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Equipment Discussions >> Reflectors

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rik ter horst
sage


Reged: 11/01/10

Loc: Ewer, the Netherlands
Re: 1/10 wave? new [Re: ausastronomer]
      #5537282 - 11/24/12 01:54 PM

Quote:

We have 4 18"/F4.5 classic Obsessions in our 3RF arsenal of scopes. One of these has a .90 strehl mirror, the other 3 have mirrors with a strehl > .97. These mirrors have been interferometrically tested. On one occasion only in the 7 years we have had these scopes in our care, have I been able to pick the lower graded mirror apart from the others and that was a feature on Jupiter which was barely detectable; and that could have even been attributed to variable seeing as I changed scopes. That having been said that mirror is still a very good mirror. It's smooth with an excellent edge and slight undercorrection. In terms of peak to valley its worst point on the mirror face is 1/4.5 waves. That mirror is in fact the mirror in my scope and it can still push 1075X under favourable conditions. Would I have gained anything with 1 of the other mirrors, yeah something barely detectable once in 7 years.

Cheers,




Interesting, your experience! I believe you should be very happy with your mirror (and you are I understand!). The mirrors have been tested with an interferometer in a laboratory under stable conditions I guess, and your particular mirror is undercorrected. Couldn't be much better because a 100% corrected large mirror tends to become slightly overcorrected during cool down because the edge of the mirror cools faster than the rest. Although your mirror has been measured to be Lambda/4.5 wave P-V undercorrected I expect it to be better than that during observation, at least as long as the temperature drops....

This might explain the almost invisible difference in performance.


Cheers,
Rik

Edited by ausastronomer (11/24/12 06:11 PM)


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dan_h
Carpal Tunnel


Reged: 12/10/07

Re: 1/10 wave? new [Re: ausastronomer]
      #5537283 - 11/24/12 01:54 PM

Quote:

I don't live anywhere near New York. In fact I have some premium optics, observe under Bortle I skies day in day out and haven't changed my opinion on this when it comes to larger optics.

Cheers




The worst mirror you referenced was a Strehl 0.90 which is already a pretty decent optic. Do you think you would see the difference between it and a 0.8 Strehl? Under excellent skies, I think it would stand out to all but the most casual of observers.

dan


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csrlice12
Postmaster
*****

Reged: 05/22/12

Loc: Denver, CO
Re: 1/10 wave? new [Re: ed_turco]
      #5537325 - 11/24/12 02:30 PM

Regarding 1/10 wave mirrors in a Diagonal for a refractor. Celestron has a 1/10 wave XLT diagonal, any idea how it compares to other diagonals?

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Asbytec
Guy in a furry hat
*****

Reged: 08/08/07

Loc: La Union, PI
Re: 1/10 wave? new [Re: csrlice12]
      #5537376 - 11/24/12 03:15 PM

With the amount of light cast into the rings with 1/4 wave spherical, one should notice a difference when seeing is calm enough and all other variables controlled. Especially doing a side by side comparison of the focused image. When seeing, cooling, and collimation are combined with better correction, IMO, you will be astounded. The better correction just gives it that nudge over the top during those moments. The difference between Strehl 0.8 and perfect 1 should be apparent while 0.95, give or take, is quite good enough to show improvement.

Edited by Asbytec (11/24/12 03:20 PM)


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ausastronomerModerator
Carpal Tunnel
*****

Reged: 06/30/03

Loc: Kiama NSW (Australia)
Re: 1/10 wave? new [Re: dan_h]
      #5537648 - 11/24/12 06:32 PM

Quote:

The worst mirror you referenced was a Strehl 0.90 which is already a pretty decent optic. Do you think you would see the difference between it and a 0.8 Strehl? Under excellent skies, I think it would stand out to all but the most casual of observers.

dan




Hi Dan,

I discussed this in one of my earlier posts.

Quote:

With larger aperture telescopes (over say 15" aperture) it gets progressively more difficult to separate them, once they are better than diffraction limited, smooth and free of astigmatism. It is certainly still easy to pick a mirror which is worse than diffraction limited (1/4 wave).





The point I am trying to make is that with larger mirrors it's easy to detect a mirror that is less than good to very good, but you need progressively better seeing and thermal stabilisation to detect any gain once the mirror gets to this level and even then the gain is very subtle.

The difference with smaller mirrors (say under 14" to 15") is a lot more noticeable and on a lot more occasions.

Cheers,


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GeneT
Ely Kid
*****

Reged: 11/07/08

Loc: South Texas
Re: 1/10 wave? new [Re: ed_turco]
      #5545658 - 11/29/12 02:46 PM

Quote:

True, under mediocre seeing, once in a great while, the seeing, by accident, becomes much better. And then you will thank your lucky stars that you have 1/10 wave mirror! Images become close to miraculous! Ed




This is where I come out.


