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[vsnet-chat 3670] Re: re Overobserving & visual magnitudes.

Hi Sebastian,
 
I was reading your item with interest - some famiiar stars in here such as T Cen, the first variable I ever observed. Which raised an intriguing point about the magnitudes used by the visual people.
 
In theory the V of the UBV system is outside of the atmosphere so that it will not match the eye's response anyway. I note that Stanton derived some corrections to transform V into v as a function of B-V. They were the opposite of what would be expected in that he found the eye sees blue stars brighter, not fainter, than one would expect. But the interesting point is that the various VS groups use V values without correcting them to 'inside atmosphere' values. Not normally a problem with +/- 20% estimates but you mention using two decimal points in your estimates. The eye also sees a little into the near UV so that at that level the colours of comparison stars become quite important. Differences between red/blue pairs 30 degrees above the horizon can easily change by a couple of tenths from the values at the zenith. 
 
So if you're going to be this accurate you need to be careful about the altitude of objects. Which is another reason why I'm a little dubious about some of the low amplitude, fast variations reported by visual observers. But for what it's worth, I used to measure Betelgeuse against Rigel (itself variable) Procyon and a couple of others in the far off days when I observed visually. It had quite noticeable variations then. This is similar to your own work with delta Sco and others.
In this I agree with Taichi Kato - noone ellse is going to systematically check magnitudes of brght stars. I had a colleague who I measures Betelgeuse, Rigel and Antares with but he was always worried that we might blow up the photomultiple tube! So when I shifted north the programme lapsed.
 
Regards,
Stan
----- Original Message -----
Sent: Sunday, October 01, 2000 1:17 PM
Subject: [vsnet-chat 3665] RE: re Overobserving

Hi, people:
                   Regarding methods, I've never been keen on Pogson's. It puts a limit to accuracy and relies on rounded off magnitudes. A star of 7.14 will be a 71 star and a 7.15 will become 72, and there is not one full Pogson step between them (furthermore, there is no difference at all).
                   I always use two decimal values for the magnitudes and make the rounding offs after the estimate was made. The delta Scorpii phenomenom was found that way. And you can see that B.Fraser photometry is almost completely the same as my visual observations. As John said it's very important which stars you are using but the method is very important too. There's no point in rounding off magnitudes, specially of bright stars which are extremely accurate: we're only making the estimate worse if we round off the values or get stuck to 0.1 mag. steps.
                  I've always been checking my own estimates and, with the naked eyes, provided a good sequence and a not-too red color, the errors are never greater than 0.03 mag.
                 But in the case of red stars - as all the miras and most of SR's discussed here-, I think this accuracy is almost impossible to get (- Never said "impossible".. :-) ) because of less availability of a proper sequence and because they are not naked eye objects. Even if they were the Purkinje effect and other color problems get worse the brighter the star: Mira may be bright but it's always a challenge....
                 And I agree with Stan in that less people (even one or two), following one star will be capable maybe to see short amplitude variations. But it's no use to make this kind of observation when results are going to be mixed with 99 other people's numbers.
                 I've just sent a confirmation for two red variables in Centaurus and Carina. If I only follow my observations I can find periodicities, trends and the like, but in the case of red stars I know that the mixed result will be dissapointing. Last months I could see the incredible one full magnitude difference between my estimates and E.van Ballegoy ones for the interesting changing-period mira R Cen. My 7.6's were his 8.6's. I thought maybe we have different (very different...) responses to red, but then I found that sometimes this difference became almost 0 and that when I watched the star fading he watched it rising and viceversa. And sometimes he observed T Cen brighter than me, so no correction for color was possible here.
               The point of "self-control" is interesting, because my 7x50's limitting magnitude here in BA is 8.5, and for red stars, you know it's brighter, maybe 8.2. So it would have been impossible for me to observe very clearly (not a glimpse but a sharp clear point) R Cen if it was at 8.6....
 
                  But it's ok, I think there hasn't got to be "rules" and that people has to do what they want..., as long as it is useful. So, it's importnat to check out how you (yourself) are doing with your estimates. People seem to like playing the "estimating game" and that would be good if the results get lost in their own folders (or PCs). But if it's not like this, one should be more careful.
 
                
>   This is an example of a caveat to non-Pogson-type (I don't know the
>exact terminology) visual estimates.

The BAA call it the "fractional method" I think.  I better not get in to an
argument on various methods... ...I _personally_ have some suspiscion re
the smoothing caused by utilising the Pogson method, as I never got good
enough to split things into tenths of a magnitude.  Sebastian as of course
shown that it is quite possible, but it is very dependant on the quality of
the sequence, which itself is dependant on the available nearby stars!

So, I don't think any scheme is better than any other, just different in
what happens when it is plotted on a folded lightcurve.

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