Question about magnitude

[DeepThought]

Hi

In order to save myself a succession of wild goose chases, could someone advise me what sort of magnitude objects I can expect to observe through a 6" scope?

I suffer some reasonable LP but hope to make it out into darker locations from time to time.

Also, how is magnitude calculated for nebulae? Often I see them listed as quite bright compared to stars, but I know they won't be available to me.

Also is magnitude the best indicator of whether I will be able to see an object or not, or should I be looking for something else?

Thanks

[Capricorn]

From a nebula the magnitude is a sum of all the light off of it. So it can be dim and big so still have a bright magnitude, M31 being the classic example. Brightest thing up there and so dim it is difficult to see, just happens to be big. A better option is to determine the surface brightness, there are lists running around the net so drag up google and start searching.

In a 6" scope I would expect something down to Mag 12 or 14, but if the light pollution matches this then whatever is simply lost. The light pollution will be the limiting factor not the scope I would say.

[wookie1965]

http://www.stargazin...wmanystars.html

According to this website 12.7

And this one says 13 so around that.

[Moonshane]

http://www.stargazin...wmanystars.html

According to this website 12.7

http://books.google....J-aCV_CQ4&hl=en

And this one says 13 so around that.

for stars yes but not extended objects like galaxies - see above. it's a case of trial end error unfortunately as optics, experience, conditions, seeing, light pollution, transparency, eyepiece quality all play a part and this can change from night to night. I live about 9 miles from Manchester and can see (or rather detect) M51 at the Zenith with my 6" scope. it's easier with my 16" scope and from a dark site easily shows spiral arms with direct vision in my 12" scope.

[themos]

http://www.telescope-optics.net/functions.htm#1.4.1._Light_gathering_power

[wookie1965]

for stars yes but not extended objects like galaxies - see above. it's a case of trial end error unfortunately as optics, experience, conditions, seeing, light pollution, transparency, eyepiece quality all play a part and this can change from night to night. I live about 9 miles from Manchester and can see (or rather detect) M51 at the Zenith with my 6" scope. it's easier with my 16" scope and from a dark site easily shows spiral arms with direct vision in my 12" scope.

Yes i should have elaborated rather than just posting what books says i never seen M51 yet and ive tried numerous times in all kinds of viewing conditions.

[acey]

A telescope doesn't see objects down to a certain magnitude - it enables your visual system to perceive a certain number of magnitudes more than you would with the naked eye. There are lots of factors and none of the models you find in online calculators etc should be taken as gospel, but roughly speaking, a 6" scope should provide a gain of about 7 magnitudes over the naked eye; i.e. if you can see stars down to mag 6 with naked eye then the scope may show stars down to about 13, but if you can only see to mag 4 with naked eye then you'd expect the scope to show stars to about mag 11. But this is only a very rough guide - in practice you've just got to find out your own personal figure.

For point-sources (stars) magnitude alone determines visibility. For extended objects (galaxies and nebulae) surface brightness and size are the two determining factors. For small extended objects, the quoted magnitude will usually be a sufficient guide to visibility, but for large objects, surface brightness is a better guide than magnitude. An object needs to have sufficient contrast against the sky (i.e. its surface brightness needs to be sufficient in comparison with the surface brightness of the sky), otherwise no amount of aperture will make it visible. The limiting magnitiude for small DSOs will probably be one or two magnitudes brighter than for stars; i.e. if you can see stars to mag 12 then you'd expect to see small galaxies of sufficient surface brightness down to maybe mag 10 or 11.

Magnitudes can be found in many different ways, yielding different results. For the visual observer, often the most reliable guide are the "visual magnitudes" found by eye by 19th-century professional astronomers. They did this by comparing extended objects with out-of-focus stars of known magnitude. Also frequently quoted are photographic magnitudes using particular filters - "V-magnitude" is the one closest to the response of the human eye, but it actually matches daytime vision better than night vision, so it shouldn't be taken as an exact match to what the eye would see. You can find the same object having lots of different magnitude figures depending which source you consult. Again, the best thing is just to try observing it, and see what you think.

If the sky is dark enough for the Milky Way to be visible to the naked eye then a 6" scope will show hundreds of DSOs. In compiling observing lists, start with what's popular - the Messiers, Caldwells, the RASC "best non-Messier" list, the Herschel 400, O'Meara's "Hidden Treasures" etc. Don't hunt for objects on the basis of pretty photographs or famous names (e.g. Horsehead), and don't compile a list on the basis of magnitude alone - otherwise you'll have a frustrating time.

[themos]

what he said.

[Moonshane]

show off

[CSM]

If you look at some of the lists of Messier objects as well as the magnitude you will see surface brightness. Thus M106 has a magnitude of 9.1 but a surface brightness of 12.4 which in even a moderately light-polluted sky will mean that a 6 inch scope would really struggle.

[Double Kick Drum]

If you look at some of the lists of Messier objects as well as the magnitude you will see surface brightness. Thus M106 has a magnitude of 9.1 but a surface brightness of 12.4 which in even a moderately light-polluted sky will mean that a 6 inch scope would really struggle.

Agree.

A combination of surface brightness value used with apparent magnitude will give you an idea of what fuzzies you can see, although use of these statistics is indicative rather than definitive. SB varies between the core and outer region of a galaxy and is effectively an average brightness figure. As a result, a large face-on galaxy with a bright core and feint spiral arms may still have a relatively low SB. M31 is a good example of this.

The targets I select are based on a combination of these two figures. The dimmest objects my five inch scope can see (from my back garden VLM 5.4) are of magnitude 11.3 but... I tend not to attempt anything feinter than magnitude 10.5 if its SB is feinter than 13.5. In other words, as an DSO's magnitude decreases, the SB required for it to be seen increases.

TBO I have only really honed my target selection through trial and error.

Initially a good start point for targets is the Messier list, which contains many of the brighter DSOs but not all of them. Beyond that, the Caldwell list and Herschel 400 will give you lots of options and hours/years of fun.

Happy hunting!

[acey]

M106 has a magnitude of 9.1 but a surface brightness of 12.4 which in even a moderately light-polluted sky will mean that a 6 inch scope would really struggle.

Don't confuse the two - magnitude has no units, but surface brightness is measured in magnitude per square degree, or magnitude per square arcsecond. The figure 12.4 means 12.4 magnitude per square degree, or 21.3 magnitude per square arcsecond. You can't say that a particular aperture is required to see something with that surface brightness - parts of the Milky Way have a surface brightness as low as 22 magnitude per square arcsecond but can be visible to the naked eye at a dark site.

DSOs with very high surface brightness typically have a figure around 16 mag/sq-arcsecond or even higher. Very low surface brightness objects are typically around 22 mag/sq-arcsecond or fainter. Most spiral galaxies are around 21 mag/sq-arcsec iin their central region but fainter in their surrounding disc (where the arms are). The quoted figure for surface brightness is usually an average taken over the entire object, whereas in practice there can be great variation, so that an observer might see the core at a light-polluted site, but not the surrounding disc. This happens a lot with M31, leading people to think it's just a featureless blob when all they've seen is the high surface-brightness core.

[Peter Drew]

This general confusion is also shared by comets, bear this in mind later this year when the promised two arrive.