Brightness and Offset

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SteveJP
Posts: 27
Joined: Sat Apr 28, 2018 8:35 am
Location: Melbourne, Australia

Brightness and Offset

Post by SteveJP » Sat Apr 28, 2018 8:45 am

Hi,

I'm using a ZWO ASI178MC camera.

There is no option to set the offset, but I have seen it suggested that this is controlled by the "Brightness" option. However, This does not appear to be the case. If I compare the resulting FITS images, or the histograms created by Sharp cap, there is a significant difference between when for example the native ZWO driver is used with Gain=100 & Brightness=50, compared to when the ASCOM driver is used with Gain=100 and Offset=50.

Can someone explain what Brightness actually does, and the relationship to offset?

I'm reluctant to complicated this question any further, but I am aware that ZWO have removed the offset from their newer Pro cameras (like the ASI071Pro which I also have), however, the ASI178MC still has the capability to set offset.

Thanks
Steve

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Re: Brightness and Offset

Post by admin » Sat Apr 28, 2018 9:53 am

Hi,

the brightness control on ZWO is the offset - I must admit I don't really look at the ZWO ASCOM driver much, but I wonder if the offset listed there has a different range to the brightness in SharpCap?

cheers,

Robin

SteveJP
Posts: 27
Joined: Sat Apr 28, 2018 8:35 am
Location: Melbourne, Australia

Re: Brightness and Offset

Post by SteveJP » Sat Apr 28, 2018 11:42 am

Hi Robin,

The "Brightness" parameter does have an offset effect, but it's not the same as when the camera is accessed via ASCOM, which of course is pretty common.

I can't see how to post images here or I'd post the histograms comparing the effect of brightness with the native driver and offset with the ASCOM driver.

With the native driver, if I take a very short dark with Gain=0 and Brightness = 50, I get a histogram, that when shown linear, looks like a normal distribution peaking at about 50 ADU's. I guess this makes sense.

With the ASCOM driver however, the same very short dark with Gain =0 and Offset=50, results in a similar looking histogram, but centered on 200 ADUs.

When the gain is not zero, the difference seem to be amplified.

The Sharpcap "brightness" and the ASCOM "offset" appear to changing the image in different ways?

Thanks
Steve

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