Understanding fits liberator

[dodgerroger]

Hi folks, please help. I am experimenting with fits liberator to find the correct offset for my camera. This is a screen shot of an image using the default gain and offset. The blue dots as i understand it are showing the clipped black pixels.So am i correct in thinking I need a higher offset? I believe i do especially when looking at the histogram. Could someone with experience of this confirm.  Thanks in advance

[vlaiv]

Your sub seems fine - it has 19ADU as minimum value (that is a bit low for light, unless it is calibrated light then it is ok).

Problem is that you set background level to 56 in linear stretch. This produces some pixels to have negative values of -0.36 (less than background level of 56 hence negative value). These are marked with blue.

Histogram is not indicative of offset issue.

To examine if you have problem with offset - it is best to examine dark subs and see if you have many pixels with minimum value. In fact, you want your offset set to high enough value so you don't get any minimum pixel values in any of your dark subs.

[dodgerroger]

Thanks for that, it is a raw image. I will check the associated dark subs I have taken. What should the background level be set to? I think fit lib is on default settings 

[vlaiv]

2 minutes ago, dodgerroger said:

Thanks for that, it is a raw image. I will check the associated dark subs I have taken. What should the background level be set to? I think fit lib is on default settings 

Why do you want to process single sub with fits liberator?

You should calibrate your raw subs and then stack them and then use some sort of software to process them further.

[dodgerroger]

I was just checking my images, it’s the first time I’ve used fits lib and thought oh why is it showing clipped pixels. I was looking at the linear histogram and thinking the offset is set to low. It has come about from trying different exposures and gains and want to make sure I have enough offset when I start changing gain values