Definitive Magnitudes???

[Paul73]

Hi Folks

A bit of a geeky question. Is there a single source of DSO information that is considered “correct”?

I have just uploaded the Herschel 400 observing list on SkySafari 6 and noticed that the magnitude figures vary from other sources for quite a few of the targets (and that they have listed NGC4752, which should be NGC4725).

Who should I believe?

Paul

[Ben the Ignorant]

They often mix up visual (mV) and photographic magnitude. But photographic magnitude depends on how much of the spectrum is taken into account. When they used chemical red-sensitive and blue-sensitive photographic plates each had its own magnitude reading. Electronic sensors have different peak sensitivities in the visual, and see various parts of the infrared as well as ultraviolet Spectrum. Plus, a full spectrum goes from radio wave photons to gamma-ray photons anyway, so the topic of magnitude is not simple.

Sticking to visual magnitude, the observed extent of the object plays a role, but how far does a globular cluster extend? Where are the most outlying parts or a nebula? Don't think too much about precise numbers, magnitude is only one among many visibility factors like humidity, light pollution, altitude, residual sunset or dawn light, telescope size, light scatter, dark adaptation, observing experience, etc. 

[John]

Just to add to the confusion, the atmosphere applies a degree of "extinction" to the brightness of targets, the lower the target is in the sky, the more extinction. Stellarium indicates this in the data it gives but I don't know how accurate it's original base data is.

 

[Paul73]

Good points. I hadn’t realised about imaging magnitude vs visual. You could also argue that mag per arc second would be a more useful measure, but for DSO with varying areas of brightness (most of them) this wouldn’t be ideal either!?

[John]

There is also integrated magnitiude vs surface (at a given point) magnitude to consider.

[Paul73]

2 minutes ago, John said:

 but I don't know how accurate it's original base data is.

 

Therein lies the issue. Is there a standard / accepted data set?

Rather like, is there a ‘correct’ Herschel 400? 

[Paul73]

16 minutes ago, John said:

There is also integrated magnitiude vs surface (at a given point) magnitude to consider.

Agreed maybe a peak mag/sec^2 would be a more useful measure. Eg. The North America nebula. High mag total, low mag/sec^2, medium peak mag around the Gulf of Mexico region. Galactic cores vs their fainter halos would be similar.

Paul

[vlaiv]

As far as I know, if band is not stated when magnitude is given it should be V magnitude and there should be a good match between visual and "photographic" magnitude in this case since V is matched against eye sensitivity. Also, magnitude system is relative, meaning it represents fraction of light compared to single source (mag5 star is x100 less bright than mag0 star - how ever you measure it - by eye or by sensor).

There are three magnitudes often quoted for DSOs - integral magnitude which relates to total light output of object in a given band and surface magnitude that comes in two flavors - arc sec^2 and arc min^2.

These three quantities are related. Surface brightness in a simple way - to switch from mag/arcmin^2 to mag/arcsec^2 just add 8.89 to magnitude value. Both are average brightness meaning integrated magnitude (total light in band) is divided by extent of target in either arcminutes squared or arcseconds squared, so if you know extent of target you can "switch" between the two. This is somewhat useful for observation as average magnitude does not account for brightness changes in target as already mentioned (galactic cores and such).

I can see various sources of discrepancy for quoted magnitudes. One would be use of specific band without mentioning it, and also there are various standards for photometry, for example:

And this is eye response to light:

You can see that V band from UBVRI is well matched to scotopic vision response, but G from UGRIZ is something completely different.

Switching between UBVRI and UGRIZ magnitudes for stellar source is not that complicated - there are finite number of stellar types and one can compute transform that will accurately switch between magnitude types (If you know stellar type and have any magnitude - you can compute any other magnitude in other bands and systems, and also by having couple of magnitudes you can pretty much narrow down stellar type from color index for example).

Problem comes with atypical sources - galaxies for example - you have know way of knowing what ratio of stellar types target contains - or even distributions (different parts of galaxy contain different generation stars, there are star birth regions and such).

I think that having a "magnitude" map is much more useful and meaningful for extended targets, but I have not come across such a resource being used. Btw, by magnitude map I mean "image" of object "posterized" with different shades of color (or gray) that represent different magnitude ranges - something like regular terrain map representing height in certain increments (0-100m, 100-200 and so on).

 

[jetstream]

This site works.

http://simbad.u-strasbg.fr/simbad/

[Size9Hex]

You mention the H400 list. I use Mark Bratton’s guide which I think seems about as definitive as it gets on the Herschel objects. For each object, it gives the photographic/blue magnitudes and the visual magnitude too if known. Galaxies largely drawn from here...

http://leda.univ-lyon1.fr/

... supported by SIMBAD which Jerry mentions above. Other DSO types are referenced from SIMBAD.

I’ve found various differences against the Sky Safari magnitudes for the same objects.

All that said, I ask myself if the precise number matters. With tongue in cheek, I find there are only two magnitudes - 1. Visible. 2. Impossible. - and the magnitude in the guide book / database doesn’t always correlate as you’d expect. The mag 12 galaxies aren’t always easy. The mag 14.5 objects will sometimes surprise you!

[Paul73]

Great resource. Now saved as a favourite. And certainly interesting to note that they give several different mag measurements as detailed by previous posters. 

Agreed, the absolute number doesn’t matter. But, It is rare that there is only one type of error in a large data set......

Paul

PS. Love the two mags approach!

 

[Ben the Ignorant]

1 hour ago, Size9Hex said:

I find there are only two magnitudes - 1. Visible. 2. Impossible.

Yeah, once I saw a mag 12.2 galaxy from a city with a 5-inch scope, but others in the same area around mag 12 or 11.8 could not be seen. You have to try them all. The detection of the very ghostly 12.2 target was achieved only because it was a marathon over several long summer nights with dark adaptation improving day by day through intensive practice. 

Don't expect a target to be easy because it has a low mag on paper, and don't expect one to be tough because it has a high printed mag. Light pollution filters change the game altogether, there is no end to the experimentation that can be done.