Making skies black in astrophotography

[botty1963]

I don't have much experience of astrophotography, so last week I decided to give it a go when we were on holiday. So, between midnight and 1am my wife and I sat on a small harbour on the Northumberland coast, very minimal lighting, and I set the camera up at my lowest f-stop (3.5) on the wide-angle lens I was using, set my shutter speed to 30s, manual focus, and tried a number of different ISO. I wasn't bothered that much about whether the stars were sharp or not so didn't do the calculation for exposure time against lens focal length, it was just a bit of a test.

Now the photos came out ok, but as with other times I have tried similar, the skies always look very blue. I have read that often images are combined with a completely black image, and I have seen Youtube clips where LOTS of post-processing is done to produce the required image. Is there some 'easy' way of getting them black or when using a camera are you always going to have to do some work to get what your eyes see?

[Shadowman]

Hi Steve,

The camera is only revealing what is there, you opened up the aperture and extended the exposure time and by doing so let in more light. If you want darker skies you can do so either in post-processing or by changing your camera settings.

[Dave Humphries]

Hi Steve,

This isn't something I have tried personally, but a few thoughts come to mind.

What WB were you using?
I'd try fixing it at Sunshine or even Cloudy, the latter would probably help reduce the blue-ness.

With regard to the overall quantity of background light, a lot will depend on the atmospherics of the night as any haze will pick up light from towns, villages, ships, oil/gas rigs or windfarms directly below the direction you're shooting. Your eyes won't see this at the time.

However, I wouldn't expect artificial lights to register as blue, more likely orange. So the other possibility is that as you're fairly far north up there and your local sunset was probably a bit later than mine, I wonder how much twilight was still in evidence at mid-night to 1am, if you see the effect diminishing towards 1am, that may be the cause.

That said, it should be fairly simple in PP to boldly move the black point so that this background light is lost (turned black) while still retaining the stars, but I'm guessing because you didn't show us a picture - if I were you, I'd do that now, so when the people with actual star shooting experience see the thread, it will be waiting for them.

Good luck, Dave

[chauncey]

I'm sorry Steve, but, in addition to what has been mentioned...PS is almost a necessity.
https://www.youtube.com/watch?v=qGp2lEIaEOY

[DanK]

I agree with Dave--it would be a big help if you posted a few of the pictures.

I doubt the problem is light pollution. Here is a 7-minute shot taken at about 11 PM under a full moon in the central Adirondacks, where there is almost no light pollution. The nearest substantial light source is a few sodium-vapor lamps 5-7 miles away, and those are yellowish, not blue:



So, without seeing your photos, my guess is that your exposure was just long enough to let the blue show up. I don't do astrophotography, but I frequently have a similar problem when I shoot flowers against a black background that turns out gray because of reflected light. My solution is to select by color the area I want to darken, make the selection a mask on a new levels adjustment layer, and make the selected area pure black.

[Manfred M]

My suspicion is that your shots are overexposed and that is why you are getting the blue.

[James G]

Steve,

Are you using a camera set white balance? If it is set for night photography it will 'enhance' the blue effect.
It should be set to neutral or daylight if you are capturing camera processed jpeg.
If you are capturing Raw make sure you are using neutral white ballance.

you may find this site, and this particular page helpful....

http://www.clarkvision.com/articles/...the.night.sky/