Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Large/Fast Newtonian Mirrors and Quality

This topic has been archived. This means that you cannot reply to this topic.
39 replies to this topic

#26 UmaDog

UmaDog

    Vanguard

  • *****
  • Posts: 2054
  • Joined: 15 Sep 2010

Posted 01 August 2012 - 04:41 PM

I'm afraid I don't know. That's the only source I am aware of for that software.

#27 Mauro Da Lio

Mauro Da Lio

    Viking 1

  • -----
  • Posts: 585
  • Joined: 12 Sep 2004

Posted 01 August 2012 - 05:13 PM

There is a Yahoo group: http://tech.groups.y.../group/roddier/

As for possible causes of tiny differences there also is dirt on mirrors. Dust scatters light (a lot) and makes sky background brighter. It is more important than surface microroughness as can be seen here: http://www.cosmo.uca...tech4_10-06.pdf (see fig. 14)

#28 JCB

JCB

    Ranger 4

  • *****
  • Posts: 333
  • Joined: 04 Oct 2004

Posted 01 August 2012 - 06:02 PM

The issue that prompted the original post is that with Apochromatic refractors, we can do star tests pretty easily and discern if the stars are pinpoint and to what degree color-free. On large/fast Newtonians, we are pushing the envelope and as a consumer, it is pretty difficult to discern if we are getting the quality we are paying for. In other words, how do we know that we are getting truly exceptional figures of revolution with smooth surfaces and no turned or uncompleted zones/edges which detract from energy being concentrated into the Airy Disk?


You can perform the star test with aperture masks.

For example, if you make a classical off-axis mask, if the central obstruction is 0.25, the resulting aperture will be 37.5% the diameter of the mirror. Thus, you will star test a much smaller instrument, in which the seeing will be less damageable and the star test easier to do.

If the mirror has spherical aberration, the defocused images will appear oval, and the star test will look like astigmatism. I think that zones, large scale roughness and turned edges will be also easier to discern, as long as you remember that only a fraction of the mirror is tested.

You can also try a Ronchi eyepiece with the aperture mask: the slower focal ratio will greatly increase the sensitivity of the Ronchi test.

Of course, since only a small part of the mirror is inspected, you have to rotate the off-axis mask in order to explore other portions of the mirror. If star testing sub-apertures doesn't give good results, there is no chance that the entire aperture will be of good quality.

Jean-Charles

#29 Mike Lockwood

Mike Lockwood

    Vendor, Lockwood Custom Optics

  • *****
  • Vendors
  • Posts: 1449
  • Joined: 01 Oct 2007

Posted 01 August 2012 - 10:31 PM

Long before a decrease in central disc intensity becomes significant scattered light does its damage to contrast transfer.

I must disagree based on my experience. The limits of concentration of energy are usually due to gross optical errors, not diffraction, and thus the system is not diffraction limited. This has a larger effect on the central intensity than one might think, and is a far greater issue than scattered light.

A mirror not forming tight star images does far more damage to the image "contrast" than a tiny amount of scattered light. I'm fairly sure plenty of people on this forum have observed with dirty optics, myself included, and have seen amazing things despite the scatter caused by the dust. However, a poor mirror will NEVER perform this well.

IMHO it's why fanatical polishing and gentle figuring work their wonders on overall performance.

While I agree with gentle figuring, I would rather spend my time tweaking the figure of a mirror rather than fanatically polishing, because in my experience it pays greater dividends at the eyepiece.

And why a smaller mirror with better contrast transfer can indeed outperform a larger one with poorer.

In my experience it's not better contrast transfer, it's usually a more accurate figure on the smaller mirror and a poorer one on the larger mirror (all other factors like equilibration, collimation, etc. being equal).

Don't get me wrong - I certainly think a smooth mirror is important, and I make sure that the optics that I make are smooth. That said, a somewhat dirty mirror with a superb figure is still going to outperform a perfectly clean mirror with a poorer figure.

