Telescope Reviews: Re: Does aperture rule in bino land?

Yes and yes. And yes

Obviously the simple answer never completely answers the question. Ed makes some good points. So let's talk about them.

A good example is that lately I've been trying to observe some very low surface brightness galaxies. I did all my recent observing in NELM 5.2 to 5.4 skies. So let's see how I did. I was able to observe several faint extended galaxies that have little to no brightening of the core (meaning the core was not significantly brighter than the extensions).

The ones I was able to see were
NGC 3628 in Leo at Sb 13.5 was pretty easy
M95 in Leo at Sb 13.6 was not as easy,
M101 in UMa at Sb 14.6 was not as easy, but it has some brightening in core,
M33 in Tri at Sb 14.0 was easy, this one also has a broad brighter core,
NGC 6946 in Cyg (near Cep) at Sb 13.8, with no core at all this was difficult

IC342 in Cam at Sb 15.0, no core, I cannot see even in the best conditions.

What can you roughly predict from this. In NELM mag 5.4 skies, with 100mm objectives I have gathered enough light that I can see faint extended objects with surface brightness 8.0 to 8.5 magnitudes fainter than NELM. Using this instrument under this sky I gather enough light to get enough contrast to see 8.5 mag fainter. That is the limit of contrast detection I can get.

A smaller aperture or a brighter sky may not allow that limit. So, as far as NELM goes, if mine were let's say 4.9, a half magnitude brighter, it would be likely I would lose sight of probably all but NGC 3628.

Now, let's try increasing magnification to darken sky background and increase contrast. This works great on stars since it doesn't decrease the brightness of the stars but it does decrease the brightness of the background sky, which is an extended object, so you get to see fainter stars. But for an extended object such as a faint galaxy, the problem you will have with trying to darken a bright background sky by increasing magnification is that you have a very narrow range where that will produce a darker sky and yet still provide a large enough exit pupil or enough light to the eye to keep the image contrast bright enough for the eye to see the very faint object.

If you increase magnification in hopes of darkening the sky background, you reduce exit pupil and you will also darken the extended light of the galaxy. If you increase the aperture too much without increasing magnification, you increase exit pupil and you will brighten the entire image and you may get a washed out image. However if you increase aperture AND increase magnification, maintaining the same exit pupil, you may improve the contrast detection of the faint extended object because you have gathered more light. Of course this will only work when sky is dark enough to permit at least the lower magnitude limit of contrast detection.

So "In higher local light pollution, magnification increases in importance relative to aperture" is always true for viewing stellar targets and is usually true also for very bright extended objects but is only true within a very narrow range for very faint extended objects.

On the other hand "while the more dark the sky, the more gain there is from increasing aperture relative to magnification." is probably always true, at least up to the limit of the eye pupil. Exceptions to this would be very small extended objects that require increased magnification to reach ODM, optimum detection magnification. (see Binoc Web Links - Mel Bartels).

How are you supposed to know what works? I guess the answer to that is go out and observe hundreds of objects in all kinds of conditions with as many different instruments as you can get your hands on. You find out what works.

edz

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