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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: 1/10 wave? new [Re: GeneT]
      #5545745 - 11/29/12 03:52 PM

I use a large enough scope that it is very rare for the seeing to actually exceed the resolution of the mirror.
What I do notice, though, is that it seems the seeing is consistently better with a better mirror. I suspect what that means is that the tightness of the star images is better on the better optic, and the occasional flashes of truly good seeing are more likely to be seen and caught because the better optic has less scattered light.
Whatever the reason, I seem to be seeing a lot more excellent seeing than I used to be since the Zambuto mirror.


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pstarr
Carpal Tunnel
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Reged: 09/17/04

Loc: NE Ohio
Re: 1/10 wave? new [Re: Starman1]
      #5545894 - 11/29/12 05:40 PM

Quote:

I use a large enough scope that it is very rare for the seeing to actually exceed the resolution of the mirror.
What I do notice, though, is that it seems the seeing is consistently better with a better mirror. I suspect what that means is that the tightness of the star images is better on the better optic, and the occasional flashes of truly good seeing are more likely to be seen and caught because the better optic has less scattered light.
Whatever the reason, I seem to be seeing a lot more excellent seeing than I used to be since the Zambuto mirror.




This has been discussed on the Zambuto mirror forum. A top notch mirror seems to cut through the bad seeing. The best way I can describe it is, you see the waves of turbulence across the image but underneath the image is still sharp.


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Asbytec
Guy in a furry hat
*****

Reged: 08/08/07

Loc: La Union, PI
Re: 1/10 wave? new [Re: pstarr]
      #5546932 - 11/30/12 10:27 AM

I think that's interesting because seeing adds to total RMS in the wavefront. In good seeing, the wavefront is pretty much planar. As seeing worsens, the wavefront is further misshapen. As it hits the optic, the timing is all off generating larger P-V errors (and ray trace no longer parallel generating speckles, as I understand it.)

I tend to think optics with very good RMS can handle a bit worse seeing conditions before the image breaks down. For example, say your scope has an RMS of 0.030 and a Strhel of >/= 0.95. It would take average seeing on the order of RMS 0.40 to drag the final image to (or below) the diffraction limit (~0.070.) Where as a scope that is diffraction at best might not produce a good image even in better seeing.

But, when seeing permits a diffraction limited scope to operate at or near it's theoretical MTF, can it be distinguished from a scope that is better corrected in those same conditions? I believe so. Both scopes would perform near their ideal MTF curve, and a better scope simply has a better curve (obstruction aside.)

Personally, I believe my own scope to be near RMS 0.40 (~1/6th or better SA) and when seeing is good, it's simply jaw dropping (smaller aperture and CO, aside.) I doubt I could tell much difference if it were 0.030 RMS or better. But, the loss of contrast could be seen if it were closer to 0.07 RMS with more light in the rings.

I do think Starman is onto something. Imagine a good scope's image in good seeing, then the same scope's image in slightly worse seeing and at the diffraction limit for the final image. The light scatter (diffraction and speckling) is better in the former and the contrast should be noticeably better than the latter.

Dunno, just an ejumecated, working hunch and a little experience with better than average seeing.


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Mike Lockwood
Vendor, Lockwood Custom Optics
*****

Reged: 10/01/07

Loc: Usually in my optical shop
Re: 1/10 wave? new [Re: Asbytec]
      #5547006 - 11/30/12 11:27 AM

It's not too complex.

A mirror is supposed to focus light into a tiny spot. A poor mirror makes a larger spot. The larger spot can happen on a mirror with transverse/slope error even though it has a small P-V error, even 1/10th wave. (A smooth mirror will not have the transverse error, though.)

Seeing further enlarges the spot in a random, time varying manner.

So, a good mirror will still make a smaller spot, on average, than a poorer one.

The difference in the tightness of star images and detail resolved on objects IS noticeable even under somewhat bad seeing, and even on large instruments.

A good reflector of any focal ratio, properly equilibrated and collimated, small or large, should produce good images on most nights, and superb images frequently.

I often hear of instruments that have "one spectacular night per year". Immediately this sets off an alarm in my mind that something is wrong.

For any poor instrument, thermal conditions will vary, but chances are that infrequently, once in a great while, those conditions will help the poor mirror perform better than it actually is!

However, the number of nights that a better mirror will perform well is FAR greater than the poorer one.

In my experience, any test claiming high Strehls should be taken with a grain of salt. Evaluation under the stars on a good number of nights, and comparisons with other instruments on the same night, under the same conditions, is a test method that I recommend for telescope owners.


