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Sky and extended object brightness vs magnification

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In my relentless quest to understand what I see and why I see them as I do , the same old question has popped up, but with a twist.

As I understand, if we up the mag (lower the exit pupil) both the sky background and an extended object dim equally. Correct I hope? The sky background brightness is made up from natural light and likely man made, even at most dark sites.

Now the question....  If there is a light dome ( light pollution) affecting the sky brightness in one direction, will the sky brightness and the extended object dim exactly the same with increasing mag as in the example with out the light dome? We are viewing over the dome for the example.

We assume using the same telescope for the observations, but with changing focal length eyepieces, that operate exactly the same in other respects.


Yes, I know...why would any one view over a light dome... :)

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I reckon Acey is the man for this job Gerry!

My thoughts are that yes, the light pollution and target will dim at the same rate, but this always comes back to surface brightness. If the target surface brightness is higher than the sky background then you should see the target in some form.

Raising the mag does dim things but also obviously makes the object larger. Image scale has an impact on the object visibility too (perceived contrast?), so larger aperture means more image scale and better view.

Light domes? LIGHT DOMES?? I live in one, don't really have a choice ;);) 

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  • 1 month later...

Just belatedly noticed this thread. Yes, magnification dims target and background equally, regardless of background brightness (e.g. presence of light dome). The crucial extra factor is the observer's contrast threshold, i.e. how much brighter the target needs to be than the background, in order for the target to be visible. This is a function of background brightness and target size. What it means in practice is that if a target is too small (mag is too low) then it is invisible, but if it is over-magnified then there is insufficient contrast (at the particular values of apparent size and background brightness) so that again it is invisible. There is some range in between where we can see the target. And it is the general case that with a dark background sky the magnification range is wider and stretches higher. When viewing against a bright sky (e.g. light dome) it is likely that the target will become invisible at a lower magnification than would be the case if the light dome were absent. Or it may just be invisible at any magnification. Also, of course, light domes are by definition low elevation phenomena, so there's the added handicap of greater atmospheric extinction. All of which makes light domes undesireable places for seeking targets, though sometimes there's no choice.

At a light polluted site we effectively have a light dome stretching right over our head. The viable magnification range can become very narrow or non-existent. Galaxies can then only be seen at low to medium power, with smaller ones of the same surface brightness being impossible to see. This can give people the impression that galaxy viewing in general can't be done at high power. But at a dark site it can be done at very high power, simply because the targets have such greater contrast to begin with.

All of this implicitly assumes that DSOs are uniform targets, which of course they aren't. Sometimes you might want to look for a galaxy's stellar nucleus or a tiny dark lane, or else you might be looking for large, faint spiral arms. Those could respectively require higher and lower magnifications. So in practice, whichever bit of sky you're looking at, you start low and work up, seeing whatever you can at different magnifications. That's why I always use a zoom eyepiece.

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Thanks Acey, there was a discussion on another site that inspired me to ask the question. Your answer confirms what I thought but I wanted to check- there seems to be many misconceptions about surface brightness/extended objects etc. I think that what some found from observing follows your information here but they "view" how it works in a different way. I just couldn't fathom how a light dome, LP sky would act differently than any other by upping the mag. Whats happens though is in your answer- they must hit a "sweet spot" with some objects by upping the mag under their LP sky.

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If you have a taste for maths it's all in this paper:


Essentially there are two limits: magnitude and surface brightness. As you raise magnification you raise magnitude limit and lower surface brightness limit. Magnitude is the most important limit for "small" targets, surface brightness for "large" ones. Most targets are somewhere in between. So we choose a magnification that optimises the two limits for the given situation.

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