What diagonal to get for use with a Celestron 80GTL?

[Meerkat]

Having just got a 2nd hand Celestron 80GTL for £62 I'd like to change the erect image diagonal for a plain diagonal as I assume erecting the image degrades the image somewhat.

Now, going with the quality of the scope, any recommendations for an equivalent diagonal?

Thanks,

Steve

[rawhead]

I just bought this one http://www.scopesnskies.com/prod/21.html for my £50 ST80...

[Milamber]

I'd say go with the best you can afford, since scopes come and go but accessories tend to stay around.

Arthur