A general feel for P/V values?

[Macavity]

Some scope manufacturers provide certificated "P/V values" for their optics. Standard issue seems to be 1/6 P/V, Research Grade 1/10 P/V -- In between values, 1/8 P/V is usually De Luxe? Basic question: Would 1/6 P/V typically be adequate for a non-imaging amateur, or should one always aspire to (spend more on!) 1/8 P/V optics if possible. I recall the historic (similar?) term "1/4 wave astro", but sense thing have improved a bit since the 70s...

Vaguely eyeing up STF Mirage MAKs... Maybe one day?

[kniclander]

I don't know the answer and for once I won't guess and pretend I do but I do remember a very interesting thread a while ago in which someone had a mirror tested by orion optics and there was a lot of discussion about the value of these tests and their results. I can't remember who started (maybe Philsail1:icon_scratch:)? it but an imaginative search should reveal it....

[johninderby]

Standard is about 1/4 PV for mass produced reflectors such as Newts by SkyWatcher or Meade etc. Specialist manufactures do go up to about 1/10 PV or so. Orion Optics UK can supply 1/20PV mirrors but they don't recommend them for general use as once you get beyond 1/10 you're not going to be able to see the difference. BTW OO gives their PV ratings as better than the stated values. A 1/4PV rated mirror will be about 1/5 PV actual, a 1/6 PV rated one about 1/7 PV actual etc.

There was a Skywatcher mirror that was tested by Orion Optics UK a while ago and that was an exceptionally good one and was about 1/5PV actual.

John

[John]

There was a Skywatcher mirror that was tested by Orion Optics UK a while ago and that was an exceptionally good one and was about 1/5PV actual.

John

How did they/we know that it was an exceptionally good one John - were a number of mirrors tested ?

If only one was tested it might have been an average one

[johninderby]

Apparently they had tested other mirrors by SkyWatcher and Meade before and Barry at OO said the "exceptionally good" SkyWatcher one was the best mass produced mirror they had ever tested.

John

[John]

Apparently they had tested other mirrors by SkyWatcher and Meade before and Barry at OO said the "exceptionally good" SkyWatcher one was the best mass produced mirror they had ever tested.

John

Thanks John - that makes sense

[kniclander]

I deleted my post cos I was being a sarcy git. sorry.

[brianb]

Mass produced Chinese optics are usually about 1/4 wave RMS which translates to probably 1 wave PV. If you get one which is better than 1/4 wave PV its performance will be noticably better than average ... going above 1/8 wave PV will only gain you anything at all when the seeing & temperature control are perfect. OTOH if you have an interest in ultra high resolution planetary viewing or imaging, and an above average site, IMO it's worth paying the premium for 1/10 wave PV "research grade" optics.

[johninderby]

I think some of mass produced Chinese dobs are great value for money, you're definitely getting what you pay for. I've got an 8" SkyWatcher dob and I'm very impressed with just how good the optics are considering the low price.

I've also got that 14" OO dob with an 1/11 PV mirror, and to be honest on most nights I'm not going to get the full benefit of the great optics, however when those rare nights with great seeing come along it really delivers with some amazing views. Definitely a very high WOW factor that makes what I spent on it well worth it.

John

[Macavity]

Thanks folks! Not too many articles around, but eventually also found:

Optical Standards

"PV 1/4λ Wavefront error, PV 1/8λ Surface error, RMS 1/15λ or Strehl 0.82" etc.

Another instance of having to be quite precise with the terminology?

[johninderby]

It's worth looking at the Optical Mechanics website as they have some good explanatory articles on the whole subject of mirrors, mirror testing, coatings etc.

http://www.opticalmechanics.com/technical_articles/index.html

John

[F15Rules]

My new 6" F12 refractor has 1/6 wave lens apparently..all I know is, on a very low and rather boiling Moon, it was razor sharp at x90. A completely unscientific way of looking at it might be Pixel rating in a camera...a few years ago 5 Megapixel cameras were the bees knees, now it's double that - but I don't see the difference in my holiday snaps!!

If my new scope is "only" 1/6 wave, I can live with that:D

cheers

Dave

[Macavity]

I think, if I have the funds, I might eventually go for something Russian (an MCT) that has "certificated" optics - Or indeed a Skywatcher light-bucket. Some of the other threads (now discovered) have been very interesting. I am persuaded that, set against my dodgy eyesight and cooling effects, I need worry rather little with (slightly better than) standard-ish optics...

Of some concern: I see some rather disquieting measurements on some of the popular SCTs. It seems fair enough that more... AFFLUENT (and/or gregarious) astronomers wheel and deal their way to finding a "good" example of a particular scope. As for the rest of us, I am rather less certain...

[great_bear]

Mass produced Chinese optics are usually about 1/4 wave RMS which translates to probably 1 wave PV.

Brian,

Is it possible to equate (brightness aside) a larger mirror with a better PV on a smaller mirror? i.e. Does a 1/8th PV mirror on a 150mm scope give it the quality of a 1/4pv 200mm mirror (as an example)

Or is it a different "type" of quality measurement?

[brianb]

is it a different "type" of quality measurement?

Yes ... a better figure will (given good seeing etc) give a "tidier" image, more aperture will give more resolution even if the image looks less crisp.