CA in Skywatcher/Helios/Meade refractors....

[GazOC]

Just a quick guide giving the expected CA levels (given by aperture/ focal ratio) in a few well know Archromatic scopes (given that everything else is equal), I've chosen the popular Skywatcher/Helios/Meade range. This list does not apply to Apos, Semi Apos (like the ED80) or Demi Apos (like the Sentinal or Revelation )

In order, with larger CA numbers being 'bad'.

150mm f5 refractor = 1.2 CA

120mm f5 refractor = 0.95 CA

100mm f5 refractor = 0.83 CA

150mm f8 refractor = 0.75 CA

80mm f5 refractor = 0.6 CA

120mm f8 refractor = 0.57 CA

127mm f 9.4 refractor = 0.53 CA

100mm f9.8 refractor = 0.4 CA

Gaz

BTW I've owned a 150mm f5 refractor and believe me 1.2 is BAD!!!!

[OXO]

Nice list Gaz, looking at the CA in my ST102 tonight on the Moon there was a green fringe on the outer edge's this does get better with more Magnifcation though. CA is most evident when taking images with the scope and thus needs processing out. The worse by far i have ever seen was with the Celestron C6-r F8 refractor totally spoiled views of Mars.

James

[GazOC]

Hi James,

The list is very general (lot of other factors to consider, inc. as you say, magnification). I don't know about you but CA on planets annoys me the most, I can handle it on the Moon, my 127mm gives a bit of yellow/purple at the edge, but I have to look for it.

If I have a scope that gives it on planets I find it very hard to ignore.

I need to talk myself out of a 6" f8 refractor, theres a cheap one up on AstroBuySell but I know theres no way it'll be that much better than my 5" f9 AND it'll have a lot more CA...it was one one of the reasons I drew the list up!

GAz

[OXO]

Hi Gaz,

I would advise against the 6" f/8 mate esp if it's anything like the one i had it really did put me off looking at Planets and being F/8 it's more a planetry scope than anything else which leaves you feeling lost with the scope. As you say there wont be much differance with your already owned 5" f/9 if anything i suspect it wont be as bad matey CA wise. Yeah i can put up with CA on the Moon no problem but when it comes to planets you dont wanna be looking a something that resembles a Smartie sweet hehe just IMHO of course. Star clusters look good though ahem......

James

[GazOC]

Cheers Mate,

You're right. I just get "greedy" with having the 127 f9, it's a great scope (cracking views of Saturn tonight, it beat my new Mak which I was gutted about!!) with not a great deal of CA on anything really....

I wish someone would make a 6" f 12....

Cheers for the "wake up call" mate

Gaz

[russ]

The thing with these chinese refractors is you have no idea what you will get. That guide gives a rough idea but for instance:

My 120 f5, an early Skywatcher model, was superb. Almost useable on the planets, very little CA on anything else. By comparison the ST80 and ST102 were bleddy awful. Forget the planets entirely and just about forget imaging too.

Worst of the bunch by a country mile was the 150 f5.....complete washout. A complete waste of space.

The 80ED is outstanding. Doesn't matter what you point it at, there's not even a hint of CA. The Sentinel is a cracking scope too. Can't quite make the same bold claim as the 80ED but it's damn fine for an achro. Only the full moon teased out some deep purple CA.

Regards the maks, my best ever views of all the planets have come from my Intes-Micro 603. In fact the only things i've looked through where i thought there was an improvement was a Tak Sky 90 and Tak FS102. The view of Saturn last year took Rob's breath away. Soon find out how a 100ED compares?

Your 127 Gaz sounds a cracker, don't ever get rid of it!

Russ

[GazOC]

Yep, Russ youre right, buying a Chinese achro is a lucky dip. The list is just a very general guide.