can you tell the difference?

[Daniel-K]

not sure if this is the right place to post so mods move if need be. so assuming SW mirrors are polished to 1/4 wave anything better than that is it possible for our eyes to detect? i have nothing to compare to but the price difference between SW and OO & John-N, can you really see anything that minute? i would like some feedback on this please as a lover of big scopes someday i would like a mirror of a high standard.

Dan

[NGC 1502]

Tough question actually, and definite answers difficult.

A genuine 1/4 wave mirror, and yes I do realise that specification is open to interpretation, will give excellent views when the scope is cooled to ambient and properly collimated.

A higher spec, 1/6, 1/8, 1/10th wave, should, under good seeing conditions, give superior views, higher contrast on the planets for instance.

Excellent conditions, like a stable and transparent atmosphere, don't happen often though.

Volumes have been written on this subject.............

Hope that helps, at least a bit, Ed.

[Moonshane]

my understanding is that at lower power/magnification of faint objects there will be little difference but at higher magnification with bright objects then there will be less light scatter with a better mirror, resulting in higher potential magnifications and better detail for planets etc.

[scarp15]

Perhaps to ascertain any visual gain using a mirror with a very good figure, would depend somewhat on using high magnification, on for example, planetary subjects in great seeing conditions, with a well collimated scope, using premium eyepieces.

Mass produced mirrors are usually of a good standard though, certainly my former SW 12" Flextube was, I have heared of a skywatcher mirror that was tested at 1/6PV. Premium mirrors figured for much larger and faster scopes i.e. F4, F3.9 would be more logical perhaps.

[John]

My Orion Optics 12" dob has mirrors that are 1/9th wave PV and a strehl ratio of .987. Not the best possible but a good mirror I think. Where I notice the difference between this scope and my previous 12" Meade Lightbridge is at high powers. 265x - 320x seems regularly usable and gives excellent results. With my Meade Lightbridge that just did not seem to be the case and 200-250x was it's "top whack" on many nights and the planetary and lunar images just seemed to lack the "bite" in contrast and sharpness that the Orion Optics gives. I have seen some things with the Orion Optics that the Meade would not show. Poor seeing conditions undermine this advantage of course but then they would impact the Meade as well. 

On deep sky objects I'd say that the advantage of the better quality optics is much less pronounced, to me anyway. Nebulae and galaxies look pretty similar in the Orion to how they looked with the 12" Lightbridge.

I can only compare the two scopes I've owned in this category of course. I have noticed Skywatcher 12" owners who have reported seeing similar details to those which my Orion Optics will show. In the past, I did own a Skywatcher 10" dob and then moved to an Orion Optics 10" with 1/6th wave PV optics and I did not notice much difference at all in the performance of those scopes so I suspect the Skywatcher optics were pretty much as good in that case.

[barkis]

Owning a perfect parabaloid is very desirable, and for sure very fine planetary detail will demand that accuracy, but other 

elements have to cooperate too, not least the seeing in our part of Europe, never the best we have to concede.

However, if you can afford the best, then go for it, but as others say, many mass produced objectives can, and do deliver

well too.  A few duffers get through the net though, so the buyer should exercise caution when thinking of buying.

Reputable dealers should be your only consideration..  

Many Amateur mirror makers  produce excellent pieces too, many have the patience to correct even small  

errors, which can take no small amount of time to do so. They have a deep pride in what they do.

Ron.

[Cath]

At what optical wavelength do these mirror wavelength accuracies apply I wonder?

The visible spectrum is around 700nm (red 430THz) down to around 390nm (violet 790THz), so nearly a 2:1 difference.

[John]

At what optical wavelength do these mirror wavelength accuracies apply I wonder?

The visible spectrum is around 700nm (red 430THz) down to around 390nm (violet 790THz), so nearly a 2:1 difference.

I don't know the answer to that although I guess it would be shown on the Zygo reports that come with my Orion Optics scopes. It would seem a bit daft for OO to provide data for wavelengths that were not visible to the eye although that might be more interest to imagers though.

