Secondary Mirror Importance

We've all seen discussions here about which manufacturer makes the best mirror and to some extent just how much better than 1/4 wave correction do you really need. How many of us are qualified to tell the difference between a 1/4 wave vs. 1/14 wave mirror? Maybe the difference is significant. I don't know because I've never looked through two telescopes side by side with know specs to be able to compare.

What I haven't seen, perhaps for lack of searching, is discussion about secondary mirrors. I've just recently purchased a used 18 Obsession and shortly thereafter noticed that the secondary has three parallel scratches along the minor axis near the center of the mirror. I don't know to what extent this affects the views through the scope and I haven't got a "perfect" secondary to campare it to. People of various levels of experience have looked through the scope without knowing about this defect, and no one has said there was any problem, but I can't help but wonder if I should replace the mirror. So, I pose this question. Should I splurge and get an Antares 1/30 wave seconday or a Protostar 1/10. Has anyone had the opportunity to do any comparisons with any types of secondary mirros that would be able to give me some guidance? All responses are greatly appreciated.

Thanking you all in advance,

Mark Klimkiewicz

The secondary mirror is every bit as important as the
primary mirror. Since they are relatively inexpensive
compared to the primary mirror, I would argue to get
one with specs significantly better than the primary.
That way you are not limited by the quality of the
secondary. For example, if the primary is 0.25 wave PTV
on the surface, then I'd say to get a secondary
0.1 wave PTV on the surface or better. The RMS error
is a better indicator of surface quality than PTV, and
could also be used in selecting a secondary, if you know
the values.

Scratches are another story. Simply having some
scratches does not make a bad mirror. It depends on
how wide (thick) they are, how long, whether they
are straight or curved, etc. A few very fine (thin)
scratches will not have much effect on anything, and
the RMS or PTV rating of the mirror will be much
more important. Say you have a 2" diagonal with
a 0.1mm wide scratch 2cm long -- this affects
only about 0.1% of the area, which will be
completely unimportant. The surface quality of the
other 99.9% of the area will be much more important.