Seeking info on the optical quality of mirrors.

richardda1st (Richard) · #1 21-01-2010, 12:25 AM

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 ?

Thanks
Richard

wavelandscott (Scott) · #2 21-01-2010, 02:50 AM

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 · #3 21-01-2010, 04:33 PM

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 (Dave) · #4 21-01-2010, 04:48 PM

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 · #5 21-01-2010, 04:58 PM

Potential starting point:

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

Google/Bing/Internet search: Strehl ratio

richardda1st (Richard) · #6 21-01-2010, 10:29 PM

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 · #7 22-01-2010, 09:58 AM

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 (Richard) · #8 22-01-2010, 08:33 PM

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?

DavidU (Dave) · #9 22-01-2010, 08:51 PM

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.

Quote:

Originally Posted by 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?

richardda1st (Richard) · #10 24-01-2010, 08:41 PM

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 (Wren) · #11 25-01-2010, 08:47 PM

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 (Richard) · #12 25-01-2010, 11:12 PM

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.

strongmanmike (Michael) · #13 26-01-2010, 01:01 AM

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

Mike

richardda1st (Richard) · #14 26-01-2010, 11:21 PM

Thanks Mike, sounds great. Will look into it.

ngcles · #15 30-01-2010, 09:24 PM

Hi Richard & Mike,

Quote:

Originally Posted by 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

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 (Scott) · #16 31-01-2010, 04:01 AM

Quote:

Originally Posted by ngcles

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.

Les D

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 (Richard) · #17 31-01-2010, 12:28 PM

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.

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,

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.

MuntiNZ (Daz) · #18 31-01-2010, 01:26 PM

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 (Richard) · #19 31-01-2010, 04:01 PM

Maaate, your a nice bundle of fun.

MuntiNZ (Daz) · #20 31-01-2010, 04:30 PM

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'???