Relationship of Aperture to Image Brightness

[ollypenrice]

The problem for me with the rule about not being able to increase surface brightness (which I'm not disputing) is that it carries with it the risk of suggesting that you can't see fainter objects in a larger telescope.

Olly

[andrew s]

Just musing but increased magnification reduces the surface brightness in a linear fashion but the eyes response is logarithmic.  So a lower sky background will reduce more than a brighter foreground on the logarithmic scale increasing contrast.

Maybe that's why a wider field with more background helps.

Regards Andrew 

This seems wrong on second thought but on third thoughts it is reducing the logarithmic compression at lower light levels compared to higher light levels. Need to think about it more a d do the sums.

Edited October 17, 2020 by andrew s

[Peter Drew]

Here's another related thought.  It is usually advised that if the telescope's exit pupil is larger than that of your eye, then the aperture of the telescope is effectively reduced.  On that basis, when observing a bright Moon, presumably the iris in your eye stops down to its minimum aperture, surely this could be effectively stopping down the aperture of the telescope?     🙂

Edited October 17, 2020 by Peter Drew
typo

[cloudsweeper]

31 minutes ago, Peter Drew said:

Here's another related thought.  It is usually advised that if the telescope's exit pupil is larger than that of your eye, then the aperture of the telescope is effectively reduced.  On that basis, when observing a bright Moon, presumably the iris in your eye stops down to its minimum aperture, surely this could be effectively stopping down the aperture of the telescope?     🙂

Yes!  (Get the polarising filter out.)

Doug.

[jetstream]

If all else is the same will a faster f ratio increase the number of photons that hit the eye per time unit?

Btw, theres a reason I only use 100 deg EP's on large nebula- actually I only use 2 now, the 21E, 20APM lunt. A wider TFOV makes a difference at least to my eyes. There is something to Al Naglers "Majesty factor" IMHO.

[andrew s]

38 minutes ago, jetstream said:

If all else is the same will a faster f ratio increase the number of photons that hit the eye per time unit?

Btw, theres a reason I only use 100 deg EP's on large nebula- actually I only use 2 now, the 21E, 20APM lunt. A wider TFOV makes a difference at least to my eyes. There is something to Al Naglers "Majesty factor" IMHO.

Aperture determines the number of photons. If the exit pupil from the eyepiece is smaller than your iris then it all gets in. The size of the exit pupil is the aperture of the telescope divided by the magnification.

Regards  Andrew 

[cloudsweeper]

42 minutes ago, jetstream said:

If all else is the same will a faster f ratio increase the number of photons that hit the eye per time unit?

 

Might be getting out of my depth here, but I reckon the answer must be Yes, thinking purely in terms of taking a photograph with a faster setting.

Doug.

[ollypenrice]

12 hours ago, cloudsweeper said:

Might be getting out of my depth here, but I reckon the answer must be Yes, thinking purely in terms of taking a photograph with a faster setting.

Doug.

Where do the extra photons come from? When you choose a faster setting you are opening the diaphragm to create more aperture.

Olly

Edited October 18, 2020 by ollypenrice

[Waddensky]

On 16/10/2020 at 23:59, scarp15 said:

 

Conversely, some objects respond more decisively with smaller aperture, fast focal ratio and very large exit pupil, when applied in the optium dark transparent sky conditions. Such an example would be Barnard's Loop; very large; 3000 light years across, emission nebula of very low surface brightness. Employing a large 6mm + exit pupil, offset by the contrast of a H-beta filter, encompassing a wide field of view may enable an observation. Larger aperture does not really gain anything at least in my own comparison between a 14" and 3.5" scope at similar exit pupil. Equally the IFN, Integrated Flux Nebula; can also, in optimum conditions be determined through moderate aperture at large exit pupil, fast focal ratio, not certain that larger aperture at the same exit pupil will gian anything related to the particular nature of this observation.

Yes, this is also true. There are two mechanisms in the visual system at work here: one is the tendency of our eyes to detect the object easier if the objects is larger, the other is that the same contrast difference between the object and the background is more easily detected on a lighter background than on a darker background. If you magnify and make the exit pupil smaller, the object becomes larger (and thus easier to detect), but the object and background become darker (making the object harder to detect). The contrast difference between the object and the background never changes, so the best exit pupil is the exit pupil where these two factors are in optimum. It depends on object brightness, background brightness and object size.

Here's a paper from Andrew Crumey on the subject. Very interesting!

