How does exit pupil and magnification effect light transmission in nebular filters?

[bendiddley]

So I have read somewhere that a larger exit pupil and getting the 'right amount' of magnification from an eyepiece can maximise light transmission of nebular filters. Is this the case? Also some people have said that 40mm gets that 'sweet spot'. Anyone found this?

[Stu]

49 minutes ago, bendiddley said:

So I have read somewhere that a larger exit pupil and getting the 'right amount' of magnification from an eyepiece can maximise light transmission of nebular filters. Is this the case? Also some people have said that 40mm gets that 'sweet spot'. Anyone found this?

Exit pupil will obviously depend on your scope, so you would need to let us know that too. In general yes, larger exit pupils do help with IHC and OIII type narrowband filters, over 4 or 5mm is probably best.

@jetstream will have some good input here, although just bear in mind he lives under some of the best skies anywhere so you need to take that into consideration.

[bendiddley]

1 minute ago, Stu said:

Exit pupil will obviously depend on your scope, so you would need to let us know that too. In general yes, larger exit pupils do help with IHC and OIII type narrowband filters, over 4 or 5mm is probably best.

@jetstream will have some good input here, although just bear in mind he lives under some of the best skies anywhere so you need to take that into consideration.

Great stuff. It's not for my scope though, I'm just interested in the subject and wanted to find out more. By the way you say exit pupil is affected by the scope too, I thought it was just the eyepiece that affected this, how does the scope affect exit pupil?

[John]

28 minutes ago, bendiddley said:

Great stuff. It's not for my scope though, I'm just interested in the subject and wanted to find out more. By the way you say exit pupil is affected by the scope too, I thought it was just the eyepiece that affected this, how does the scope affect exit pupil?

Exit pupil is eyepiece focal length divided by focal ratio of scope.

 

[vlaiv]

No effect on transmission what so ever.

There is real effect on observation but not related to filter and transmission - but rather our perception of the image and ability of eye/brain system to detect / recognize image.

Larger exit pupil (or lower magnification for same aperture size) concentrates more of the sky onto our photo sensing cells in our eyes. This means more photons hit our "eye pixels". Target is easier to detect.

Problem with above is light pollution as it also has photons and lower magnification also concentrates those photons so background sky also becomes brighter.

Relative change is roughly the same (not quite as our vision is not linear - that is why there is "sweet spot" exit pupil for given conditions).

When you add filter - you remove much of the light pollution. More LP you remove, or rather darker the background - more benefit you will see from lower magnification or "concentration of the light".

[bendiddley]

45 minutes ago, vlaiv said:

No effect on transmission what so ever.

There is real effect on observation but not related to filter and transmission - but rather our perception of the image and ability of eye/brain system to detect / recognize image.

Larger exit pupil (or lower magnification for same aperture size) concentrates more of the sky onto our photo sensing cells in our eyes. This means more photons hit our "eye pixels". Target is easier to detect.

Problem with above is light pollution as it also has photons and lower magnification also concentrates those photons so background sky also becomes brighter.

Relative change is roughly the same (not quite as our vision is not linear - that is why there is "sweet spot" exit pupil for given conditions).

When you add filter - you remove much of the light pollution. More LP you remove, or rather darker the background - more benefit you will see from lower magnification or "concentration of the light".

thanks, so is it due to the lower magnification of the eyepiece that the nebular benefits from it? Sorry maybe my terminology light transmission is wrong. Guess I meant can you get more out a nebular filter by having a lower magnification eyepiece? Stu says that larger exit pupils help with nebular filters.

[vlaiv]

5 minutes ago, bendiddley said:

thanks, so is it due to the lower magnification of the eyepiece that the nebular benefits from it? Sorry maybe my terminology light transmission is wrong. Guess I meant can you get more out a nebular filter by having a lower magnification eyepiece? Stu says that larger exit pupils help with nebular filters.

