What are good optics? - Page 3

koputai (Jason) · #41 30-12-2007, 09:30 PM

Thanks to everyone for their replies. I didn't intend it to be the sh*tfight it turned out to be! I've learned a bit, and need to learn a lot more.

Thanks,
Jason.

Starkler (Geoff) · #42 30-12-2007, 09:42 PM

As long as you had your question answered Jason.

Peter Ward · #43 31-12-2007, 03:34 AM

John,

So are you saying test data provided by (skilled) opticians is some sort of random number that in no way correlates to the quality of the optic?

As I mentioned earlier there are many ways to test optics, and I'd certainly agree some are more subjective than others, but I would not suggest that various quantitative methods are in some way divergent.

An excellent optic makes itself apparent no matter how you look at it.

To paraphrase Ceravolo, interferometry is not necessary to make excellent mirrors, he in fact uses the Foucault test to actually make the optics, but then quantifies the results with interferometry to ensure the optic meets the required performance as it is less subjective than the star test and easily quantified (by him at least)

In fact Ceravolo did a piece for Sky and Telescope (March 92?) where telescopes with mirrors of varying (but known) quality were used side by side to see if anyone could tell the difference visually.

In this case the numbers were not just relevant, they demonstrated perfectly the difference between "bad, fair, good and excellent optics"

Quote:

Originally Posted by ausastronomer

Peter,

I don't agree with this in any way shape or form.

Unless the numbers for each different optic are calculated by the same person using exactly the same test equipment under identical test conditions with identical adjustments and numerical manipulation, the numbers themselves are meaningless. .....

..... The raw numbers IMO are pretty meaningless for comparison purposes, but, if the test methods and conditions are known, they do give you an indication as to the overall quality of the scope. ie. bad, fair, good, excellent.

Cheers,
John B

Aster · #44 31-12-2007, 11:06 AM

One of the funniest and most useless threads I have read since the comparing of the EQ6 to the Losmandy Mounts.

Jason, How good is your eyesight, or anyone elses for that matter ?

Unless you want to see your name in print and tell everyone within hearing distance how good your equipment and DSImages are, even if that is so, then any of the Known Manufacturers Optics will be OK. Yes, there is a chance that you could get a lemon but most will exchange or give you are refund.

Mind you, there are also lemons produced by Manufactures who charge ten times of what the everage person spends on his hobby.

More then 90% of the populace wouldn't know the difference between 1 wave optics or 1/50 wave optics. More then 90% of the time you can't even use the full potential of 1/4 wave optics due to atmospheric seeing. Then there are heaps of people who believe anything some people say. Then there are other people who have heaps of money to waste and will tell you the only car to drive is a Rolls.

I don't even know why Mark, who has more experience in mirror making and testing then 99% of the people in this forum, even bothers to get involved in discussions like this. It's a nowin useless exercise. . Unless you have actually tested a mirror yourself, with home made or the most modern equipment, you have no idea what you are talking about. Reading about it makes you an armchair expert, which is worse then a nonsmoker who gave up smoking.

Mirror making was a hobby of mine for near on 30 years, in that time I made somewhere around 250 plus mirrors from 4 1/4" to 16" including 16" RC and SC. I had also the priveledge to meet some of the top mirror makers in that time, and even work with one. All my mirrors were tested in this order, Ronchi, Knife Edge, Null. The only test which gives the same results, temperature controlled, is the Null Test. Any other test is subject to the interpretation of the person and equipment used doing the testing. I have had the priveledge to be present when three highly experienced opticians tested the same mirror with the same equipment in the same locallity and finished up with three different results, mind you, the were still still within % points of each other but still different. The Null Test gave the same result by all three. Strehl is a good rating for a mirror but nowhere near perfect. Mirror with a slight turned edge still had a Strehl of better then .9. It was clearly visible with a Null Test and rejected.

There are that many variables in optical bench testing, setup, quality of equipment, person doing the test, etc. that the only real test is the instrument performance in the field, and that is a Star Test AND How Good Is Your Eyesight.

casstony · #45 31-12-2007, 11:58 AM

Quote:

Originally Posted by Aster

More then 90% of the populace wouldn't know the difference between 1 wave optics or 1/50 wave optics. .

Probably true, but if you set up a sharp scope beside an observer who is accustomed to using his soft scope, he will be able to see the difference.

Stephen65 · #46 31-12-2007, 12:32 PM

Quote:

Unless you want to see your name in print and tell everyone within hearing distance how good your equipment and DSImages are, even if that is so, then any of the Known Manufacturers Optics will be OK. Yes, there is a chance that you could get a lemon but most will exchange or give you are refund.

Depends on where you draw the lines between good/OK and OK/lemon. I've read a few horror stories on CN about people buying SCTs from the big two manufacturers only to discover by star testing that they received scopes are that are, generously, very mediocre optically. But provided the scope passes the low threshholds set by those manufacturers then they will not exchange them.

