Binning

Hi Marlin,

Nice setup.  The Meade 16 works well, and the ST-10 is SBIG's most sensitive camera. What is your typical seeing?  The usual guideline is 2-3 pixels per fwhm.  This means if your seeing is 2arcsec, then you would want a pixel size somewhere around 0.7-1.0arcsec.  With your image scale, that would mean binning 2x2 or 3x3.

There are some caveats.  SBIG does not change the gain of the camera when binning, so you don't get the maximum potential from binning - that is, the analog/digital converter will still be at 2.5e-/ADU, and the maximum number of detected electrons will be 2.5 x 65536 = 164Ke-, even though 3x3 binning means you are summing 9 pixels, each capable of 77Ke- full well, or 693K electrons.  So when the ADC maxes out, each native pixel will only have ~1/4 full well.  You will get more dynamic range by running in native 1x1 binning, but you will lose by oversampling, so that the image size is bigger than necessary, it takes longer to download, and the readnoise per star will be higher.  I'd certainly run in native mode for exoplanet transits or any very high signal/noise application.

What some people do is use the 0.5 or 0.63 focal reducers to decrease the image scale so that you don't have to bin as much (or at all).  Since you already have the camera and telescope, adding a focal reducer is simple and should be considered.

Arne

Thanks, Arnie

     I neglected to mention I do have an installed 6.3 Meade focal reducer inserted into the focuser, an STF-S.  Further, right now I am doing photometry on variables but would like to switch over to exoplanets.  Seeing is 2- 3 arcseconds at 100 M elevation in a small city.

Hi Phil,

I did see that article by Arne. It says nothing about my “concern” of potential saturation of native pixels, so perhaps there is nothing to that.

My camera is the ST9xe (20u pixels) purchased about 10 years ago. I have been using it very successfully for those 10 years in native 1x1 mode. I chose this camera because the pixel size is right on the money in that mode with my f/10 8” Meade LX 200. The image scale is 2.06 arc”/pixel. Using 4” seeing, the star images fit nicely into the 4 pixel Nyquist sampling. In fact, that was a big reason I bought the ST9 instead of the ST8 with its 9u pixels. Yes, I do recall wondering back then about buying either the ST7 or ST8 and to achieve the matching pixel size for the scope by binning 2x2. It was then that I read about the full well capacity issue I mentioned previously. Somehow that stuck with me which is why I sent my original post. Then I read yesterday that full well capacity actually does increase with 2x2 binned  pixels – hence my confusion.

You and others who are doing photometry with small pixel camera are obviously doing very well with binned smaller pixels and binning seems to have no effect on the quality of your photometry.  You said that to achieve proper sampling you both bin and use a focal reducer. So apparently the important thing is to adjust the scope/camera system to get the image scale as close as possible to achieve Nyquist sampling . If binning does, in fact, increase full well capacity then binning will not be an issue.

Cheers,

Keith

Binning involves several steps.

- the native pixel accumulates charge (converted photons).  These pixels have a specific full well capacity depending on the sensor.

- during readout, each row is shifted to the serial readout register.  The serial register "pixels" actually are bigger than the imaging pixels, and so typically have twice the full well capacity of an imaging pixel.  This means you can bin by 2 in the vertical direction (transfer two rows into the serial register before readout) without losing any charge.

- then the serial register is transferred to the summing well.  This well is like an even bigger pixel, typically with 4x imaging pixel capacity.  This means it can hold a bin-by-2 row binning into the serial register, followed by a bin-by-2 column binning of the serial register, without losing any charge.

- finally, the summing well charge is amplified (gain) and sampled by the analog-to-digital (ADC) unit.  If the gain is adjustable, it may be set so that 2x2 binning yields 1/4 of the gain of a 1x1 situation, so that the ADC does not overflow.

So the questions for any given sensor and camera are: (1) are these "pixel" capacities typical, so that 2x2 binning does not overflow the summing well?  and (2) does the system adjust the amplification gain depending on the binning, or is there only one fixed gain for any situation?

If only one fixed gain, then you will overflow/saturate the ADC and you can only expose each native pixel to about 1/4 of its maximum capacity without causing saturation.  If the gain is adjusted, then you might sample with 10 electrons per count rather than 2.5 electrons per count.  While that permits exposing to the true full well depth of each native pixel, it means that a count for a binned image is different than a count for an unbinned image.  Perhaps more important, since the gain changes, the bias frame may also change.  You should take biases and darks for each binning situation.

Binning is ok, and entirely approprate for oversampled cases such as small pixels on long focal length cameras.  On OC61 for example, we bin the native 12-micron pixels into 24-micron pixels (even then, the scale is 0.55arcsec/pix!).  On BSM_NH, for another example, we're using a ZWO 183 camera with 2.4 micron pixels, which yields 1.1 arcsec native resolution, below the Dawes Limit for these telescopes.  So we bin 2x2, and defocus to yield 1.5-2.5 pixels per fwhm.  Binning helps for the 183 in many ways, including making the 12-bit ADC into effectively a 14-bit ADC, because for a CMOS detector like the 183, the binning is done in software.  I won't talk about CMOS again in this post.

Focal reducers are a different issue.  Again, if you have a long focal length telescope, you may want to use a focal reducer to keep from oversampling with a given sensor.  However, you are adding an optical element to the optical path.  Focal reducers tend to only correct human-visible light; how well they perform form UV or NIR light is unknown.  They also can "ghost" so that bright stars might have secondary images elsewhere in the field of view.  They also can increase the vignetting.  They are a reasonal solution to the oversampling case, but you need to be aware of their potential interference when doing photometry.

Regarding binning, what I highly recommend is to do a linearity test on your sensor.  Usually regular CCD cameras like the SBIG line will have a straight line up to a certain count value, and then curve over, asymptotically approaching some value which may be considerably below the ADC maximum count.  For an ST-7XME, for example, deviation from linearity may occur around 48K counts.  If you see this kind of curve, then you are sampling to full well on your sensor, even in the 2x2 mode.  If you instead see a straight line up to 65535, then most likely you are not sampling to full well on your sensor, and instead are being limited by the ADC.  For most anti-blooming-gate (ABG) CCD sensors, because of their inherent nonlinear behavior near full well, vendors typically increase the gain so that the linearity curve is linear up to the full range of the ADC, and so this test for 2x2 gain doesn't work as well.

I hope this helps!

Arne