Exit pupil size is determined by used magnification - in this case they are the same thing as exit pupil = aperture / magnification

Less magnification you use - larger exit pupil.

Yes, it is magnification that helps here - or lack of it. There is limited number of cells in the back of our eyes and limited number of photons coming from nebula

If you spread light from nebula over more light sensing cells - each cell will get to sense smaller number of photons. Image will be fainter. You can easily see this effect when observing the moon for example - pump up the magnification and image will become fainter - as light is spread over more cells and each cell gets less photons.

Using lower magnification does opposite - makes image brighter. Problem is that it does the same with background light pollution - again you can see this - use low mag eyepiece and sky will look bright - use high mag EP and it will turn black (or at least darker - depending on how much LP you have).

Filter removes much of the LP and once light falls below certain threshold - we no longer see it. This is the key - make nebula brighter by using lower magnification and at the same time - make sky dark by use of filter.

[jetstream]

19 minutes ago, bendiddley said:

Stu says that larger exit pupils help with nebular filters.

Stu is absolutely correct.  Larger exit pupils increase eye illumination.

If your interested in the short answer as to what works use an exit pupil of 4mm-5mm with an OIII or UHC for diffuse nebula such as the Veil, North American nebula etc.

The parameter "exit pupil" is a universal one applicable to any scope- using this number and disregarding magnification simplifies things, at least for me. If you run the numbers using different scopes you'll see what I mean.

Btw, planetary nebula can use much lower exit pupils than stated above.

Edited March 25, 2021 by jetstream

[jetstream]

4 minutes ago, vlaiv said:

This is the key - make nebula brighter by using lower magnification and at the same time - make sky dark by use of filter.

Yes, except contrast of an extended object is fixed between the surface brightness of the object and the sky brightness. I know exactly what you mean Vlaiv 👍 but for the OP I thought I'd mention that we can't make the object brighter IMHO.

[vlaiv]

8 minutes ago, jetstream said:

Yes, except contrast of an extended object is fixed between the surface brightness of the object and the sky brightness. I know exactly what you mean Vlaiv 👍 but for the OP I thought I'd mention that we can't make the object brighter IMHO.

Contrast in linear light is fixed and for most part we follow that with our vision. There is Weber's law - or Just Noticeable Difference - that is about 10% for human vision. This means that if surface brightness of nebula is 10% higher than sky - we will be able to detect it.

Except this does not work in very low illumination levels (or very high).

(above image taken from here: https://engineering.purdue.edu/~bouman/ece637/notes/pdf/Vision.pdf)

With using filters and varying exit pupil or magnification - we are aiming for that sweet spot - where sky is dark enough to be in photon noise domain where our brain will effectively filter it out - but nebula bright enough for us to see it. Perceived contrast increases because of that.

[jetstream]

11 minutes ago, vlaiv said:

Perceived contrast increases because of that.

Ah yes perceived contrast   - a favourite term of mine! Another 100% agree!

11 minutes ago, vlaiv said:

This means that if surface brightness of nebula is 10% higher than sky - we will be able to detect it.

Can you please explain how I can see some features of IFN?  another filtered example is a very thin line off the end, side of the Eastern Veil that does not show in some images. Ive reported on it in the past- do you have an image of the Eastern Veil to identify what I mean Vlaiv?

11 minutes ago, vlaiv said:

we are aiming for that sweet spot

100% true and I've taken years to find the "sweet spots " In my experience minor differences in exit pupil can make differences on many objects, mostly very faint object features. Mind you it could also involve the eyepieces used to achieve the minor differences so probably a combination.

Edited March 25, 2021 by jetstream

[vlaiv]

12 minutes ago, jetstream said:

Can you please explain how I can see some features of IFN?

I can try to. I don't know much about IFN - but let's see what can be found online.

Lowest estimate of JND that I've found is about 7% for visual. That is 0.07 as ratio or 2.77 magnitudes of difference.

Dark natural sky is SQM 22 - this means that IFN needs to be brighter than sqm 24.7 to be visible against background sky.

