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> On behalf of the charts group at the AAVSO, I'd like to thank Mr. Otero for > pointing out some errors on a chart. As has been stated time and time > again, corrections and updates are an ongoing process. > I see no need however, to rant. When Mr. Otero's charts are flawless, and > he has several thousand charts of varying ages, qualities and sources, maybe > then I'll take him seriously. Until such time, there is a less public forum > for pointing out errors. I suggest he use it. And in a more professional, > respectful manner.
You can take me seriously or not. Meanwhile, you'll be plotting wrong stars
four times in two days.
I think that the addition of new errors has nothing to do with an ongoing
correction process.
It has nothing to do with quality or perfection.
When you mention different sources, this also doesn't mean that there aren't
better sources than others.
Those sources don't seem to be chosen.
Two consecutive plotting errors for two stars mean a careless attitude.
You had my imperfect -of course- but well-plotted charts for comparison and
a correct identification of AG, T, etc.
However, the careless and "unprofessional" attitude continued.
The other day I found that a comparison star in one of my charts was
variable. I had to update the chart in a hurry. It can happen. But I am not
talking about it.
I think that if there is not a Henden-Sumner sequence ready to use or
something already done at hand, the resulting chart won't be good. It
shouldn't be like that.
Besides wrong identifications, the confussion of Hp and V magnitudes, the
use of variable stars that could be worked out with some research and a lack
of priorities regarding the accuracy of the different catalogues is also a
mater of concern.
Hipparcos and Tycho-2 are included as if they were equally accurate and for
most of the stars with Hipparcos (and PEP from the ground) data availbale,
those data weren't used.
I think that the huge amount of work in existence is always a justification
to keep doing it the wrong way right now.
About the publicity of these kinf of things, I certainly think that it's
important to alert all the people regarding those errors.
I ususally met people totally n love with AAVSO charts, even those that
contain this kind of mistakes.
Mu point is trying people to judge and think by themselves about a chart's
quality, even one of mine or De Bernardini's charts.
Regards, Sebastian.
Hello all. Sorry, but I felt the need to throw in my two cents.
I am a visual observer for the AAVSO, and have yet to tackle any kind of CCD observing or other electronic methods. I've been reading these latest entries with some interest. I am also a little concerned about the tone the e-mails are taking.
It seems to me that any project as intricate as producing charts for thousands of variable stars cannot be error-free, and that these errors take some time to find and correct. And yes, it's logical to assume that in the correction process, a few more errors could be introduced accidentally.
In spite of that, isn't it accepted that as long as the observer logs which comparison stars were used and which chart was used, any magnitude estimate of a variable star can be corrected once any errors in comparison stars are found and corrected? (Unless of course a comparison star is found to be variable?)
It just seems to me that the important thing is to make the estimate and get all of the information down. The corrections, if need be, will come in time.
Paul Zeller (ZPA) Indianapolis, IN
>> In spite of that, isn't it accepted that as long as the observer logs which comparison stars were used and which chart was used, any magnitude estimate of a variable star can be corrected once any errors in comparison stars are found and corrected? (Unless of course a comparison star is found to be variable?)
It just seems to me that the important thing is to make the estimate and get all of the information down. The corrections, if need be, will come in time.
Paul,
the problem is that when the variable star can not be plotted correctly, no
correctrion is possible!! You'll be estimating the wrong star!
I don't want to bother anyone, just to ask for more care when marking
variable stras in the charts!
Sebastian.
Paul wrote:
>I am also a little concerned about the tone the e-mails are taking.<
I understand Sebastian's exasperation with this particular issue. He has had good charts for eta Car published for quite some time. He is more familiar and experienced with observing this particular field than most observers, and more than likely, the chart team that assembled the latest AAVSO charts.
The errors he pointed out in the first post were not correctly addressed, even after pointing to the url for his charts, so it appears that his offer to help and the reference to his charts was rejected out of hand. The discrepancies noted were obvious to me upon a cursory examination of the Otero charts and the revised AAVSO charts.
Sebastian does make excellent charts. I can tell you from experience that his chart and sequence for V Hya makes a lot more sense to me visually than the AAVSO chart for this beautiful red star.
>In spite of that, isn't it accepted that as long as the observer logs which comparison stars were used and which chart was used, any magnitude estimate of a variable star can be corrected once any errors in comparison stars are found and corrected? (Unless of course a comparison star is found to be variable?)<
This is stated over and again to be the case, but in the end I think it will be found to be more difficult than is thought, or impossible in some cases all together because of the nature of some of the errors that exist on the charts.
