2" Prism Diagonal

[ccdscopeman]

Is there any advantage in using a 2" prism diagonal over a 2" TeleVue mirror diagonal on my Tak FS128?

Andy

[faulksy]

i think im right if you use a prism diagonal it corrects the image to right way up. usefull for land viewing

[haitch]

i think im right if you use a prism diagonal it corrects the image to right way up. usefull for land viewing

But extra glass in the pathway for astro and also tends to put a nasty diffrection spike through the view.

[ccdscopeman]

Better off sticking with the TeleVue then. I thought I read somewhere that a good one on a refractor can enhance the viewing but this obviously can't be right. The article was on an Amici roof prism diagonal. Glad I checked on here first,lol.

[John]

There was talk on another forum that prism diagonals could improve the colour correction of certain ED doublet refractors. Your FS128 will be extremely well corrected though as it's one of the very best of it's type made so you can safely stick with a conventional mirror diagonal I think.

[ccdscopeman]

Thanks guys. Just as an aside, would the said prism perform this correction on an ST80 F5 short tube?

[John]

I don't think so. The type of scope the prism diagonal corrected (and it was only a modest improvement as I recall) was a certain type of ED doublet. The ST80 is a fast achromat and is bound to show a fair amount of chromatic aberration. Not a problem for the intended use of these scopes though, ie: low to medium power, wide field views.

If there was a low cost way to create an apochromat from an achromat I reckon it would be all over the web by now !

[haitch]

I don't think so. The type of scope the prism diagonal corrected (and it was only a modest improvement as I recall) was a certain type of ED doublet. The ST80 is a fast achromat and is bound to show a fair amount of chromatic aberration. Not a problem for the intended use of these scopes though, ie: low to medium power, wide field views.

If there was a low cost way to create an apochromat from an achromat I reckon it would be all over the web by now !

A shame Skywatcher never mass produced a Chromacorr.

[ccdscopeman]

Wasn't trying to convert an achro into an apo John! ! I have a crappy diagonal at the mo and was just trying to improve on the diagonal quality.

My ST80 does show a little false colour but it is surprisingly low and it seems to disappear depending on what angle you hold your head at. Sometimes a very,very pale violet, if I move my head slightly it turns to a very pale yellow before disappearing. The optics must have some sort of correction or are these colours so pale because it is a faster F5 scope? I've experienced CA on longer focal lengths before and it has always been a lot deeper and richer in colour.

Andy