Seeking info on the optical quality of mirrors.

[richardda1st]

I would appreciate some info on the optical difference between really good quality Newtonian mirrors and the general run of the mill stuff.
Please steer away from the battle between Skywatcher and Meade LB mirrors. I'm more interested in knowing if it's worth upgrading my mirrors to achieve the best possible results. I have a 10" LB.

Can the difference be Quantified ? :shrug:

Thanks
Richard

[wavelandscott]

You are posing an interesting question and the answer is yes, the differences between a "premium" mirror and a mass market one can be quantified.

The related questions (and I think more important) is can "I" tell the difference and is the difference worth it to "me".

And the answer to this is probably...

If you are a serious viewing and get lots of eyepiece time and are looking for and able to differentiate small differences in the view then you will appreciate the difference (in my opinion)...

Having said that, the optical train is only as strong as the weakest link in it. I'll paraphrase (poorly and in an exagerated manner) what other people have said previously.

$50 eyepiece with a $5,000 mirror = $50 view
$1,000 eyepiece with a $500 mirro = $500 view
$1,000 eyepiece in a $5,000 mirror = Priceless view

A lot of the mass market stuff is pretty good and "nearly" (85-90%?) as good as the top end...depending on your viewing habits and targets you may not find the difference worthwhile.

The other consideration is size...a mass market 16 inch versus a premium 10 inch...who wins? And every other variation you can imagine...

The best advice that I can give you is try it and see...got to a star party and look through some high end gear and judge for yourself.

[Garyh]

Hi rich,
Depends on what you use your LB for? Do you do high power planetary viewing or more into looking for faint fuzzies etc?
To really see the difference from the average mass produced and a top figured mirror you will need excellent seeing for starters which doesn`t happen very often and high powers to utilise it. I don`t think many people would be able to tell the difference from a 1/4 PV mirror to a 1/20 using low powers on Deepsky stuff and in average seeing.
Saying that but, the chances of getting a below average mass produced mirror would be say 30% with zones, roughness, dog biscuit etc, 50% pretty good figure around 1/5- 1/6 PV and the remaining 20% could get you to 1/10 or better.
It`s a bit hit and miss.
Like Scott mentions, if you can get to a star party and have a look through some different scopes and maybe get someone to check out your mirror and go from there.
If it comes out with a nice smooth figure, you should be happy but if it comes up with zones etc then it might be worth looking at something better or even getting it refigured.
cheers Gary

[DavidU]

Richard, why not send the mirror to Mark Suching (Sachmo) and get him to test it and re figure it to a very high standard and just bolt it back in the OTA.

[Coen]

Potential starting point:

http://www.bbastrodesigns.com/ratemirrors.html

Google/Bing/Internet search: Strehl ratio

[richardda1st]

Thanks all.

I'm not saying that there's anything wrong with my mirror, but if there's a noticeable improvement, then I would way up the cost against this improvement.

Yes Scott, I do plan to go again to the Snake Valley star party in March, if all goes well at home. I will try and look through some scopes if the opportunity arises. If there's a 10 to 15% difference then I think I will look into it further.

Garry, I take advantage of any clear night, if a planet is available I will concentrate on that for a while. I spent hours searching for the moons of Uranus, a couple of months ago, which I think that maybe but not sure, did I see them, yes - no - yes! Just that bit more light gathering from my mirror may have made all the difference. Of course, always trying for faint fuzzies.

David, I may do that after I have some more facts. It is something to look into.

Coen, thanks for that link, I have read a few of those sites, a bit beyond me I'm afraid, but at least I get the jist of it.

A fact of life is that I will do most of my viewing from my light polluted back yard, like most of us I suppose.:(



Thanks again:)
Richard

[pgc hunter]

I've got a 10" GSO dob and a 12" dob with a premium quality hand figured mirror (by one of the best optical craftsmen out here). I've noticed tiny low-contrast details on Jupiter are much finer with the 12". And I've seen stars close to 16th mag with the 12" from my outer suburban backyard, and galaxies down to nearly 15th mag, I have also seen 3 of Uranus' moons. I've also noted detail in NGC 3918 (the Blue Planetary) which appears on Hubble Space Telescope images... at over 700x with this mirror - whereas with the 10" GSO these details have eluded me.

