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Skywatcher refractor


dawson
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Apart from the longer focal length, are there any optical differences between skywatchers startravel and evosyar models?

I just ask as there is a startravel 120 on ASB for £135. I hear lots of praise for the evosyar, but never heard the startravel mentioned.

Thanks.

James

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Startravel series are achromatic refractors whereas all the evostar are doublet apochromatic I think, so yes there's a difference optically, on my tablet so you'll have to Google difference between achromatic and apochromatic

Edit: Unless only the ED ones are apochromatic, I don't know without researching

Edited by Eddy
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Erm sorry eddy but evostars arent all apos im afraid

Startavels are short focus scopes, to make them portable, travel - getit:-)

Evostars are longer focal length.

Therefore optically they are conciderably different

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The lenses will likely be similar but They will have a shorter focal length (ergo shorter tubes) than the evostars so CA will be more apparent. Not sure if they'd use different types of glass though. You'd need to do a bit more research ti answer that.

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Just to clarify a point made above about CA, in case you don't know James it stands for Chromatic Aberration

.In optics, chromatic aberration (CA, also called achromatism or chromatic distortion) is a type of distortion in which there is a failure of a lens to focus all colors to the same convergence point. It occurs because lenses have a different refractive index for different wavelengths of light (the dispersion of the lens). The refractive index decreases with increasing wavelength.

If you did know all this, sorry for wasting your time.

:)

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The glass in both scopes is the same, crown and flint elements which make up the achromatic doublets used in the fraunhofer design airspaced doublets which these scopes use. However because the startravel is a short focus F5 and the evostar is long focus, F7 or whatever. The startravel has more curves on the lenses to bring the focus of the light to the shorter focal length. This means that chromatic aberration is more evident in the shorter focal length scopes because it is more difficult to get the separate colours R, G and B to come to a focus.


 


That's achromatic scopes for you. And no, before you ask :D, putting a barlow to artificially increase the focal length in a short focus scope wont improve the CA all it does is magnify the CA and makes it twice as bad for a 2x barlow or three times worse for a 3x etc.


 


Therefore if you wish to get a fairly colour controlled SW achromatic scope it is better to go for the evostar. But if your into DSO visual and aren't bothered about CA, because you don't get much colour in DSOs then the startravel may be for you.


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Thanks tobias, daz and phil; i dosn't know about CA so that was really useful.

I won't persue this scope any more..

Out of interEst, if one was imaging in mono, either pure b&w or using coloured filters, does the CA fault still make an appearance?

Cheers and hopefully see some of you on friday.

James

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Guest Kheldar

What does the ed stand for? Whats the dofference?

 

Can't remember what it stands for but it's result is an "almost" APO scope, there's the barest minimum of CA with the ED scopes which is why they are recommended as a good first "proper" imaging scope so often (the ED80 that is.)

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Guest Ely Ellis

Is it something like Extra Dispersion or Enhanced Dispersion glass.


 


James, if I make it to Wymsewold on Friday, I will bring my scope, it is the Celestron version of the Evostar 120, then you can have a look.


 


Cheers


 


Martin


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Guest Ely Ellis

Ah, Kim got in there before me. ;)


 


At least I was on the right track, I must have learnt something! :)


 


Cheers


Edited by Ely Ellis
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Guest Tweedledum

Had an st 120, it was a great scope, accept its limitations with a hint of ca. Brilliant grab & go, snow, sand and dog slobber proof. Travelled in boot, roof box and back seat, just usable on az3 but great on az4.


 


For a cheap scope it punches above cost, and if it breaks on travel then you have not lost a great deal, consider the cost of a 2" uhc / Fringe killer at 80 quid, 55 more and you have the scope.


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