Iso v sub length

[leemanley]

I am sat here guiding away on a nice widefield image of M42 and running man, a quick question while I'm at it.
When I just tried out a 10 min sub of it, the preview on cam was pretty much washed out with lightness, so, is there any usable data there to stack ?
I'm using iso800, would I be better off dropping it to 400 ?
Just thought I'd ask the question before I waste any more of this delicious starry night !!!

[Leveye]

You might consider a light pollution filter. M42 will come out great with lower iso's give it a go!

[leemanley]

Got a 2"LP filter, going to stick it on in a mo and drop it down to 400, if it doesnt like it it's only a 10 min experiment !

[Physicist13]

As long as no pixels are saturated, the info is still there. changing the ISO in fact makes not that much difference to the SNR.

[m37]

The middle tends to get burned out pretty easily, seen lots of articles and posts about shooting several different lengths and ISOs and taking the best bits from each i.e. longer exposures for the outer bits and shorter/lower ISO for the center.

Keep meaning to try that!

[laser_jock99]

Always shoot in RAW mode as well - sometimes subs are not as washed out as they might appear and the extra colour depth of the RAW file has usable data still there.

Example - washed out RAW sub (1200 seconds @ ISO3200)

The same processed sub (with flat frames)

I don't recommend over exposing by any means - but all is necessarily lost.

[Russe]

Well put - when you expose your subs you can check it's histogram - some people say to keep the bulk of the data somewhere around 25% - this prevents over and underexposure.

[dph1nm]

Well put - when you expose your subs you can check it's histogram - some people say to keep the bulk of the data somewhere around 25% - this prevents over and underexposure.

Don't put too much reliance on the histogram. On my Canon it drops the top two bits of the RAW file (a factor of 4) , so if the histogram is completely over to the right, the RAW image is only exposed to  1/4  of the saturation level!

NigelM

[wxsatuser]

The histogram is of an in camera jpeg and will most likely not be anything like the RAW histogram.

The 25% is a guide to the correct exposure which is limited by your sky conditions.

The range of 25>40% is most likely ok, if you expose longer you will lose highlights.

[Russe]

I should have said - look at a sub in your laptop and check a histogram in your image processing software of your choice. That's what I meant.

lol

I've never managed to make out anything on my Canon's histogram. Forgot there is one...

[leemanley]

I have had a look at the resulting stack of subs, all was not lost apart from some lousy plane trails !!!
10 mins is definatelt going to be the way forward, trying it out now on the Cone neb,,,,,,,,,,,,,,,,,,,,,,

[wxsatuser]

I should have said - look at a sub in your laptop and check a histogram in your image processing software of your choice. That's what I meant.

lol

I've never managed to make out anything on my Canon's histogram. Forgot there is one...

Check the histogram in your camera.

This is a guide to the correct exposure, if you go past the values I mention the detail will be lost.

If your not to worried what your images look like, then by all means expose longer, longer is better than shorter.

The 25% gets your data out of the camera read noise, which is most important.