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A 600sec sub using a DSLR under a major Cities light pollution ( even with a CLS filter is going to be absolutely full of noise ( read noise, shot noise and dark current ) Was the moon up as well ?
I image in Sydney with my 600D @ ISO 800 and a 90 sec sub is my limit even during the new moon phase
DSLR’s have a poor quantum efficiency at around 40% so pushing 600sec subs is multiplying the noise and reducing your signal to noise ratio ( SNR ) Dedicated cooled CMOS Astrophotography cameras have a QE of 80% and higher
Try the following -
Choose a different easier target like a globular cluster
Take 60 sec subs, and lots of them ( 40 to 60 )
Take darks , at least 25
Stacking reduces noise so use a free staking software like Deep Sky stacker
Dither your sub frames, dithering helps reduces noise
Good processing software will remove any residual noise gradients from your final image
There’s a good YouTube clip on Astrophotography and Noise by Craig Stark , it’s worth looking at to understand the fundamentals of Noise in Astrophotography
Cheers
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