How can I tell if/whether my 4.5" mirror is spherical/parabolic

[ColHut]

And does it matter at f8. The tube is a Bushnell but the manufacturer is Synta I think. Are the Skywatcher f8 4.5" mirrors parabolic or spherical.

cheers

[Blue Skies]

At f/8 the mirror is probably spherical and no, it doesn't matter because of the long focal length. I have a table that give the min focal length for a 4" mirror to be usable as spherical as f/7.0 and a 5" as f/7.8 so your being in between those two aperture and longer in focal length than the 5" would easily be so. This is why these scopes and mirrors are so cheap - they don't need the specialised skills needed to make it parabolic!

[Merlin66]

Fastest and easiest way to check is using a Ronchi grating focussed on a star with 3- 4 lines visible. If the lines are straight is parabolic; if they curve then its spherical.