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Benjam

25mm or 18mm - Lunar / Solar

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I’m having trouble on deciding between these two and wish for some feedback. I currently own, 40, 12 & 9mm eyepieces. The 25mm has a similar fov to the 40mm, though the 18mm fits the target perfectly.

I’m kinda swaying for the 18mm, though interested to hear from others. 

 

18mm 

943751C9-8EA3-4EF4-A46D-04263652A176.thumb.png.431e395a4706972b516adaffbaa526b0.png25mm

8060EA4B-0C77-47DE-B7CE-7F2C3FAADA24.thumb.png.e8b97e6e74db122335961e7e274f0539.png

40mm 

567B5692-E775-4D0C-B82E-359210F2433B.thumb.png.cf6ebc95995a36dff3af78ac0b32dd35.png

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I measured the effective apparent field of view (what you plug into equations to get true field of view) for my 18mm Meade HD-60 (reportedly same optics as Celestron Xcel LX) to be 59 degrees rather than 60 degrees based on my measured effective field stop of 18.5mm.  As such, the moon's image might be clipped a bit.  It would also depend on the focal length of your SCT at your focus position for your primary.  If you have a long path (2" diagonal, filter wheel, rear Crayford focuser, etc.), you could be extending your focal length by a significant amount when moving the primary forward to reach focus.  This would clip the image further.

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