[Aavso-photometry] V0567 Oph

[Sebastian Otero]

> It is untransformed (as is currently the case with most amateur
> data, as most observers currently are just using a V filter).  It is
> calibrated in the sense that it uses the Tycho2 catalog for its
> zeropoint, and so is close to the standard system except for extreme-color
> objects.  However, I haven't seen a good comparison of recent ASAS
> photometry vs. standard photometry to see how well the zeropoints
> match.  Pojmanski says 0.05mag errors for the zeropoint offset,
> with 0.01mag differential error on well-exposed stars.  I recommend
> reading Pojmanski, G., 2002, Acta Astronomica, 52, 397 for more
> details.

Remember that the original reduction process has been changed. Originally 
the zeropoints were really good but now they can't be off by 0.05 mag. You 
will always have a GCPD star in a given field to calibrate the zeropoint. 
The zeropoint offset depends on th field so you can't make a single 
correction, you need to work on an individual basis. Some fields in Lepus 
are okay but generally we are talking about a differnece of 0.02 or 0.03 mag 
in the sense ASAS-3 V is fainter than Johnsons' V.


> This can be an issue, but it won't go away and will only grow worse
> as more and more data are released from various surveys.  Amateurs
> did not stop observing when NSVS released a year's worth of ROTSE-I
> data.  They did not stop observing when SDSS released their Stripe 82
> data.  The main thing to note is that each survey covers a certain
> parameter space, and visual observers may need to move to a different
> parameter space.


Well, we can't compare a living database like ASAS, with data coming online 
almost daily with surveys that were active only for a limitted time span.
But even then, I think that visual observers will always have targets to 
follow. It is not the same now to observe a 10th mag. southern mira than it 
was before but that's not a bad thing. Now we need to focus on other things. 
We will never run out of targets.


> With respect to ASAS, the northern hemisphere data
> has not been released; it saturates at 8th magnitude; observations
> much fainter than 12-13mag have the same error as visual estimates

It depends on exposure time. Since 2003 the limit got brighter, around V= 
6.5 or so (With care even brighter for some fields and dates, but again, 
nothing straightforward and in the time series mode the limit is fainter 
again due to different exposure time)


> the large pixels mean severe blending in crowded fields such as
> Saggitarius; its cadence is not sufficient for transient objects,etc.
> I think it is improper to hide survey data from the observers;


The data are not hidden. One can go to the source. I prefer to do that to be 
able to work with all the flags and vagaries of each database. But it's a 
matter of preference. Some will like having all at one place. For this kind 
of thing I prefer not.


> Actually, ASAS has been used for searching for new variables; there
> are some 50,000 variables in its on-line catalog.  You always need
> more variables - knowledge of a given field guards against the
> use of a variable star as a comparison star; statistical samples
> of specific classes can be drawn; finding the rare pathological cases
> requires the discovery of lots of common variables.  A survey gives
> you both: discovery of variables, plus monitoring of all variables
> within its parameter space.

There are far more ELL or ACV variables than we had thought. There are 
probably new types hidden behind other classifications. Finding double-mode 
galactic pulsators, DPVs and other interesting stars is now easier looking 
at the very large numbr of light curves. But I don't think Stan was saying 
the opposite. It is more a question on mixing ASAS and the rest of the 
observations in the database.

Best wishes,
Sebastian.