{"id":391720,"date":"2024-10-29T11:33:37","date_gmt":"2024-10-29T11:33:37","guid":{"rendered":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/rms-spot-size-interpretation\/"},"modified":"2024-10-29T11:33:37","modified_gmt":"2024-10-29T11:33:37","slug":"rms-spot-size-interpretation","status":"publish","type":"topic","link":"https:\/\/innovationspace.ansys.com\/forum\/forums\/topic\/rms-spot-size-interpretation\/","title":{"rendered":"RMS spot size interpretation"},"content":{"rendered":"<p>&lt;p&gt;Hello everyone,&nbsp;&lt;br&gt;&lt;br&gt;I recently tried to tolerance a spectrograph, looking at the rms spot size as criterion. First I found that there is no &#8220;ignore lateral color&#8221; option, so the spot size in dispersion direction is something like 20mm which is the spectrum length. It would be really great to add the &#8220;ignore lateral color&#8221; as it is done in the merit function wizard.&nbsp;&lt;br&gt;&lt;br&gt;Nevertheless, I created a merit function with &#8220;ignore lateral color&#8221; to work around the problem. However, what I find now is that while my rms wavefront plot gives values between 8 micron and 12 micron:&lt;br&gt;&lt;br&gt;<img decoding=\"async\" src=\"https:\/\/innovationspace.ansys.com\/forum\/wp-content\/uploads\/sites\/2\/2024\/10\/29-10-2024-1730200966-mceclip0.png\" \/>&lt;\/p&gt;&lt;p&gt;&nbsp;&lt;\/p&gt;&lt;p&gt;the merit function produces a value of 3.9 micron. Looking at the merit function, it says it adds the rms x and rms y values, but under &#8220;value&#8221; I get negative numbers. How can this be as rms values should always be positive? While I understand that radii are always positive, explaining the higher values in the plot above, I do not understand why the values in the &#8220;value&#8221; column which are rms(x) + rms(y) are sometimes negative:&lt;br&gt;&lt;br&gt;<img decoding=\"async\" src=\"https:\/\/innovationspace.ansys.com\/forum\/wp-content\/uploads\/sites\/2\/2024\/10\/29-10-2024-1730201126-mceclip1.png\" \/>&lt;\/p&gt;&lt;p&gt;If &#8220;value&#8221; is just the coordinate in x\/y for a given wave, field and pupil position, the negative values make sense. So in this case the rms happens somewhere else not visible in the tolerance editor, and the contribution is calculated by this. However, it is still not clear for me why I only get 3.9 microns where the plot indicates 8-12 microns.&nbsp;&lt;\/p&gt;&lt;p&gt;Thanks in advance for any clarification,&nbsp;&lt;br&gt;&lt;br&gt;&lt;br&gt;Jurgen&lt;\/p&gt;<\/p>\n","protected":false},"template":"","class_list":["post-391720","topic","type-topic","status-publish","hentry"],"aioseo_notices":[],"aioseo_head":"\n\t\t<!-- All in One SEO 4.9.10 - aioseo.com -->\n\t<meta name=\"description\" content=\"Hello everyone, I recently tried to tolerance a spectrograph, looking at the rms spot size as criterion. First I found that there is no &quot;ignore lateral color&quot; option, so the spot size in dispersion direction is something like 20mm which is the spectrum length. 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