[Aavso-photometry] Differential transformation

[Chuck Pullen]

Geir - it is a valid approach, but I disagree with Radu in that I think you 
either have to use fields at the same airmass, and have photometric all sky 
conditions, or you introduce the effects of extinction.  Perhaps we are 
quibbling over a small amount of error, but it is so much less work to use 
a cluster, in my opinion.

Note that I have posted an airmass calculating spread sheet to the AAVSO 
web site that graphically shows the airmass of a number of standard 
fields.  So, you could easily plan your observations of multiple fields 
with a wide variety of colors at the same airmass during a photometric 
night, making the extinction issue moot.

Again, I'd suggest V and I rather than V and R, if you have an I filter.

Perhaps Arne Henden could chime in with his sage advise...?  :-)

Chuck


At 11:59 12/29/04 +0100, Geir Klingenberg wrote:

>Hi Chuck - Yes, I am trying to avoid extinction calculations due to my
>less than ideal site. I was hoping to accomplish this by only using
>*differential* magnitudes between stars on the *same* image. So the
>algorithm goes something like this:
>
>1. Take V and R images of a standard field.
>2. Calibrate and reduce, and find the differential magnitudes between
>all suitable stars. This gives a bunch of  (v1 - v2) and (r1 - r2)
>measurements.
>3. Record this along with the standard values V and R to a spreadsheet
>or similar.
>4. Go to another field and repeat step 1 to 3 until done.
>5. Use this data along with the equations from the first mail to
>estimate the coefficients.
>6. Go to bed.
>
>Does this sound a valid approach?
>
>Radu, that's and interesting web page, I will have a closer look even
>though I am a happy Windows user ;-)
>
>Geir