For example, if the comp stars are not assigned values that are in fact sequential from brightest to faintest no transformation process will yield reliable data. Garbage in, garbage out. The number of charts with this specific problem is disappointingly great.
And as Sebastian pointed out, if the observer actually estimates the magnitude of the wrong star there is no way to recover any useful information. Again, there are in fact many charts with variables mis-identified to this day.
>It just seems to me that the important thing is to make the estimate and get all of the information down. The corrections, if need be, will come in time.<
On the other hand, it seems to many observers that to wait for corrections to charts that could be improved now with existing photometric data in order to maintain the integrity of a database is more a political decision than a scientific one. I think that is what Sebastian was referring to when he wrote "I think that the huge amount of work in existence is always a justification to keep doing it the wrong way right now."
You may disagree with his methods or his tone, but it is hard to argue with his results. A light curve of delta Sco based solely on Otero's observations will prove to be more coherent and scientifically useful than the AAVSO data for that star. I'd venture to guess the comparative data for eta Car for the time he has been observing it will show the same results.
From what I have heard and read, eta Car is a particularly challenging target. Good luck to you all who observe it. Perhaps a few pointers from Sebastian would prove more interesting and positive than the re-hashing of the old chart/sequence debate.
Regards, Mike Simonsen
Re: chart problems (was: More and more and more .)
> [...] > Besides wrong identifications, the confussion of Hp and V magnitudes, the > use of variable stars that could be worked out with some research and a lack > of priorities regarding the accuracy of the different catalogues is also a > mater of concern. > Hipparcos and Tycho-2 are included as if they were equally accurate and for > most of the stars with Hipparcos (and PEP from the ground) data availbale, > those data weren't used. > [...]Having seen this kind of confusion so many times, I see there is a need for some sort of accrediting system (though I don't want to mean an authorization system, but I am inclined to provide some "self-check" points) for chart makers. The similar things would be valid for authors of astronomy software packages (e.g. star plotting software, photometry software etc.).
[Things should be considered before any eyepiece check or release]
* Do you correctly recognize the difference between Hp and V magnitude?
* Do you correctly recognize the influence of color index on visual
photometry?
* Do you correctly recognize the limitation of Hipparcos/Tycho photometry?
* Do you correctly recognize the limitation of the GCVS?
* Do you correctly recognize the limitation of the GSC?
* Can you discern primary and secondary catalogs?
* Can you discern catalogs which should be avoided?
* What sources or catalogs of photometry can you use?
* Have you been carefully following the recent information of the target
objects from various sources of information, including electronic
discussion groups?
* Do you have sufficient astronomical understanding of the target objects?
* Do you know alternative methods to check the correct identifications
of certain variable stars?
* How many methods do you use to exclude potentially variable objects
from comparison stars?
* What method do you use to exclude duplicity-induced potential errors
for comparison stars?
* What criterion do you use to confirm that the sequence selection
is adequate for observers?
* What kind of efforts for charts do you make to avoid observer's errors?
* Can you be careful enough to check as many thinkable points as one can?
* Do you have sufficient time to regularly undertake the above jobs?
[Follow-up]
* Do you maintain the backlog system for recording past errors?
* Can you promptly respond to requested corrections?
* What kind of continous improvement of the quality are you trying to
make?
* What action do you take if there is a complaints in your charts,
or delayed updating process? Make an excuse or something else?
[Any additions are welcome]
Regards,
Taichi Kato
> Having seen this kind of confusion so many times, I see there is a need > for some sort of accrediting system (though I don't want to mean > an authorization system, but I am inclined to provide some "self-check" > points) for chart makers. The similar things would be valid for authors of > astronomy software packages (e.g. star plotting software, photometry software > etc.).Taichi,
I think it would be useful to make some suggestions "a la Brian" regarding
catalogues heirarchy.
I have been comparing different data for a long while now and the different
sources are really different.
Any of them has some advantage at some point and that is what makes the task
even more difficult. But noone said it would be easy... ;-))
This is mostly for bright stars since those are my targets.
1) PEP photometry from the ground. The GCPD is a great source of data but VizieR can include even more if needed (For bright stars it's generally not the case, but in some examples just like the variable comparison star in the AAVSO "e" chart of eta Car, it can help). Of course, Arne's photometry and the great LONEOS when you go fainter.
