Eyepiece, Barlow & Filter Tool

How Celestron groups these accessories

Celestron sells these in four catalogue categories, and each plays a different optical role — so the tool treats them separately, then combines them.

① Eyepieces

The element you look through; sets the base magnification = scope focal length ÷ eyepiece focal length. Shorter focal length = more power, narrower field. Your Plössls (6–25 mm) and the 4 mm are standard astronomical eyepieces; the 20 mm erecting eyepiece is a terrestrial/daytime piece.

② Barlow lenses (factor > 1)

Negative-lens amplifiers in front of the eyepiece that multiply the system focal length. Your 2.3× and 3× Barlows increase magnification, narrow the true field, shrink the exit pupil and slow the f-ratio. They work with any eyepiece.

③ Erecting (image-correcting) pieces

Celestron groups these with daytime/terrestrial use. Your 1.5× erecting piece is an erecting Barlow — it both amplifies (1.5×) and flips the image right-way-up, with a small contrast/light cost. The 20 mm erecting eyepiece does the same as an eyepiece.

④ Filters

Threaded glass that changes spectral transmission only — no effect on magnification, field or exit pupil. Celestron splits these into Neutral Density / Moon (your ND .9) and colour / planetary (your No. 58, 25, 12, 21, 80A, 56). Matched to the target, not the power — shown in their own panel below.

Combining the optics: effective focal length = 2032 mm × Barlow factor (stacked Barlows multiply). mag = effFL ÷ eyepieceFL · true FOV ≈ AFOV ÷ mag · exit pupil = 203 ÷ mag · f-ratio = effFL ÷ 203. Filters leave every one of these unchanged.

Your inventory

Toggle any item to include/exclude it from the permutations below.

Eyepieces

Use	Eyepiece	Focal length	AFOV	Native power	Notes

Barlows & erecting pieces (amplifiers)

Use	Item	Type	Factor	Effect	Notes

Filters

Use	Filter	Category	Transmission	Best targets

Scope

Seeing (atmosphere)

Sort permutations by

Options

Include stacked Barlows

Hide “too much” rows

Include daytime/erecting

Permutations: — Good Borderline Too much

All magnification permutations

Every eyepiece × every Barlow/erecting combination on your 8SE.

Eyepiece	Amplifier	Mag	True FOV	Exit pupil	f/	Optical verdict	Best for

Filters — separate panel

Filters don’t change magnification, field or exit pupil. Pick by target, then use any eyepiece/Barlow combo above. Stronger colours dim the view, so favour the brighter combos (larger exit pupil) when stacking a deep colour.