Auto exposure and Altair 290M

[MrAstroBen]

Nothing in the manual about this. Tick the box but the histogram still shrivels away as light fades on my all sky dome.

What should it be doing / how is it activated and set up?

Thanks Rob

[MrAstroBen]

Update. I bought a ZWO 224mc

The auto exposure works well with this. Self adjusts between bright light and darkness.

Seems to be an issue with Altair cams as see another user of a gcam has the same problem.

Rob

[admin]

I thought I replied to your post but maybe it was one of those replies that gets half done and then lost when you close the browser - it happens sometimes, sorry.

Basically the Altair auto-exposure was limited to about 0.3s. I was planning on making a change to set the limit to about 15s, but I seem to have lost that too! It was obviously a bad day!

cheers,

Robin

[MrAstroBen]

Thanks Robin. If you could post when you get around to this i will try the 290M again. Cheers Rob

[admin]

Hi,

this change is in the latest 3.2 build that I uploaded last night. Just to note that during my testing I found that if a significant area of the image was well exposed then the auto-exposure would stay unchanged even if other parts were strongly under/over exposed - this meant that sometimes the auto-exposure would not respond to changes as quickly as you might expect.

cheers,

Robin

[MrAstroBen]

Thanks Robin, that sounds good for All Sky use. Especially in day time or if the moon is up average exposure for the majority of the fov works best. Speed of change is less critical.

Out of interest, how does it choose an exposure value, from the mid point of the histogram after any user limits are set?

Rob

[admin]

Actually I don't know, since the auto-exposure is managed by the Altair SDK, sorry.

cheers,

Robin

[MrAstroBen]

Robin,

Put the 290M back in my dome last night. Downloaded the latest version of 3.2

Set auto exposure. It chose 13.1 sec which gave a reasonable histogram at about 10pm. Started a 12hr capture of .ser

Re checked at 11pm. Histogram was low, exposure not changed from 13.1s

Stopped sharpcap. Restarted. This time is chose 24s. Higher than the max 15s level you advised had been set.

It would then not respond to attempts to uncheck auto exposure or move the exposure slide.

Restarted. Auto Exposure of 14.8s started capture.

This morning it was still running at 14.8s and a white out.

Restarted, it chose 159ms. Image ok but over exposed. Exposure time was moving about by 10ms or so.

Unchecked auto exposure, manually set gain from 200 to 100. Dropped auto exposure brightness level from 60 to 50. Rechecked auto exposure. Still running around 159ms and histogram clipping.

In comparison, ZWO 224mc auto exposure runs well and works to 30s+

So still an error somewhere with the Altair. On a dark night i think 15s will not be enough. Assume set to limit noise or amp glow? If so that is not so critical for all sky recording a .ser file.

Thanks Rob

[admin]

Hi,

that sounds a bit like my experience with it - at times it would work and then at other times it would get stuck and leave the image either too dark or too bright but then eventually adjust.

I just had a look at AltairCapture and I can't find a way in that to increase the range of the auto-exposure beyond the default limit of 350ms which is what you were hitting anyway. I suspect that means that the auto-exposure built into the SDK has never been tuned to work with longer exposures.

Without implementing auto-exposure in SharpCap itself I don't think much more can be done to improve the responsiveness of the Altair camera here.

cheers,

Robin