Photographing Stars

Hi All,

For the past week I’ve been trying to capture the stars on my K20D. Having reads lots of articles on the subject and watched various YouTube videos, I was getting an increasing number of conflicting and confusing advice. Some articles suggest high ISO with a 30 seconds exposure but fail to mention the aperture, others suggest low ISO, large aperture and long exposure, etc, etc… The option was to start experimenting myself.

My first attempts were terrible. I went out into the country late at night to take a few shots but all I go was images that were either too dark or too grainy.

I continued to experiment however and last night there was a clear sky. I set the camera up in my garden (full of urban light pollution) pointing up at the stars. With the K20D in bulb mode, aperture around 4 and at ISO 100, I took an approx 3 minute exposure (using a lockable shutter cable). To my surprise and joy, it worked!

So they my not be the greatest shots, but I think it’s a good start.

[IMG]http://img209.imageshack.us/img209/122/imgp7229.th.jpg[/IMG]
Large

[IMG]http://img710.imageshack.us/img710/7138/imgp7231i.th.jpg[/IMG]
Large

Matt

Doesn't look much wrong with them, from what I can see (although, they are a little small).
I've not been too successful with digital, but had some half decent results with film. My best star trail was a 30 minute exposure.I accidentally got my camera dead centred on Polaris, which created a nice circular pattern.

Hi,

I realised you had to click large to see the images. Well done!

Fun isn't it. Away from town light pollution is best.

Divide 600 by the 35mm focal length for the maximum to avoid light trails from the rotation of the earth or the other way round when they will start.

The ISO level depends on the sensor and in camera software. My KR is very happy at 800 when I need it, your otherwise wonderful K20D might struggle. Finding infinity with modern lenses is difficult unless you have the moon to focus on.

My best night sky to date (avoiding precession trails) is a 48 second at F4 with a 10mm (15mm equivalent) at 800 ISO in Greece last May with the Milky Way cloud visible.

Just keep playing, especially at the moment with clear nights.

I'm in my "thick" stage, could you explain this a bit more please

regards
Bernard

I couldn't work out how to post the larger images on the forum without a link, hence the 'large' link underneath.

Thanks for the advice, all very helpful!

I quite like the look of star trails so I'll try to find time (and patience) to do some 20-30mins exposures.

M.

Because the earth spins, the stars appear to move, and quite quickly too.
This equation ( I presume) will give you the maximum exposure time to avoid the motion blur ( star trail) unless, of course, that's what yuo are trying to achieve.

Looked at them again in large this time. They're not half bad! I particularly like the second one as you can still make out the constellation ( Orion ).

Hi,

a 300 mm lens will get earth motion blur after two seconds

a 10mm lens after 60 seconds

This is very interesting; something I'd not read before (I must be reading the wrong books!). Do you know why 600?

Hi Matt,

With digital is now much more common to take a series of short exposures and use software to stack them on top of each other. This avoids both the increased noise in high ISOs and the problems DSLRs have with very long exposures.

There's software called RegiStax which seems the most popular free option: link.

An example of this is here (87x 20.sec exposures): link

John.

Hi, I read it while browsing online to learn. I can't find which site I read it. Looking again some photographers play safe and use a 400 rule but the principle applies so the wider the lens the longer you can leave it open to avoid trails. Conversely a telephoto lens will produce trails easier.

I personally have got away with 48 seconds on my 10-20 Sigma at 10mm. Pixel peeping there might be a tiny precession trailing so maybe slightly shorter works better for the purist. This blog is typical of the help you can find and questions 600 http://blog.alex-kunz.de/2011/08/night-skies-and-time-lapses.html

Having noted the comments following my "thick" post I sat and thought about it and scribbled a few sums and I come up with the following :-

A 50mm (SLR) lens has a field with of approx 27deg [horizontal 40 degs]
Earth rotates at 15 deg/hour

so it takes 27/15 [40/15] hours for a point to transit the width [length] of a negative
that is approx 6500 [9600] seconds

If we divide by 600 then 0.04 [0.06mm] of the negative is exposed in 11 sec [16 sec]

Not a visually discernable amount but when enlarged to say 12" x 8" the track becomes nominally 0.35mm [0.5mm] which is, so I conclude this is probably where the rule of thumb 600/focal length comes from

DLSR lenses have different fields of view so a different rule of thumb can be determined as above.

regards
Bernard