[raymo]

On the brighter galaxies such as NGC 253, and M83, it will do well, but on fainter ones you would need to up the ISO in order to keep your sub lengths
acceptable. I haven't followed your progress closely, but if you have mastered
long exposure guiding, you can work wonders with that scope.
Just saw Martin's post, 900mm is not enough focal length if you want to get seriously
into galaxies, unless you are prepared to stick with only the largest ones. Many quite
bright ones are too small even with my 1000mm f/l. 900 is also virtually useless for planetaries.
The simple answer is that no one scope fits all.
raymo

I have added a pic of NGC 3918 taken with 1,000mm + 2x Barlow =2,00mm and enlarged to about 10,000mm.