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Asbytec
Guy in a furry hat
*****

Reged: 08/08/07

Loc: La Union, PI
Re: 1/10 wave? new [Re: Mike Lockwood]
      #5547154 - 11/30/12 12:50 PM

Mike, when you say larger "spot", you are not referring to the airy disc?

To me that's a gray area I don't fully comprehend. It's easy enough to see how at least one ray could miss the Airy disc or some target "spot", but really the Airy disc forms when the entire wave comes to focus. And if one ray is not cooperating (the wavefront P-V), then it get's sent to the diffraction rings when the diffraction pattern comes to focus. Right?

The Airy disc is determined by aperture, not figure. A poor 6" figure does not make an Airy "spot" larger than 0.92" arc, it redistributes light throughout the pattern. Is not the result of a poor optic a normal Airy pattern with more light spread into the rings? Or does that offending ray actually show as blurring somewhere in the first minimum?

Edited by Asbytec (11/30/12 12:52 PM)


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ausastronomerModerator
Carpal Tunnel
*****

Reged: 06/30/03

Loc: Kiama NSW (Australia)
Re: 1/10 wave? new [Re: Mike Lockwood]
      #5547807 - 11/30/12 08:21 PM

Quote:

It's not real complex.

The difference in the tightness of star images and detail resolved on objects IS noticeable even under somewhat bad seeing, and even on large instruments.

A good reflector of any focal ratio, properly equilibrated and collimated, small or large, should produce good images on most nights, and superb images frequently.

For any poor instrument, thermal conditions will vary, but chances are that infrequently, once in a great while, those conditions will help the poor mirror perform better than it actually is!





Hi Mike,

It's more complex than you give it credit. I can't agree entirely with your comments. Your comments really only apply to areas that do not experience dramatic temperature changes from day to night.

Let me give you this example:-

Yesterday in Sydney the temperature peaked at 29.1 C at 1:40pm (84.4F) and fell to a low of 23.5 C (74.3 F) at 2:40am this morning. That is a very low temperature change of .4 C / hour or .8 F / hour. I have no doubt that those people who observed in Sydney last night, with scopes of all apertures, had really nice views, provided they had decent mirrors. All the high grade mirrors would have shone like beacons.

Two weeks ago when I observed at Coonabarabran with a group of 22 Canadian and Asian observers who were eclipse visitors, the temperature change from 3pm Sunday afternoon to 1am Monday morning was dramatic. The temperature dropped from 33 C (91.4 F) at 3pm to 8 C (46.4 F) at 2am. this is a drop of 2.5 C / hour or 4.5 F /hour.

I can guarantee you at no stage did any of the scopes deliver exceptional images. My 14" with a thin Zambuto mirror was certainly the least affected and gave the best planetary images, because it was the smallest scope on the field with the thinnest mirror (1.3"); but the 18" and over scopes with 2" plus thicknness mirrors never had a hope of catching up and stabilising at any stage of the night.

In Australia (and I am sure in some parts of the US) we frequently observe under these types of conditions. It's not uncommon in many places to go from high 30's C to well under 10 C at night. I observed one night at about 3000 feet elevation where the temperature dropped from 42 C (108 F) at 5pm to 4 C (39F)at 1am. A fall of 4.75C or 8.6 F per hour, over 8 hours. The rapid temperature drop was sensational for the body and soul, pretty ordinary for the telescopes and local seeing conditions. Granted with people like you and Mark Suchting who are now capable of producing top quality large thin mirrors, things have improved dramatically. But irrespective of mirror thickness an 18" plus aperture mirror can still struggle to stabilising under rapidly falling temperatures. It just does a lot better than a thick one of the same aperture. These rapidly falling temperatures also have an effect on local seeing conditions as well as on thermal equilibration of the mirror.

You also need to keep in mind that in regards to the number of 18" plus aperture mirrors running around the world, there are an infinitely greater number of them 2" thick at minimum, than there are < 2" thick. For that matter there are also a lot more mirrors in the 12" to 18" aperture class mirrors which are 2" think running around the world than there are which are less than 2" thick. 2" thick was the norm with 10" to 20" mirrors for decades. At one time it was even a selling point to have a "full thickness mirror". Thin mirrors and "honeycomb" mirrors are a recent innovation, thick mirrors are still in proliferation.

Quote:


In my experience, any test claiming high Strehls should be taken with a grain of salt. Evaluation under the stars on a good number of nights, and comparisons with other instruments on the same night, under the same conditions, is a test method that I recommend for telescope owners.




