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Mark Peterman
super member


Reged: 08/07/12

Loc: Texas
My Antares Secondary Interferometer Report new
      #5555511 - 12/05/12 11:17 AM

I just purchased a 2.6", 1/20th wave secondary from Antares Optics. These secondary mirrors come with an Interferometer report and a reflectivity chart.

I trust that the mirror is what they say it is, that is not what this post is about. I'm curious to know how to read the report.

I think the actual Wave PV of this mirror is 1/22?

'Trimmed: 5'? The test did not include the entire surface?

The Oblique Plot looks like a chunk of the mirror is missing.

Anyway, thought that I might learn something by throwing this out here.

Thanks,

Mark

To be readable, the image exceeds the CN limits. Please click below to see the report.

Interferometer Report


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MKV
Carpal Tunnel
*****

Reged: 01/20/11

Re: My Antares Secondary Interferometer Report new [Re: Mark Peterman]
      #5555524 - 12/05/12 11:30 AM

Mark, yes the report claims the mirror is 1/20 wave PV. Your RNS is listed as 1/167 wave. if this is true, this is an overkill which is an excuse to charge more money. Unless your primary can match the secondary it's a waste of money, imo.

You say the mirror is trimmed? Interesting. I wonder why. I detect a small curvature on the middle fringe at the left edge. The surface seems smooth, but they should test mirrors uncoated. It gives better contrast.

The only way you can know how good it is is to have it tested independently. Why? Read my tagline.

Mladen


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Mark Peterman
super member


Reged: 08/07/12

Loc: Texas
Re: My Antares Secondary Interferometer Report new [Re: MKV]
      #5555556 - 12/05/12 12:06 PM

Quote:

Mark, yes the report claims the mirror is 1/20 wave PV. Your RNS is listed as 1/167 wave. if this is true, this is an overkill which is an excuse to charge more money. Unless your primary can match the secondary it's a waste of money, imo.

You say the mirror is trimmed? Interesting. I wonder why. I detect a small curvature on the middle fringe at the left edge. The surface seems smooth, but they should test mirrors uncoated. It gives better contrast.

The only way you can know how good it is is to have it tested independently. Why? Read my tagline.

Mladen




The trimmed comment was referring to a parameter on the report that says, "Trimmed: 5". Just curious what that meant and was taking a stab that it meant some of the area was not taken into consideration.

I understand your tagline. My reason for purchasing the "1/20" secondary was because my mirror maker claims my primary will be "1/10" wave PV wavefront (or better) and I have read that it is desirable to have a secondary that is twice as good as the primary. So, 'in theory', I have covered my bases.

Thanks for the feedback.

Mark


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greenglass
scholastic sledgehammer
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Reged: 01/22/06

Loc: Hamilton. Ontario, Canada
Re: My Antares Secondary Interferometer Report new [Re: Mark Peterman]
      #5555769 - 12/05/12 02:21 PM

certainly they tested the whole surface, their master flat I read is 12 inches diameter and 1/100 wave. So a smaller part of it used for your 2.6 inch is probably 1/200 wave. My guess of trimmed is, set at zero the surface picture would look too flat so they cranked it to 5 to exagerate the slopes to make a better pic of the slopes high to low just for graphing sake.

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Napersky
Carpal Tunnel
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Reged: 01/27/10

Loc: Chicagoland
Re: My Antares Secondary Interferometer Report new [Re: greenglass]
      #5562084 - 12/09/12 09:48 AM

1/22th PV is most likely surface error not wavefront. Wavefront on a mirror is double the surface or 1/11th wave.

Noone is going to have 1/100th wave PV surface. Zygo sells their top commerical reference sphere at 1/50th wave. More common are 1/10th, 1/15th, and 1/20th. You cannot measure better than your transmission sphere.

Mark


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greenglass
scholastic sledgehammer
*****

Reged: 01/22/06

Loc: Hamilton. Ontario, Canada
Re: My Antares Secondary Interferometer Report new [Re: Napersky]
      #5562177 - 12/09/12 10:45 AM

Your right, "Our new 12 inch phase measuring interferometer is 1/30 wave PV over the entire 12 inch aperture. If you measure a small 2.00" diameter on its reference flat, it measures better than 1/50 - 1/100 wave PV. More than adequate to measure a 1/20 or 1/30 wave PV on a little secondary mirror" I just seen the 1/100 wave part and that stuck in my head.

