Observation by Nick James: M51
Uploaded by
Nick James
Observer
Nick James
Observed
2021 May 09 - 00:09
Uploaded
2021 May 09 - 07:19
Objects
The Whirlpool Galaxy (M51)
Planetarium overlay
Constellation
Canes Venatici
Field centre
RA: 13h29m
Dec: +47°11'
Position angle: +0°39'
Field size
0°22' × 0°16'
Equipment
Exposure
88x30s
Location
Chelmsford, UK
Target name
M51
Title
M51
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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Comments
Nick: Good image, but how do you bin a OSC without loosing the colour information? Sorry if this is a dumb question.
Cheers
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.
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?
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.
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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