I have always been told this is a no no because of the increase of focal ratio, but take a look at this thread on SGL. Makes very interesting reading:
https://stargazerslounge.com/topic/298212-julian-shaws-sombrero-with-a-surprise/
Carole
Thats very interesting Carole..
That was an interesting read and I made on reply on that thread.
With low read noise cooled CMOS cameras I can see this approach (i.e. Refractor+Barlow) being quite viable. It wouldn't work well for DSLR cameras though because the thermal noise would end up becoming dominant.
Although this approach does increase the versatility of a reflector, in the end you'll get the same quality image much quicker by using a large aperture reflector (Newtonian/SCT/RC)with the same effective focal length.
Mark