I've now looked up Herschel's Ghosts article on Mel's website (here is link: http://www.bbastrodesigns.com/Herschels Ghosts.html) and we agree on this calculation, here is quote:

Quote

Alan Sandage analyzed the brightest portions of the IFN to be 24.5 mag/arcsec^2. If you observe from 21.4 mag/arcsec^2 skies, then you have a 6% contrast ratio, sufficient to see the IFN and dimmer detail when magnified to fit a very wide angle apparent field eyepiece. My observations suggest that only the brightest portions of the IFN can be seen when the sky is at 21 mag/arcsec^2. IFN visibility depends critically on very dark, contrasty, transparent skies. Here are two sketches of IFN near M64, one at SQM 21.1 and one at SQM 21.4, illustrating the overwhelming impact of dark skies.

But I have problem with this. We can easily hit 25-26 mag / arcsec^2 in our images. Outer parts of galaxies are that bright.

Have a look at these:

or for M110

Both of these have features that are 25mag or above - we regularly capture them in images.

Why don't we capture IFN regularly in our images? - it is rather hard to capture it.

For example - image of M31:

Mel's sketch of M31:

Is it really IFN or some sort of transient sky effect that people are seeing?

What is your experience - do you always see the same things?

Looking around the internet - features near M31 are not IFN - but rather Ha nebulosity much closer to home - according to this website:

http://www.deepskycolors.com/archive/2017/01/01/Clouds-Of-Andromeda.html

It is still interesting that we can't normally find them in images (means very faint) and that Mel sees them.

[jetstream]

8 minutes ago, vlaiv said:

What is your experience - do you always see the same things?

I do in one place in particular, around the Double Double and Vega, I can confirm seeing these truly faint shade edges in both the SW120ED, TSA120 and the 200mm F3.8.  I dont however see the things exactly the same as Bartels. Who knows what it really is.

If you have an image of the Eastern Veil I can show you another feature to discuss, I dont have an example image anymore.

Edit: forgot to say I also see the Pleiades Bubble every time conditions allow, this is the easiest one IMHO.

Edited March 25, 2021 by jetstream

[vlaiv]

1 hour ago, jetstream said:

If you have an image of the Eastern Veil I can show you another feature to discuss, I dont have an example image anymore.

This is probably the deepest Veil image that I could find:

But it is narrowband - if the feature you are referring to is not narrowband in nature - it will likely not show in the image.

By the way, author of this image recently released stunning mosaic of the milky way in NB - 12 years, 234 panels and 1.7 gigapixels

[Stu]

5 hours ago, bendiddley said:

Great stuff. It's not for my scope though, I'm just interested in the subject and wanted to find out more. By the way you say exit pupil is affected by the scope too, I thought it was just the eyepiece that affected this, how does the scope affect exit pupil?

You can calculate exit pupil in two ways.

First is to divide the aperture by the magnification being used.

The second is to divide the focal length of the eyepiece by the focal ratio of the scope.

From this you can deduce a couple of things. One, the same focal length eyepiece will give the same exit pupil in scopes of the same focal ratio, no matter what the aperture. This is simply because the focal length increases with aperture for the same focal ratio, which means the mag also increases and the exit pupil stays the same.

Conversely, if the focal length stays the same, exit pupil increases with increasing aperture. So, take my 200mm f8 dob which has a focal length of 1600mm. A 24mm eyepiece would give a 24/8 = 3mm exit pupil. Put that same eyepiece in a 400mm f4 dob which has the same 1600mm focal length and you would get a 24mm/4 = 6mm exit pupil. To illustrate the first point, if the 200mm scope was an f4 too, then it would have an 800mm focal length and would also show an exit pupil of 6mm with the 24mm eyepiece as the lower focal length would compensate for the decreased aperture by also decreasing magnification.

I hope that makes some sense.

Stu

[vlaiv]

10 minutes ago, Stu said:

First is to divide the aperture by the magnification being used.

