Originally Posted by prova
so what would looking through a 4" refractor be the equivelent of a 4" newtonian, is that how it works?
to be precise, im looking at the skywatcher 100ED pro opposed to an 8" reflector ..
if it's a case of seeing a little less with the 100ED but getting more clarity on the image .. fine
Hi Provo - I think my thing about refractors is an "oldies" phenemenon more than anything else: the 8" newt would definitely outperform the 4" refractor of whatever lens system (achro/apo.)
However, the 4" refractor would definitely outperform the 4" newt: forgetting about other aspects that could start a raging "war" if I go too far here, there is the simple issue of "clear aperture." Simply, the refractor doesn't have to contend with a secondary mirror in its' light path.
Refractor afficianados will speak of clarity of definition/contrast, especially re planets: I have a 6" f8 refractor, only an achromatic (2 doublet objective lens cells, adjustable) but even without the Baader "semi apo" filter, CA (chromatic aberration) was almost non-existent - with this filter in, I can confidently say it is virtually non-existant. And I've seen my fair share of achro refractors with their share (and more) of CA!
The Synta job I have gives me absolutely great views (I call him "Long Tom") and I am almost about to christen a special pier I have just about finished for him: there is a review of one of these particular refractors (not mine!) in the "Articles, Reviews etc" section of IIS where the frequently used description of these refractors as "planet-killers" is used to describe its capabilities!
Long Tom really works a treat on Jupiter and Saturn (and no, with the Baader in I get no CA with Venus) and I darn well reckon it'd give most 8" newts more than a run for their money imho!
ED's and "true apochromatics" are wondrous sharp and clear in their definition: just ask some of the lucky owners of these units what they think!
The longer focal length refractors (and if they're only achromats the general consensus is that longer f/l reduces CA) are a bit of a pain when obbing the zenith (you're on your hands and knees almost) but my special ergonomic designed pier (Pierre!) will sort that all out.
As I said, I think it's because when I was young the only refractors were cheap and really nasty "department store" types or hideously expensive astro units: I could (and did) grind my own mirrors and make a newt of good aperture (6" & 8") but there was allways that allure of owning a quality decent sized achromatic refractor. (ED's and true apochromatic weren't invented then!)
Notwithstanding any of the above, price and light-gathering/aperture competitions with reflectors leaves the refractors for dead, particularly if you want state-of-art apochromatics: however, this old fella is still going to purchase another "Long Tom" in the near future (along with an upgrade from HEQ5 to EQ6) for his already designed 6" achromatic binoculars....!