[Aavso-photometry] Comps for VZ Sgr

[Jim Roe]

arne wrote:
> Giorgio Di Scala has been kind enough to send me calibrations for
> three of the stars in the AAVSO sequence, based on a number of clear
> nights and Landolt standards.  For those who are following this
> RCB star as it fades, here are some bright comparison stars.  Use
> the old 'e' chart for their identification.
> ID       V            B-V          V-Rc         Rc-Ic        V-Ic
> L 109  10.98 (0.016)  0.33 (0.02)  0.20 (0.02)  0.24 (0.02)  0.44 (0.02)
> N 115  11.58 (0.016)  1.22 (0.02)  0.64 (0.02)  0.55 (0.02)  1.19 (0.02)
> P 117  11.78 (0.016)  0.28 (0.02)  0.19 (0.02)  0.22 (0.02)  0.41 (0.02)
> 
> The (B-V) colors on this calibration agree nicely with David Kilkenny's
> photoelectric calibration in 1983.  The V magnitudes are 0.04 fainter
> than Kilkenny; my guess is that this is due to the crowded nature of
> this field.  Use Di Scala's values for now for CCD work, and place
> "Di Scala" in the comment field of your submission.

Thanks for these, Arne.  I will apply them as soon as I resolve another 
problem.  I'm not satisfied with the state of my flat field corrections 
- maybe someone here can set me straight?

I'm using MaximDL (4.6, I think) and the whole calibration process is a 
mystery - take your bias images, your dark images and your sky flats and 
leave everything to MaximDL?  (I've posted this problem on the Maxim 
list, but no one deemed to respond.)

To test the flat field correction efficiency, I took the sky flats, used 
the Set Calibration routine to create a master flat and then applied the 
calibration to the sky flats.  I expect to see a flat, uniform image - 
but I don't - at least with the V filter (the Ic filter result is not 
too bad).  I get a bright ring structure which a little probing shows to 
be 3-5% brighter than the rest of the image.

My concept of a flat correction is to take an image of a uniformly lit 
field, normalize it and divide it into the light image.  By dividing 
such a flat image by itself, I expect to get the average intensity 
across the board.  Actually, the image doesn't have to be of a uniformly 
lit field - any image divided by itself should give 1.  What could be 
happening here and how do I fix it?  Please?

Jim Roe [ROE]