Sensor and Sensor Analysis Queries

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Kopa
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Sensor and Sensor Analysis Queries

Post by Kopa »

Hi All

I have bought ZWO asi224mc and at its default setting and a exposure of 30s (cap on in dark) I have this extreme brightness on top right corner.

Is this normal or is it a defect in my sensor?

also it has 2 white pixels visible. Is this normal for this camera?.
Zw0ASI224 default30sStretched.jpg
Zw0ASI224 default30sStretched.jpg (844.22 KiB) Viewed 351 times


I have also done sensor analysis on it as shown below.
    On the top right corner Sensor Linear to: 88.4% but on another test it was showing 61%. Would this 88.4% be adequate for this camera?.
    Zw0ASI224Sensor analysis16raw.jpg
    Zw0ASI224Sensor analysis16raw.jpg (129.48 KiB) Viewed 351 times

    Basically I am trying to ascertain if this camera is OK as I can still return it for an exchange.

    Just for comparison the SVbony SV305 Camera does not show such a drastic glow at one corner at its defaults and 30s exposure.
    SV305 default30sStretched.jpg
    SV305 default30sStretched.jpg (896.3 KiB) Viewed 351 times

    Kind regards

    Vince
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    Re: Sensor and Sensor Analysis Queries

    Post by admin »

    Hi Vince,

    in order to get a good comparison between these two cameras you need to make sure that they are using equivalent gain settings.

    For the SVBONY camera, at the maximum gain of 30, the image is 30 times brighter than it would be at the minimum gain. Basically, for this camera the gain control value tells you how many times brighter the images than the baseline minimum.

    For the ZWO camera, at the gain value of 500, the image is ~300 times brighter than it would be at minimum gain. For this camera, the gain value is measured in units of 0.1 dB, so increasing the gain by 200 increases the brightness of the image by a factor of 10.

    The combination of these two means that to put both the cameras into roughly the same settings, you should either set the gain on both to the minimum value, or set the gain on the SVBony camera to 30 and that of the ZWO camera to ~300. I suspect you will find that the glow brightness is very similar once you have the cameras both set to the same gain level as well as the same exposure.

    Also, be sure to make sure the stretch levels in the display histogram are the same when you are trying to compare images from the two cameras.

    Cheers, Robin
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    Re: Sensor and Sensor Analysis Queries

    Post by admin »

    Oops, I failed to notice that the SVB305 actually uses the 290 sensor not the 224, so would not be expected to be comparable.

    I have an Altair 224 here, and it shows distinct glow at the right side on 30s exposures at 30x gain :
    Capture.JPG
    Capture.JPG (323.74 KiB) Viewed 339 times
    I suspect that this is normal for the 224 sensor.

    cheers,

    Robin
    Kopa
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    Re: Sensor and Sensor Analysis Queries

    Post by Kopa »

    Thank you for your quick response Robin.

    Since the distinct glow on the right hand side seems normal I shall keep the camera.

    On the issue of Sensor Analysis results from SC do the figures look reasonable?. if so

    I would like to be confident in my sensor analysis result, and for now I am assuming that the linear percentage of 88.4% is good enough.
    In your opinion what minimum linear % could be considered OK. I had 61% , 80% prior. or what to look out for generally.

    Kind regards Robin, response very much appreciated.

    Vince
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    Re: Sensor and Sensor Analysis Queries

    Post by admin »

    Hi Vince,

    ah, yes, sorry, got so focused on the glow that I forgot about the linearity thing.

    In general the results look fine. The linearity measurement is very dependent on steady illumination while the measurements are taking place and unless you have a specially set up constant light source you should probably ignore the figure. If you are (for instance) using natural light then a cloud going past during the measurement could easily cut have a big effect on that value.

    cheers,

    Robin
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    Re: Sensor and Sensor Analysis Queries

    Post by Kopa »

    Thanks Robin

    Very much appreciated.

    Vince
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