2) Hipparcos magnitudes properly transformed to V. (stop saying Tycho is better. It is not. Excepting for M stars when no convertion is possible)
3) Tycho-1 for stars brighter than 8. (always transformed to V, not the Vt mag.)
4) Tycho-2 for stars fainter than 8 and brighter than ~9.5. Tycho-2 magnitudes (transformed to V) suffer for some systematic color -term that makes white and yellow stars to appear with a whiter B-V color (peaking around B-V 0.5 with a difference of 0.06-0.07 mag.) and white and blue stars to appear with a V mag 0.01-0.02 mag. brighter than Johnson's V. In Tycho-1 this effect is less pronounced and thus negligible, but accuracy falls steeper beyond 8th magnitude.
5) (Only now!) ASAS-3 data (PROPERLY CHOSEN according to aperture and star's magnitude) for stars fainter than 9th magnitude
6) UCAC-1 data transformed to V when there are not ASAS-3 data and correcting for zero-point errors, but with uncertainties up to 0.3 mag. sometimes (although I usually find results better than that)
7) I think a visually look at the field through an experienced observer's eyepiece would be more reliable than the catalogues left..... ;-))
I agree with most of your suggestions concerning what one must have in mind before mking a good sequence.
Regards, Sebastian
>> 4) Tycho-2 for stars fainter than 8 and brighter than ~9.5. Tycho- >> magnitudes (transformed to V) suffer for some systematic color -term that >> makes white and yellow stars to appear with a whiter B-V color (peaking >> around B-V 0.5 with a difference of 0.06-0.07 mag.) and white and blue stars >> to appear with a V mag 0.01-0.02 mag. brighter than Johnson's V. >> In Tycho-1 this effect is less pronounced and thus negligible, but accuracy >> falls steeper beyond 8th magnitude.Do you have some plots demonstrating this with respect to primary standards (specifically the Landolt and Cousins E-region standards)?
What transformation are you using to get Johnson V from VT? Bessell? The "official" Tycho-consortium transformation is not adequate at the level you're talking about, and might introduce just the errors you are noticing.
\Brian
Another point to be considered in creating charts for variables-
Be aware of the previously existing sequences for a particular star and the ramifications of wholesale changes to the sequences on the historical data.
Of course, adding this to the list of "qualifications" for accredited chart makers narrows the list of potential candidates to a select few, who are probably professional astronomers that would have little or no interest in such a job. That is why much of this work has fallen to dedicated amateurs.
(I've always thought the charts for visual use should be made and checked by experienced visual observers, which would eliminate most professional astronomers too.)
For the most part, chart makers have done an admirable job with the information available. The charts served the needs of most observers with modest equipment, and a lot of good work has been done using them.
However, we have now come to a point where the needs and equipment of serious amateur observers has far outstripped the utility of the more antiquated charts. Most of the telescopic sequences only reach into the 13th or 14th magnitude, while many of us routinely observe stars visually in the 15's and 16's. And the accuracy of 14th mag comps based on pv magnitudes is suspect at best.
The majority of these charts are nearly useless for CCD observers, mainly because they were never intended to be used for this, so they do not match the needs or potential accuracy of CCD observers. Most have not even been updated with accurate magnitude information for the bright end of the sequences where good photometry does exist.
Unfortunately, little significant improvement will ever be made until there is an accurate source of magnitude and color information for the whole sky down to at least mag 16, and investing the time and money to rectify the chart situation is deemed a priority.
Mike
Brian,
I am using Hog (2000) tranformations as stated in the catalogue:
V= Vt-0.09*(Bt-Vt)
B-V= 0.85*(Bt-Vt)
The relation is for ALL stars I used for ALL sequences. (I don't have this specific difference recorded in my files) No zero point differences. V values are from GCPD data and show a ery clear trend. This table is a good example:
Star Vmag B-V / Tycho-2 Vmag B-V mu Col 5.17 -0.28 / 5.141 -0.255 eps Lup 3.37 -0.18 / 3.355 -0.165 iot Lup 3.55 -0.18 / 3.517 -0.165 ome1 Sco 3.95 -0.05 / 3.934 -0.049 eps Aqr 3.77 0.00 / 3.757 0.009 lam Boo 4.18 0.08 / 4.173 0.079 alp Vol 4.00 0.15 / 3.995 0.148 n Cen 4.26 0.22 / 4.255 0.197 eta Cru 4.14 0.35 / 4.144 0.305 gam Tuc 3.99 0.40 / 3.994 0.362 gam Pav 4.22v? 0.49 / 4.230 0.431 ups And 4.09 0.54 / 4.099 0.496 bet Hyi 2.79 0.62 / 2.793 0.588 del Vol 3.97 0.78 / 3.973 0.738 d Vel 4.06 0.87 / 4.054 0.843 eps Oph 3.23 0.96 / 3.219 0.960 del Crt 3.56 1.12 / 3.556 1.116 eps Crv 3.00 1.33 / 2.994 1.355 eps1 Ara 4.06 1.45 / 4.063 1.463 0.06 mag. bluer B-V at 0.5. 0.02 mag. brighter V for blue stars.Is there some transformation that corrects this difference?
