Jump to content

Banner.jpg.b89429c566825f6ab32bcafbada449c9.jpg

Magnitude is misleading?


Recommended Posts

I believe I'm learning the hard way that a magnitude 8 star cannot be compared to a magnitude 8 galaxy. I was reading on another forum something about total magnitude vs. surface magnitude?

Last night I set out to find M51...I know this galaxy is a magnitude 8.1 and I know that I have seen magnitude 10 stars through my scope, so I thought it would be no problem. However, after an hour I could not find anything that resembled a fuzzy. Is it correct to say that you cannot compare the magnitudes of differently "shaped" objects (e.g. stars vs galaxies)? 

As to not end the night with complete failure I took a nice long look at M13.

Link to comment
Share on other sites

Messier 33 - stated magnitude 5.7 and the 4th brightest galaxy in the sky but any light pollution or moon light and it becomes very tricky or imposible to spot even with large apertures and low powers.

It is correct that point sources magnitudes are a much more reliable guide to their actual visibility than the integrated magnitide figures quoted for exented objects.

M51 is better than some being quite compact. M101 and M33 are spread over a much wider area and therefore rather thinly. It's quite possible to be looking straight "through" them without even realising it !

 

Link to comment
Share on other sites

I think you've already answered your question and I've been in exactly the same situation as you. Images of M51 look spectacular and it's easy to think, "I can't miss that one".  However it is hard to see until you've managed your expectations, especially this time of year (assuming you're Northern hemisphere). The trick is to make sure your scope is 'on it' and then keep looking through the ep (averted vision really helps). 

The first time I used setting circles to view M97 (owl nebula) I looked through the ep expecting to see it and was disappointed not to. I knew I must be very close so started moving the scope around and noticed that a patch of gloom in the middle of the ep wasn't quite as gloomy as the rest and after a bit of looking at it, the features of M97 started to appear. 

Belive it or not, that's part of the fun of observational astronomy. 

Happy viewing. 

??

Link to comment
Share on other sites

Hi. I find seeing conditions and a dark site is everything when searching out the faint fuzzys. And this effects the so called magnitude as in real life observing any type of light pollution dampens down the true brightness of an object when viewed through the scope. Obviously a large aperture scope helps tremendously but without a dark site and clear sky's still makes life challenging.

Link to comment
Share on other sites

3 hours ago, SacRiker said:

I believe I'm learning the hard way that a magnitude 8 star cannot be compared to a magnitude 8 galaxy. I was reading on another forum something about total magnitude vs. surface magnitude?

Last night I set out to find M51...I know this galaxy is a magnitude 8.1 and I know that I have seen magnitude 10 stars through my scope, so I thought it would be no problem. However, after an hour I could not find anything that resembled a fuzzy. Is it correct to say that you cannot compare the magnitudes of differently "shaped" objects (e.g. stars vs galaxies)? 

As to not end the night with complete failure I took a nice long look at M13.

The critical thing is the difference between magnitude and surface brightness

When talking about extended objects it is more useful to talk about surface brightness rather than magnitude, because the luminosity is spread over a certain area of the sky and not not in a point source like a star. In a star, as seen from Earth, all light seems concentrated in a point, in an extended object all light is spread into its area and therefore the luminosity per unit area is much smaller. Consequently, the contrast with the background sky brightness is less marked resulting in extended objects being more difficult to observe. Hope this makes things more clear!

Link to comment
Share on other sites

The best analogy for this is a torch beam. When focus to a small spot it appears bright, when spread out over a larger area it becomes a lot fainter per unit area.

Surface brightness is a key criteria to understand in establishing which targets will be visible and which won't. The other important factor is sky brightness. I started using an SQM (Sky Quality Meter) which gives a reading for sky brightness. From this you can get a better idea of object visibility. Obviously it's not as simple as that because objects are not uniform in their brightness, and other factors such as transparency come into play but it helps.

The thing with galaxies is that they generally don't respond to filtering (not 100% true but near enough). That means that the best option for seeing them is the darkest sky possible, and most likely a big scope for the smaller ones.

Some useful links

http://www.users.on.net/~dbenn/ECMAScript/surface_brightness.html

http://www.unihedron.com/projects/darksky/NELM2BCalc.html

http://www.bbastrodesigns.com/VisualDetectionCalculator.htm

http://www.unihedron.com/projects/sqm-l/

Link to comment
Share on other sites

There are two ways of describing "brightness". One is apparent magnitude (measued in a unit called magnitude), the other is surface brightness (measured in magnitude per square arcsecond). Apparent magnitude is essentially the amount of illumination received from a source. Walk away from a streetlight and you get less illumination: its apparent magnitude is lessened. But the lamp itself doesn't look fainter, just smaller. What has stayed the same is its surface brightness. So when it comes to extended objects (e.g. galaxies) surface brightness is a better way of judging how bright something "looks". But there's still a catch. Two streetlamps might have the same surface brightness, but one of them is so far away you can only see it with binoculars. Because its apparent size is so small, its apparent magnitude is beyond the limit of visual detection. Simlarly, with galaxies you might have two with the same surface brightness, but one is 10th magnitude, the other 15th. So really, to judge visibility, you need to know both the apparent magnitude and the surface brightness; or equivalently, you need to know one of those things, plus the target's angular size.

In the same way that we speak of the sky's limiting magnitude, there's also a limiting surface brightness. A telescope improves the magnitude limit (shows fainter stars) but does not improve the surface brightness limit (if you can't see the Milky Way then a telescope won't show galaxies with the same surface brightness as the Milky Way, which is most of them). For a target to be visible at all, its surface brightness needs to be sufficiently greater than the surface brightness of the background sky. If there isn't enough contrast then no amount of aperture will make it visible (you can't see galaxies in daytime). If enough contrast exists, it's then a case of magnifying it sufficiently so that it becomes visible.

DSOs are never completely uniform targets, so it's impossible to predict exactly. In practice you've just got to look, and see if the target (or at least the brightest part of it) is visible.

Link to comment
Share on other sites

As others have explained the magnitude of a star is focused onto a bright point whereas a galaxy has that same magnitude spread across its surface area, making it much fainter. Magnitude is a relatively pointless indicator of what you'll see when it comes to galaxies. There are ones like M101 which I struggle to see even though it's just 7.8, yet I've seen several faint NGC galaxies down to 12.5 just as clearly. I started off by not bothering to look for what I believed to be faint galaxies as I thought I'd no hope, but a bit of experience has taught me that it's worth having a try for anything at all that's in my atlas. I might see it, I might not.

It's not the best time of year to be looking for galaxies though, the sky never really gets dark enough. As it's the first time this year that I've been able to look into Virgo, I had a brief foray into it the other night but found the sky wasn't offering up sufficient contrast to make it viable. Instead I split doubles, like yourself enjoyed the miracle that is M13, and got very fine views of Mars and Saturn.

Link to comment
Share on other sites

Yes - magnitude is "integrated" over the area of the extended object, so is not really very helpful, especially for largish objects.  Surface Brightness - or brightness per unit area - is a better indicator of what you can see.  And this is where Exit Pupil comes in, since SB is proportional to Exit Pupil squared.  

In other words, for faint objects you want your Exit Pupil close to the max it can be, depending on your eyes - typically 5 to 8mm.

(There are other considerations too.)
 

Doug.

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.