Which magnitude?

[Guest]

I'm trying to find my way around to start variable star observing. I picked a variable star called mu. cep from the AAVSO website and printed out the map and the photometry data. There is only one comparison star nearby. mu. cep is listed as having a magnitude of 3.43 - 5.1 V. I know that the magnitude is different depending on the wavelength band and that V mean the visual range.  The comparison star is listed as having a magnitude of 6.080V

So I started looking at the simbad database and ended up using a facility called ALADIN Lite which allows you to click on star to find information about it. Under Aladin the same star is given a magnitude of 6.088V  This comes from the simbad database.

I know that 6.080 is close to 6.088 but I expected them to be the exactly same so why the discrepancy? And if I wanted to do the comparison which magnitude should I use.#

Cheers

Steve

 

[vlaiv]

It is just down to measurement error.

Difference is mag 0.008 which translates to ~ 0.74% of difference in brightness - or less than one percent.

If you want to do comparison - use either as you won't be able to tell much larger difference than less then one percent of brightness difference.

[Guest]

Thanks. I realise that the difference is very small but I assumed that there would be internationally agreed values for the magnitudes of comparison stars. In both cases the magnitude is quoted to 4 significant digits so I guessed they had some confidence in the values they quoted otherwise there's no point in quoting to that many significant digits.

But I take your point that it won't make much difference to anything I come up with.

I don't understand how these various astronomical databases work  at the moment. From what you say the data comes from different sources. I had some idea that the data was somehow aggregated from different sources.

Cheers

Steve

  

[globular]

49 minutes ago, vlaiv said:

Difference is mag 0.008 which translates to ~ 0.74% of difference in brightness - or less than one percent.

Doesn't each 1 mag represent increase in brightness of 2.5119 times?  (5th root of 100; so mag 6 is 100 times brighter than mag 1)
If so, doesn't that mean that 0.008 of a mag equal to about 2% brightness difference (2.5119 * 0.008 = 0.02)?
What am I doing wrong?

Either way, I agree you won't notice the difference with just your eyes.

Edited November 28, 2020 by globular

[Mr Spock]

It takes a very experienced variable star observer to detect a difference of 0.1 mag. I wouldn't worry about 0.008 

[andrew s]

If you want to do photometry I would use the AAVSO or BAA charts.

The magnitude in catalogues vary as they were measured under varying conditions at various times. Getting precise magnitudes is very difficult and requires excellent skies.

That's why most amateurs do differential photometry which is much easier.

Regards Andrew 

[Xilman]

16 minutes ago, globular said:

Doesn't each 1 mag represent increase in brightness of 2.5119 times?  (5th root of 100; so mag 6 is 100 times brighter than mag 1)
If so, doesn't that mean that 0.008 of a mag equal to about 2% brightness difference (2.5119 * 0.008 = 0.02)?
What am I doing wrong?

Either way, I agree you won't notice the difference with just your eyes.

"What am I doing wrong?"  --- you are multiplying and not  taking logarithms.

To a very good approximation, and for fairly small values of x, a difference of x millimags corresponds to a 0.x% difference in brightness.  So 8 millimags is about a 0.8% difference.  0.74% is indeed about 0.8%.

For those who did A-level mathematics, consider the power series approximation to ln(1+x).

[Xilman]

12 minutes ago, Mr Spock said:

It takes a very experienced variable star observer to detect a difference of 0.1 mag. I wouldn't worry about 0.008 

True for visual observers.

I aim for between 0.01 and 0.03  most of the time, rising to 0.1 below magnitude 18 or so, and 0.001 to 0.003 for exoplanet transits.  I use a CCD camera for those.

Edited November 28, 2020 by Xilman
Clarification

[globular]

OK, so it's ((100^(1/5))^6.088) / ((100^(1/5))^6.080) - 1 = 0.07395 ~0.74%
Thanks @Xilman

[Marvin Jenkins]

32 minutes ago, woodblock said:

Thanks. I realise that the difference is very small but I assumed that there would be internationally agreed values for the magnitudes of comparison stars. In both cases the magnitude is quoted to 4 significant digits so I guessed they had some confidence in the values they quoted otherwise there's no point in quoting to that many significant digits.

