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[-=CROMAN=-]

Mornin' folks....

I have a question about Limiting Magnitude in regards to different scope sizes. If I can get a 12inch dob with a limiting mag of 15.1 for 1299.00 what justifies $1900.00 for a 16" lightbridge with a limiting mag of 15.5? Is .4 really worth the extra $600?

xx14i Limiting mag = 15.4
xx12i Limiting mag = 15.1
16" Lightbridge Limiting mag = 15.5.

Perhaps I am a bit confused as to what limiting mag is? I mean i know what it is, but am i missing something? Would the views be that much better with the 16" as opposed to the 12" with a great eyepiece?

My current Zhumell Z10 has a limiting mag of 14.92, a tad bit less than half a magnitude from the 16. This is so very confusing....

[patrickm]

I think you are paying not only to increase the number of faint stars that will become visible but also to pull in more photons for those that are visible. The difference in 1.25" when viewing a faint NGC galaxy at Conway was significant, though the limiting magnitude of the two scopes was not significantly different. Probably somebody will give us a scientific explanation and perhaps correct me. I know that the increase in light gathering as aperture increases is MUCH greater than linear.

[patrickm]

If you're gonna buy a scope in that size range, I think the most important question is the limiting hassle factor more than limiting magnitude. How big a scope would actually get regular use is a more practical question, and everybody is different on that one.

[Chicago Astronomer - Astro Joe]

Rich,

There comes a point where the diminishing returns outweigh the cost/size of a scope.

The limiting magnitude is a simple guide to just detection. Is that faint star there or not? In a light polluted environment like our, it's not a big enough factor to split hairs about. For the Hubble or Keck...yes.


The light gathering abilities of a LM 15.5 scope will brighten tangible objects considerably. Maybe squeeze out that extra faint band on Jupiter, tease out a small craterette, or deepen M15 with more star haze. For fainties, in this area, won't do much to resolve the object a whole lot. Light gathering aperture is exponential from inch to inch...I forget the formula. But we all know bigger is always better for brightness, clarity and magnification.

But

$600.00 for that small percentage of LM......not worth it I say.

[patrickm]

Interesting stuff from this article, plus a lot of equations, but the things that stand out are:

Often times, light-gathering power of a telescope is expressed in terms of limiting stellar magnitude detectable. The apparent stellar magnitude - usually denoted by m - is a measure of apparent brightness, with the difference of 5 magnitudes corresponding to 100 times difference in brightness; thus the difference in one magnitude implies difference in brightness of 2.512 (from 1000.2), and any nominal difference in magnitude "x" implies 2.512x difference in brightness.

It implies that "y" times difference in brightness converts to x=2.5log(y) magnitude differential.

While the apparent sky background brightness is inversely proportional to (E/X)2, field tests show that the resulting limiting magnitude gain or loss changes at a significantly slower rate. The rate is also faster for bright background, when gain in limiting magnitude can approach half of the nominal change (darkening) of the background within a range of several magnitudes. With dark sky background, the gain can be less than half as much for the same nominal change in background brightness (most of this effect is likely caused by the background brightness falling below perception threshold of the eye). This implies that higher magnification mainly compensates for the limiting magnitude loss due to light-polluted sky.

So, more aperture pays bigger dividends when viewing in the city!

Stellar magnitude is the basis for integrated magnitude, which represents total brightness over an area expressed in terms of stellar magnitude. Thus, for given magnitude, an extended object is always fainter than a star (that is, with lower surface brightness), and limiting magnitude for extended objects is generally brighter than limiting stellar magnitude (it also varies depending on the intensity distribution over the object). Considering that surface brightness of the telescopic image diminishes with the square of magnification, it is always lower - at least for the transmission loss at the minimum useful magnification, and more at higher magnifications - in the image seen through a telescope, than in the image seen by the naked eye. However, telescopic image on the retina is much larger, and the total of light energy - or total brightness - much higher. That is why the Moon appears so much brighter in a telescope, despite its surface brightness being normally (due to magnification) much lower.

So huge, bright galaxies have their brightness spread out over a very large area, making their apparent brightness decrease drastically.

Also, each human eye is different and thus:

Consequently, limiting magnitude of a particular telescope will also vary up to a magnitude, or more, from one individual to another.

Does any of this help? Maybe not, but you still don't wanna be the guy with an expensive 16" scope sitting in the closet because it's too much of a PITA to use and so you keep going back to your trusty 10" dob.

If your observing is not going to be done in anything approaching a dark sky, you will not see anything near a 14.0 magnitude anyway. Having a 15.1 or 15.5 will make little difference on that end of things, but as we just saw, the aperture will brighten the things that you can see, and apparently even more so under LP skies.

The question then sorta becomes how and where are you gonna be using this monster scope? A 16" hardly seems portable to get it to a dark sky. Will you have a way to leave it set up near home or wherever you will be observing? If so, then I'd say go for the most aperture that you can come close to affording and the rest of this is kinda irrelevant. Then you're just talking about the type of scope you want for the money. If the scope needs to be portable to get it to a dark sky, then you come back to what Joe and I said about Limiting Hassle Factor. :-)

[dapple]

[Disclaimer: I have not had much exposure to larger telescopes. This is a purely theoretical discussion.]

I think Joe hits it right on the head. For a mathematical discussion of what a larger aperture can do for you, check out Roger Clark's "Visual Astronomy of the Deep Sky." Basically, a larger scope with the same eyepieces will allow you to magnify the image more without losing as much image brightness. Assuming it's just bright enough to be seen, the eye is more sensitive to tiny brightness fluctuations when those fluctuations are big. At a dark sight, that means more interesting views, and in light pollution, you can see something, anything easier. Or at least in theory -- I just read this, and intend to put the equations to the test!

On the other hand, if your passion is about detecting really faint stuff regardless of how boring it may look (the Berkeley list of globular clusters perhaps?), then every magnitude deeper you can get means an exponentially larger number of sources to just barely see. For example, the number of galaxies brighter than a limiting magnitude of m is roughly N(<m) propto 10^(0.5m) . If I do my math right, your 0.4 mag increase in the limiting depth should give you a fractional increase of 58% in the number of faint fuzzies detectable -- ignoring all the effects I mentioned above from Clark. That'd keep you busy!

[patrickm]

This whole article is awesome!
www.ayton.id.au/gary/Science/Astronomy/Ast_Telescope.htm

Especially this part:
www.ayton.id.au/gary/Science/Astronomy/Ast_viewing.htm#Sky%20transparency:

Also answers the perennial favorite, "Why is the sky blue?" more clearly than I remember it previously.

Apparently, they have similar problems down in Australia even though they are hanging upside down. You would think all the atmospheric particulate matter would just fall off into space like shaking a rug!

[-=CROMAN=-]

Wow guys ..... This was a lot to take in. The 16" dob is probably overkill and will never , ever leave the garage. Aperture fever is a son of a beeotch!

I did a bit of soul searching and came to the conclusion that I want to get into astrophotography. So here is what I am thinking now... 8" or 10" SCT with an Canon EOS Rebel T3 to begin with. Celestron advance series or the Meade LX series? GEM mount or alt/az with a wedge? Of course there are the doublet and triplet refractors.

Decisions... Decisions...

[patrickm]

If going with SCT, you might wanna check out my 9.25 EdgeHD on CGEM. No wedge necessary and the field is truly flat all the way out. The main downside is that as of now, the promised focal reducer has not appeared. There is a third party fr for the 14", but it doesn't work on other scopes. You are then left to image at F/10 or F/2 via Fastar off the front of the SCT. For a beginner in AP, there is plenty to image at F/2 and Celestron are still promising the FR by year's end.