I couldn't agree more. You need to remember that I am more than capable of properly star testing a telescope to confirm its optical quality. In addition, a very skilled optician is a friend of mine and I have seen some of these mirrors on the test stand, under bench test conditions.

You and I both know the accuracy of the interferometer results is totally dependent on the honesty of the optician. In some cases the skill of the optician to use his test equipment and the software properly also comes into play. There are 101 ways to fudge the interferometer results as you and I both know. Clipping the edges of the fringes and/or removing astigmatism are two pretty common ones. There are plenty of others. I have seen several examples of it over the past couple of years, where the interferometric test certificate and calculated numbers were clearly inconsistent with the "actual" mirror quality. I am not going to comment any further on that, but I have no doubt you have seen it many times too.

Cheers,


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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: 1/10 wave? new [Re: ausastronomer]
      #5547836 - 11/30/12 08:40 PM

John's post brings up the fact I recently went from a 2" thick mirror with 1 fan to a 1.25" thick mirror with 3 fans, so I am at the ambient temperature a lot more.
This time of year, my observing site goes from a daily high of 58F degrees (15C) to a low night time temperature of 15F (-10C)
And it does so in the first 3 or 4 hours of the night, then stabilizes to a 1F degree per hour fall after that.
If I'm seeing better images, the thermal characteristics of the mirror have a lot to do with it.


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Pinbout
Postmaster
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Reged: 02/22/10

Loc: nj
Re: 1/10 wave? new [Re: Starman1]
      #5547991 - 11/30/12 10:35 PM

Quote:

I recently went from a 2" thick mirror with 1 fan to a 1.25" thick mirror with 3 fans




how did that effect your balance?


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Bill Weir
Carpal Tunnel
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Reged: 06/01/04

Loc: Metchosin (Victoria), Canada
Re: 1/10 wave? new [Re: Pinbout]
      #5548040 - 11/30/12 11:26 PM

Quote:

Quote:

I recently went from a 2" thick mirror with 1 fan to a 1.25" thick mirror with 3 fans




how did that effect your balance?




Different scopes go along with those different mirrors. He now has a 12.5" Teeter and used to have a 12.5" Discovery Truss. Or do you mean does Don fall down now? ; )

Bill


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Mike B
Starstruck
*****

Reged: 04/06/05

Loc: shake, rattle, & roll, CA
Re: 1/10 wave? new [Re: Bill Weir]
      #5548048 - 11/30/12 11:33 PM

Quote:

Or do you mean does Don fall down now?




No, i think he was asking about his checking account since getting the Teeter.


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Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: 1/10 wave? new [Re: Mike B]
      #5548092 - 12/01/12 12:07 AM

Quote:

No, i think he was asking about his checking account since getting the Teeter.







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Starman1
Vendor (EyepiecesEtc.com)
*****

Reged: 06/24/03

Loc: Los Angeles
Re: 1/10 wave? new [Re: Mike B]
      #5548153 - 12/01/12 01:18 AM

Quote:

Quote:

Or do you mean does Don fall down now?




No, i think he was asking about his checking account since getting the Teeter.




Amen.


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Mark Harry
Vendor
*****

Reged: 09/05/05

Loc: Northeast USA
Re: 1/10 wave? new [Re: Starman1]
      #5548356 - 12/01/12 07:14 AM

Agree with John; +1. There are several favorite objects I frequent, and one of them I've seen certain resolution, contrast and definition only once in the last 3 years on a spectacular transparent night when everything cooperated. Several have said it was theoretically impossible- but later substantiated as doable with fine conditions and excellent equipment.
M.


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ausastronomerModerator
Carpal Tunnel
*****

Reged: 06/30/03

Loc: Kiama NSW (Australia)
Re: 1/10 wave? new [Re: Starman1]
      #5549309 - 12/01/12 07:36 PM

Quote:

What I do notice, though, is that it seems the seeing is consistently better with a better mirror. I suspect what that means is that the tightness of the star images is better on the better optic, and the occasional flashes of truly good seeing are more likely to be seen and caught because the better optic has less scattered light.
Whatever the reason, I seem to be seeing a lot more excellent seeing than I used to be since the Zambuto mirror.




Hi Don,

We both mention one of the main reasons for this in subsequent posts. That is the better thermal equilibrium of the new scope with much thinner mirror and a better fan setup.

The other reason is the fact that the Zambuto mirror is exceptionally smooth and totally eliminates any micro ripple or surface roughness, which contribute to increase light scatter.

While your new Zambuto mirror is likely better than 1/10th wave, I have little doubt you would still see a greater frequency of nights with "better seeing" if you had a Zambuto mirror that was only 1/5 or 1/6 wave.

Cheers,


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