But I take your point that it won't make much difference to anything I come up with.

I don't understand how these various astronomical databases work  at the moment. From what you say the data comes from different sources. I had some idea that the data was somehow aggregated from different sources.

Cheers

Steve

  

I am not an expert by any means but in this day and age a lot of us think that with modern technology, we have all this stuff nailed down. 
It is one of the things I love about astronomy, we are a lot closer to the beginning of our understanding than the end.

You only have to see the wide differences in distances of stellar objects between the different data bases, many many light years some times. This is far from an exact science.

Marv

[Guest]

I completely understand that it's a small difference in magnitude. The issue I had was the way these data bases work. When the AAVSO quotes a magnitude for a star is that based on a single measurement by a single observer? Since the star is not variable I assume that it's brightness must have been measured several times and that the quoted figure was some kind of an agreed average. In the photometry data from the AAVSO site it lists 48 source references. Presumably these are other catalogues. I tried to query some of  them but didn't have any luck. But I'm guessing that they somehow used these sources to work out a working figure for the magnitude of 6.080. I would have thought that any database would have a very close value.

Edited November 28, 2020 by Guest

[Marvin Jenkins]

I would advise to not look too far down the rabbit hole. I queried a load of data about the distances of quasars in relation to gravitational lensing and ARP’s ideas of incorrect redshift measurements. Wow it was quite a ride, got messy and no one came out the winner.
That’s astronomy, you have it down to three decimal places, that’s a good thing.

Marv

[Xilman]

16 minutes ago, woodblock said:

I completely understand that it's a small difference in magnitude. The issue I had was the way these data bases work. When the AAVSO quotes a magnitude for a star is that based on a single measurement by a single observer? Since the star is not variable I assume that it's brightness must have been measured several times and that the quoted figure was some kind of an agreed average. In the photometry data from the AAVSO site it lists 48 source references. Presumably these are other catalogues. I tried to query some of  them but didn't have any luck. But I'm guessing that they somehow used these sources to work out a working figure for the magnitude of 6.080. I would have thought that any database would have a very close value.

8 millimags is a very close value.

It is extremely difficult to discover, measure and account for differences at the 0.1% level.  The formal precision estimated from SNR considerations may be that low but systematic errors are a different kettle of fish.

[vlaiv]

1 hour ago, woodblock said:

I completely understand that it's a small difference in magnitude. The issue I had was the way these data bases work. When the AAVSO quotes a magnitude for a star is that based on a single measurement by a single observer? Since the star is not variable I assume that it's brightness must have been measured several times and that the quoted figure was some kind of an agreed average. In the photometry data from the AAVSO site it lists 48 source references. Presumably these are other catalogues. I tried to query some of  them but didn't have any luck. But I'm guessing that they somehow used these sources to work out a working figure for the magnitude of 6.080. I would have thought that any database would have a very close value.

Ideally, you want at least number of measurements, average value and standard deviation to be able to tell how accurate some measurement is.

Generally published catalogs don't often include that information, but if you search scientific sources, you'll get bunch of additional information on measurement in particular catalog. For example, here is screen shot of VizieR search on random mag 6 star that I picked (SAO 55330) - Hippocaros Main catalog:

Same star in ASCC:

http://vizier.u-strasbg.fr/viz-bin/VizieR-5?-source=I%2F280B%2Fascc&recno=462590

Shows error in V magnitude to be 0.008mag:

[Guest]

As a matter of interested I posted a comment about this on the AAVSO forum and had a reply which said that the entry for the star in the AAVSO data base had an uncertainty of 0.1  I think that means that the magnitude is 6.080 +/- 0.1  It seems silly to quote to 3 decimal places when the uncertainty is 0.1. My correspondent mentioned that the entry is old and probably needs revisiting.  They would probably not use this star as a comparison star these days because of its high uncertainty.

He also said that the source of the data was called The Bright Star Catalogue which he described as 'ancient' and no longer used.

This star was the comparison star and not the variable star. The comparison star information is stored in the variable star database (VSD). Access to this is restricted and is not available through VSX which is where I was looking.

Hope that is useful

Cheers

Steve