F/9 or F7 Query

[Sully]

I have got a skywatcher 130m, and am currently looking at the back page of Decembers Astronomy Now. It has advertised the 130m at F/900 and the 130P at F/650. I have also been reading Warthogs piece on ideal EP's that says focal ratio =(Length/aperture).

My question is two fold, firstly what is the focal ratio of my telescope, does being parabolic make a difference to focal ratio. Secondly, as I'm considering buying a Tal x2 Barlow and a couple of GSO plossl's and want to make sure I don't over magnify my scope or waste my money, will I notice any significant difference given the scope I've got??

thanks

Sully

[umadog]

Your telescope has a focal length of 900 mm and an aperture of 130 mm, making its focal ratio equal to 6.92 (since that's a ratio so it has no units). Being parabolic or not is irrelevant since that doesn't alter the focal length or the diameter, which are what define the focal ratio. Strictly speaking, a telescope mirror should be parabolic but at longer focal ratios you can just about get away with a spherical cross-section. Grinding a spherical cross section is much, much, easier so manufacturers sometimes do this to save a few bob. Incidentally, SCTs have spherical mirrors but the corrector plate, well, corrects for this (amongst other things). They have a clever way to easily mass-produce the correctors, thus keeping prices under control.

I can't answer the second question since you don't say what magnifications you plan on using. Just follow these guidelines and you'll be good: Useful Magnification Ranges for Visual Observimg - How To

[Sully]

Cheers Umadog, I thought it was F7 the wording in the advert made me unsure. As for EP's I was thinking of a 6mm and a 15mm to go with the 10 and 25 I've already got for just now.