What is a good mirror ?

[Bobbyoutback]

Your about to buy a newt & know enough that the better the mirror is made the better it will perform , you see a vendor offering 1/12 wave specs .
Wow you think that must be good hey !

But what does that actually mean ?

1. Is it peak to valley ?
2 . Wave length at surface plus or minus ?
3 . Or expressed as RMS ( root mean squared criterion )

Below is from Mel Bartels



Star test standard: immaterial differences between intra and extra focal images, diagonal breakout on either side of focus occurs at about the same distance, and there is no turned edge. The star test is qualitative only. Popularized by John Dobson and his protoge Bob Kestner (optician for the COBE Hubble correction optics at Tinsley Labs), this method relies on slightly defocusing a test star, first inside of focus, then outside of focus. By noting how the light from the entire mirror de-focuses, a rating in the form of: excellent, good, average, poor, or unacceptable, can be given to the mirror.

Here is Bratislav's star test scale: from 1 to 10 :
1. Can't find anything wrong with it, absolutely perfect: expletive' Yet to see one after ~25 yrs.
2 . Defects visible only in extrafocal images, and only after extensive star testing in best seeing conditions (1/10 wf): Can count these on fingers of one hand.
3 . Extrafocal defects readily visible, but really minor (1/10 wf): 'Excellent' Best examples of best commercial telescopes (Zeiss, AP, Tak, etc). Best examples of homemade optics.
4 . Extrafocal defects fairly obvious, but in focus image still essentially perfect (1/10 - 1/6 wf): 'Very good' Majority of current 'best commercial telescopes'; best examples of mass produced scopes.
5 . Large defects visible on extrafocal images, in focus image suffers only slightly (1/6 - 1/4 wf): 'Good' selected examples of mass produced telescopes, most well made amateur optics; some examples of 'best commercial scopes' can still be found here.
6 . In focus image visibly suffers (~1/4 wf): 'Acceptable' good mass produced scope, most good large/fast mirrors I've seen.
7 . Image deterioration serious, clearly beyond 1/4 wavefront: 'Light bucket' majority of older generation mass produced scopes, special purpose telescopes (astrographs).
8 . It's difficult to determine when scope is in focus at all ( 1/2 - 1 wf): 'If you're happy with it ...' unfortunately, not that difficult to find !
9 . Usable only at very low magnification ( ~1 wf): 'I don't want to have anything to do with this one'
10 . Absolutely useless: expletive' unlike 1, I've seen these :-)
99% of all scopes I've seen fall into '4-10' bracket.

Are you going to be happy with a 5 or 6 ? :sadeyes:

Then there is the secondary mirror , most mass produced can have astigmatism !

Hope to get the more knowledgeable to enter this thread then me .

Bobby .

[astro744]

In no particular order you can get superb mirrors from http://www.loptics.com and https://zambutomirrors.com See also https://zambutomirrors.com/zambutoopticalca.html

Go quartz if you can but not an absolute requirement.

Locally there is Mark Suchting but I'm not sure if he's still making mirror. He refigured my 10.1" f6.4 and it is exquisite and I am very happy with the result. this was many years ago around the 2003 opposition of Mars which I remember provided me with some superb views.

There are a few other quality mirror makers. Note the rest of the 'scope can and will impact on performance. See http://www.sdmtelescopes.com.au for some quality telescopes.

After saying all of that you can get a very good quality mirror from a mass produced manufacturer. The only difference is that with a custom manufacturer you will be guaranteed to get quality every time but the QA of the mass produced makers means some poorer quality mirrors slip through.

The other day I had some exceptional views of the crescent moon (5-6 day old in twilight with mirror at ambient; makes a difference) with resolution I had never seen before on the lunar surface. Which mirror do you ask... a 16" f4.5 Meade mass produced mirror that I am very happy with so much so that the 10.1" has been retired...sorry not selling either.