[Waddensky]

14 hours ago, jetstream said:

If all else is the same will a faster f ratio increase the number of photons that hit the eye per time unit?

No, the number of photons is determined by the apterture. I'm not an astrophotographer, but imagine two telescopes with the same focal length but with different f-ratios (say f/6 and f/4). The reason the f/4 collects more light per time unit, is because with the same focal length (and therefore the same 'field of view'), the faster scope has a larger aperture than the f/6.

The human visual system doesn't work this way, that's why focal ratio doesn't make a difference visually but aperture does. A larger scope allows visual observers to magnify more with a larger exit pupil, making it easier to detect dim objects.

[ollypenrice]

15 minutes ago, Waddensky said:

No, the number of photons is determined by the apterture. I'm not an astrophotographer, but imagine two telescopes with the same focal length but with different f-ratios (say f/6 and f/4). The reason the f/4 collects more light per time unit, is because with the same focal length (and therefore the same 'field of view'), the faster scope has a larger aperture than the f/6.

 

Yes. If the telescopes have the same focal length but one has a faster F ratio it must, by definition, have a larger objective. And that alone is why it catches more light and gives faster exposures. The reason F ratio becomes contentious and confusing in astrophotography is that it is also possible to change a telescope's F ratio by using a focal reducer. In this case the aperture has not been increased so there are no new photons. (The photons which there are, however, are concentrated onto fewer pixels, trading resolution for speed.)

Olly

Edited October 18, 2020 by ollypenrice

[jetstream]

I want to say that what has come out of this thread is how I was taught and how I believe ie making the object larger at a proper exit pupil, making objects easier to see. My 24" f4.1 is a good example.

I have read Crumeys (Acey) paper as well as these in the links:

https://clarkvision.com/visastro/omva1/

http://www.bbastrodesigns.com/visual.html

From Bartels:

"So why then is aperture the dominant factor? If exit pupil or sky background brightness is kept constant then as aperture increases so must the magnification."

[jetstream]

However, what do you guys think of this:

"what is a HET (High Etendue Telescope)?

Imagine a water pipe. Etendue is the total amount of water flowing through the pipe. How can we get more water? Well, we can enlarge the pipe allowing more water to flow through and we can turn the valve on more to increase the flow of water. Enlarging the pipe is equivalent to increasing the apparent field of view while turning on the value more is equivalent to increasing aperture. Etendue is the amount of light flowing through the field of view into the eye.

To calculate etendue, use this simple formula: etendue = aperture squared * real field squared.

For example, a 12 inch f4 with a 21mm Ethos eyepiece has an etendue close to 1700 cm^2deg^2
where as a 12 inch f3 with the same eyepiece and coma corrector has a greater etendue close to 2200 cm^2deg^2.

The increased etendue allows me to see lower contrast objects. In dark skies with my High Etendue Telescopes I can get down to ~3% contrast (ratio of object+sky to sky). I have an object contrast calculator here. And if the real field is held constant, then greater etendue allows me to see fainter stars.

Looking at the equation again, we see two factors that drive etendue: aperture and real field. Compared to a conventional RFT, a HET (High Etendue Telescope) has either increased aperture for the same field or increased field for the same aperture."

 

https://www.bbastrodesigns.com/HET.html

[andrew s]

@jetstream we came up with Etendue after a similar long discussion on here. I fully support your analysis.  It applies equally well to visual and ccd/CMOS imaging.

With visual you have to check you eye doesn't reduce the true field with the iris becoming the field stop.

Regards Andrew 

[jetstream]

3 hours ago, Waddensky said:

The human visual system doesn't work this way,

Thinking conventionally I agree, however I think, but don't know that some other factors come into play. It might be possible that under the right stimulation that the brain can "fill" in information correctly, after repeated observation.

What triggers this interests me.

[cloudsweeper]

HET/Etendue

Is an HET then essentially a fast telescope (large aperture and exit pupil) plus a WA eyepiece?

For stars, brightness depends on aperture; for extended objects (and given EP and objective focal lengths) surface brightness also depends on aperture.

What part does FOV play then?  If it is increased, the integrated light throughput will also increase.  But surely that would not make any difference for individual stars and EOs?

Drat!  Just when I thought I'd begun to understand a little bit about telescope physics!  😉

Doug.

[Littleguy80]

Etendue is interesting but I think we have to be careful when including the eyepiece. When comparing a Lunt XWA 9mm (100 degree) vs 9mm BGO (42 degree), the BGO goes deeper. The Lunt will have the higher HET value but the BGO is higher transmission/contrast. 