Now, the 10" GSO has very nice optics, but I always get that "edge" with the hand figured 12" mirror.

if it's within your means, don't hesitate to get your mirror refigured.

Infact, I am planning on putting this mirror into a premium dob structure, as the existing one is quite inadequete for serious observation.

[richardda1st]

When you say to re-figure the mirror, what is actually done. I would assume removing the original coating and replacing with new coating of a higher quality. Would the mirror need to be reground? I suppose it would have to be tested first. Can anyone give me an indication of cost?:shrug:

[DavidU]

Usually a finished mirror like yours is "almost done" as it were. A mirror is chemically stripped of it's Aluminium coating then they will have a look at the "figure" with various optical tests.
Now they can create a polishing tool out of pitch (a type of pine tar) the mirror is pressed onto the surface to form a reverse of the mirrors shape. Then the tool will have channels cut into it. Now for the hard bit !
An extremely fine abrasive slurry is put on the tool and the mirror is VERY finely polished to a parabaloid shape and re tested many times until the optician is happy with the final figure and surface smoothness.
Then off to be re vacuum coated.
The figuring removes a minute amount of glass.
The surface error measurement is in fractions of a wave length of light !
( millionths of an inch).
Having done all this my self I have a massive amount of admiration for people who can turn out very high spec mirrors.

[richardda1st]

Thanks for that explanation Dave, very informative. It is something that I will definitely be doing, but I'm in no hurry.

PGC Hunter, you sound convincing. You don't thing that it's the 12" over the 10" that's giving you the extra detail do you?

I will have to get some quotes as at this stage I have no idea as to the cost.

[Greenswale]

Richard,

When I was looking for a new scope, I looked at commercial products, and then sought info on custom equipment.

The outcome was that I have an 8" that cost about three times more than the commercial items. I went for 8" because of ease of transport and handling.

So the question - is the scope worth the extra cost? In 8", definately! And there is the bonus that I contributed to its construction, so there is a fair chunk of ownership and feel-good there. And it is unique!

Is it better than a 12" or 14"? One would need to define 'better'. I am at peace with my scope, it suits me well, and I use it a lot. Further, I do not aspire to a larger scope (yet). If I did go bigger, I would save my pennies and buy premium again, because of the little bit extra that makes so much difference.

Maybe we will meet at Snake Valley?

[richardda1st]

Hi Wren.
I hope I can make it to Snake Valley. :)
Hey I think we spoke at the last camp, sitting outside the dining room on the bench on Sunday morning, not sure though, I tend to get confused in new surroundings.:lol::lol:

[strongmanmike]

Astronomik has released a new product - a Ronchi eyepiece, with it you can easily test any scope you care to put it in, for all the major aberations that can plague an optic, so it will be very usefull and easy to check any telescopes optics :thumbsup:

Mike

[richardda1st]

Thanks Mike, sounds great. Will look into it. :thumbsup:

[ngcles]

Hi Richard & Mike,



This may be a new product to Astronomic, but they have been available for donkey's. You can make one if you like with a bit of 1 1/4 brass pipe and a grating. I bought one about 15 years ago from Jack here:

http://schmidling.com/ez-testr.htm

when they were about $16 and I figured it would cost me nearly that much to make it.

A simple but effective tool that will pretty easily show all but subtle aberrations like localised dog-biscuit or lemon-peel that are best seen with a knife-edge. If the mirror is a true dud, this thing will show it pretty quickly. Gross errors are pretty easy to see.

If you'd like to become the most un-popular person at your next star-party, take a wander around the field at night and ask to look through various people's 'scopes. After a quick peek at what they're looking at, tell them you bought an eyepiece of your own and ask if you can I use it -- or you can slip it in when they're not looking and put it on a bright star.