The second is to divide the focal length of the eyepiece by the focal ratio of the scope.

In both equation same three numbers figure - focal length of telescope, focal length of eyepiece and size of aperture.

First approach combines focal lengths into single entity - telescope_fl / ep_fl = magnification and then adds aperture

Second approach combines telescope focal length and aperture into single entity - F/ratio = telescope_fl / aperture and adds eyepiece FL into the mix.

That is why these two approaches are equal - same three numbers are used to calculate exit pupil.

Actual equation is simple proportion which says aperture : exit_pupil = telescope_fl : eyepiece_fl (ratio of input and output pupil is same as ratio of focal lengths), but above two equations are much simpler to remember.

[jetstream]

37 minutes ago, vlaiv said:

it will likely not show in the image.

It does, see the detached blue little triangle patch off the tip in top of its image? follow this left on the image and there is a very faint really thin thread there, sharp in nature- this is the feature I'm talking about. Btw that triangle detached patch is easy. I dont see the wider more diffuse stretch that the thin thread sits in however.

Ill have to check that MW image, thanks Vlaiv

[vlaiv]

3 minutes ago, jetstream said:

Ill have to check that MW image

there are couple of threads on it - one in Astro Lounge, one in imaging section. Here is link to actual website:

https://astroanarchy.blogspot.com/2021/03/gigapixel-mosaic-of-milky-way-1250.html

[Don Pensack]

On 25/03/2021 at 03:58, bendiddley said:

So I have read somewhere that a larger exit pupil and getting the 'right amount' of magnification from an eyepiece can maximise light transmission of nebular filters. Is this the case? Also some people have said that 40mm gets that 'sweet spot'. Anyone found this?

Take the f/ratio of your scope and multiply by 2.5.  That will be the highest power, shortest focal length, eyepiece to use with nebula filters in your scope.

Multiply the f/ratio by 7.  That will be the lowest power, longest focal length, eyepiece to use with nebula filters.

On an f/5 scope, for example, that would be eyepieces from 35mm down to 12.5mm.

Generally, the lower power end of that range will be better than the upper end of that range, simply because that will yield a brighter image (albeit smaller).

Why such low powers?

Nebula filters work by dimming the background sky about 2.5 to 3 magnitudes while only dimming the nebula 0.05-0.1 magnitude.

As you raise the power, the background in the eyepiece gets dimmer anyway.  By the time the background is already quite dark, the efficacy of dimming the background further is significantly reduced.

Then the 0.05-0.1 magnitude dimming of the nebula starts becoming more important.

So above a certain point, the nebula filter just isn't helpful any more, and that occurs around a 2.5mm exit pupil (about 10x/inch of aperture).

[Don Pensack]

As to how faint we can see visually, the eye can see some prodigiously faint stuff if you are fully dark adapted and observing in dark skies.

Several posters on cludynights.com have done experiments to see how deep one can go, and it seems to hover around magnitude 28 per square arc second.

At that point, the brightness of the field stop begins to match that of the field.

Which explains why so many visual observers see IFN that takes extremely long exposures to catch in an image.

Pannekoek visually drew a sketch of M31 that went out to 5° in length, yet I have seen exactly 1 image that showed the galaxy that large.

Me Bartels regularly sketches IFN around many famous objects that he sees visually in his 25" f/2.6 scope.

And I can tell you that, visually, if the sky is sufficiently dark, a good nebula filter will reveal nebulosity from one side of the Veil Nebula to the other, something I've never seen in a photograph.

And M16 and M17 are merely bright points on the same nebula--something you can see visually yet is shown only in 1 in a 1000 images of either nebula.

In skies of magnitude 22 per square arc second,  seeing out to magnitude 25 isn't easy, but it's also not that hard. 

The sizes of galaxies are typically gauged on the mag.25 isophote, and I'd say that's not far off the visual size.

The key is simply to have dark skies, say mag. 21.5 or darker.