10 x 5 minutes ISO 800 vs 5 x 10 minutes ISO 400

[bamus]

Hi All

Finally got my guiding setup working and I can choose whatever exposure time I want J I have done test session last Sunday and was shooting M31. Have done 10 x 5 minutes subs at ISO 800, but after processing in DSS and Gimp I was less than satisfied with the results due to huge amount of noise.

I was wandering if I would get better results using 10 minutes subs subs but with lower 400 ISO. Not sure if it will change anything. Total sub time will be same as in 10 x 5 minutes subs, ISO 400 will produce less noise (not sure about that as I will be using twice as long exposure time), but I will have only 5 subs, and stacking will not reduce noise as much as 10 subs stacking.

I will try to do some tests, but perhaps someone has done it before, or got knowledge how noise increases/decreases in different ISO/Time settings.

And almost forgot to mention, my camera is canon 1000D

Best regards

Adam

[Ags]

I have heard that thermal noise becomes in DSLRs when exposure time goes over 5 minutes.

[bamus]

I have found this:

http://stargazerslounge.com/imaging-discussion/87319-what-iso-3.html

Think I will go don to ISO 200 and keep 5 minutes subs. Cant wait to see results!

[Sparrow]

At 10 x 5 minute subs at ISO800 your image will still be noisy. You should be able to arrive at a good result with 5 minute subs at ISO800, but you need many more of them - so far this just adds up to 50 minutes of data. Ideally you need to be aiming for about 4 hours of data. This will dramatically increase the signal and reduce the noise.

There's nothing wrong with ten minute subs but you run the risk of losing more subs through guiding errors/satellite trails/ plane trails. Not so bad to write off a 5 minute sub but annoying to have to write offr a ten minute sub.

Regards

John

[Merlin66]

The longer the subs the better.

ISO above 400 IMHO is just upping the internal gain setting in the camera, doesn't add anything to the data in the RAW image.

[themos]

The answer depends on the quality of sky you have.

Why? because the golden rule is "read error must be small compared to shot-noise of background sky" for each sub.

If your sky is very dark (or you use narrowband filters) that shot-noise will be very low so you will need a longer exposure to overcome the read error.

Additionally, read error changes at different gain settings (ISO settings, that is). So does the dynamic range.

That makes it quite complicated.

But don't worry, your exposures are usually limited by your tracking accuracy. Aim for 5 minutes (I don't think you are shooting narrowband). And keep ISO to 800, look at the subs and check if the bits you want to capture accurately are saturated. If they are, go to ISO400. If not, you could try ISO 1600. I avoid 1600, myself, but others like it.

[shaunster]

10 x 10mins would be best...

[themos]

Have a look at this interactive experiment to get a feel for how the different variables work.

Nikon MicroscopyU | Interactive Java Tutorials | CCD Signal-To-Noise Ratio