Observation by Nick James: M51

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Nick James


Nick James


2021 May 09 - 00:09


2021 May 09 - 07:19


The Whirlpool Galaxy (M51)

Planetarium overlay


Canes Venatici

Field centre

RA: 13h29m
Dec: +47°11'
Position angle: +0°39'

Field size

0°22' × 0°16'

  • ASI294 + HD11



Chelmsford, UK

Target name




About this image

Another colour image while I'm temporarily without my big mono camera which has gone off (literally) to meet its maker. I'm binning this camera 2x2 but that still gives a fairly small pixel size of 0.66 arcsec. The seeing wasn't too good this morning, in fact one of the advantages of a USB3 CMOS camera is that you can see stars twinkling in real time when focussing on the video. The FWHM of stars in this image is 3.0 arcsec, I did get some images with 2.1 arcsec yesterday when I was setting this up.

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Tim Haymes
Tim Haymes, 2021 May 10 - 13:03 UTC

Nick: Good image,  but how do you bin a OSC without loosing the colour information? Sorry if this is a dumb question.


Nick James
Nick James, 2021 May 10 - 19:04 UTC

Tim, you can bin Bayer matrix sensors in multiples of two and retain the RGB info without interpolation since each binned pixel contains R, G and B sub pixels. for 2x2 each pixel contains one each of R and B and two G. This is called a super-pixel in some places. For 4x4 binning you have four super-pixels so 4 x R, 4 x B and 8 x G. All binning in CMOS sensors takes place in software after conversion so you can have a complex map of pixels to bin. They don't need to be adjacent as they are with on-chip CCD binning.   

Tim Haymes
Tim Haymes, 2021 May 10 - 21:32 UTC

Nick, So is the binning specific to the software, and whether or not the specification say "2x2" supported.   Ive only seen the ASI 224mc say it supports 2x2 bin.  What software are you using for colour  2x2 processing?

Nick James
Nick James, 2021 May 10 - 23:11 UTC

Tim, yes the binning is specific to  the camera driver and post processing software you use. In a CCD charge is shifted along columns and rows and accumulates before being turned into a number by an external ADC. In a CMOS array each pixel has an amplifier and the voltage from each pixel is then converted into a number by multiple on-chip ADCs. This means that binning in the CCD sense isn't possible but you can have the same effect by adding together pixels in software. In CMOS cameras each binned pixel has multiple, uncorrelated amounts of read noise compared to the single read noise component in a CCD but since the CMOS sensor has much lower read noise to start with that doesn't often matter. I have my own software to do all this but I'm sure commercial software does this too. 

Nick James
Nick James, 2021 May 11 - 06:48 UTC

Tim - I managed to get a better image of M51 last night here using the ASI294MC. Processing was done in the same way.

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