I think what matters is what difference you can actually see when you use the scopes which is what the OP asked

[Mike73]

I agree with Ed, I think we are in a hobby which is governed a lot more by our skies and seeing conditions than the equipment we use, we buy and strive for the best equipment possible so when those brief moments of good seeing or dark nights we know we have seen it at its best.

Unfortunately we very rarely get perfect seeing conditions in the UK, most of us observe in or near towns and cities so we may never see the real benefit of a 1/10th Pv mirror!

[Earl]

As far as know

Seeing, Light pollution then Quality with diminishing return.

[astronymonkey]

I think the premium mirror question is more of a guarantee of quality rather than what you can see. As seeing is so variable then you mau never see the true benefit but for a premium scope or a large aperture I personally feel a premium mirror, with a certificated level of accuracy is essential.

Imagine paying £7k for a large aperture dob, you would expect a certain level of quality in the optics but you may not expect the same standard in a £500 scope but you might end up with it just by chance.

also with the premium mirrors that are hand figured, its not a case of quality control being able to discard 1 in 6 of them and only allow the good ones through, this simply isnt an option with the money and time already invested so the mirror goes from the test bench back for further refiguring.

so for day to day viewing my 16inch meade mirror does just fine, but if I decide to go bigger, and the cost increases ill be moving into the premium mirror territory so I know im getting the level of quality im after.

mind you, I always say just buy what you can afford and enjoy it ... I still love showing my kids jupiter through my 35 year old 60mm tasco refractor, it was my first scope and for me not even the Hubble comes close !

[Daniel-K]

really good info here thank you. the plan is when we have done the work on the new house is, to get my self my dream scope something that will keep me satisfied. im forever bouncing round chopping and changing and to be frank does my head in. the only scope i have had for over 6 months was my meade LB 16". i have the 14" Flexi tube ATM but guaranteed a few months down the line i'll be itching for something else possibly another 16". my dream scope would be what east wing has his 18" F3.6 or.9 i think  is something i would want to hold onto knowing the mirror is about as good as us amateurs can get. TBH i dont think i can see myself getting back into AP either since i have the 14" ive been able to get out a few times all though it may only be upto 1HR at a time but ive been having more enjoyment from the hobby. with AP what the heck is 1 HR its nothing takes you that long to get focus and guiding sometimes. 

[Laurie61]

At what optical wavelength do these mirror wavelength accuracies apply I wonder?

The visible spectrum is around 700nm (red 430THz) down to around 390nm (violet 790THz), so nearly a 2:1 difference.

OO use 632.8nm. My own experience is a 1/8-1/10 w optic will show an improvement over a scope with standard 'mass produced' optics with the proviso of comments above. The question you might have asked instead is 'How good is good enough' but this will also throw up different but perfectly reasonable answers dependant on the individuals priorities.       

[John]

OO use 632.8nm. My own experience is a 1/8-1/10 w optic will show an improvement over a scope with standard 'mass produced' optics with the proviso of comments above. The question you might have asked instead is 'How good is good enough' but this will also throw up different but perfectly reasonable answers dependant on the individuals priorities.       

Good post 

My approach has been to get the best optical quality I can afford for all aspects of the optical train. It seems to me that astronomy is an exacting hobby where we are often trying view things at the edge of whats possible for the aperture / conditions / observer ability and I want every "edge" I can get, even if it's a small one

When I see "will I notice the difference ?" type questions I tend to feel that the honest answer is probably "much of the time, probably not, but with more experience you will chase more challenging targets and thats when the difference tells and you are glad you went for the better quality".

[chiltonstar]

It would be interesting to know what the figure of a mirror is when it's actually mounted, tipped at an angle etc, particularly modern thin mirror designs? I suspect that 1/8 wave might often be a bit less in reality.

Chris

[russ.will]

Quite. I've mooted the issue of newtonian OTAt/focuser mounting area flex when you've got one of the current massive 82 & 100 deg UWA EPs in place, especially if you've added the coma corrector to that for a bit of extra leverage. 

