture fast F ration is better given 2" eyepiece constraints.

I am sure not why the longer fl eyepiece does not darken the sky background they same if the magnification is the same as we were discussing.

Regards Andrew