[Aavso-photometry] Bright star wide field CCD photometry

[David Boyd]

Hello Wolfgang,

About 2 years ago I made a series of measurements of AT Dra (V~5.7) over a few
months using a 35mm lens mounted directly on a HX516 with a V filter between the
lens and the camera. I needed an exposure of 60sec to get around 50% saturation
on AT Dra. The wide fov (about 8x6 deg) was necessary to get suitable comparison
stars on the frame.

I backed off one stop from full aperture on the lens to improve star image
quality. The image scale was 44 arcsec per pixel so I needed to defocus to get
star images to cover more than one pixel. It is particularly important to spread
star images over more than one pixel on the HX5 because of its physical
structure (interline transfer with microlenses). However with this mid-quality
lens I could only defocus so much before the star images started to flare
unacceptably. The result was a compromise at about 2 pixel FWHM.

30 images taken consecutively on one night typically resulted in a distribution
of differential magnitudes relative to a V=6.60 comparison with a std dev of
0.02-0.03 mag and so a theoretical precision (NB not accuracy!) of the mean of
these measurements of around 0.005 mag. Over this 30 minute period the star
centroid would typically drift over at least 1 pixel so gross effects of the
pixel structure were averaged over although there could still be an issue
because of the correlated drift of the variable and comparison over the pixel
grid.

Taking flats using a white screen didn't seem to be a problem. I used a diffuser
over the front of the lens.

Regards,
David Boyd

----- Original Message ----- 
From: "Wolfgang Renz" <w_renz at onlinehome.de>
To: "AAVSO-PHOTOMETRY" <aavso-photometry at mira.aavso.org>
Sent: Friday, January 14, 2005 5:45 PM
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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