My first telescope was a Saxon 1309EQ; a 130mm, F6.9 reflector. I still have it, though use it less frequently now that I've upgraded. When I bought it, it was advertised as having a parabolic mirror; on the website, the box etc. However, I'm aware that the Skywatcher 1309EQ, a VERY similar scope with the same aperture and focal ratio has a spherical mirror. Since it appears that Saxon and Skywatcher both source their optics from Synta, it would be a reasonable assumption that they are in fact the same telescope, raising the question as to why one claims to have a parabolic mirror while the other has a spherical mirror.
As a result of this, I emailed Saxon and was told that while the scope used to have a parabolic mirror, the latest batch has a spherical mirror, though they could not confirm whether mine (bought early 2012) was parabolic or spherical. Soon after, the entry for the scope on their website was changed to remove the claim of a parabolic mirror, though some retailers have retained it. All a bit odd...
Does anyone know anything else to do with this? I realise Saxon scopes are rather more obscure than other brands, but I'd really like to clarify this, if only to know what to say if I ever sell it off (I doubt many people would be interested in a spherical mirror reflector). I'd also feel a bit cheated to have received a telescope with a claimed parabolic mirror that did not in fact have one.
Aaron,
There's an easy way to verify it.
Just use a fine grid - Rochi screen 100 lines per mm - a piece of fine mesh will also work.
Point the scope at a bright star, remove the eyepiece and place the grid over the end of the focuser. You should see a series of fine lines. As you re-focus, and get closer to the focal point of the mirror, the number of lines will reduce, and their width increase.
When you have 4-5 lines, look at their straightness - if straight, the mirror is parabolic, if curved then it's spherical.
HTH
Kevin,
That only applies when you're testing a mirror at the centre of curvature...using parallel, collimated, starlight gives straight bands with a parabolic mirror.
The lines would look fairly straight when looking at a star with a spherical mirror of this spec as the spherical aberration would be less than 1/4 wave so it would be hard to tell if the mirror were a bad parabola or a good sphere !