Then tell them exactly what is wrong with their optics. Repeat with the next person ... and the next etc etc.

Pretty soon you'll be the most un-popular person at the party ...


Best,

Les D

[wavelandscott]

Les, I am impressed with your idea...I had not even considered the use of that tool in the manner that you described (and I'm disappointed that I did not think of it first). What a "balloon busting" night that would be...

[richardda1st]

No, I definitely don't need any special tools to become the least popular guy at the party, I'm quite capable of doing that all by myself, thanks.:question:

While I don't think that this tooool will indicate the quality of the mirror, is it actuality worthwhile for testing collimation and ep aberations, :confused2:

I have been getting some really bad views of Mars and Saturn lately, just not achieving a sharp image. I'm hoping it's the seeing conditions.:sadeyes:

[MuntiNZ]

Mate are you any good of an observer?
Theres guys out there with zillion $$$ every thing and they cant observe for crap!!
I could see more thru the bottom of a coke bottle than they could with all there top gear and I dont call myself a real expert.
You should find some one with the gear your thinking about buying and then using it to see if its even worth the money for you to up-grade to it.

[richardda1st]

Maaate, your a nice bundle of fun.:P

[MuntiNZ]

Just trying to help thats all.
Lots of clowns spend up large on the best and its all wasted coz they loose interest.
Not calling you a clown but.
See if you can try out the flash new gear at a star party or some where just to see if its worth it to you to buy the same stuff or not.
You know like they say 'Try before you buy'???

[Satchmo]

Les, Yes this does look like a sure fire way to become unpopular..Nothing worse than somebody trying to tell you your mirror which you know to be excellent has problems ;)

I would advise anybody new on this path to do lots of testing and understand the way telescopes behave out in the 'wild' , before telling anyone that their telescope is has genuine problems . Collimation , internal seeing , atmosphere and thermal inertia can all make a telecope look defective in terms of surface smoothness, and spherical aberration and astigmatism. I know some large dobsonians can show significant over and under correction while cooling depending on their design and some nights will never reach their equilibrium state. I've seen the same optics perform superbly on other nights.

Ronchi gratings at focus in the presence of inclement seeing make it particularly hard to judge any correction defects better than 1/2 to 1/4 wave for faster than F5 scopes, though will certainly weed out lemons. Even then the bowning of the lines can be very subtle, and very difficult to determine the differnce between a mediochre one and a superb one. Seeing and tube currents usually limits the ditance from focus to four to six bands bands. In the workshop with good seeing I find a single band is about right for seeing correction errors that seperate the average from the excellent.

There are ways to increase sensitivity like barlow lenses or higher frequency grating , but these can also amplify the swimming effect of seeing conditions . So tread very carefull before declaring a mirror either a gem or a lemon with a casual ronchi test under the real sky , it may take prolonged observation and further star testing on a few nights before making any meaningful declarations particularly with faster mirrors . At a casual glance most telescopes will show rudimentarily straight bands ...

[richardda1st]

Thanks Daz.

I'm just a novice amateur and a bit frustrated with all the bad conditions lately.

Hey, going from a 4" Tasco to a 10" Mead LB :eyepop: is it for me as far as flash gear goes, for a long time to come. I will try to improve things to get the best out of it. That is why I am interested in the possibility of improving the optics. The rest is okay or stuff that I can make or fix my self.

Cheers
Richard

[richardda1st]

Hi Mark, PM sent.

[MuntiNZ]

Or you will be a Sep!!!
Seriously all they ever do is star tests!
Most of them wouldnt even recognise the moon if you showed it to them.
Thats coz when ever they have looked at it before they were to busy looking for CA and telling you why your EPs are no good coz they dont have Televue written on them!!! :lol:

[JethroB76]

Mark, in relation to another reply in this thread about refiguring mirrors, do you or anyone else in Australia offer mirror refiguring services of mass produced mirrors?
And is it cost effective considering the price of such mirrors?

