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What is offset and how do I decide what it should be?


StuartT

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The offset is an analogue DC voltage added to the analogue signal read from each pixel before it goes to the A-D (analogue to digital) converter in the camera. The offset needs to be large enough that no black clipping occurs on the output from the A-D, that is, no pixel has a digital value of zero. This is especially important when taking bias and darks as this will lead to calibration errors which can't be fixed. Increasing the offset effectively increases the 'brightness' of the image and so sets what level corresponds to actual 'black' in your image.

If the offset is too large then you will lose some dynamic range in the image as the number of discrete digital levels available to represent the image signal reduces. When CMOS cameras came out there was some discussion on making the offset as low as possible to avoid loss of dynamic range. The ASI1600 when it first appeared recommended changing the offset depending on the gain used, but this was quickly dropped and the offset was recommened to be set to a fixed level where black clipping was not an issue at any gain.

On the latest cameras this is usually set to quite a conservative value, and you could probably lower it a little without issues but in reality the loss of dynamic range due to it being set a bit too high than necessary is not significant.

For the 16bit ZWO cameras like the ASI2600, an offset increase of 1 increases the ADU output by 10, so the recommended setting of offset 50 means the 'black'  output from the camera is around 500ADU. Clipped stars which correspond to 'white' are at 65535 ADU so you're left with 65035 levels to represent your image which is fine. 12 bit cameras like the ASI1600 only have 4096 levels to represent the image anyway, and they still give good results. 🙂

Alan

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I had a similar question having just obtained my first CMOS camera (ASI 290MM) and that reply really helps - thanks Alan. In particular, there's quite a lot of information on the web about changing the offset every time the gain is changed, which sounds like a nightmare... so its good to know that it can be set at a fixed value with a reasonable certainty that black clipping won't occur. I have mine set at 25. I also tend to use unity gain only, no doubt suboptimal but I'm finding it gives me results close to what I get with a CCD and an absence of stress!

Martin

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4 hours ago, Martin Meredith said:

I had a similar question having just obtained my first CMOS camera (ASI 290MM) and that reply really helps - thanks Alan. In particular, there's quite a lot of information on the web about changing the offset every time the gain is changed, which sounds like a nightmare... so its good to know that it can be set at a fixed value with a reasonable certainty that black clipping won't occur. I have mine set at 25. I also tend to use unity gain only, no doubt suboptimal but I'm finding it gives me results close to what I get with a CCD and an absence of stress!

Martin

Martin,

Changing the offset means you need to take a set of darks for every different offset value you use. 😲 Just not worth it. The easiest way to check you aren't black clipping is to take a bias frame or a short dark frame of a second or two and look at the histogram and image statistics. Here's the master bias from my ASI6200 below. As you can see the peak in the signal occurs at ADU 500 on the histogram, due to the offset value of 50 used as I mentioned above. Due to the noise distribution there are actual ADU values varying from 452 to 618 ADU shown on the statistics. Individual bias frames will vary to some degree around these figures. I only use the camera at one gain setting of 100, where the HCG (High Conversion Gain) mode kicks in. I could use an offset of say 20 and that wouldn't create any black clipping on this camera, but for the tiny increase in dynamic range, it's not worth changing from the default offset 50 .

Some CMOS cameras, the ASI1600 being one, had different internal processing on exposures less than a second compared to longer exposures, so the bias calibration frame was not representitive of the bias contribution to a long exposure image so the image would not calibrate correctly. This meant that darks had to be used for calibration, rather that just using bias, like with CCD cameras. Although amp glow with many CMOS cameras made darks necessary anyway.

My ASI6200  doesn't exhibit this behaviour with short exposures and has no noticeable amp glow, similar to the ASI2600, so bias frames could be used as calibration frames, although by habit I still use darks. 🙂

The ASI290 seems to be a fairly new camera and has a HCG mode kicking in at around gain 60. Unity gain is the way to go as well, for the best overall performance, at least when starting out, and the ASI290 unity gain is at 110 so no problem there. If you wish check your bias frame and a 1 sec dark to see if they are noticeably different in peak value and noise spread, unlikely I would think on recent cameras. As your camera is uncooled though you would need darks for long exposures anyway. For planetary I don't bother with calibration frames, though you could use a bias or matching short exposure dark if you wished.

471284612_BiasHistogram.thumb.png.7c69354f9e0ac0203a3c07cb1c099056.png

Alan

Edit: Offset isn't a feature peculiar to CMOS cameras, CCD cameras have an offset value too for the same reason. It's just fixed, as well as the gain, by the manufacturer so isn't user adjustable.

Edited by symmetal
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