thumb|Plate scale can be derived using this image assuming that the angle is very small (so that tan(theta) is approximately theta). The '''plate scale''' of a telescope connects the angular separation of an object with the linear separation of its image at the focal plane.
If focal length {{mvar|f}} is measured in millimeters, the plate scale in radians per millimeter is given by angular separation {{mvar|θ}}, and the linear separation of the image at the focal plane {{mvar|s}}, or by simply the focal length {{mvar|f}}: <math display=block>p =\frac{\theta}{s} =\frac{1}{f}\ ,</math> since <math display=block>s =f \theta\ .</math>
Plate scale is usually expressed in arcseconds per millimeter: <math display=block>p \approx\frac{206265}{f},</math> where {{mvar|f}} is in {{abbr|mm|millimeters}},<ref>{{cite book |title= An Introduction to Modern Astrophysics |edition= 1st |last=Carroll |first=Bradley |author2=Ostlie, Dale |page= 163 }}</ref> or expressed in arcseconds per pixel after further division through the pixel scale.
==Plate scale on JWST FGS/NIRISS== The pixel scale of the James Webb Space Telescope component Fine Guidance Sensor and Near Infrared Imager and Slitless Spectrograph is about 0.066 arcsec/pixel.<ref>{{Cite web |title=JWST Near Infrared Imager and Slitless Spectrograph - JWST User Documentation |url=https://jwst-docs.stsci.edu/jwst-near-infrared-imager-and-slitless-spectrograph#JWSTNearInfraredImagerandSlitlessSpectrograph-Observationalcapabilities |access-date=2026-05-27 |website=jwst-docs.stsci.edu}}</ref> It uses a {{resx|2040|2040}} pixel array with a pixel size of {{val|18|u=microns}} per side with a field of view of {{resx|2.2′|2.2′}} <ref>{{Cite web |title=JWST Near Infrared Imager and Slitless Spectrograph - JWST User Documentation |url=https://jwst-docs.stsci.edu/jwst-near-infrared-imager-and-slitless-spectrograph#JWSTNearInfraredImagerandSlitlessSpectrograph-Opticalelements |access-date=2026-05-27 |website=jwst-docs.stsci.edu}}</ref>
==See also== *Photographic plate
==Notes== {{Reflist}}
Category:Astrophotography Category:Equations of astronomy
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