[Starkler]

I have been keen to check out the optics on a 16" lightbridge but have never seen one at thermal equilibrium :rolleyes: Like a big heat bank.

[ngcles]

Hi Satchmo & All,



Yep, and I only mention it because I have actually seen it happen, many moons ago (back in the early-days) at a well-known star party you and I have attended many times. No names, no pack drill; it wasn't a member of this forum (that I know of) but he is a highly experienced amateur and an experienced telescope maker who has made at least a dozen mirrors and other optics -- maybe even twenty. I look up to him a great deal ... and he is no fool. He has a reputation for being both "candid" and quite "direct" (in the same sense that an exocet missile could be described as "candid" and "direct") in his comments. I think you'll probably work out who it is from that all that.

He had a home-made ronchi eyepiece at the star-party and was doing precisely that. It was fascinating to see the muttering, cursing trail of disgruntled individuals he left in his wake, proffering all manner of sundry advice touching upon the subject of other darkened places he ought to consider inserting his ronchi eyepiece. From memory, there wasn't a single Schmidt-Cassgrainian that he gave a passing-grade to. He did it to me too ... on my old 10" f/6 (AOS mirror) and pronounced it excellent (whew ...). But I already knew that.



Yes very much correct -- I agree for what it's worth.



Re; "Bowing of the lines can be very subtle ..." Yep, but this is for a Ronchi used approximately at the centre of curvature as in an optical bench-test, not at the focal-plane like this device. Used at the focal plane a ronchi ep examining a good optic shows ruler straight lines, not curved ones -- you don't have to make an assessment of whether a curve "looks right". All curves, any curves are wrong! The ronchi will be an easier-to-read test in mediocre and average seeing that a full-on star test that requires very good to excellent seeing, though the star test, in the right conditions, is undoubtedly a more demanding examination.

As you say "... will certainly weed out lemons". I don't think it really pretends to be a high-precision optical test but as you say will certainly show a dud and pretty quick. Personally, I'd generally be lothe to condemn an optic based on a 10-second assessment with a ronchi eyepiece -- but it is an excellent indicator that further investigation at least is warranted if the result looks consistently dodgy. Re the comment about f/5 yep certainly less accurate at f/5 than say f/8, but if you have a barlow of known excellent quality, you can effectively make an f/5 an f/10 and get a more meaningful reading assuming good seeing and an optic at or near thermal equilibrium.




True, but as I said personally I don't think it is a be-all and end-all. It is a very good indicator if used properly, and for just a few bob (in context) will tell you a good deal about the state of the optics. :thumbsup:


Best,

Les D

[Satchmo]

Not really sure who you mean but Its certainly not a good example of how not to behave on the observing field.




Of course I was talking about the ronchi test on a star which is the topic of this thread. The very subtle bend of the bands showing spherical aberration is easily masked if the bands are in turbulence, on a faster mirror . Using a barlow lens increases sensitivity but also makes the mirror look twice as far away which along with much stronger diffraction effects can make the picture harder to read.

I've calculated a ronchigram here for an F3.7 mirror ( becoming more popular these days) . It shows 1 wave wavefront of primary spherical aberration which can refocus to 1/4 wave at the eyepiece, and also 1/2 wave primary spherical which can refocus to 1/8 wave at the eyepiece.

I demonstrate it here in the way the Schmidling Ronchi kit demonstrates. 133 lines per inch and six bands . You can see how subtle the bending is and imagine that it can be pretty hard to see when the bands are undulating about. Four to six bands is a good compromise as the closer to focus you go the the bands will swim about, effected by internal and external air currents .

What I maintain is that the beginner is more likely to see what look like superficially straight bands in most telescopes they test, and be tempted to declare their scopes `excellent' , and the test in the hands of the inexperienced is far less likely to detect medicochre levels of correction in faster scopes.



