Help with blemish

[woldsman]

I've noticed a blemish appearing on images. Photo uploaded. Would really appreciate help identifying what is causing this and what I can try to do to try and fix it. 

[woldsman]

PS am using a Canon DSLR 100d (modded). 

[happy-kat]

I can't see the blemish using a mobile phone, blemishes can often be dust bunnies, how they look can help indicate where they are.

[GalaxyGael]

Are you using flats in the processing, or is it a single sub or full stack? These often 3D looking divots can come from imperfect flats with various levels of over and under correction especially for dust bunnies. Helps to know where it in image train the dust might be, on filter, camera window etc. Is your image train too finnicky or tuned to bother taking it apart? If so, then perhaps getting new flats might be useful. 

[tooth_dr]

Looks like the dust has moved between the light and the flat, so it appears kinda 3d.

[woldsman]

Many thanks for the replies. The image is just a sub. For imaging, I take flats and darks and these seem to take care of the blemish in stacking. Must be a dust bunny in the camera as it’s there in the subs regardless of lens. I wondered how common these issues are.