ADU Values


I am new to the group and some help and advice would be very much appreciated I have a ZWO1600mmc which is new to me. I have taken my first subs last night with an zwo ha filter ( moon was big) My maximum ADU shows as 255 i was doing 10 min exposure with a mean ADU of 25 Camera gain at 200 scope skwatcher ed80 The images look okay when starched with Pixinsight. but the when i tried doing flats using and ipad as i light source, i can not get above a maximum ADU of 255 using the HA filter and now matter how long my exposure is it still a very dark sub (looks like a dark frame)

What am i doing wrong???

Cheers In advance

Try doing a flat without the filter and a very bright light source. If you are still getting a max ADU of 255 then you are maxed out on the dynamic range of the camera for that gain setting. I mentioned on another post that when you increase the gain of this camera above zero you are losing dynamic range. From my perspective there is no advantage to ever run above zero gain. Any perceived improvement in noise is because it is throwing away the lower bits where the noise exists. I have this camera and with narrow band and 15 minute exposures I am getting max values in the thousands.


Thanks for the reply and your experience with the camera.

I do have a couple more questions.

Is it okay to use non filtter flats with all my filtered subs?

Also i am doing 600sec subs with my narrowband filters @ gain 200 ( really beacause of what i have seen that jrista has done). If i go with 0 gain. How long would my exposures have to be to get the same results.

When i say i am doing 600sec subs. What i mean is i have done that once as i have only used the camera once. Moved from a canon 80d


Flats have to be done with the filter because you need the same optical system for corresponding lights and flats. There is likely some dust on the filter that the flat will pick up.

With the same exposure and zero gain the dynamic range will be greater although the highlights will be at a lower value. I choose to do 15 minute exposures because that is where I start to see amp glow and I don’t want the amp glow to overwhelm the image at yet longer exposures.


Thanks for the input. I will try lowering my gain and see what happens. Think its going to be clear in my area tonight so a can get a couple of hours imaging in. Going to to try the o111 tonight . Now if i shoot o111 at a different gain then stack them do you think i will be able to combine the ha@200 gain and o11@ 0 gain to produce a colour image?


Depending on the software you are using maybe it will work but to be safe I would use the same gain for the entire stack.

Maybe I am missing something in the conversation above, but have you checked the driver settings to make sure that it’s configured for RAW16 and not RAW8?


i worked it out this afternoon and yes you are correct i was shooting @ 8 bit. the subs look good to me just that the adu values were 255 max.

Thanks for the reply though… I am imaging at the moment and the adu values look correct…