Visibility of M31

[DaveS]

Last night, having a look around with my 7x50s I was looking at M31, and could clearly see it as an elongated patch, not just the star-like core.

Now I was thinking, Sidgwick says that an extended object like M31 cannot appear brighter telescopically than with the naked eye, and I get his math. With my old eyes, and under my less-than-dark skies I'm probably not even getting the full exit pencil of my 7x50s.

Therefore, does this mean that I *should* be able to see M31 naked-eye given that it won't appear with greater contrast in the bins than by eye?

As they say in examinations, please read the whole of the question before anwering, I'm not asking:

"Should I buy bins with a smaller exit pupil"; Possibly, but not just now.

"Should I buy a dob"; No, I'm 90% imaging, and don't like alt-az anyway.

"Should I move to a darker site": Yes, it's on my to-do list.'

If I *could* see M31 naked-eye that would mean I had Bortle 7 skies rather than the borderline 8 that I've assumed. Time was, I *could* see M31 naked-eye in Acton, even further in to London .

[Mr niall]

I don’t understand that (my fault prob not yours!) I caught glimpses of it with av last night but had oodles more contrast with my 5” - I thought that’s how the whole aperture thing worked? Have I got the wrong end of the stick?

[IanL]

In short, yes you should be able to see it. There are two key factors at play here:

1. Using the 7x50s, M31 will have a lower surface brightness that with the naked eye, but it will appear larger (covering more of your retina). The decrease in brightness is an indisputable physical fact as you realise from reading Sidgwick.*

2. As you magnify the object, it occupies a larger area on your retina. This makes it more perceptible to your visual system, even though it is actually dimmer than with the naked eye.

The one wrinkle in Sidgwick is that I'm not sure exactly what he means by "Also, the sky appears less bright through a telescope than to the naked eye." It appears to suggest that the physical contrast (difference in brightness) between the object and the background sky increases with increasing magnification, but this is patently not the case:

- One can think of the background sky as an extended object subject to the same dimming rules as M31 or any other extended object. In fact once the magnification is sufficient, M31 would occupy the whole field of view. So at this point you're considering the contrast between the core, the spiral arms and dark lanes between them, i.e. it is one extended object.

- Instead, I think he means (but does not explain) that the combination of the overall dimming of the scene, including the background sky, plus the enlarged image makes it easier to perceive less bright extended objects.

* In answer to anyone who leaps on and says "Why are larger apertures better then?", firstly at a given magnification the view of an object through a larger aperture will be brighter than a smaller one, but it will always be less bright than with the naked eye. Secondly, larger apertures allow more magnification which makes the object bigger and more perceptible, provided the exit pupil of the eyepiece is not larger than the pupil of your eye. Thirdly, when imaging rather than viewing, the rules are different.

One issue you may be facing is that when looking through the 7x50s, the surrounding scene is blocked out. If you are under a lot of light pollution, this helps a lot. So firstly a lot of the local glare is blocked out, making it easier to see the faint objects. Secondly, the dimming of the skyglow through the 7x50s will allow your eyes to dark adapt better - although it takes 30+ minutes to properly dark adapt, I find that even a few seconds of darkness lets my ageing pupils dilate enough to make a difference.

As an experiment, maybe try using a couple of longish cardboard tubes (paper towel centres for example) as a pair of 0x30 binoculars - I'd be willing to bet that you would be able to pick out M31 through them more easily than with the naked eye just by virtue of blocking out most of the skyglow and nearby glare.

[DaveS]

Thanks Ian.

I'll give the cardboard tubes idea a go whem I have a couple.

Maybe my skies aren't quite as dire as I had thought.

[jetstream]

3 hours ago, IanL said:

firstly at a given magnification the view of an object through a larger aperture will be brighter than a smaller one, but it will always be less bright than with the naked eye.

Nope.

3 hours ago, IanL said:

larger apertures allow more magnification which makes the object bigger and more perceptible, provided the exit pupil of the eyepiece is not larger than the pupil of your eye.

Yup, kind of.

[jetstream]

10 minutes ago, DaveS said:

Thanks Ian.

I'll give the cardboard tubes idea a go whem I have a couple.

Maybe my skies aren't quite as dire as I had thought.

The binos made the object bigger, therefore easier to see.

[IanL]

29 minutes ago, jetstream said:

Nope.

Care to explain?

Before you do, suggest you read "Amateur Astronomer's Handbook By John Benson Sidgwick, R. C. Gamble", page 31, section 1.3, Apparent brightness of extended images, specifically where it says:

"It can readily be seen that a large aperture gives a brighter extended image than a smaller one using the same magnification, and that the large instrument can use a higher magnification than the smaller one in the production of an equally bright image."

Seems indisputable to me.

34 minutes ago, jetstream said:

Yup, kind of.

Not kind of, precisely what I said.  Again if you think differently would be happy to hear why.

[jetstream]

From CN

"   #5 Ernest_SPB

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Posted Today, 08:50 AM

Brightness of image (background or diffuse nebula foreground) does not depend from scope aperture... only from exit pupil diameter. Aperture allows to rich more or less magnification."