Don't get too hung up over the numbers. A quality mirror maker will guarantee a first class mirror whereas mass produced makers can put whatever spin they want on the numbers. Don't get too hung up on Strehl either. I've seen a Rhonchigram of a mirror showing a clear defect yet reportedly had a very good Strehl! It's just another way to spin the numbers.

[Bobbyoutback]

Thanks Astro744 ,
Your reply post had some good information 'especially that last part !
Do you remember the cost of getting your 10" f6.4 refigured ?

I particularly like the way John Dobson reads a mirror in his video , starts at 1.19:55 https://www.youtube.com/watch?v=snz7JJlSZvw

Cheers
Bobby .

[Atmos]

I don’t believe that Mark Suchting has made mirrors for quite some time but the two mirror makers mentioned are considered to be the go-to for high quality dob mirrors.

The cheaper end of the market has made leaps and bounds in recent years in how repeatable better quality cheap mirrors are made. Both Mike Lockwood and Carl Zambuto talk about it being more than just the optical figure but also the quality of the finish. You can have a really well figured mass produced mirror with a not so great finish and that’ll I torusce considerably amounts of contrast killing scatter.

[billdan]

The quality of an acceptable mirror also depends on its purpose.
For visual use where you may want to go 500X magnification for planetary, you need the best mirror you can buy.

For imaging with a camera you are using the mirror at its lowest magnification, given by the formula :-

Aperture(mm) / 7.

So a 200mm (8in) mirror is about 29X mag and a 16in about 58X.

So for imaging a mass produced mirror is OK, for visual a custom built mirror may be required.

[Bobbyoutback]

This is about visual ' not imaging .

I want to see the image real time through the eyepiece ' not a cooked up image show time example of who has the best gear !

Bobby :)

[Wavytone]

Bobby this is an area about as clear as mud.

Mirror quality is often expressed as 1/X wavelength, this generally refers to P-V values (ie peak-valley) not RMS values which are often smaller by a factor of 10-20.

Conversely when you see claims like 1/20 or 1/50 wave this is most likely RMS, and the P-V value could be barely ¼.

It originates from the Raleigh criterion, which proposed a wavefront error not exceeding ¼ wavelength. For a newtonian primary that translates to a surface error not exceeding 1/8 wave, although that assumes the secondary is essentially perfect.

A few companies test the complete scopes and provide the test results. For example Intes scope generally have a test certificate of the complete scope and Intes guaranteed wavefront error less that 1/6 P-V for their maks, or 1.8 for the deluxe models. These scopes however command a premium even secondhand - and for good reason - optically they are exquisite.

Another measure is Strehl, where 1.000 means perfect. 0.9 corresponds to ¼ wave P-V error, 0.95 corresponds to 1/8 wave, 0.975 corresponds to 1/16 wave.. To measure Strehl requires an optical test lab that can run a double-pass interferometry test (DPAC).

FWIW my scope is 0.965 strehl... visually on bright stars in good seeing it shows a very tight text-book perfect diffraction pattern if you crank the magnification up to 600X.

A really, really hard test is Dawes criterion - you can google that for yourself. Basically it requires magnification around 3X per mm of aperture and very few scopes can run to that, usefully.

[Bobbyoutback]

Good stuff Wavytone ( Nick ) :thanx:

Below is Mel Bartels :

Magnification roll-off: Another qualitative method, though it does demonstrate the MTF (modulation transfer function). A low contrast object such as Jupiter is selected, and a series of magnifications are run through. At some point, the image will 'roll-off' beginning to lose it sharpness. Dividing this magnification by the aperture gives a rating, for example, a mirror is good to 50x per inch of aperture.

Here is my scale:
Mirrors that can sustain 35x to 50x per inch of aperture I rate as excellent.
Mirrors that sustain 25x to 35x per inch of aperture I rate as good.
Mirrors that sustain 25x per inch of aperture are acceptable.
Mirrors that sustain 15x to 25x per inch of aperture are poor, usable only at lower powers.
Mirrors that fail at 12x per inch of aperture are plain just not finished.

For smaller slower mirrors, push these numbers higher, for large fast mirrors, push the numbers lower.

Bobby :)