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4" secondary mirror test result

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#1 astro_alex80

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Posted 28 June 2022 - 03:01 PM

Hi all,

 

likely the hundredth question on mirror test results but I am somehow having difficulties in regards to secondary mirrors and their estimation of performance.

 

Received following Zygo test result of my new 4" (100mm) secondary.

 

Measured wavelength 632.8 nm, approx. 1/16 inch (2mm) edge have been masked and not taken in cosideration for the results.

 

Surface results:

PV 0.167 wave (1/6)

RMS 0.029 wave

Power - 0.024 wave

 

Below the surface plot.

 

The secondary is supposed to be in use for an DIY 14" F3,8 newtonian. Whats your straigtforward opinion about the secondary looking at the results?

 

Thanks for your help and feedback,

Alex

 

 

100mm_flat.png


Edited by astro_alex80, 28 June 2022 - 03:04 PM.


#2 ngc7319_20

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Posted 28 June 2022 - 03:06 PM

I would not use this mirror for a planetary scope.  It has bad astigmatism, like 1/3 wave on the wavefront.  For a deep-sky light bucket, yes it would be OK.  In green light error is even worse -- like 0.38 wave PTV wavefront. 

 

The PTV error is not confined to a tiny spot, but is part of a fairly smooth roll-off along opposite edges.  If all the PTV were at one small spot I might be more forgiving.


Edited by ngc7319_20, 28 June 2022 - 03:10 PM.

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#3 Jim Waters

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Posted 28 June 2022 - 03:13 PM

IMO - for a secondary mirror its below average.  From what I have seen 1/10 wave is standard.

 

EDIT - My 3.1" Obsession mirror was 1/12 wave.


Edited by Jim Waters, 28 June 2022 - 03:14 PM.

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#4 astro_alex80

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Posted 28 June 2022 - 03:51 PM

Thanks a lot Gentlemen.

ordered my last but way smaller secondary (2,6“) for my 12“ Dobson at Antares some time ago with 1/15 PV and very happy about it.

 

Had the chance to get this 4“ but seems not to be what I expected, thanks for confirmation.



#5 Steve Dodds

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Posted 28 June 2022 - 04:03 PM

Well other than 1/6 wave of astigmatism, it's pretty flat.


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#6 ngc7319_20

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Posted 28 June 2022 - 04:21 PM

I now notice in your signature line that this is perhaps for a "Astrograph (in build)".  It might be OK for that.  The errors in the Airy pattern might be hidden below the pixel size.  Unless the pixel size is smaller than about 1/4 arcsecond, you would not see the errors.  But if you are trying to capture planet detail visually at high power, then I would skip this mirror.


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#7 astro_alex80

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Posted 28 June 2022 - 04:44 PM

In fact the mirror was specified to be 1/8 λ Wavefront, which I didn’t mentioned initially and it’s clear the mirror is not according to specs but thought it might be still a decent enough secondary also giving the quite low RMS. Also intention is not to do any blaming/finger pointing as already contacted the dealer as in good contact with him anyway and maybe the mirror has just been confounded or what ever.

In fact will do same with the main mirror and give it for an independent test even though its coming with an analysis.

 

Pixel scale is 0.57 arcsec so could be okay but on the other hand as you mentioned forget about planetary, which I am not willing to even it’s not an planetary optimised scope.

 

On top I am investing bit by bit a quite important amount for an high quality main mirror, carbon tube, large focuser (3“ Pegasus Prodigy), custom made mirror cell, heavy duty spider, decent corrector and so on basically aiming to build an „once for all“ scope were I can be sure not to have to have even the smallest doubt or worry about it’s components (knowings it’s not a $$$ Planewave CDK, etc,).


Edited by astro_alex80, 28 June 2022 - 04:53 PM.


#8 davidc135

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Posted 28 June 2022 - 04:46 PM

Vla Sacek in Telescope-Optics.net on page 8.1.3 gives the wave front error as x1.4 the difference between the depths across the major and minor axes of the diagonal so in this near cylindrical case about 1/4 wave astigmatism, not so good.

 

If the contours were concentric with the edge of the diagonal there would be only a focusing error.

 

David


Edited by davidc135, 28 June 2022 - 04:47 PM.

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#9 ccaissie

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Posted 28 June 2022 - 08:05 PM

There was discussion around an astigmatic primary, and one suggestion was to find an astigmatic Secondary to cancel it.  I don't know how feasible this really is.   I note you are acquiring a quality primary. If you get good interferometry data on this primary,  any chance of values cancelling out in your system?


Edited by ccaissie, 28 June 2022 - 08:08 PM.


#10 ccaissie

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Posted 28 June 2022 - 08:16 PM

Had the chance to get this 4“ but seems not to be what I expected, thanks for confirmation.

Long wavelength used in the tests. 0.125 (1/8 wave claimed) vs. 0.167 (1/6) listed is pretty close.   Flats are usually measured by surface PV.


Edited by ccaissie, 28 June 2022 - 08:19 PM.


#11 davidc135

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Posted 29 June 2022 - 01:39 AM

In post8 I had forgotten to take the fact that the diagonal is larger than needed to illuminate only the centre of the field. So, if the 'footprint' has a minor axis of say 3'', the actual astig will be reduced, if the figure is smooth, as it is.

 

Which takes the wave-front error down to around 1/7 or 1/8 wave astig. If that's the case your slightly outside the spec diagonal won't be distinguishable in practise from a perfect one. Given the aperture. You might just spot it in an indoor test with perfect conditions.

 

All assuming that the primary mirror/support isn't contributing astigmatism of the same orientation.

 

I'd say the diagonal is fine.

 

David


Edited by davidc135, 30 June 2022 - 01:30 AM.

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