Affiliation
American Association of Variable Star Observers (AAVSO)
Wed, 08/15/2018 - 15:28
Hi, This relates to SeqPlot.
Just brought up a plot to make a seq for the Mira EU Aqr, and discovered two instances of what is clearly one star being shown as two (arrowed in the attached screen capture) since the chart in VSP showed that both of these stars were single. What causes this, and can it be 'cured'? Also, what happens in those cases where there is a genuine equal double (I'm thinking theta Ser off the top of my head) - how do we tell the true doubles from the fake ones?
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Hi Michael,
Try turning off catalogs so that you have only one showing at a time and see if the problem goes away.
I suspect that the "fake" doubles you are referring to are caused by slight differences in the star positions from one catalog to another. When you request to show data from more than one catalog, SeqPlot simply overlays them on top of each other. It makes no attempt to resolve differences.
I hope that helps.
-Sara
While a fantastic tool for the sequence team, what seqplot is not is a planetarium program.... the dots are shown only in a relativistic sense and do not reflect all stars in any given fov.
Those of us who do sequences realize that it is important for us, when considering a specific comparison star, to check a DDS image to avoid crowded doubles or triples; Aladin is a very rapid tool for this process while the VSP’s DSS images can also be used and to advantage as, if option selected, they will show all the variable stars, whereas, for several reasons, seqplot may not.
Another problem with the uninitiated, when using seqplot, is that the observer sometimes seems to not understand what the various survey limits are in terms of brightness and faintness.
Tim Crawford, Sequence Team
tcarchcape@yahoo.com
Thanks for that Tim. I have been in contact with Tom over this issue. It would be nice to have some sort of user guide for the uninitiated. For instance I know not to use Tycho mags for stars below about 11m but it then becomes a matter of realising the various vagaries of all the other catalogues!
I am just wondering how many non-team members use seqplot. Maybe it's easier and less trouble just to request a sequence via VSP!
I use SeqPlot regularly in search of suitable ensembles of comparison stars for my DSLR observing program. I never use the graphic, but download the table instead. As SeqPlot does not list star IDs in other catalogs, I usually need to cross reference to a planetarium program (I use Guide9) to get a catalog ID. Then I can use VizieR to obtain other information like Tycho BT and VT, spectral class, luminosity class and, if luminosity class is not available then parallax from GAIA DR2 so I can do a crude estimate. SeqPlot is useful primarily as a mechanism for identifying potential candidates.
Since I am optically limited to about 10V, Tycho2 data (from the 2000Hog+ reduction) are my preferred choice. For spectral classes and luminosity classes within the scope of Bessel's paper, they are very accurate. I realize that for dimmer stars and other luminosity classes the Tycho2 data are pretty much useless, but like I said that is not a problem for me, fortunately.
One thing that I have found curious is the occasional differences between data in the SeqPlot and VSP tables. Until I started doing DSLR work I routinely used the VSP tables - and still do, mostly, for visual work - without ever bothering to question the validity of their content.
One thing that I have found curious is the occasional differences between data in the SeqPlot and VSP tables. Until I started doing DSLR work I routinely used the VSP tables - and still do, mostly, for visual work - without ever bothering to question the validity of their content.
Small differences are most probably due to the fact that seqplot contains the most recent APASS data release (DR9), which typically will contain more nights of data, while the VSP's photometry table may very well have been sequenced with an eairler version of an APASS data release with fewer nights of data. Remotely, but still possible is that a few sequences were made using UCAC4 APASS data that had a limited number of nights of APASS data for some FOV's.
Going into the future, it is my most recent understanding, that with the release of DR10 all exisiting Sequences will also be updated with the DR10 data and that should then eliminate any small differences between what is noted in Seqplot vs the photometry table within the VSP.
Tim Crawford, Sequence Team