View Full Version here: : Brightness in top left of frame, any clues as to what might be causing this?

24-07-2012, 07:17 PM
Ok, so first up please ignore the photo quality itself, and apologies if this is way too basic to be posting. I'm fairly astounded by the quality of the photos in the 'Beginners Astrophotography' forum, and feel a little out of place, but I have nowhere else to ask so sorry!

I had most settings completely wrong when I took it hence the shocking quality. What I am wondering is what might be causing the bright area in the top left corner of the image?

I'm keen to see what I can squeeze out of my Canon S5 in the way of wide field, time lapse, star trails, barn door, etc. The maximum exposure on the camera is 15 seconds, however I have loaded CHDK onto it which opens up long exposures, intervalometer, etc.

I took this photo during last weeks Aurora, I hastily set the camera up and just left it continuous shooting 50 second exposures. Every frame looks like this, with the blown out area in the top left. I've since tried this at home and I get this whenever I shoot in CHDK mode, if I shoot 15 second exposures with the default firmware I don't see it. One other thing is I have the cameras noise reduction turned off in CHDK, could it be this?

24-07-2012, 07:29 PM
is this shot on a normal tripod?
50 seconds on a normal tripod would show significant star trailing, Malc.
Can you see trailing like blobby stars with a streak in the original photo?

I don't know CHDK - but it doesn't seem to keep the shutter open as long as you told it to.

I don't understand enough but I assume the chip in your camera is damaged in that part.
DSS could potentially help. ... try using DSS (Deep Sky Stacker) and stack one photo with darks and flats while setting the exposure time to those dubious 50 seconds, again. Maybe DSS can get rid of the light area?

24-07-2012, 07:32 PM
must have been a wonderful experience, that aurora. beautiful green and red... I've never seen one and admit I am envious


24-07-2012, 07:39 PM
Looks like amp glow to me Malc, especially if the camera wasn't designed for long exposures like that. I would recommend to leave the noise reduction on. Its a pain to lose that time while its taking the autodark but it might take care of most of that amp glow. You could shoot manual dark frames of the same exp length and temp and subtract them later. I think though that the autodarks might do a better job

allan gould
24-07-2012, 08:11 PM
Agree with Paul, amp glow is the best explanation and removed with flats and darks.

24-07-2012, 08:32 PM
Paul, thanks. I'll try leaving the NR on and see if that cures it.

Silv, yes there is some trailing visible on the originals. I'm not bothered about trying to salvage these photos, but more interested in the cause of the glow. The exif data on the images shows the wrong exposure time, I assume that CHDK doesn't overwrite the cameras settings.

It was awesome to see the aurora, and was a first for me. It certainly wasn't anything like in the photos though. Thankfully there was someone just around the corner from me on the Christchurch Port Hills on the night who knew what he was doing, I'll be keeping his shots as a memento ;)


24-07-2012, 10:32 PM
Malc, try shooting back to back in normal firmware and CHDK at the same exposure times, so 15secs.

15 in normal and 50 in CHDK won't tell us a lot.

If they are both fine at 15secs, but a problem at 50, then it is most likely amp glow as people have suggested but if it still shows up at 15secs but only in CHDK mode, then there is something wrong with that specifically.

25-07-2012, 05:30 PM
I think this is almost certainly amp glow. Shooting 15 secs with the lens covered in CHDK with auto darks turned on removes it. Same settings with the auto darks turned off and it's back.

I think I'll initially take Paul's advice and leave the auto darks on before I try and do my own darks etc.

Thanks everyone for the help, wouldn't have been able to work it out otherwise.