Ring in center of my photos?

[mozeyonover]

Last night I went outside and took some photos of the aurora lights.
I noticed that all my photos had a ring (pretty much in the center) that spread outwards.
I just want to know how to get rid of the ring and what causes it, Thank You.

I was shooting on bulb between five and twenty seconds.
Camera: Nikon D7000
Lens: Tamron 18-50 2.8

I added a photo to this thread.

 

[480sparky]

Newtonian rings.

 

[mozeyonover]

Newtonian rings.

Thank you!

 

[KmH]

I bet you have a "protection"filter om the lens.
"Protection" filters usually cause more problems than they solve.
Newton's rings

 

[astroNikon]

I thought it would have been Aliens.

But the above 2 answers are probably much more accurate

 

[Designer]

Ooo...Weeee...Ooooo!

 

[KmH]

Probably UVerian Drcvclmstorn nutoids from Gamma Doradus in the Eta Corvi system. They're only 60 light years away.

 

[dennybeall]

Was it a long exposure and did you cover the eyepiece?