Im with you on that one robin, when i saw 8mm i said WTF... you do have to remember that a 10" dobsonian at F4.7 is pretty quick as it is. Being that it is actually a dobsonian and ment for viewing not astrophotography, they reduce the size of the secondary for ultra contrasty views furthermore they make the spider vanes super thin to reduce diffraction. I think the central obstruction on a 10" in its standard guize is 21% which is very low. with the new 70mm its upto just under 30% which still gives great contrast. RC's and the like are up near the 40-50%...
Just out of intrest, i tried to define how contrast is determined. and from what i could see its about light scattering and lightening the dark airy diffraction ring and darkening the light airy diffraction rings (used in star testing). hence the overall image is some what normalized.
You are right about the 2" focuser vingetting some of the light cone, but thats where things get better. some people think that you must allow upto the 75% ray cone to pass to the sensor/ocular, but this isn't the case, my 75% ray cone is nearly 100% vingetted, but it means nothing because the 100% covers the chip! and in any case who would want less photons hitting the sensor?
Just a side note, the standard SW 254mm Dob actually vingettes the lightcone from the front aperature! by a long way, to gain the full 75% i would have to open the ID of the tube to 310mm from its present 283mm!
Grab newt 2.5 and start putting your values in, you will be serprized.
Rob, I am a uni student, it is rare that i get much more than a hundred bucks or so to do something. so everything i do is done as economically as i can do it!