#30 mark cowan

mark cowan

    Vendor (Veritas Optics)

  • *****
  • Vendors
  • Posts: 8580
  • Joined: 03 Jun 2005

Posted 02 August 2012 - 09:50 PM

Post deleted by mark cowan

#31 Bob S.

Bob S.

    Mercury-Atlas

  • *****
  • topic starter
  • Posts: 2652
  • Joined: 14 Jul 2005

Posted 03 August 2012 - 06:09 AM

Thanks Mike and Mark for adding your professional expertise to this discussion. We amateurs really benefit from seeing into the "mind-sets" of opticians and how they go about creating the kinds of high-quality mirrors we are looking for. I think Jarad, one of our moderators will agree that nobody is blowing their horn here but legitimately trying to help us amateurs understand what the ingredients are that distinguish a good mirror from a great one. Keep the posts coming to help us understand the processes. I hope even other opticians will chime in and share some of their wisdom so as consumers we can make better/more informed decisions. Bob Schilling

#32 Jarad

Jarad

    Fly Me to the Moon

  • *****
  • Posts: 6393
  • Joined: 28 Apr 2003

Posted 03 August 2012 - 07:23 AM

The dirt issue was suggested by Mauro, not Mark. I think both Mark and Mike were focusing on the mirror surface, with slightly different phrasing.

I think you are both actually making the same point - Mark's example of a TDE does scatter light elsewhere, but of course that light comes out of the central peak so it also reduces the concentration of light there (as Mike pointed out). You can't do one without the other, they are two sides of the same see-saw. Both of you are emphasizing the role of divergence of the mirror surface from the ideal parabola as the root cause of that effect, with Mark emphasizing the "smoothness" aspect, and Mike emphasizing the "figure" aspect.

"Tastes Great!" :gve: "Less Filling!"

Jarad

#33 Mike Lockwood

Mike Lockwood

    Vendor, Lockwood Custom Optics

  • *****
  • Vendors
  • Posts: 1449
  • Joined: 01 Oct 2007

Posted 03 August 2012 - 02:38 PM

The dirt issue was suggested by Mauro, not Mark. I think both Mark and Mike were focusing on the mirror surface, with slightly different phrasing.

Yes, I'm talking about the surface, but I was using a dirty mirror (because people can relate) to approximate the effects of a rough microsurface.

I'm not saying Mark is wrong - I'm just saying the "rough" mirrors I have seen have always had significant figure issues, so to me the figure is the more important problem. I've never seen a rough mirror that had an excellent figure. I have seen smooth mirrors with poor and excellent figures.

As Mark indicated, fixing the figure with good polishing/figuring techniques will generally smooth out the surface, too.... unless things are REALLY bad.

I think you are both actually making the same point - Mark's example of a TDE does scatter light elsewhere, but of course that light comes out of the central peak so it also reduces the concentration of light there (as Mike pointed out). You can't do one without the other, they are two sides of the same see-saw. Both of you are emphasizing the role of divergence of the mirror surface from the ideal parabola as the root cause of that effect, with Mark emphasizing the "smoothness" aspect, and Mike emphasizing the "figure" aspect. "Tastes Great!" :gve: "Less Filling!"

Right. However, Mark chose to quote my statement about contrast and in essence make a "correction" to it, so naturally I disagreed and stated my reasons for my statement being correct.

We can agree and disagree here provided we provide facts and reasons for the position we take. No malice is intended, though the debate may be "spirited". :sumo:

My main point is: A high-quality large telescope, properly collimated and equilibrated, should produce sharp images at moderate or high power fairly often, not on every night, but on a lot of them, more than 5-10%. On some nights it should amaze you. If this is not the case, (i.e. it never performs, or only does so very infrequently) something is amiss. The focus on a good, large instrument should snap, providing tight star images, even if the seeing is not excellent. If it never does, something is amiss.

