Clipped RGB subs

whzzz28 (Nathan) · #1 24-07-2012, 07:37 PM

I decided to do some unbinned color M8 grabs while at Peter's place on the weekend.
I've been trying to process it all but all the RGB subs are useless.
The histogram on each frame is very tiny and for 5min subs, there is a lack of any information.
The histogram is hard up on the black levels, and when stacked in DSS the histogram curve cuts past the absolute limit for blacks.
The histogram looks very similar on each color frame. For instance i would of thought Red would have more data than Blue, but the histogram looks almost the same. There is however more data in the red frame once stretched.
Attached is an unstretech histogram view of a single blue sub. The histogram begins at around 95%, not 0. The lum frame does begin at 0.

I tried stacking and then processing anyway and the results were... well crap.

Luminance subs appear to be ok, but the histograms on the lum subs also look funny - i would of expected a larger histogram for lum but the data appears to be there, a bit faint though.

I don't really know what i have done wrong. I did some M33 subs directly after this, but binned x2 for the RGB. The histogram starts a decent way to the right in these frames.

The images were taken in MaximDL and i used the LRGB template, just changed binning to 1x (and obviously number of subs etc). Everything else was pretty much stock standard.

I've uploaded a blue sub and lum sub. Is anyone able to take a look and see if there is anything obvious?
Did i screw it up when taking the subs with Maxim?
I double checked the FITS headers on the files. Everything appears to be in order.

http://core-au.net/astro/m8.zip

Thanks.

-edit-
I just remembered that i took some 60second shots as well. I checked these - they too are clipping. Even worse, luminance is also clipping in these shots. There is next to no data in the actual images except for very bright stars.

RickS (Rick) · #2 24-07-2012, 08:19 PM

Nathan,

You're not imagining it. The Blue sub has a huge number of zero value pixels.

I presume you haven't calibrated the Blue sub? If you had a problem with the master bias or dark it could possibly cause something like this to happen.

Have you set the gain and offset for your QHY9? A bad offset could probably cause a problem like this too. It's a long time since I did this for my QHY12 and I don't really remember the details, but this might help: http://www.cool.id.au/astronomy/Neb_...d%20Offset.pdf

Cheers,
Rick.

whzzz28 (Nathan) · #3 24-07-2012, 11:08 PM

That's a straight exposure, nothing was done to it by myself.
I know the Find DSO profile takes an auto-dark and subtracts it in Maxim, but the LRGB profile should not.
I do have the gain set, and after i imaged M8 i went straight to M33 and began that. The only thing i did was change the binning for RGB.
M33 RGB frames are fine.

Maybe there was a dark frame being applied to the image, i'm not sure. I'll try and get back out this weekend and take a few more shots at 1x binning.