[Aavso-photometry] Cluster Photometry

[bailyhill at aol.com]

Hello Arne;



As you know, we have 12 "Standards Stars" in our CCD Program, for folks 
to use.  They are nicely spaced in RA, so that there is roughly one 
every 2 hours.  These are there for AAVSO and other folks to use for 
various purposes.



I think these are Landolt sequences--but I am not sure.  Could you 
comment on their PT sequences?



If observers are not familiar with these, click on:  
http://www.aavso.org/observing/programs/ccd/standards/



Thanks

Gary













-----Original Message-----

From: Arne Henden <arne at aavso.org>

To: Eric.Broens at skynet.be

Cc: Aavso-photometry at mira.aavso.org

Sent: Fri, 25 May 2007 1:47 am

Subject: Re: [Aavso-photometry] Cluster Photometry













On 5/24/07, Arne Henden <arne at aavso.org> wrote:

> On 5/24/07, Eric Broens <Eric.Broens at skynet.be> wrote:

> > Hi Arne,

> >

> > Isn't NGC6811 a candidate?

> > I don't know if you could observe it on a sufficient number of

> > photometric nights from SRO during the campaign.

> > If so wouldn't these observations be suitable for determining the

> > transformation coefficients?

> >

> There are lots of potential candidates for "standard" fields.

> I've observed many fields on hundreds of nights, for example.

> Such fields are good for rought calculation of transformation

> coefficients, but the problem is how well do they actually

> match the Landolt standard system.  There are a number of

> tests that need to be made before you can trust the absolute

> quality of the data.  I haven't done all of the tests to satisfy me

> for more than a few such fields.

> Arne

>

I ought to explain this comment in more detail.  When dealing

with transformations, there are 3 things to remember:



1.  It is actually the Landolt System, not Johnson/Cousins.

While Arlo tries extremely hard to match the results from

Johnson and Cousins, it is not a perfect match (including

systematics in their original papers).  However, *everyone*

then uses Landolt's measures for determining their own

transformation coefficients.  Hence, we are trying to

emulate Landolt.



2.  Landolt never observed any clusters - he does single stars

scattered all over the sky, using a photometer with a fixed

aperture size.  So all cluster work is coming from someone

else, such as Henden, who is trying to emulate Landolt.

In general, we are not trying to do true standards work, so

are not using nonlinear fits to the transformation coefficients,

not doing extinction with as much care, or were using

less-than-perfect nights for our work.



3. CCD measures of wide fields are great for getting magnitudes

and colors for large numbers of stars, but such observations

have their own problems.  The dynamic range of the chip means

bright stars are saturated and faint ones underexposed.  Imperfect

flatfielding means you might have systematic errors across a field.

Blending of stars means an object might appear in one catalog

with one magnitude, but another catalog with a different magnitude

depending on resolution and aperture size.  You should also note

that red stars in clusters are usually intrinsically red, whereas the

red stars from Landolt are usually reddened objects that don't

have the molecular features.   Using them for transformations will

introduce small systematic effects.



So in general, you have to hand-inspect your data if you want it

to be truly standards-quality.  You want to compare your results

with others who have observed the same field.  These things

take time, and so I don't do it often.   Using "secondary standard"

clusters will work for most transformation determination needs,

but you should always be aware that such determinations usually

are systematically different than the Landolt system.  They may

be the only way you can get coefficients with your sky conditions,

and so don't avoid clusters, just consider using the primary

Landolt calibrators if conditions are favorable.

Arne

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