Edited October 19, 2020 by Littleguy80

[cloudsweeper]

2 hours ago, Littleguy80 said:

Entendue is interesting but I think we have to be careful when including the eyepiece. When comparing a Lunt XWA 9mm (100 degree) vs 9mm BGO (42 degree), the BGO goes deeper. The Lunt will have the higher HET value but the BGO is higher transmission/contrast. 

As interesting as Etendue, Neil?  (Sorry!!)  😉

Doug.

[Littleguy80]

3 minutes ago, cloudsweeper said:

As interesting as Etendue, Neil?  (Sorry!!)  😉

Doug.

Haha I’m gonna put that down to autocorrect or perhaps I hadn’t quite woken up when I wrote that 😴

[jetstream]

6 hours ago, cloudsweeper said:

But surely that would not make any difference for individual stars and EOs?

In practise the HET is used primarily on very large very faint objects, but it also works well on large not so faint ones like Meissa,etc. Note that I can see some IFN near the Double Double and also up by Vega using my SW120ED/TSA120/42mm LVW @2.9deg. My SV90mm falls short on this stuff despite the larger TFOV ie the scope has to be capable of showing these objects- they all don't.

[niallk]

Rather than exit pupil, I have always thought about it as at the same image scale, larger aperture => brighter image on extended objects.  Hence good for O-III.  Or, one can choose to magnify further before dimming below the image achievable in a lower aperture.  Please let me know if I've misunderstood!

Globs are amazing in larger aperture - each star being a point source.

[cloudsweeper]

13 hours ago, niallk said:

Rather than exit pupil, I have always thought about it as at the same image scale, larger aperture => brighter image on extended objects.  Hence good for O-III.  Or, one can choose to magnify further before dimming below the image achievable in a lower aperture.  Please let me know if I've misunderstood!

Globs are amazing in larger aperture - each star being a point source.

I believe that the surface brightness of an extended object depends on exit pupil squared.  Now a larger aperture usually (but not necessarily) means a smaller focal ratio, which in turn (for a given eyepiece focal length) produces a larger exit pupil.  So - it is generally correct to say that the surface brightness of an extended object increases with aperture.  (I hope.....)  Contrast against the sky also has to be taken into account, plus there are other issues with increasing exit pupil.

It's different for a point source, where dependence on aperture is straightforward.

Doug.

 

[jetstream]

52 minutes ago, cloudsweeper said:

Now a larger aperture usually (but not necessarily) means a smaller focal ratio,

Exit pupil is one factor that is absolutely important IMHO. I like bright objects in my 24" such as M81/M82 because I can use a large exit pupil while still having a large image scale- the results under a dark sky are very impressive. The large exit pupil also might contribute to the hocus pocus phenomena above(HET).

For reference I can see pink under the wings of M42 in the 200mm f3.8, good for a 200mm-the 15" f4.8 and the 24" f4.1.

The 24" f4.1 also shows blue...

Regarding another thread- my brain does not "fill in" blue in the other scopes after repeated observations.

One thing that puzzles me is the PCII- it allows more contrast in the 24 f4.1 on the Veil (test target) eventhough it pushes he effective f ratio to f4.7, must be Suiters wobbly stack thing.

[jetstream]

On 15/10/2020 at 18:40, vlaiv said:

Surface of aperture changed by factor of x4 and also area that light gets spread over increased x4

Learning about the effect's of telescope parameters is interesting- like what I quoted from you now. It might be possible that this increased spread of light due to magnification will engage more of the eye, and when done with larger exit pupils and could be object size dependent- the result might be a brighter image in the brain.I think this and other factors contribute to getting brighter images.

Years ago I observed and calculated object size in in EP (arc min) for different exit pupils using many objects.

What I kind of figured is that each object size needs a certain scope to provide best views ie illumination at the entrance pupil of the observer and enough aperture and speed to make it happen. Then there is the fact that many of the enlarged objects now show detail...which needs more again...

I would love to observe through a 40" f2.5, with CC.

[PeterW]

....the curse of Bartels “one eyepiece, many scopes” philosophy! Of course if you “open the other eye” your brain can now do clever stuff with the extra information- reduce noise, improve contrast... even Mel  is moving this way (albeit in his super sized, super fast way), should enable him to dredge up yet more IFN for the rest of us.

Peter