SW 102ED vs. SW 120ED

[emadmoussa]

It's a simple as that, the 100ED has a maximum practical power of 300x, while the 120ED can achieve up to 350x - I think.

The question remains, in terms of actual magnification under the UK skies - wouldn't 300x be the useful maximum? Which makes you wonder what's the obvious advantage the 120ED have over it's smaller brother the 100ED if our skies are the limit?

# NOTE - title should read 100ED, not 102ED..couldn't edit it - sorry!

[ollypenrice]

There's more to resolution that magnification. You can keep to a fixed power and still see considerable differences between the resolution of one scope and another. I wouldn't expect to see a great improvement between two diffraction limited scopes of 100 and 120mm but I'd expect to see a little. The main difference would be in detectng faint light.

The Dawes limit for 100mm aperture is 1.16 arcseconds and for a 120 it's 0.97 arcseconds. There is a calculator here; http://www.asterism.org/clubact/calc06.htm

Olly

[emadmoussa]

Thanks, Olly. That makes a lot of sense. But I'm assuming as well that this little difference will manifest itself under either steady, high visibility sky conditions or a dark site.

Since you own a Takahashi, may I ask...comparing to a 100ED what does the Takahashi FS102 NSV offer - imaging aside?

[John]

I used to own an ED100 which was then replaced by a Vixen ED102SS with a faster focal ratio but broadly similar performance. I've since added an ED120 to that so I compare the two reasonably regularly.

The 100 / 102mm's top out at around 200-225x most of the time although you can push further towards 250x with binary stars. 300x really seems to me to be over the top with these scopes. WIth the ED120 the regularly useful max goes up to 225x - 250x with 300x being used on the binaries and sometimes for fine lunar details under really good conditions.

As yet, I've not had the opportunity to try my two ED fracs against a Tak but I'd be interested to do so someday

[ollypenrice]

Thanks, Olly. That makes a lot of sense. But I'm assuming as well that this little difference will manifest itself under either steady, high visibility sky conditions or a dark site.

Since you own a Takahashi, may I ask...comparing to a 100ED what does the Takahashi FS102 NSV offer - imaging aside?

I'm afraid I have no idea. I use a pair of FSQ106Ns, one mine and one Tom O'Donoghue's. They are used for imaging where their enormous flat field and excellent colour correction into the blue-violet sets them apart from cheaper instruments. My hunch is that, in visual use, you'd be payng a fortune for a tiny improvement. Mechanically no, the Taks are are in a different league, as they should be for the price. But would I expect a quantum change at the eyepiece between an ED100 and an FS102? No. In a blind test I'd be slightly nervous of getting it wrong! Handling and owning the scopes would be difrerent, as might be the verdict of my children in thirty years' time. (I have an old TeleVue. It is perfect.)

Olly

[SiriusB]

Another dfference to consider is the weight, 120 refractor is (suprisingly) significantly heavier than 100mm, if you want easy grab & go then maybe 100mm better? I use my st102 more often than my st120 for just this reason.

[dmahon]

The 100 may have less chromatic aberration. The 120 will gather data quicker if used for photography.

[emadmoussa]

I'm also looking at William Optics 110mm apo -- as far as I know there are two types of lenses used in these telescopes: TMB and the other one....can't remember what it's called. Do you know?

[emadmoussa]

TEC optics..this one

[Naemeth]

I think with either you will greatly miss 11" of aperture. Up to you of course, but it's not what I'd do.

Sorry I can't be of much more help .

[emadmoussa]

Well, once I have managed to get an observatory I can always buy another one...if finances allow of course

[ollypenrice]

The TEC optics of the earlier 110s should be superb but you have to beware of the terrible focusers that afflict some WO offerings.

Olly