[Aavso-photometry] V0567 Oph

[arne]

Stan Walker wrote:
> Hi Shawn,
> 
> I read your post with interest. Two questions/thoughts.
> 
> Firstly, is ASAS3 calibrated? Arne has stated, I think, that it's just V 
> filter photometry but uncalibrated and untransformed. So if this is the 
> case it may well contain errors amounting to the slope of the response 
> plue the colour difference. Could this issue be clarified and referred 
> to in all future posts?
> 
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.

> Secondly, and this is more in Arne's territory, what is the point in 
> including ASAS3 data in the International Database? The implications are 
> that this is a repository for amateur measures. ASAS data is available 
> on line from their website in a rather more detailed manner than it can 
> be published in the ID. It has limitations which are detailed in their 
> database but it's impossible to carry the nature of these across to the 
> AAVSO area.
> 
The International Database is a repository of *all* variable star
data, whether amateur or professional.  Right now, we're archiving
everything we can find.  In the future, with the larger surveys coming
on-line, we may be using more Virtual Observatory links to access
photometry from other sources.

> As a user of both I am a little frustrated that I have to remove the 
> ASAS data - and throw it away - from measures I download from the IB. 
> The IB contains some good transformed BVRI measures which tend to be 
> submerged in the mass of slightly lower quality ASAS V measures.
> 
You must then be throwing away the visual observations too, along with
much of the single-filter CCD observations from other observers.  I don't
think it is a matter of ASAS or not-ASAS; it is what kind of data,
out of all the possibilities that are stored in the International
Database, that are most useful to you for your current research.

> Still on this topic, will not the inclusion of ASAS data tend to 
> discourage visual observers?  Being supplanted by a large scale 
> mechanical operation it is not a positive way of encouraging visual 
> measures. And the V of the Johnson UBV system is not equivalent to 
> classical visual measures which define the behaviour of variable stars 
> over the past century or two. I think both you and Arne need to think 
> much more seriously before these measures are added to the database!
> 
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.  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;
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;
I think it is improper not to present the best scientific
data for researchers.  The goal is to show all observers that
their observations are worthwhile, whether visual, PEP, CCD,
spectroscopy or whatever, and to provide guidance in pursuit of that goal.

> I'm not decrying ASAS's role but I think that it would be much more 
> useful in searching for new variables - do we need any more? - and 
> working on the low amplitude objects such as SRs with half magnitude 
> ranges. For the larger amplitude objects visual observing is adequate 
> and fits the historical methods much better. Why mess about with 
> something which isn't broken? Maybe you should get out behind a 
> telescope more?
> 
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.

I won't deny that most of us should get out behind a telescope more!
Arne