Higher end scopes are not completely immune from optical defects but the variability in quality is much less and accordingly your chances of getting one that's subpar are considerably lower.

ausastronomer (John Bambury) · #47 31-12-2007, 01:01 PM

Quote:

Originally Posted by Peter Ward

John,

So are you saying test data provided by (skilled) opticians is some sort of random number that in no way correlates to the quality of the optic?

I am saying the numbers are a lot less relevant and meaningful than many people perceive. They are good to compare optics tested by the one person, under identical test conditions. They are not good to compare optics made and tested by different people under different conditions, because of inconsistency in test methods and test conditions.

Quote:

Originally Posted by Peter Ward

In fact Ceravolo did a piece for Sky and Telescope (March 92?) where telescopes with mirrors of varying (but known) quality were used side by side to see if anyone could tell the difference visually.

The article did appear in the March 1992 edition of Sky and Telescope. It was in fact a joint article by Peter Ceravolo, Terence Dickinson and Douglas George. I have read it many times.

Quote:

Originally Posted by Peter Ward

In this case the numbers were not just relevant, they demonstrated perfectly the difference between "bad, fair, good and excellent optics"

Of course they demonstrated things perfectly. All the mirrors were made and tested by the same skilled person (Ceravolo) on the exact same equipment, using the same methods and conditions. This is the exact point I am trying to make and that Aster, Mark and others also referred to, and you are missing.

Cheers,
John B

Peter Ward · #48 31-12-2007, 02:34 PM

John,

I clearly am missing the point, as it seems like you are having an each way bet here.

The fact there is variation comes as no surprise, but a skilled practitioner will know from one of his measurements what to trust and in what circumstances.

I would not trust an optic that was only subject to one test. The opticians who I deal with make as many tests as possible....which is a very different regime to mass produced optics which I have seen first hand to be a cursory inspection at best.

I suspect therein lies the rub: testing=time=money.

Skilled observers *can* see the difference between average and excellent optics, and simply saying an optic is "diffraction limited" or 1/4 wave will shed no light on whether an optic is excellent or not.

When Jones, Ceravolo Christen et al. tell me an optic has a Strehl of 0.96, while you may not accept it, I tend to believe them.

Plus it beats flying to Chicago or Atlanta to perform a star test on the instrument before they ship it out

Quote:

Originally Posted by ausastronomer

I am saying the numbers are a lot less relevant and meaningful than many people perceive.........

.........they demonstrated things perfectly. All the mirrors were made and tested by the same skilled person (Ceravolo) on the exact same equipment, using the same methods and conditions. This is the exact point I am trying to make and that Aster, Mark and others also referred to, and you are missing.

Cheers,
John B

ausastronomer (John Bambury) · #49 31-12-2007, 03:42 PM

Quote:

Originally Posted by Peter Ward

John,

When Jones, Ceravolo Christen et al. tell me an optic has a Strehl of 0.96, while you may not accept it, I tend to believe them.

Peter,

I also believe them as it relates to a relative measure with all the other items they each have tested and certified. My point is this. Is a Ceravolo optic of strehl .98 better than a Christen Optic of .97 strehl? Be rest assured you don't know, I don't know, and most importantly, we are unlikely to get seeing conditions good enough to tell them apart more than once a decade, if we are lucky. The only way to tell them apart is if the one optician was to test both optics under the same conditions and on the same equipment. Consequently, as a relative measure the numbers don't mean much.

I am about to order a mirror from Mark and I won't have a clue what the strehl number is; and I don't care. Suffice to say that I have looked through enough of his mirrors and mirrors from a lot of other premium opticians, to know that he doesn't make any "bad" ones. In fact he is easily in the top 10 glass pushers on the planet and that is more comfort to me than a piece of paper with a number certified under variable test conditions. Similarly, if I buy an optic made by Christen, Zambuto, Steve Kennedy or Peter Ceravolo, I don't need to look at the strehl number to know that it's as good as hands can make it and I am happy to buy it.

Cheers,
John B

Peter Ward · #50 31-12-2007, 04:16 PM

No argument from me there! You gave me the impression much larger values would go un-noticed.

To ruminate on a 0.01 difference would be ludicrous.

BTW Ceravolo just e-mailed me he never Star tests (!) ....does a whole bunch of other stuff... but commercial constraints require him to produce certified optics even when its raining or the seeing is crappy!

Agreed there are many producers of excellent optics out there, with well deserved reputations....and, Strehl ratios aside, perhaps that is how a novice might best find his or her dream telescope.

Have a good one!

Quote:

Originally Posted by ausastronomer

Peter,

....Consequently, as a relative measure the numbers don't mean much.

....I buy an optic made by Christen, Zambuto, Steve Kennedy or Peter Ceravolo, I don't need to look at the strehl number to know that it's as good as hands can make it and I am happy to buy it.

Cheers,
John B

ausastronomer (John Bambury) · #51 31-12-2007, 04:45 PM

Quote:

Originally Posted by Peter Ward

Agreed there are many producers of excellent optics out there, with well deserved reputations....and, Strehl ratios aside, perhaps that is how a novice might best find his or her dream telescope.

