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ZWO ASI 120 MC (colour) sensor size


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Can anyone tell me the dimensions of the ZWO ASI 120 MC sensor?  The spec says it's a 1/3" CMOS AR0130 CS (colour) sensor, 1.2 Mega pixels resolution (1280 x 960), pixel size 3.75 microns.  I thought it might therefore be (1280 x 3.75) width x (960 x 3.75) height microns or 4.80mm x 3.60mm, which seems to correlate with Wikipedia's information on Image Sensor Format? However, visually looking at the sensor it looks much bigger! 

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Thanks, waiting for answer from ZWO forum.

Whilst on the subject, what is the pixel size for a Canon 700D, I have tried Google but without success?  Given the sensor is 5184 x 3456 pixels using RAW and the size is 22.3mm x 14.9mm and taking the equation: width of sensor in millimetres divided by image width in pixels multiplied by 1000, I make it 4.3 microns = seems very small?

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Hi Grobarts,

Not sure about the 700D but the 600D has a pixel pitch of 4.3 um. So doesn't sound unreasonable for the 700D to have the same.

In regard to the sensor size, dont forget that the bayer matrix takes up some space too, so that might account for a chip appearing larger than calculated. So maybe a couple of microns extra for an OSC per pixel ( just a guess ).  

In fact i was reading something the other day about including the size of the bayer matrix ( even though it is essentially dead space between pixels) in any calculations for working out focal length, image size or the like. Whether it makes a huge difference in the real world i don't know but for a small sensor like the asi's have then it shouldn't be a massive error in any calculations.

HTH

Mark

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Note that pixels aren't the only thing on the silicon wafer - each pixel is surrounded by other electronic components so you get a grid o pixels. Also around pixel grid there are other components used when the data is read out from the sensor. 1/3" is the diagonal size, and it's not  a very precise for telling what the imaging sensor area is.

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  • 3 years later...

This is an old thread, but I signed up for an ID with SGL explicitly to respond to it because I spent a while trying to find the sensor dimension for the ASI120MM in order to accurately represent it's view of the sky in SkySafari.  The link posted above to the ZWO user group is dead so perhaps this information was present there at one time, but no longer.

TLDR; I think it's 4.83 mm x 3.63 mm

According to the ZWO ASI120 manual, all versions of the camera use the AR0130CS sensor:
https://astronomy-imaging-camera.com/manuals/ASI120 Manual EN.pdf

According to Figure 5 in the AR0130CS sensor spec, the array of optically active pixels is 4.86 mm x 3.66 mm (1296 x 976 pixels):
http://www.onsemi.com/pub/Collateral/AR0130CS-D.PDF

But, it shows that some of the pixels are used for environmental correction etc, not for the actual image.  The sensor's output format is 1280 x 960, but for other reasons there are actually 1288 x 968 pixels used for the actual image and that area measures 4.83 mm x 3.63 mm.

You can go a step further and see that the pixel size is .00375 mm square:
3.63 mm / 968 pixels = .00375 mm square = 3.75 micrometers (microns)
4.83 mm / 1288 pixels = .00375 mm square = 3.75 micrometers (microns)

3.75 microns is in agreement with the (more easily found) pixel size spec and seems to confirm that when they define a pixel to include any supporting wire/transistors/structure etc around an individual optical photon bucket, not just the optical photon bucket itself.

In this case I can tell from the diagram in the spec sheet that the the sensor dimension I found is exactly that of the optically active pixels used for the image, but there are other pixels (optically or in other ways active) surrounding this area on all four sides being used for black level and noise correction etc.  In the case of this sensor, there are an additional 52 pixels above or below the active area and an additional 116 pixels left or right of the active area.

I assume when spec sheets plainly define a sensor dimension size (unlike that of the AR0130CS) it is measuring just the area of the pixels used for the actual image and not the other pixels surrounding that, but I'm not certain...if they include the other pixels then you'll be a bit off if you try to calculate, for example, pixel size based on resolution and sensor size.

I noticed that SkySafari also has fields under the camera definition for the sensor's X and Y-offsets.  If you include offsets it moves the projected outline of the camera's capture area off center relative to where the OTA is pointed.  I suspect this is to account for the fact that the active pixels area is surrounded unevenly by those other pixels.  For instance, the AR0130CS is down and left from center relative to the total pixel array due to an additional 28 inactive pixels on the top and an additional 104 inactive pixels on the right (after canceling out for the 6 inactive pixels on the left and the 12 inactive pixels on the bottom).  I suppose whether or not it's off-center relative to the OTA ultimately depends on whether the sensor was mounted directly center in the camera or if it was mounted already compensating for the offset of the optically active pixels, and that I don't know how to tell.

Assuming it's mounted dead center to the sensor as a whole and I did the math right, then:
X-offset = .00375 mm * 104 pixels = .39 mm
Y-offset = .00375 mm * 28 pixels = .105 mm

I hope this helps someone!

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Offset will depend on how the manufacturer, who will be aware of these offsets, decided to mount the sensor in the camera. They may or may not allow for the offset.

If you align the scope using the sensor any offset is automatically taken into account.

I don't recall where I got the figures I have in Stellarium from , but the size, pixel size and number of pixels agree with those you give for the active area.

image.png.d6c1bed7863e3aa830e4cef831fcdb72.png

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