Quote:
Originally Posted by Emuhead
So I'm looking directly at the RASA 8 now. 
7x faster than the 100ED, and 12x faster than the 120ED.
Can you get nice round coloured stars with this scope? Any glaring reasons not to go the RASA? |
Hi EH,
No- no glaring reasons, but since you appear to basing your decision somewhat on the numbers, and there is nothing wrong with that, that's all part of any decision, there are a few perhaps hidden issues to consider,
when looking at how fast one system is relative to the other, since you are looking at the mathematical square of the aperture ratio, small differences may matter.
The RASA 8 is a 400mm focal length optic with a 203mm front objective OUTER diameter. By the common definition that makes it a 400/203=1.97 or f/1.97 optic,rounded up Celestron call it an f/2 optic. Rightly so, as this typical definition goes, but if one is looking at a comparison between Reflectors and Refractors, and how much light is truly collected then it pays to consider how much light is lost/uncollected over the central obstruction. (Here I'm not considering any contrast loss, just how much light is not collected over the central obstruction and how that might equate something one could call the effective f-ratio.
The INNER diameter of the front objective (the central obstruction)is somewhat large at 93mm, so whilst the system is advertised as f/2,
in light gathering terms it is a little slower, circa f/2.22. That is the number I would consider using in any comparison of exposure or light gather.
Another difference to consider is that the RASA 8 has a 32mm wide "Usable field" of which it defines 22mm as the Image circle, compared with 2 of the other telescopes you mentioned: The SW Espirit 100ED and 120ED, which have 40mm and 42mm image circles respectively. Of course this means you can use larger sensors, if you so desire, but also, as you've already noted you can use reducers to increase effective speed on an appropriate sensor.
So perhaps take that in to consideration when looking at how fast one system is relative to the other, for a given sensor.
Taking one of the examples you suggested, i.e: using the 0.65x reducer on the SW Espirit 100ED f/5.5 that would make it ~f/3.57 and reduce its image circle from 40mm to ~26mm. NOW comparing that with the RASA-8 at f/2.22 effective, that would make the RASA-8 about 2.6 times "faster" if using the entire 32mm "Usable Field" of the RASA-8. If using the 22mm Image circle of the RASA-8 things would be much closer, with the RASA-8 still being about 1.3 times faster, based on the reduction in area alone.
Best
JA