Ideal Exposure for subs - Clipping stars?

[DuaneDibbley (Phil)]

Hi all,

Should my star pixel values be clipping in my light frames? I would assume not as a pure white value isn't super useful.

I got about 3hrs of exposure on Carina, my first target, only to realise after integrating all the data that the vast majority of my stars are 255 values and clipped.

I was using a EOS 70D at ISO400 and 3min exposures. I'm unsure whether it would be best to drop ISO to 100 and leave exposure time as it is OR leave ISO400 and drop time and just take more frames.

Any thoughts or suggestions?

[Karlzburg (Karl)]

I run my Canon 700d at 800 iso while doing 5 min subs and get great quality images, I rarely change the iso.

[xelasnave]

This may help.
http://www.iceinspace.com.au/forum/s...d.php?t=117010
Alex

[Mickoid (Michael)]

I would imagine your white stars will be clipped but any star that exhibits colour should hold an RGB value of some degree. Are you saying your Raw files show every star has a value of 255? The star colour is usually subtle, it may be your exposure that it causing the issue because it won't take much to blow out the colour. It's certainly a juggling act in capturing faint nebulosity and correct star exposure. Maybe you could combine a few subs of lesser exposure for the star colour and add them to your longer exposures.

I know DSS can clip your stars, so I usually make curve adjustments to the highlight end of the histogram to maintain the colour. This means my autosave file is no longer linear but that doesn't worry me. This won't work though, if your Raw subs are clipped to start with. It sounds like a little experimentation may be necessary to achieve the desired results.

[Atmos (Colin)]

I used to use ISO200-400 with a Nikon D7200 and rarely had issues with star clipping except on very bright stars.
If you're clipping at 255, are you saving your files as JPEG, TIFF or RAW? 255 suggest that you're working in 8-bit images which would most likely be JPEG.

[DuaneDibbley (Phil)]

Originally Posted by Atmos

I used to use ISO200-400 with a Nikon D7200 and rarely had issues with star clipping except on very bright stars.
If you're clipping at 255, are you saving your files as JPEG, TIFF or RAW? 255 suggest that you're working in 8-bit images which would most likely be JPEG.

Working in RAW and in Pixinsight. I think I misspoke with quoting 255. When I look at the raw files in my regular photographic raw viewer many stars are reported as clipping. Pixinsight readout mode, however, reports all values (RGB+L) below 1.0 for most of the 'overexposed' stars

Originally Posted by Mickoid

I would imagine your white stars will be clipped but any star that exhibits colour should hold an RGB value of some degree. Are you saying your Raw files show every star has a value of 255? The star colour is usually subtle, it may be your exposure that it causing the issue because it won't take much to blow out the colour. It's certainly a juggling act in capturing faint nebulosity and correct star exposure. Maybe you could combine a few subs of lesser exposure for the star colour and add them to your longer exposures.

I know DSS can clip your stars, so I usually make curve adjustments to the highlight end of the histogram to maintain the colour. This means my autosave file is no longer linear but that doesn't worry me. This won't work though, if your Raw subs are clipped to start with. It sounds like a little experimentation may be necessary to achieve the desired results.

I think I misspoke and misremembered... Now I'm actually sitting in front of pixinsight again I can see many of the stars I thought were clipped have <1.0 RGB+L values. They're pretty high - up around the 0.9 in a lot of cases.

Originally Posted by xelasnave

This may help.
http://www.iceinspace.com.au/forum/s...d.php?t=117010
Alex

Unsure if that applies to CMOS cameras? Shall have a read none the less. Ta


Edit: On a slightly off topic note can I just say how frustrating it is that Pixinsight costs nearly $400AU yet is so poorly documented the only way to really learn it is with tutorials that may or may not be a few point releases behind? Such a comprehensive application needs documentation!