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Napersky
Carpal Tunnel
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Reged: 01/27/10

Loc: Chicagoland
Re: My Antares Secondary Interferometer Report new [Re: greenglass]
      #5562293 - 12/09/12 11:46 AM

I would be very very happy to have a 1/30th wave PV reference flat. Just ordered a flat for my home lab that's supposed to be 1/20th. 1/30th is exceptional.

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Mark Peterman
super member


Reged: 08/07/12

Loc: Texas
Re: My Antares Secondary Interferometer Report new [Re: Napersky]
      #5562563 - 12/09/12 02:28 PM

So on this report, are they saying 1/22 wave at the surface or at the wavefront? (PV 0.022 wave, right above date and time)

I guess if it is at the surface, then it is really 1/11 wave at the wavefront, correct?

Their website simply says 1/20 WAVE PV. They have a whole FAQ about quality measurements, surface vs wavefront, but I can't find anywhere where they state which it is that they quote on their mirrors !


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mark cowan
Vendor (Veritas Optics)
*****

Reged: 06/03/05

Loc: salem, OR
Re: My Antares Secondary Interferometer Report new [Re: Mark Peterman]
      #5562770 - 12/09/12 04:51 PM

Quote:

have read that it is desirable to have a secondary that is twice as good as the primary.




Where did you read that? The way secondaries are usually manufactured, their predominant error is power, and most of that focuses out (most of the time, not all of the time). Since the secondary surface error is increased by about 1.4x on 45 degree reflection, in practice if the secondary's surface spec is anywhere close to the primary's wavefront spec you're good to go.

Looks like an excellent flat, BTW.

Best,
Mark


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Starman1
Vendor (EyepiecesEtc.com)
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Reged: 06/24/03

Loc: Los Angeles
Re: My Antares Secondary Interferometer Report new [Re: mark cowan]
      #5562936 - 12/09/12 06:37 PM

You can't always believe what you read.
My 2.6" Antares secondary reads:
Wavefront map:
P-V error 0.033 wave
RMS 0.006 wave
power -0.009 wave

Surface profile:
P-V error 0.02 wave
RMS 0.004 wave

Nearly all the error is at the edge, and 0.1" of the edge all the way around is masked by the secondary holder.

Nice specs.
BUT:
--how many points were sampled? It would have to be a lot for any level of confidence those figures were anything other than fictitious.
--what reference machine allowed measurements of that level?
1/250 wave RMS? Really? Come on.

Do I believe it's a good secondary? Yeah, it performs well in the field.
Do I believe it's that good?
No. I don't believe in the Tooth Fairy, either.


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ausastronomerModerator
Carpal Tunnel
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Reged: 06/30/03

Loc: Kiama NSW (Australia)
Re: My Antares Secondary Interferometer Report new [Re: Starman1]
      #5563092 - 12/09/12 08:31 PM

Quote:

Do I believe it's a good secondary? Yeah, it performs well in the field.
Do I believe it's that good?
No. I don't believe in the Tooth Fairy, either.




Hi Don,

I am a bit like you in this regard. The last 2 scopes I have had built were built by Peter Read at SDM telescopes. The 14" has a Zambuto primary and the 10" has a primary by Mark Suchting, which is every bit as good as the Zambuto mirror.

The 14" scope has a 2.6" protostar quartz secondary and the 10" scope has a 1.83" protostar quartz secondary, for an 18% CO in each scope. Both scopes have the protostar heated secondary holder, which is excellent. I didn't even bother asking for the test reports on the secondaries, knowing the protostar quartz secondaries are invariably of a very high standard and any test report numbers are academic and could be manipulated in any case.

It just isn't worth fussing over the numbers for mine when you buy a premium secondary mirror from a reputable source and you know it is going to perform at a level consistent, or above, the level of your premium primary optic.

Similarly I wasn't worried about any test reports when I ordered the primary optics. The reputation of the mirror maker was good enough for me and subsequently confirmed with the supplied products.

Cheers,


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mark cowan
Vendor (Veritas Optics)
*****

Reged: 06/03/05

Loc: salem, OR
Re: My Antares Secondary Interferometer Report new [Re: Starman1]
      #5563116 - 12/09/12 08:51 PM

Quote:

--how many points were sampled? It would have to be a lot for any level of confidence those figures were anything other than fictitious.
--what reference machine allowed measurements of that level?
1/250 wave RMS? Really? Come on.




Yup. Yup. Probably down around the error level of the Zygo unless it was a special edition. I suppose you could always just measure the interferometric image itself, you know, see if you can detect where it deviates from ruler straight.

If a true depiction of the flat's performance it's overkill in practice.