#34 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 38823
  • Joined: 23 Jun 2003

Posted 03 August 2012 - 05:08 PM

I'll back up Mike's comments.
For years, I'd looked through tons of 20"-32" scopes and been singularly unimpressed by the image qualities.
Since those scopes had had a range of f/ratios, I figured that all big scopes were basically trade-offs: to get large aperture and the concomitant light grasp, you had to sacrifice the sharpness of image I see all the time in my 12.5".

Fast forward to a night at Mt. Pinos, where Steve Kennedy was showing Mike Zammit of Starstructure Telescopes a 28" f/4.2 scope (Spica Eyes with full GoTo on board). We all took turns climbing the ladder to get a peek at something or other (don't remember what).

What I saw simply blew me away. Sure, there was a little more residual coma in the f/4.2 (using a Paracorr I) at the edge of the fields of view than in my f/5 (also using a Paracorr), but overall, the images were smashingly good--tiny little pinpoints over most of the field and a large aperture version of the optical quality I'd come to expect only in smaller apertures.

Since then (that was several years ago), I've seen a number of large scopes with superb star images and image quality. I think that the mirror makers producing large mirrors have gotten better and their standards are very high. That doesn't mean EVERY large mirror I've seen was up to that standard, but certainly image quality and size don't have the inverse relationship I once assumed.

Currently, my only objections to the short f/ratios becoming common are that the depth of focus is very short, making focus variability in mediocre seeing a bit more of a problem than in longer f/ratios, and that most eyepieces don't perform as well at the edges at f/3 as they do at f/5, even when both are Paracorred with the latest Paracorr II at the correct setting.
But, that being said, I would still unhesitatingly choose a fast f/ratio at the really large sizes of scope simply because it's easier and safer to stand on the 3rd step of a step ladder than it is to stand on the tenth (!), and I've done that in a 36" f/5.

Along the way of large scope progress have come better cells, thinner mirrors, better fans, and better collimation tools.
Put those all together, and the performance level of the large scopes seems now to be only limited to the mirror qualities, and there are makers of large mirrors now who put the same quality into their mirrors as some of the better makers who stop with much smaller sizes.

I truly wish many of you had seen the poor quality large scopes over the years that I have seen. If you had, you'd realize how we truly live in the Golden Age of Astronomy right now.

#35 John Kuhl

John Kuhl

    Ranger 4

  • -----
  • Posts: 330
  • Joined: 10 Nov 2005

Posted 03 August 2012 - 05:22 PM


I'm with you Don. The large scopes and mirrors we observe with today are wonderful. They are head and shoulders above what we used in the past.

Best, John

#36 NHRob

NHRob

    Fly Me to the Moon

  • *****
  • Posts: 7146
  • Joined: 27 Aug 2004

Posted 03 August 2012 - 07:47 PM

I'm still amazed at high quality f/3 mirrors!!

#37 Mike Spooner

Mike Spooner

    Vendor (mirrors)

  • *****
  • Vendors
  • Posts: 503
  • Joined: 06 Aug 2010

Posted 03 August 2012 - 09:07 PM

I truly wish many of you had seen the poor quality large scopes over the years that I have seen. If you had, you'd realize how we truly live in the Golden Age of Astronomy right now.


Hi Don,

Couldn't agree more! I remember talking to some in the past who wanted a larger scope no matter what the quality. I always suggested it was easier to transport and use a smaller scope of quality - yes the larger optic would be brighter overall but the contrast was so limited that a smaller quality optic would show more.
To respond to Bob's original observations, I've noted several times over the years where one night one scope would seem to outperform another only to see the reverse on another night. I can't explain it but might offer thermal, eyepiece or support differences (or even how our eyes work on a particular night) that may cause the effect. It also could all be in my mind but I've done hundreds of hours of side by side viewing with different scopes. I've also seen Pyrex blanks as small as 10" that had differing thermal aspects when the temp changed while bench testing - fairly rare but can be frustrating when it happens. Probably an inaccurate anneal. Still at the quality levels being discussed here, it's amazing how far things have come in the past 20 years and I feel that quality scopes are a relative bargain compared to what it took in the past reach the levels we see today. It takes a really solid atmosphere and several tens of power per inch to examine the Airy disk for trace amounts of astigmatism and correction, especially with scopes so far above the 6 or 8" sizes so common years ago.