Ernest knows his stuff.

[jetstream]

4 minutes ago, IanL said:

Apparent brightness

This is "perceived brightness"?

 

[Piero]

27 minutes ago, IanL said:

Care to explain?

Before you do, suggest you read "Amateur Astronomer's Handbook By John Benson Sidgwick, R. C. Gamble", page 31, section 1.3, Apparent brightness of extended images, specifically where it says:

"It can readily be seen that a large aperture gives a brighter extended image than a smaller one using the same magnification, and that the large instrument can use a higher magnification than the smaller one in the production of an equally bright image."

Seems indisputable to me.

Not kind of, precisely what I said.  Again if you think differently would be happy to hear why.

 

 

This thread might help: 

 

[ronin]

Keep hearing of this but it seems to get dragged up at times, then everyone goes back to simply observing and seeing all the dim object they cannot see without a scope.

Considering Messier listed the fuzzy not-comet objects of his time if they were brighter by naked eye he sure missed a lot of them like The California Neb in Perseus, The Rosette Nebula to the left of Betelguese. If the naked eye is better why were these not catalogued amongst many others?

They are both circumpolar so visible all year, both a good size and by the arguement he should have catalogued these almost ommeduatly, not missed them off.

[IanL]

1 hour ago, jetstream said:

Brightness of image (background or diffuse nebula foreground) does not depend from scope aperture... only from exit pupil diameter. Aperture allows to rich more or less magnification."

Sidgwick, section 3.3:

d = D / M

Where d is the diameter of the exit pupil, D is the diameter of scope aperture and M is the magnification ( D / d = F / f = M) where F and f are the focal lengths of the scope and eyepiece.

So:

80mm / 50x = 1.6mm

200mm / 50x = 4mm

So yes, the brightness of an extended object does indeed depend on the diameter of the exit pupil, that is not disputed (provided the exit pupil is smaller than the eye's pupil and the eye is placed so the exit pupil coincides with the eye's pupil).

But, the diameter of the exit pupil, for a given magnification (50x) depends only on the aperture of the scope. So larger aperture = larger exit pupil = brighter extended object as I previously stated. Ernest may 'know his stuff', but he may be confusing cause and effect here if we're being uncharitable, or to be kinder he hasn't presented the whole picture including magnification.

59 minutes ago, jetstream said:

This is "perceived brightness"?

No, "Apparent Brightness" is the amount of light per unit area, i.e. the illumination due to optical properties of the telescope-eyepiece-eye system.

Perceived brightness is something different and depends on the reaction of the retina to that illumination and the eye-brain system's response to that stimulus in forming a perception of the image. I don't have a good reference for this, but it seems to be accepted that a larger object of a given apparent brightness is more readily perceived than a smaller one. If it wasn't the case, we wouldn't need telescopes to see the faint fuzzies, because we have (I hope) now established that telescopes can only make extended objects dimmer, not brighter, so it is magnification that allows us to see them (with larger apertures reducing the amount of dimming at a given magnification, and by the same token larger apertures enabling more magnification for a given brightness).

[jetstream]

8 minutes ago, IanL said:

brightness of an extended object does indeed depend on the diameter of the exit pupil,

The brightness of an extended object is fixed and contrast is the difference between this and the sky darkness. More focal length at a given (appropriate) exit pupil will make objects bigger while preserving eye illumination. If a 7mm exit pupil is used as a reference this chart shows the dimming effect of using smaller ones eventhough contrast between the extended object and the sky remains the same.

If using a telescope and eyepiece combo that matches your max exit pupil then the telescope does not dim the object.

Yes, the increased mag using a telescope at an appropriate exit pupil enhances or enables the view. The OP's binos did this eventhough the contrast stayed the same naked eye as did the objects surface brightness.

 

[DaveS]

The exit pupil of my bins would be just over 7mm (50/7). I doubt that the entrance pupils of my eyes were anything like that, possibly no more than 5mm, giving an effective aperture of 35mm, age and light pollution being to blame.

So the upshot of this thread is that If I can see M31 using my compromised bins then I should be able to see it naked-eye. Possibly.

[DaveS]

Well I won't be checking it tonight. Although it's "clear" I can barely make out the Square of Pegasus through the high-level murk.

Imaging (Sorry) The Pacman with my triplet apo and 3nm HII filter.

[DaveS]

Update:

I was out imaging (Sorry again ) last night, and while the subs were coming in took the opportunity to eyeball the sky, I had my old 7x50s with me too. The new LED streetlights do seem to have had an effect, as with averted vision (Averted imagination, LOL?) I could *just* make out M31 naked eye, the double cluster too.

Note: I knew where to look, and knew what I was looking for but even so it was at the limit of what I could call vision. Through the bins it was very clear. I *may* have glimpsed a very slight brightening of the sky where the Milky Way runs through the zenith but I'm quite willing to put that down to imagination.

Someone with eyes 40 years younger than mine might well have seen more.

[newtyng5]

Nice discussion. Very Enlightening. If I ever get into astrophotography, I must get an eyeball implant (would save time with stacking all those images) . Sorry ignore me, please continue.