DSLRs have a front bandpass filter, a filter that allows a certain band of wavelenghts of light to pass thru to the sensor and block everything else.
This is because...the unwanted wavelengths..the wavelengths we cannot see ...still posses energy (even though we cannot see them) which will be registered (captured) but the camera sensor.
Thus the camera will be taking pics of wavelengths we cannot see converting them to images we can see. Thus the sensor will (can) "bloom" the image...that is become overwhelmed by the amount of energy reaching it making images "foggy/blurred".
Another consideration is that these unwanted wavelengths come to a differing focus wrt light we can see..thus the camera will convert this unfocussed energy to foggy images.
However...camera mfgrs are a little conservative when it comes to the band-pass filters they put on their DSLRs...so many do not capture some of the wavelengths we want....like Hydrogen Alpha and hydrogen beta. Which are abundant in the universe.
Thus users modify their DSLR to put in band-pass filter
that allows these Ha/Hb wavelengths thru...or even leave out their filters to allow all wavelengths thru..like infra-red.
edit:
beaten!