How to Get Less Noise

[JohnSadlerAstro]

Hi,

So will I actually get less noise with my 1000d at ISO 400? Or will it be basically the same as at 1600? If I took, say 10*45 sec subs at ISO 1600 instead of 2*225 sec ones at ISO 400, would it really make all that much difference? I'm really confused! ?

John

[michaelmorris]

3 hours ago, JohnSadlerAstro said:

Hi,

So will I actually get less noise with my 1000d at ISO 400? Or will it be basically the same as at 1600? If I took, say 10*45 sec subs at ISO 1600 instead of 2*225 sec ones at ISO 400, would it really make all that much difference? I'm really confused! ?

John

Hi John

The answer to your first question is, it depends on the camera!!  ISO in  DSLRs is NOT directly analagous to ISO in film cameras.  Noise drops with ISO setting, but on DSLRs this doesn't tend to be a straight line or even a smooth curve.  In many cameras there tends to be cliff where as ISO rises, noise suddenly increases.  On the 1000D the sweet spot between sensitivity and noise seems to be around ISO 800.  In some more modern cameras this sweet spot can to be at 1600 or even 3200.

As far as the length of sub exposure goes, the longer the better, but in reality this has a number of limitations

• light pollution
• length of accurate tracking of your mount
• the amount of noise from the camera
• the amount of background noise from thermal signal

Whilst this might make it seem that taking lots of short exposures is the answer, unfortunately every time a camera chip downloads the image to memory, this process introduces noise (known as 'bias' or 'shot noise').  The net result of this combination of factors is that the sweet spot for exposure length under a dark sky with a 1000D is going to be around 300 seconds.  Take 20 or so of them with darks, flats and bias calibration frames chuck the whole lot into Deepskystacker and you should be able to get some nice images.

I know, this imaging lark can get pretty confusing, but it all starts to make sense eventually (so I'm told).  I thoroughly recommend you look at Astropix.com for more information.

[alacant]

On 17 September 2018 at 23:06, michaelmorris said:

Take 20 or so of them with darks

Hi. I found that dark frames introduced more noise on my 700d. I suppose the difficulty of not being able to control the temperature of camera sensor is a contributory factor. But very much agree with the number of light frames. 20 300s exposures will usually get you something worthwhile if you can keep the telescope pointed at the same spot for that long. Do the same again e.g. on the next clear night and you then get even more noise reduction. I think that with a dslr you're gonna be fighting noise with anything less than 2 hours of light frames.

HTH

[michaelmorris]

Two extra tips for DSO imaging I have picked. 

Dithering - this really helps with reducing fixed pattern noise and hot pixels.

Dark Master software. Helps match your darks to your light frames before bunging them into DSS.