Best,
Mark


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Mark Peterman
super member


Reged: 08/07/12

Loc: Texas
Re: My Antares Secondary Interferometer Report [Re: Starman1]
      #5563847 - 12/10/12 09:51 AM

Quote:

--how many points were sampled?




Please excuse my ignorance, but I thought this method of testing tested the whole surface and that is the reason it is touted over the foucault test?


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

Reged: 06/24/03

Loc: Los Angeles
Re: My Antares Secondary Interferometer Report new [Re: Mark Peterman]
      #5563964 - 12/10/12 11:10 AM

The Foucault test does allow you to see the entire surface. You can mask off zones to get a better idea of zonal defects, but the typical Foucault test allows you to see very very small ripples and bumps on a mirror's surface.
Typical interferometer tests sample a limited number of points. Otherwise the calculations get too intense.
One example I saw where a 12.5" mirror was tested sampled only a little more than 1 point per square inch.
The most revealing test I've seen is the Lyot surface test. Though it's not quantified, it can reveal surface errors at nearly the size of the glass spall itself.


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Pinbout
Postmaster
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Reged: 02/22/10

Loc: nj
Re: My Antares Secondary Interferometer Report new [Re: greenglass]
      #5565760 - 12/11/12 11:25 AM Attachment (8 downloads)

Quote:

Your right, "Our new 12 inch phase measuring interferometer is 1/30 wave PV over the entire 12 inch aperture. If you measure a small 2.00" diameter on its reference flat, it measures better than 1/50 - 1/100 wave PV. More than adequate to measure a 1/20 or 1/30 wave PV on a little secondary mirror" I just seen the 1/100 wave part and that stuck in my head.





wouldn't this math be correct? using a 12in 1/2~ and getting a 4in 1/6~?


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mark cowan
Vendor (Veritas Optics)
*****

Reged: 06/03/05

Loc: salem, OR
Re: My Antares Secondary Interferometer Report new [Re: Starman1]
      #5565891 - 12/11/12 12:49 PM

Quote:

Typical interferometer tests sample a limited number of points. Otherwise the calculations get too intense.




Ah...no. Many points can be acquired, it depends on the methodology, not the computation which is pretty straightforward. The problems with dense sampling techniques arise mostly during phase unwrapping and that's what can lead to computational bottlenecks, for poorly optimized analyses anyways. SFAIK, YMMV.

Best,
Mark


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Mark Harry
Vendor
*****

Reged: 09/05/05

Loc: Northeast USA
Re: My Antares Secondary Interferometer Report new [Re: mark cowan]
      #5567058 - 12/12/12 07:04 AM

Danny, the deflection if everything is perfectly smooth and spherical, varies as a -CUBE ROOT- for accuracy of the smaller aperture. The analogy: considering a concave fit of the parts- you are dealing with a -VOLUME- of space, not an area,(squared) or ordinary linear. So the IF will show exactly this relationship, along with taking the reference optic into account it can vary.
So if the smaller aperture is 4" diameter, and your reference is a half wave, the smaller should be able to be tested to 1/27th wave surface accuracy which would be the contribution of the reference error. In IF, 2 fringes = 1 wave surface error. The lines will appear straighter across the 4" above with an actual test. Seems not to be correct, but it is.
M.


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Mike I. Jones
Post Laureate
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Reged: 07/02/06

Loc: Fort Worth TX
Re: My Antares Secondary Interferometer Report new [Re: Mark Harry]
      #5567146 - 12/12/12 08:46 AM

My 20 millidollars - modern phase-shifting Fizeau interferometers are upgrading to HD-resolution cameras to image the fringe patterns at each of the piezo steps. Processing fringe imagery with Fourier transform interferometry allows very high spatial resolution. The profile wavefront accuracy is set by (1) the number of bits the camera quantizes to, and (2) the spatial sampling frequency across the measured pupil. Going HD pretty much solves the sampling frequency issue, with FFT's at 512x512 or 1024x1024 possible. A 12-bit camera will give you roughly 1/30 wave accuracy, and a 14-bit camera can give roughly 1/120 wave accuracy. To exceed this accuracy, you have to switch to multi-pass fringe-sharpening interferometry using Fabry-Perot cavities. The LIGO fold mirrors were measured to roughly 1/1000 wave accuracy using this method.
Mike


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mark cowan
Vendor (Veritas Optics)
*****

Reged: 06/03/05

Loc: salem, OR
Re: My Antares Secondary Interferometer Report new [Re: Pinbout]
      #5567454 - 12/12/12 12:14 PM Attachment (12 downloads)

This is the comparison:

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