Best,
--Mike Spooner

#38 a__l

a__l

    Viking 1

  • -----
  • Posts: 805
  • Joined: 24 Nov 2007

Posted 03 August 2012 - 09:16 PM

Doubt as a quality mirror, send it to the interferometric testing in an independent company. (book D.Kriege, A Practical manual ... page 93).

#39 Bob S.

Bob S.

    Mercury-Atlas

  • *****
  • topic starter
  • Posts: 2652
  • Joined: 14 Jul 2005

Posted 04 August 2012 - 04:37 AM

I truly wish many of you had seen the poor quality large scopes over the years that I have seen. If you had, you'd realize how we truly live in the Golden Age of Astronomy right now.


Hi Don,

Couldn't agree more! I remember talking to some in the past who wanted a larger scope no matter what the quality. I always suggested it was easier to transport and use a smaller scope of quality - yes the larger optic would be brighter overall but the contrast was so limited that a smaller quality optic would show more.
To respond to Bob's original observations, I've noted several times over the years where one night one scope would seem to outperform another only to see the reverse on another night. I can't explain it but might offer thermal, eyepiece or support differences (or even how our eyes work on a particular night) that may cause the effect. It also could all be in my mind but I've done hundreds of hours of side by side viewing with different scopes. I've also seen Pyrex blanks as small as 10" that had differing thermal aspects when the temp changed while bench testing - fairly rare but can be frustrating when it happens. Probably an inaccurate anneal. Still at the quality levels being discussed here, it's amazing how far things have come in the past 20 years and I feel that quality scopes are a relative bargain compared to what it took in the past reach the levels we see today. It takes a really solid atmosphere and several tens of power per inch to examine the Airy disk for trace amounts of astigmatism and correction, especially with scopes so far above the 6 or 8" sizes so common years ago.

Best,
--Mike Spooner


+3 !!!

Mike S., You are certainly one of the optician's in my book that has helped raise the bar over the past 20 years. Your quiet work on superb, very high performing mirrors has always been a comfort when I have owned them. Your ability to produce pinpoint stars with superb contrast in refractor-like 6-8" mirrors is something that has been etched in my memory banks as an absolute reference standard for beautifully figured mirrors. Bob Schilling

#40 Mike Lockwood

Mike Lockwood

    Vendor, Lockwood Custom Optics

  • *****
  • Vendors
  • Posts: 1449
  • Joined: 01 Oct 2007

Posted 06 August 2012 - 11:28 AM

Doubt as a quality mirror, send it to the interferometric testing in an independent company.

Well that works in theory, if the testing is done competently and honestly. The point in this thread, though, is to help observers determine if there is an issue based on their observations.

To respond to Bob's original observations, I've noted several times over the years where one night one scope would seem to outperform another only to see the reverse on another night. I can't explain it but might offer thermal, eyepiece or support differences (or even how our eyes work on a particular night) that may cause the effect.

I think that as various issues in telescope construction have been addressed and improved, much of the inconsistency has fallen away.

The anneal of glass is generally better and is checked for, edge supports have improved, thermal management is better understood, coma correctors have improved, collimation is done better and the tools for it have improved and evolved, and the internet has allowed people to communicate this knowledge.

When you add in good optics, this is indeed a golden age as Don mentioned.

The key is that knowledgeable telescope owners have identified and accounted for many of the variables that can cause night-to-night variations. Now, putting properly set up telescopes side by side, observers can see the differences that are generally caused by the optics and thermal issues. Doing these comparisons over multiple nights will start to separate those differences, too.


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics