Charts and Sequences

Hi, all,I wanted to share some experiences about the charts and sequences for our project.The first thing is that VSP-generated charts have a size limit (15º) and that is not enough for our purposes. Specially if we want more than one variable to be included on the same chart.We will probably have to make them with a planetarium software and then customize them to our needs, adding the comp stars magnitudes and information on the variable stars.(That is an issue we need to discuss: do we want data on each star on the chart or not? I vote for yes, at least magnitude range and type of variability. I wouldn't include period so people look at them without being biased by knowing what the star should look like)If charts are not going to be directly made using VSP, this means sequences doesn't strictly need to be entered in the Comparison Star Database in order to do the charts.I am consulting the Northern chart makers to see exactly how they dealt with this.About sequences.Revising a star sequence or choosing the best stars for this project is not so easy.The stars that I think ought to be chosen in this case may not be the same as the stars selected to make estimates by experienced observers.The main point is the ability to detect small brightness differences. While choosing comp stars with small brightness differences between them will shrink the resulting error for experienced observers, beginners may find too difficult to detect 0.1 or 0.2 mag. differences. Thus, some aditional comp stars will be selected.E.g.: l Car varies between 3.32 and 4.12, thus a 33 comp star would be enough. But even though a 33 and a 35 comp stars could do it (estimating l Car as = 33), a brighter comp star (in this case a 30) will help newcomers to see the brightness differences between stars more clearly.Other factor that has to be taken into account to select the comp stars is the star's colour. It's always better to have comp stars of the same colour and -if possible- of the same colour of the variable star.Sometimes Mother Nature won't let us get away with that and if we want to have the same colour for all of our sequence stars we'll end up with some gaps in the sequences. We have to choose to lose accuracy by using comparison stars of very different magnitude or using comparison stars of different colours.AAVSO policy is to avoid using red comparison stars. They could be variable and every observer may have a different color response.However, when the variable is red, estimating it with blue stars will cause a systematic deviation from the standard V-scale. It's all about choices.What is very interesting is that in the l Carinae area we have a lot of potential comparison stars (between mag. 3 and 4) but several of them turn out to be variable so we can't use them!There is a 33 that sometimes can be a 32 or a 34, even a 35...There is a 34 that is an irregular giant of low amplitude but we can't be sure that the amplitude won't grow...There is another 33 that is not catalogued as variable but... it actually is. You can find several of these facts by researching to create a sequence.Actually this potential 33 comp star, omega Carinae, has varied by only 0.02 mag, which would make it acceptable for isual use (we couldn't detect such a small variation) but the problem is that this is a Be star and the slow 0.02 mean magnitude trend that I found means the star ha a disk around it and this in turns means thet the possibility of a larger amplitude outburst can't be ruled out. So, I had to drop two 33 comp stars.A 35 comp star that varies 0.04 mag. was retained in the sequence because its nture doesn't suggest further variability.You have to check the star's spectral type or variability type. A Be star can surprise you but other types of star won't. For example there is a 40 comp star that varies 0.02 mag. every 22 days, and this won't change in the future so if I wanted to include it, it would be a safe choice.So selecting 4 or 5 stars is not so easy sometimes!Now, back to the end product, I think that we shouldn't go deeper with these issues (e.g. colour) by now. We don't want to scare people away so let's keep it as simple as possible.If there are some problems during the observations we can then discuss them and go a step further. Let's cross that bridge when we come to it.So please, take a look at the charts in the Northern tutorial:http://www.aavso.org/sites/default/files/10startutorial.pdfand see if you want our charts to be like those or with more information included.Cheers,Sebastian.