I think they are a great tool for comparing the state of your telescope both internally and seeing on an on going basis. They are very good at showing errors of strong slope such as rolled or turned edges. I think they take skill and practice with a single telescope to make any judgments about if a telescope is just average or a gem, because you have to learn how it behaves in different situations .

I've seen some 20" plus scopes that at some times of the year never show a good wavefront due to the mirror box design and thicker mirrors. If the scope has obvious problems in the star test then it easy for the grating to tell the owner if the mirror is under or over corrected.

There are no magic bullets available for $16 but definitely recommended accessory as it will tell you a lot about the state of your telescope , with all factors of collimation , seeing and cooling combined.

Cheers

Mark

[MuntiNZ]

Yeah but your comparing a 10 incher to a 12 incher.
The bigger one will usually be better even if the big one has optics not as good as the smaller one.
Apature rules mate.

[Satchmo]

Larger apertures only ( for example) show fainter stars only if the light is properly focussed in to the Airy disc. Theory shows, that at a wavefront error ( primary spherical ) of 2/3 of a wave / 1/3 surface accuracy , a superb telescope of _half_ the aperture will have the same visual magnitude limit.

Bright diffuse objects will benefit from extra aperture even't if it isn't quality, but the higher contrast of a smaller more accurate system will really help in spotting faint galaxies.

[ausastronomer]

Hi Les and all,

Indeed Les is correct. These devices have been around for ages, I know, because like Les, I have had one for ages :).

Jack Schmidling in the US has been making them for a long time as have a few others. Jack Schmidlings' units can be purchased directly from him via the link Les gave you or through a couple of other dealers like Scopestuff and Agena Astro products.

Similarly, Orion Optiks in the UK (not to be confused with Orion US) have been producing a ronchi eyepiece for many years.

http://www.orionoptics.co.uk/ACCESSORIES/miscellaneouspag.html

This is the unit I have. It is quite expensive compared to some of the others, but it is worth noting that it uses a 250 lines per inch (lpi) grating compared to some others which only use a 133 lpi grating. They all work well, the good ones with 250 lpi are a little more accurate than the cheaper ones. The new astronomik unit is also a 250 lpi unit (10 lines per mm).

This page on the Orion Optics website is also quite helpfull

http://www.orionoptics.co.uk/OPTICS/telescopetestpag.html

To be truthful I rarely use my ronchi eyepiece these days. For the simple reason it doesn't show me anything I can't detect in a star test. Whilst I may not be as good a star tester as some of the experts, I can see what I need to see in terms of the principal things which will concern the quality of the optics, like spherical aberration, astigmatism, zones, turned edges and smoothness. The problem is of course, you don't learn to star test a telescope properly overnight either.

It is also worth noting as Mark and Les have discussed, unless the seeing is good and the scope has reached thermal equilibrium the ronchi test doesn't tell you much.

Cheers,
John B

[richardda1st]

Thanks everyone.

I know this is stating the bleeding obvious, but the quality of the mirrors is paramount. All other attributes and add-on accessories are important and nice, but it's all a waste of time and money if the mirrors are below the level of quality you can afford.

If a good quality 10" mirror is able to perform as good/better than say a 12" mass produce mirror I would choose the 10. A 10" is big and heavy enough for a portable telescope thank you.

So my next major expense is going to be a new set of mirrors, as soon as I can manage to sort out the best way to acquire the best mirror for my scope that I can afford.

Regards
Richard

[gb_astro]

I was going to buy a Ronchi eyepiece until I read
Siuter's "Star Testing.." book.

Suiter says what Marks says in that the variation in pattern produced by the Ronchi grating
used near focus is too subtle to produce a result suitable to distinguish between a so-so mirror and an excellent one.

The test is ok for course optics like a camera lens but not
for diffraction limited optics like telescopes. Some mirror makers use it as a gross error check during manufacture.

Suiter says Ronchi himself abandoned the test in the 1920's because of it's limitations.

gb.