Then again, when a OO UK 1/10 wave Newt cropped up, I snapped it up because even if it makes no difference for observing, I now KNOW any issues aren't going to be with the mirror. It removes a variable.

Russell

[Dude_with_the_tube]

hmm,so far no definite answer about improved views of higher wave mirrors apart of confirmed that they will improve  the views on higher mags what means the scope will be great as planetary instrument only and the extra cost will be some sort of "guarantee" of quality. looking at all the options will it not be more beneficial to get a good quality long focal length refractor for that?

Orion Optics can produce a mirror beyond 1/10th wave for extra cost.So far i have seen 1/12.49 wave mirror out of them.Was built for a dedicated planetary observer and many of yours know this chap.

[AlexB67]

I suppose when these things matter more or less also depends  what you are looking at as well, what is the main purpose of the scope, say planetary detail, splitting doubles a good mirror is important but also slower focal ratio will help on wider separations where you want to minimise distortions and mirror shape design are important factors too.   On the other hand when chasing down a fuzzy galaxy,  when you are pretty much working at the detection limit, if that is what mostly matters, perhaps a 12 inch 1/6PV is better than a 10 inch 1/10 PV, the extra aperture versus PV rating would be a tradeoff well worth having I guess. 

Of course cost aside I am sure in an ideal world we'd all like the better quality mirror but at the same time, as John said, even in the 10 inch most of the time the diffs are small, and with my level of experience there is so much work and observing to be done with a standard 1/4PV SW mirror at that aperture,. I can safely say it is not limiting what I want to do at this stage. Perhaps in future these things will be nice to have with long term experience, when that day arrives my mirror is probably well down the line and ready for its n^th recoating, perhaps a higher quality replacement mirror would an option then,  but right now, for me at least,  I feel it is one of those things to easily get hung up on rather than being essential in my scope. Sure if you can afford it and feel you really need it and know the reasons why you need it, all good of course   

[swamp thing]

It would be interesting to know what the figure of a mirror is when it's actually mounted, tipped at an angle etc, particularly modern thin mirror designs? I suspect that 1/8 wave might often be a bit less in reality.

Chris

Not sure about large EQ mounted optics so I will not comment on them.

In large dobs most use a fully floating mirror cell with support sling (either a strap or wire). This gives first class support to a mirror regardless of what altitude the scope is pointed.

We can even use computers to help us calculate the exacting requirements of our own individual mirrors. We can then tailor make a mirror cell for our own mirror.

The mirror is supported on contact pads when in place in the mirror box. As the scope is brought lower towards the horizon the support sling takes more and more of the weight.

For thinner mirrors we can add support pads to the cell until we reach adequate support.

My larger scope has a pretty thin mirror (1.5"), for this reason I opted for a 27 point cell as opposed to the more usual 18 on a scope of this size. For an even thinner mirror we would simply add more contact points.

Here is my homebuilt 27 point fully floating mirror cell.

I would think for thinner mirrors the problems would be more in the actual figuring process itself than the cell.

[chiltonstar]

Looks impressive....are there any actual measurements of a mounted mirror about though (I ask in innocence as a non-dob person...)?

Chris

[estwing]

Who's this east wing guy? Sounds like he's got a great scope!!! Anything above 1/6 is a decent mirror and let's be fair here some of the sw/Meade gso mirrors although mass produced give some cracking views...the only major issue I've heard of was mike73's mirror and that was sorted out within days. We are really lucky when you think how much we are paying now for a 16" truss dob!!

[Rustysplit]

One thing you could do is get a Ronchi eyepiece. Pop this in the focuser and defocus slightly on a fairly bright star. It will give you a quick indication of the general state of your mirror.  Knowing what I know now since grinding my mirror, if I were to buy any reflecor, I would check it out with a ronchi ep. A quick test to see if the optics may be less than described. The question I guess then is, is it the primary or secondary that is at fault??

As has been mentioned, the skill involved to hand figure a premium mirror, is that extra guarantee and assurance that your optics are up to scratch. Mounting and collimation in reality probably pay as big a part.