I own a Nikon Coolpix P900. I'm still fairly new to photography but I'm interested in learning about photographing the stars. I've read that the best settings to do this are to have an aperture of f/2.8, a shutter speed of at least 20", and an ISO of between 800 and 1200.
I'm having some trouble getting my camera to these settings. At an ISO of 800 my camera will only allow me a maximum shutter speed of 2". How can I change this?
I don't have the P900 camera, but the bigger Reference manual PDF for it is at http://download.nikonimglib.com/archive2/KMeWX00JKxMZ01nkeDM84gfnob78/P900RM_%28En%2903.pdf
It has two sections, a regular pages 1-122, and then a Reference Section pages 1-124. In Reference section page 22, it says the control range of shutter speed is:
ISO 200 8 seconds
ISO 400 4 seconds
ISO 800 2 seconds, etc
That does not sound good for your use.
Unless you mount your camera on an equatorial motor drive tripod that keeps track of the stars as the Earth rotates, you should not expose for longer than a few seconds. The stars will become star trails if you use an ordinary tripod. Your pictures may look fine when you don't magnify it too much but it's not going to give you great results. Faint stars won't be visible when they would be visible when mounted on a motor drive tripod, because the light from stars is now smeared out over many pixels and then it falls below the noise floor.
The solution to this problem when using an ordinary tripod is to use image stacking. If you would need to expose for 20 seconds but you can only expose for 2 seconds, then that means that the signal to noise ratio is too high by a factor of no more than 10. In the ideal case where there is only read noise and the noise doesn't increase with exposure time, exposing for ten times longer would boost the signal by a factor of ten while keeping the noise the same. In reality the noise will also increase.
To reduce the noise by a factor of ten using image stacking requires taking 10^2 = 100 images and averaging over them. In practice you need less than a stack of 100 to get great results, but of course, the larger the image stack, the better the results will be.
If you are limited in how long you can expose for, then I recommend using a higher ISO, such as ISO 1600 maybe even 3200, use short exposures of around 30 seconds, and get many of them. I would say get at least 16x, however 25x or 36x would be better.
The reason you want to use a high ISO is the higher the ISO, the lower the read noise in most cases. The reason you want to get many frames is you can stack multiple frames together after "registering" them (which aligns each one on the stars). By stacking 16 frames, you reduce noise by SQRT(16), or 4x. By stacking 25 frames you reduce noise by SQRT(25), or 5x. By stacking 36 frames you reduce noise by SQRT(36), or 6x. You could stack 64 frames to reduce noise by 8x, but without tracking that is likely to result in funky star trailing in the corners (you'll get a little of that with 36 frames even, but it will be small enough that you could crop it out.)
You can register and stack the frames with a free tool called DSS (Deep Sky Stacker). DSS will take all the frames, align them with each other, then stack them together to average out the noise. Stacking like this is called "integrating the exposure". This should get you a decent result using very short exposures, even without tracking. Once you have a stack, you will usually need to "stretch" it to reveal fainter details, enhance contrast, etc. Think of this as basically the same as pushing shadows with a high DR scene.
If you want to get better results with much longer integration times, you could look into getting a small tracker. A tracker is a simple astrophotography mount capable of holding a DSLR and a small lens, maybe up to 135-200mm in length. They will usually track the sky for up to 2 hours, which is usually more than enough to get lots and lots of frames to stack for a very clean result.
there are rigid limitations to the P900 and long exposures - its virtually impossible to do a long shutter release, particularly for starlight.