Sebastian.
Some time ago, Arne Henden provided these transformations to the TASS group. He has stated recently that they aren't as good as possible, but they are the best I have at the moment.
Given color -0.2 < (Bt - Vt) < 1.8
B = Bt + 0.018 - 0.2580*(Bt - Vt)
V = Vt + 0.008 - 0.0988*(Bt - Vt)
As a bonus, he threw conversions to U, R and I (though of course
these are less accurate than the others, since they extrapolate
quite a distance across the spectrum).
You can read his message explaining the conversions at
http://stupendous.rit.edu/tass/mailarchive/2001-02/msg00055.html
Michael Richmond
Since it appears the (probable) "best" transformation of the Tycho photometry to Johnson V and B-V needs to be sent around again, here is Bessell's look-up table, followed by a very useful polynomial for the VT-to-V conversion from Mike Linnolt.
Briefly, Bessell used the E-region primary standards data for his analysis, showing that the standard Tycho-consortium transformation is not very good and is easy to improve. He published only a look-up table, and not an analytical function, but Linnolt has recovered this at least for getting V. The linear consortium transformation is the source of the systematic errors that Sebastian has noticed.
\Brian
===============================================================================
Corrections to put Hipparcos/Tycho data on the standard V and B system
from: 2000PASP..112..961B
BESSELL M.S.
Publ. Astron. Soc. Pac., 112, 961-965 (2000) [July 2000 issue]
The Hipparcos and Tycho photometric system passbands.
TABLE 2
RELATION BETWEEN BT-VT AND HIPPARCOS/TYCHO DATA FOR B-G
MAIN-SEQUENCE STARS AND K-M GIANTS
BT-VT V-VT del(B-V) V-HP
-0.250... 0.038 0.031 0.066
-0.200... 0.030 0.021 0.051
-0.150... 0.022 0.011 0.036
-0.100... 0.015 0.005 0.021
-0.050... 0.008 0.002 0.006
-0.000... 0.001 -0.005 -0.011
0.050... -0.005 -0.010 -0.025
0.100... -0.012 -0.017 -0.038
0.150... -0.018 -0.020 -0.048
0.200... -0.024 -0.021 -0.058
0.250... -0.029 -0.023 -0.069
0.300... -0.035 -0.025 -0.079
0.350... -0.040 -0.025 -0.087
0.400... -0.045 -0.026 -0.094
0.450... -0.050 -0.030 -0.101
0.500... -0.054 -0.035 -0.108
0.550... -0.059 -0.045 -0.114
0.600... -0.064 -0.051 -0.120
0.650... -0.068 -0.060 -0.127
0.700... -0.072 -0.068 -0.131
0.750... -0.077 -0.076 -0.134
0.800... -0.081 -0.085 -0.137
0.850... -0.085 -0.094 -0.142
0.900... -0.089 -0.104 -0.147
0.950... -0.093 -0.113 -0.151
1.000... -0.098 -0.122 -0.155
1.050... -0.102 -0.131 -0.158
1.100... -0.106 -0.142 -0.157
1.150... -0.110 -0.154 -0.160
1.200... -0.115 -0.166 -0.162
1.250... -0.119 -0.178 -0.164
1.300... -0.124 -0.189 -0.166
1.350... -0.128 -0.199 -0.166
1.400... -0.133 -0.210 -0.165
1.450... -0.138 -0.222 -0.164
1.500... -0.143 -0.234 -0.161
1.550... -0.148 -0.245 -0.157
1.600... -0.154 -0.256 -0.153
1.650... -0.160 -0.266 -0.148
1.700... -0.165 -0.277 -0.143
1.750... -0.172 -0.288 -0.137
1.800... -0.178 -0.299 -0.131
1.850... -0.185 -0.309 -0.125
1.900... -0.191 -0.320 -0.119
1.950... -0.199 -0.331 -0.112
2.000... -0.206 -0.342 -0.106
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