The Canon 350D's peak sensitivity lies almost exactly in the blue-green portion of the spectrum, which by happy chance is where some nebulae emit strongly - at the O-III and H-beta wavelengths.
Using an H-alpha filter with an unmodified DSLR would be like pushing rocks uphill, because the IR filter severely reduces the strength of the H-alpha signal. Not impossible but long effective exposures (a big stack) would be required.
Using an Oxygen (O-III) filter on an unmodified DSLR plays right to the strength of the camera. If you want to try narrowband imaging with an unmodified DSLR then an O-III filter should be your first choice.
If you want to bring out some of those hidden wispy lines of detail then an O-III filter may be the go for you. I'm very pleased with the results of this test.
Many of my astro pics, taken without filters, have large areas of blue-green nebulosity that are clearly emitting a lot of light at the O-III and H-beta wavelengths. So, I bought an Astronomik O-III filter to see if the detail in images (i.e. wispy clouds, etc..) is similar to those ghostly H-alpha shots that we all like. I have to say the results are very pleasing.
An H-alpha sensitive (modded) camera is going to be better all round, and it will capture more nebulosity. It will also let you use H-alpha narrowband, but you've got to think this O-III shot on an unmodded camera is pretty cool.
Check this out.
I'm calling it the Freak (or Boogey Man) Nebula. See if you can spot the skull.
The image is just a stack of three x 4-minute exposures at ISO1600.
- does it have a NGC/IC no???
Just looked at the pics closely.... disregard.... 
