[Aavso-photometry] Bright star wide field CCD photometry

Fri Jan 14 20:37:10 EST 2005

Wolfgang, I have done experients with (somewhat) widefield photometry.  I
was using a 135 mm telephoto lens and an ST-7 camera.  There are numerous
up-and-running projects using short focal lengths.  E.g. ROTSE (in its
earliest incarnation), TASS, ASAS, Stardial.  The latter uses a 50 mm FL
lens.  Check out their web pages and publications for more information.

I did some "validation" tests, comparing my photometry to Tycho.  My data
matched Tycho magnitudes down to V=10 or so.  Below that, my scatter between
repeated measurements was less than the difference from Tycho, which
suggests that the Tycho mags are inaccurate (though there are colour issues
too).  Of course, it is well known that Tycho mags start being much more
uncertain somewhere below V=10.

Surprisingly, the critical determinant of how deep you can go (and get
reliable photometry) seems to be determined by focal length.  This is
because FL determines the plate scale (the size of area on the sky, in
arc-sec, captured by each pixel).  Once the pixel area becomes larger than
the PSF, the signal recorded does not increase but the noise does (as more
sky area is integrated).

Also, be careful about which filter band you choose to use.  CCD pixels have
intra-pixel (within the pixel!) sensitivity variations that are wavelength
dependent, especially e.g. the Kodak E chips.  The E chips are pretty
uniform at R, less so at V, and quite non-uniform within the pixel in B.  My
experiments did not evaluate this effect (I was using a V filter), but the
Kodak literature documents this (though not in the context of photometry!)

A very short FL is, of course, attractive for the wide field, allowing
numerous bright potential comp stars to be captured in the image.  However,
I think you will find that flat fielding errors will limit the accuracy of
your results.  For example, nowhere on the twilight sky is sufficiently flat
to make a "twilight flat" for a FOV that is 20 or 30 degrees wide!

A solid state photometer (SSP-3) is probably the best tool for bright stars,
as you can employ dimmer comps by simply integrating longer on them. Also,
as you go to very wide fields, you may need to consider extinction effects
within your CCD photometry.

Gary Billings

----- Original Message -----
From: "Wolfgang Renz" <w_renz at onlinehome.de>
To: "AAVSO-PHOTOMETRY" <aavso-photometry at mira.aavso.org>
Sent: Friday, January 14, 2005 10:45 AM
Subject: [Aavso-photometry] Bright star wide field CCD photometry

> Hello
>
> Has anybody ever tried to do photometry from images taken with
> short FL camera lenses ?
> Say 50 mm FL or even down to 8 mm (= real all-sky photometry ;-) ) ?
>
> Has anybody ever dared to try high precesion photometry on bright
> variables like gamma Cas, beta Lyr, alpha Ori, beta Per, alpha Sco,
> delta Sco, alpha Tau, ... using a CCD ?
>
> The brightest CCDV measure in the AAVSO DB I've found up to now
> was by Doug West on the SRB star W ORI (5.6 mag).
>
> The AAVSO PEP charts use comp and check stars that are often
> several degrees away and several mags fainter than the variable.
> To get such a wide field it would require a FL of ~ 100 mm or less
> for the ST-8 or ST-10  or  ~ 50 mm or less for the ST-7.
>
> The FOVs for the SBIG ST-10 are:
> f/mm   hor/°    vert/°   diag/°  "/pix  available
>    8  85.9224  64.0108  96.5576  175.00 *  photo lense
>   14  56.0387  39.3076  65.3100  100.00    photo lense
>   20  40.8607  28.0725  48.3235   70.00 *  photo lense
>   28  29.7991  20.2493  35.5351   50.00 *  photo lense
>   35  24.0330  16.2602  28.7565   40.00 *  photo lense
>   50  16.9494  11.4212  20.3465   28.00 *  photo lense
>   70  12.1501   8.1712  14.6082   20.00    photo lense
>   85  10.0180   6.7329  12.0513   16.50    photo lense
>  100   8.5213   5.7248  10.2541   14.00 *  photo lense
>  105   8.1169   5.4526   9.7682   13.40    photo lense
>  120   7.1051   4.7719   8.5520   11.70    photo lense
>  135   6.3173   4.2422   7.6048   10.40 *  photo lense
>  180   4.7401   3.1823   5.7072    7.80 *  photo lense
>  200   4.2666   2.8642   5.1373    7.00    photo lense
>  300   2.8451   1.9097   3.4262    4.70 *  mirror & photo lense
>  400   2.1340   1.4323   2.5699    3.51 *  photo lense
>  500   1.7073   1.1459   2.0561    2.81 *  mirror photo lense
> 1000   0.8537   0.5730   1.0281    1.40    8" f/10 SCT with f/0.50 reducer
> 1333   0.6404   0.4298   0.7713    1.05    8" f/10 SCT with f/0.67 reducer
> 2000   0.4269   0.2865   0.5141    0.70 *  8" f/10 SCT at prime focus
> All data except pixel size calculated via:
> http://www.mat.uc.pt/~rps/photos/angles.html
> Pixel size (and plate scale) calculated via:
> http://home.earthlink.net/~stanleymm/CCD_topics.html
>
>
> Clear skies
>   Wolfgang
>
> --
> Wolfgang Renz, Karlsruhe, Germany
> Rz.BAV = WRe.vsnet = RWG.AAVSO
>
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