[Aavso-photometry] Differential transformation

Thu Dec 30 10:00:20 EST 2004

Arne and Chuck - I'm sorry I keep on nagging but I'm not sure I get
it. How does extinction affect the results as long as I don't mix
stars from different fields (that is for v1 - v2 both star 1 and 2 is
from the same image)? I understand that there can be extinction
effects between the top and bottom of an image but in that respect
there's no difference between using one or many fields.

I agree with Radu that there are pros and cons here. On one hand it is
important to get stars with great color variation and also high SNR.
But that is difficult in crowded fields and I get only a handful of
"good" stars from M67 and NGC 7790 due to my large FOV. So it makes
sense to pick the best from many fields.

On the other hand one should account for extinction effects. But the
error should not be too large if one stays at low arimasses. And it
might be that this is a lesser evil than getting extinction
coefficients with large errors due to a not-so-photometric site.
Errors in the extinction coefficients will of course in turn affect
the transformation coefficients.

Geir

On Wed, 29 Dec 2004 09:01:56 -0700, Arne Henden <aah at nofs.navy.mil> wrote:
> Chuck's answer is essentially complete, and Radu has an interesting
> technique, but I thought I'd toss in a couple of more comments.
>   First, even if you are using a single field, don't forget that
> you cannot always neglect extinction.  At high airmass, there is
> a significant airmass (and extinction) difference between the
> top and bottom of even a small field.  What is "significant" depends
> on the desired accuracy.
>   Second, please don't use my normal field calibrations for
> determining your coefficients.  Please use Landolt (and Cousins)
> standards only.  While I am a careful observer, I will not guarantee
> that my field calibrations are exactly on the Landolt system, and
> if you use my calibrations, you basically remove yourself one more
> step from being on the true standard system.  My field calibrations
> are primarily for determining the magnitude and color zeropoints
> for differential photometry.  I know I am pretty darn close, and
> for rough values you will be fine.  For precise measures, a reviewer
> will question the use of Henden photometry in this manner as systematics
> have not been investigated.
> Arne
> 
> 
> Radu Corlan wrote:
> > On Tue, 28 Dec 2004, Chuck Pullen wrote:
> >
> >
> >>Geir - in an ideal world, your approach would be perfect.  However, for
> >>determining transformation coefficients, you want a wide range of color as
> >>well as stars of about the same magnitudes so they have about the same
> >>signal to noise across the color range.  And, of course, the stars need to
> >>be non variable, and calibrated against a known data set, such as Landolt
> >>fields.  So, the odds of finding all of these requirements in your 15' by
> >>15' image for a given part of the sky are pretty slim!
> >
> >
> > Chuck - one can use Geir's approach and combine stars of different colors
> > from different fields, they don't need to be all on one field. If you have
> > enough stars with good photometry (for instance some of Arne's sequences,
> > which are spread across the sky) the errors reduce pretty well.
> >
> > I use that approach (see http://astro.corlan.net/gcx/html/node9.html ) for
> > a description and some example graphs.
> >
> > Radu
> >
> >
> >
> >>The good news is that there are fields that do meet all these requirements,
> >>such as M67 and NGC 7790.  No extinction calcs are necessary, and you can do
> >>your BVRI images on the same pointing.  And yes, you can get better
> >>statistics by calculating your coefficients over several nights, and
> >>averaging them together.  This is because transformation coefficients don't
> >>change all that much over time, unless you change something significant in
> >>the optical path or in your CCD. Data for these fields is available from
> >>Arne Henden's FTP site at
> >>ftp://ftp.nofs.navy.mil/pub/outgoing/aah/sequence/.
> >>
> >>They can be crowded for amatuer focal lengths, so pick your stars carefully.
> >>And note that some are variable, so look for ones on the edge of the fields,
> >>with the most observations and least associated error.
> >>
> >>By the way, if you have an I filter, you might get better color data with
> >>V-I rather than V-R.
> >>
> >>Chuck Pullen (PCH)
> >>
> >>----- Original Message -----
> >>From: "Geir Klingenberg" <geir.klingenberg at gmail.com>
> >>To: <aavso-photometry at mira.aavso.org>
> >>Sent: Tuesday, December 28, 2004 2:23 AM
> >>Subject: [Aavso-photometry] Differential transformation
> >>
> >>
> >>
> >>>Hi
> >>>
> >>>The methods I have seen for estimating the transformation coefficients
> >>>involves using instrumental magnitudes in a LS solution. But - if one
> >>>is only doing differential photometry and don't need to be concerned
> >>>with extinction - is there anything wrong in using a differential
> >>>approach? More specifically, if we ignoring extinction the relation
> >>>between the stars magnitude, instrumental magnitude and color can be
> >>>written in the usual form as
> >>>
> >>>V = v + a*(v - r) + b
> >>>V - R = c*(v - r) + d
> >>>
> >>>This is for the V and R band - capital letters are magnitudes and
> >>>small letters are instrumental magnitudes. So, subtracting magnitudes
> >>>of star 1 and 2 and rearranging we get
> >>>
> >>>(V1 - V2) - (v1 - v2) =  a*( (V1 - R1) - (V2 - R2) )
> >>>(V1 - R1) - (V2 - R2) = c*( (v1 - v2) - (r1 - r2) )
> >>>
> >>>which can be used to estimate a and c.
> >>>
> >>>Since all instrumental magnitudes in these equations are on
> >>>differential form  between stars on the *same* image, won't the
> >>>solution be more robust against sneaking cirrus clouds and changing
> >>>atmospheric conditions? And can measurements from different nights be
> >>>used to improve the coefficient estimates?
> >>>
> >>>Geir Klingenberg
> >>>_______________________________________________
> >>>Aavso-photometry mailing list
> >>>Aavso-photometry at mira.aavso.org
> >>>http://www.aavso.org/mailman/listinfo/aavso-photometry
> >>>
> >>>
> >>
> >>
> >>_______________________________________________
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> >>
> >
> >
> 
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