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Are you considering buying a 40mm Plossl eyepiece ? Read on to find out why we would recommend going with the 32mm Plossls instead.



40mm Plossls vs 32mm Plossls

Date: 11 Jan 2010

If you're interested in a 40mm plossl eyepiece, read on. I would not recommend getting it, but go for the 32mm eyepiece instead.

Here are two graphical representations of the views you will get. Simulating views of the Hyades cluster through an Orion ShortTube 80 (FL 400mm), the left one is the 32mm view, and the right is 40mm . Here are the 2 key differences.

  •  32mm view is larger than the 40mm view (greater magnification)
  •  40mm view sees slightly more stars (within the circle) than 32mm (larger Field of View)

The conclusion seems like a no-brainer to me :-) !!



Here are the maths for those who are interested :

The AFV (Apparent Field of View) of the 40mm is 43 deg, and for the 32mm is 52deg. Assuming your telescope focal length is F :

  40mm 32mm
Power F/40 F/32
AFV (Apparent Field of View) 43° 52°
FOV (True Field of View) 43/(F/40) = 1720/F 52/(F/32) = 1664/F

Here are some examples with telescopes of 600mm and 1900mm.

F=600mm 40mm 32mm
Power 15.0x 18.8x !!
FOV (True Field of View) 2.9° 2.8°


F=1900mm 40mm 32mm
Power 47.5x 59.4x !!
FOV (True Field of View) 0.9° 0.9°

Bottom line, you'll the see the very nearly the same amount of stars with both eyepieces, but the view will be bigger with the 32mm !!

Hope this helps, and do let me know if you need more clarification. In any case though, the choice is certainly yours depending on your application !

Joo Beng



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