Sensor cleaning Canon 700D

[almcl]

I noticed some swirl marks on a sub of the Iris nebula a little while ago (1st image below - running down from just below left of centre).  At first I thought they were cloud, but they turned up again a week later (image 2).  Then at the weekend they appeared on a widefield image of another target (image 3) - same camera and clip filter but using a camera lens rather than a scope.

Tried imaging without the clip filter to see if they went away and the result was inconclusive - the light pollution that the clip filter helps combat is so bad that a sub of more than 40 seconds without it is overwhelmed by sky glow.  So tried reversing the clip filter, hoping to see the swirl pattern change sides.  It didn’t.  Still in the same place.  It doesn’t show (as far as I can see) on the flats, darks or bias frames, and isn’t always visible on the subs, either, but still shows up at intervals on some.

Reluctantly, I conclude that there’s something on the Baader filter in front of the sensor that isn’t being removed by my Hurricane blower and that sterner measures are needed.

Has anyone any advice (apart from ‘don’t’) about cleaning the Baader filter attached to the sensor or otherwise combating the problem?

[Alien 13]

My own preference is the Lenspen sensor Klear it works well without spreading any greasy deposits that solvent based systems do.

Alan 

[almcl]

Thanks for that, Alan.

I feel one of those may well be in my future - hadn't thought of using a magnifying glass or lens to look at the sensor (http://www.lenspen.com/how_to_use/dslr-sensor-cleaning/) might try that before sending off for the lens pen...