Optical Quality and Effects of Secondary

Dead right there Michael, the p-v value is not any guarantee, if you had 2 mirrors both one sixth wave but one had a wave front diagram that looked like it had been blasted with a shot gun while the second mirror had only a small area of high deviation, then those 2 mirrors are not going to perform equally

Dave_Lillis wrote: it had been blasted with a shot gun

That would make for an interesting mirror Dave...
For the record, IFAS wishes to denounce all violence carried out on mirrors, OTAs and all other astronomical-related equipment, including their owners...

I Just like to say I’m no expert on optics and have no formal training just an interest in telescope optics as part of this hobby so anything I say take with a huge grain of salt.
The telescope in question was purchased second hand and cost me no more than the price of a good/premium eyepiece, but it had a test report and that’s something my other scopes did not have.

OO.UK, state on paraboloids they electronically remove the central section of the mirror on the final test
The first test is full aperture with no central section removed (as indicated on top right panel)

So yes it looks like primary mirror only test, I assume that the secondary is of equal quality but you know what they say about assume, I suppose I could check with them.

From what I can gather the RMS figure is of multi point, if not the whole mirror, which as you say is more accurate than P-V. Were a single or couple of points tested could give an unfair mirror reading either way good or bad.

I think its worth checking out Orion Optics Web site as it explains it much better than I can.
www.orionoptics.co.uk/OPTICS/optics.html

Looking again at the first link posted StrehI Ratio Table
The table states a mirror of .992 strehl and RMS 0.014 is equal to 1/20 Wave. Orion Optics only states that the mirror is equal to 1/10 or better.

What I found very interesting in the first link posted is the relationship of secondary quality and size when taken into consideration gives a more accurate and overall rating. When I put my figures into this formula I get a value of just over 1/9 wave for the complete scope.

You might be interested in this site were you can run your own test report.
I posted my results here a couple of years ago on a Celestron 100mm f9 ED refractor, it’s a bit finicky to use but worth the effort, and its fun, I can post results again if you wish.

New WinRoddier Version 3 User Manual - Compubuild

As someone once said at the end of the day it’s a hobby and most people can’t discern ¼wave from 1/8 wave when at the eyepiece. Having said that, I think I would rather have 1/8 wave scope.

Tony

tony h wrote: I Just like to say I’m no expert on optics and have no formal training just an interest in telescope optics as part of this hobby so anything I say take with a huge grain of salt.

Ah no, too late for that disclaimer now Tony, start as you mean to continue...

tony h wrote: So yes it looks like primary mirror only test, I assume that the secondary is of equal quality but you know what they say about assume, I suppose I could check with them.

The secondary is usually of a higher grade.
It's small and flat so easier to polish accurately.
Often 1/20th wave.

tony h wrote: From what I can gather the RMS figure is of multi point, if not the whole mirror, which as you say is more accurate than P-V. Were a single or couple of points tested could give an unfair mirror reading either way good or bad.

AFAIK, it should be quite a few points spread right across the mirror.
Of course, if one wanted a good result, I'm sure one could be selective in what points are used...
However, I don't think OO would do such a thing as they would be found out quite easily and it damage their reputation. I have heard only good things about their mirrors.

Worth downloading the program Aberrator:
aberrator.astronomy.net/html/mdibeta.html

MIchael.

Hey,
the RMS value if done properly is usually very indicative of the mirror quality, I can remember that the number of points for my own mirror was a 4 figure value (I cant find the diagram), and that the rms value was very low, much lower then the p-p value, indicating a mainly smooth mirror with small areas of deviation.
and tbh I wouldnt get too hung up on this, most people under most conditions wouldn't notice any visible improvements beyond 1/6th wave accuracy, possibly 1/4th wave.
There are far bigger things to worry about in a dob such as tube currents. My scope is a truss scope so it doesn't have a tube, but I can put on a shroud which has the effects of a tube.
For planetary viewing, putting on the shroud usually has a very obvious deleterious effect on the view, so when I use the shroud I have about 2-3 feet above the mirror unshrouded when viewing the planets..
This is one of the reason the SAC 12" dob has a hatch just above the primary mirror, allowing warm air rising off the mirror to escape the tube, works a charm.

What Dave said about tube currents....I have big problems with them on my SCT, it can last hours.
OTOH I have the F6 version of Tony H's scope and am nowhere near as plaqued with currents. As long as it's within an asses roar of ambient it will get down to it rapidly enough. That's also a 1/10 wave scope.
In the war between SCT and Newt, collimation is the achilles heel of the newt, still, easily overcome with a god quality collimation kit.