Have a good one!

Funnily enough I was led to believe that the people that cared enough about the quality of their optics to order large thin parabolic mirrors from people like Mark Suchting, Steve Kennedy and Carl Zambuto without strehl certification, had progressed past the novice stage. Each to their own.

John B

Bassnut (Fred) · #52 31-12-2007, 07:28 PM

This has been a very interesting thread, I have learnt much, thank you gentlemen.

If a University/Gov dept/Professional Obs/Military purchased a premium scope, surely they would require absolute certification of some sort for an approval process. Seems from yr discussion, organisations such as these would not rely on reputation or a relative/subjective test, so then wouldnt a many 1000 point strehl test be the most comprehensive of all of them, that can be described in absolute terms. It seems to me a glass pusher, such as some here, would have access to/intuatively understand/require different specifications to a commercial maker selling to a competative global commercial market.

Aster · #53 01-01-2008, 09:46 AM

Quote:

Originally Posted by Bassnut

This has been a very interesting thread, I have learnt much, thank you gentlemen.

If a University/Gov dept/Professional Obs/Military purchased a premium scope, surely they would require absolute certification of some sort for an approval process. Seems from yr discussion, organisations such as these would not rely on reputation or a relative/subjective test, so then wouldnt a many 1000 point strehl test be the most comprehensive of all of them, that can be described in absolute terms. It seems to me a glass pusher, such as some here, would have access to/intuatively understand/require different specifications to a commercial maker selling to a competative global commercial market.

Sure they need certification and usually will require the optics to be of a certain specification. But it doesn't matter which way you look at it, the are still only a set of numbers. It's like "The average wage, etc. is....! Numbers are Numbers, there to be manipulated.

The top opticians didn't get their reputation by producing .99999 Strehl Test on a testbench but by the actually use of their optics in the field.

How good the optics - equipment will actually perform can only be tested in the field, Not on a testbench, which only will give you an indication of how well they are supposed to perform.

Peter Ward · #54 01-01-2008, 11:04 AM

So are you saying there is absolutelty no correlation between test bench figures and field performance?

Quote:

Originally Posted by Aster

Sure they need certification and usually will require the optics to be of a certain specification. But it doesn't matter which way you look at it, the are still only a set of numbers....

How good the optics - equipment will actually perform can only be tested in the field, Not on a testbench, which only will give you an indication of how well they are supposed to perform.

Aster · #55 01-01-2008, 11:11 AM

Quote:

Originally Posted by Peter Ward

So are you saying there is absolutelty no correlation between test bench figures and field performance?

Read the last paragraph of my post.

Peter Ward · #56 01-01-2008, 11:24 AM

O.K. the numbers give an indication of how they are *supposed* to perform.

How much of an indication?

Quote:

Originally Posted by Aster

Read the last paragraph of my post.

Aster · #57 01-01-2008, 01:09 PM

Quote:

Originally Posted by Peter Ward

O.K. the numbers give an indication of how they are *supposed* to perform.

How much of an indication?

What do you want Peter, a definition of the word indication?
I am afraid you have to pick your own, the same applies to numbers.

If you can't get it through your head that numbers alone are not the final result of good optics then I am wasting my time.

Peter Ward · #58 01-01-2008, 03:57 PM

Similarly, if you think a bad optic will somehow pop up up with a 0.99 Strehl then I am wasting mine.

I have star tested only a few scopes with the above spec. They were indeed wonderful.

Quote:

Originally Posted by Aster

If you can't get it through your head that numbers alone are not the final result of good optics then I am wasting my time.

Alchemy (Clive) · #59 01-01-2008, 06:14 PM

who would have thought this would be still running.

what would happen if the perfect telescope were taken out in poor- average seeing conditions.

is it true that the worst part of a scope is the atmosphere?

wavelandscott (Scott) · #60 01-01-2008, 06:22 PM

I have followed this thread with some interest and I am glad it has remained civil.

It seems from the discussion that there is no "single" consistent measure currently taken that can be compared across a variety of manufacturers. Due to differences in testing and testing equipment direct comparison can be difficult.

I'll ask a slightly different set of questions that I think is still in line with the thread...

Can you have a bad mirror with a .99 strehl (or .95 or .96 etc. etc.)?

Is it reasonable to assume that anyone that could/would create a mirror with such a high strehl mirror would automatically check for these other defects that would cause such a high strehl mirror to be "bad"?

Just looking for some helpful guidelines/rules of thumb on this as I now beleive that this may have been the original intent of the first post...and I do recognize that this may be a situation where "if you have to ask, you aren't experienced enough to see the difference"...

I stand by the implication in my earlier post that not everyone "needs" (desires/deserves/can afford) ultra premium gear but that should not disqualify them from enjoying 97.8734% of the things to see in the hobby. Like an automobile, sometimes a Kia is sufficient...