[Aavso-photometry] binning considerations

[Wolfgang Renz]

Hi David

Although the pixel sizes of the ST-7/8/10/2000 are much smaller than
optimum, you can still do good photometry with them on longer focal
length sopes.

When you image bighter stars or take long exposures of fainter ones
with a KAF NAGB camera that can bring you close to ~ 2/3 of the full
well of single pixels (= ~ 2/3 of the saturation ADU count in unbinned
mode if the camera saturates within the ADU range of the A/D con-
verter), go unbinned no matter how oversampled your images are.
Here you are always in danger that you will leave the linear range of
your camera und if you go binned (that would require an accordingly
shorter exposure time) you won't be able to notice it with certainty
(especially if one goes undersampled). In this situation, IMO the only
disadvantage of the unbinned mode is the longer download times
and the larger file sizes, but its not too bad for small chips like one
in the ST-7. I always take some unbinned images in the different
filters (while focusing) to ensure that all stars of interest stay in the
linear range before I switch to binned mode (requires some minor
math and knowledge of the behaviour of the camera in binned mode).
When imaging fainter stars (that won't get close to non-linearity) there
is no reason why not bin 2x2. IMO its always better to go a bit over-
than undersampled.

When you image undersampled (with your refractors) you can improve
your photometry by slight defocusing so that the FWHM is at least 2
pixel. This has the advantage to avoid most issues due to intra-pixel
variations (E and ME chips) and micro lens focus (ME chips). When
trying to go faint with an undersamples setup (especially when doing
pretty picture imaging with color camera at sharp focus), sub-pixel
dithering will also improve the results. On this one you can find a good
example at:
- Christian Buil - Canon EOS-DSLR (false colors for point aspect stars
  without Canon anti-IR and anti-aliasing Filter)
  http://astrosurf.com/buil/baader/eval_us.htm
  http://astrosurf.com/buil/baader/dipher0.jpg
  Crop of the NGC 7000 field. Single 120 s exposures. Original scale.
  Left, frame #1 of the sequence (note false colors for many point aspect
  stars). Middle, frame #2 of the sequence (the telescope pointing is not
  the same and the artifact are not the same also).
  Stack of 18 randomly dithered frames. Now, the star colors are more
  representative of the real spectral types.

Does your ST-7E also saturate at ~ 32k ADU unbinned ?
Where does it saturate in higher binning modes ? At ~ 64k ADU  ?
SBIG sets the gain in the firmware of their cameras (with USB cams it
might be changed, but this is not suported by imaging software up to
now). E.g. with the old parallel ST cameras the gain might change bet-
ween unbinned and higher binning modes. With the ST USB models the
gain is fixed and optimized for the unbinned mode (ideal just for faint
targets that never get close to saturation).

Clear skies
 Wolfgang

-- 
Wolfgang Renz, Karlsruhe, Germany
Rz.BAV = WRe.vsnet = RWG.AAVSO



----- Original Message ----- 
From: "David Trowbridge" <David at tinyblue.com>
To: <aavso-photometry at mira.aavso.org>
Sent: Sunday, June 10, 2007 10:34 PM
Subject: [Aavso-photometry] binning considerations

> When doing photometry, is it okay to freely use 2x2 or 3x3 binning to
> achieve the desired image scale?
> 
> Where I live, the seeing is generally not better than 2 arcseconds I
> don't think. 
> The photometry I've been doing so far has been with an SBIG ST-7E
> camera on a Takahashi FSQ-106 4" refractor which has been giving
> me 3.5 arcseconds/pixel at f/5 or 2.2 arcseconds/pixel at f/8. 
> Now I plan to upgrade to a larger scope for greater light grasp and a
> new camera for faster download times. I'm considering the Takahashi
> Mewlon 250 and an ST-8XME camera. Using 1x1 binning this scope
> would give only 0.62 arcsconds/pixel at f/12 and 0.80 at f/9.2. I think I'll
> need bigger pixels. 
> 
> Will it work just fine to use 2x2 or 3x3 binning?
> Or should I consider an ST-9 instead? 
> 
> Also, will going to a camera with microlensing have any detrimental effects
> on photometry measurements?
> 
> Are there any other issues I should be aware of?
> 
> David Trowbridge
> Tinyblue Observatory
> Whidbey Island, WA