My first problem – gradient across the sky

[Pierre_C]

Hi people.

I am new to astrophotography and have just started using a DSLR on a tracking mount for the first time.

Can you tell me what is going on with the bright blue gradient across this picture? I was expecting it to be all the same background colour.

This issue became apparent when I adjusted the photo by turning the brightness setting down from 50 to 0 to bring out the starry detail.

I used a Nikon D7000 DSLR with a Nikon Nikkor 18-105 zoom lens on a Sky-Watcher EQ3 mount with RA drive. The lens was set for a nightscape at 18mm and the exposure was 10 seconds at ISO 1600.

We are looking south along the meridian.

There is some Milky Way running vertically through the centre of the photo (Crux with the Emu Head are just below centre, and eta and theta Carinae are above centre) but the bright bit to the right seems excessive. The top right corner is also very dark, which seems odd.

The city is 25km to the west – to the right of the photo. The picture was taken after astronomical twilight. There was no moon. I did not notice any clouds.

Could it be due to something to do with the zoom lens or the camera sensor? Or something else?

Thanks.

[sharkbite]

Hi Pierre...

I'm not sure what the 'brightness' setting is that you are referring to,

but using a wide angle lends near a city is always going to show some gradient due to light pollution.

"stretching" the image in processing by increasing the exposure level, and then also increasing the contrast will make this gradient more apparent...
you could try using something like graxpert to smooth it out but i'm not sure how much it would help

The lens you are using is known for light drop off in the corners, and i think i see a bit of vignetting in the top right corner? This could be a factor as well.

You make also have a bit of stray light hitting the front element - you could try a lens hood?

you may not have seen any cloud, but it sure looks like you had some....

I'd also try stacking more than one image - single images are really hard to get right

[JA]

Hi Pierre,

You don't say if you used the 18-105mm f/3.5 wide open or at what aperture; however, it's important to remember that as an all round lens it must try to do many things and that there are some compromises. Light fall off is one compromise especially wide open or thereabouts. This is quantified in this Lenstip Review link , which reports a 43% light fall-off at 18mm f/3.5 and 37% at f/4. That's quite significant especially when the image is then stretched to bring out dark detail. Couple that with light pollution from city lights and an apparent local stray light source (yellow glow) near the house at the bottom right and hey presto, you have some reasons for the vignetting and gradients seen in the image.

Best
JA

[Nikolas (Nik)]

Light pollution always produces gradients as seen in your image.

[Pierre_C]

Thanks for the very helpful advice and information.

It sounds like there were quite a number of things going on in the image.

The gradient being lighter on the city side matches the potential light pollution.

The aperture was wide open at f/3.5, so that matches the zoom lens vignetting.

There were some street and neighbour lights nearby hitting the lens, so that could have contributed.

I will try some of your suggestions and see what difference it makes.

[Leo.G (Leo)]

Pierre if it helps, while I don't live in a dense city we still have bright LED street lights but I find some places in my back yard where I'm in less light than others and my imaging improves by my moving into the darker areas of the yard. I also have significant tree coverage from neighbouring properties which can be both a cure and a curse (80% of stuff I want to photograph is behind those trees).

Of course this only helps if you have a yard and in Melbourne would be like when I lived in Sydney, too much ambient light everywhere.
Perhaps filters may help which I'm sure would have been suggested already.


This is why I purchased the Astro-Trac so I can take it to a friends farm 15 minutes away with pitch black skies and no street lights, the portability will be a huge bonus when I get to it (health and weather).

[Pierre_C]

Thanks, Leo.