Which big low/mid power achromat?

[casstony]

The recent sale of an NA140SS on IIS has got me thinking about a largish refractor for scanning along the milky way. I'm considering the 150mm f/8 Skywatcher or the Vixen NA140SS. Can anyone offer experiences in comparing these two scopes?

In addition, how would the Skywatcher 120ED compare to the above scopes for low/mid power views - I'm assuming the larger aperture of the achromats would be better than the higher quality ED optics for low/mid power views?

[rmcconachy]

The 150mm F/8 is a heavier scope than the other two you mention (~10kgs for the 150mm versus ~6kgs for the 120ED and ~7.5kg for the Vixen). The 150mm is also much more nose heavy than the other two scopes making it a pain to mount. If you want to look at objects near the zenith with a 150mm F/8 scope you'll need a tall tripod/pier if you want to avoid getting close to the ground (4ft long OTA balanced about 1/3rd back from the objective).

I had a Synta 150mm F/8 and its 120mm F/8.3 sibling and much preferred the smaller scope for its ease of use so I sold the 150mm OTA. Subsequently the 120mm Synta made way for a Vixen 4" F/9 ED scope. I like the Vixen fit an finish (and focuser) much more than the Synta but between a 120ED and a NA140SS I'd probably take the 120ED, sometimes I'd want to bump the magnification up and today the false colour bugs me. Happy observing with whatever you pick!

[casstony]

Thanks for your insights Ricky. Hard to decide. Might end up placing a wanted ad for an NA140 and ED120 and see what turns up.

[rmcconachy]

Sounds like a good plan. Another cheaper option might be the Synta 120mm F/8.3 achromat I mentioned above. I bought mine second hand as a bare OTA for <$400.

[Luke Bellani]

I've had a Skywatcher 150mm F8 achromat for about 8 years and I wouldn't part with it.

I replaced the focuser with a WO unit (a GSO one would just as good) and although there is some colour fringing, the views are fantastic.

With a minus violet filter, I can't see any fringing at all.

If they're still made as good as mine. You should consider one at least.

Cheers,
Luke

[casstony]

Thanks for your input Luke. I ended up ordering the Skywatcher 150mm f/5 achro today ($600 + shipping). I'm planning to use it for widefield below 60x magnification so I'm hoping chromatic aberration won't be an issue.

[anj026]

Widefield below 60x is wonderful in 6"f5 refractor. I have been enjoying the secondhand Jaegers I purchased last year for that purpose. There is still a little purple haze on the brightest stars but for the majority of milky way sweeping it is not a problem at all. I mainly use just the 31mm Nagler which gives a 3 degree field at about 25x. Occasionally zoom in with a 17mm for about 45x. I fitted a GSO focuser and shortened the drawtube as much as practical in an effort to reduce vignetting of the light cone. I have read a lot about this type of scope in other forums and people either love them or hate them. Low power widefield is their forte.

[casstony]

Nice to hear Andy. If the scope meets expectations I might splash on a 31 Nagler since I'll be in the US later this year.

The light cone is down to 2" at 500mm behind the objective, leaving 250mm; subtract about 100mm for the 2" diagonal light path, leaves about 150mm for the maximum focuser drawtube length, assuming a full 2" clearance inside the drawtube. Are the original or GSO drawtubes longer than 6 inches?

[anj026]

Hi Tony

I am away from home at the moment but I can refer you to this thread I started on Cloudy Nights which may help. Rather than me post a long complicated link please do a search.

It is in the ATM, Optics and DIY forum, titled Using wide field eyepiece in f5 refractor, 17/12/2009. My user name on CN is Plyscope.

Let me know if you can't find it I will try another way to get you the info.