Celestron Omni 4 mm - 1.25"

The Omni 4mm is designed for high-magnification observing of bright, small targets. It excels at revealing details on planets like Jupiter and Saturn, splitting close double stars like Albireo (Beta Cygni), and examining the structure of planetary nebulae such as the Ring Nebula (M57).

In an 8" (2032mm focal length) SCT, the 4mm eyepiece will produce a magnification of 508x (2032mm / 4mm). This is extremely high power, exceeding the generally recommended maximum of 400x for an 8" scope. It will only be usable on nights with exceptionally steady atmospheric seeing. For most nights, a 6mm or 8mm eyepiece will provide more consistently sharp views.

The 6mm eye relief is very short and is a typical trade-off for high-power eyepieces of this design. Most eyeglass wearers will not be able to see the full 50° field of view while wearing their glasses. To see the entire field, you will need to remove your glasses and press your eye close to the lens, using the telescope's focuser to correct for your vision.

Higher magnification spreads the same amount of light over a larger area, which naturally results in a dimmer image. This is a fundamental principle of optics. The Omni 4mm's multi-coated lenses help maximize light transmission, but the high power will always produce a less bright view compared to an eyepiece like a 25mm or 32mm.

Yes. The 1.25" barrel of the Omni 4mm eyepiece is threaded to accept standard 1.25" astronomical filters. This is useful for adding planetary filters to enhance specific features (like using a blue filter to see Jupiter's Great Red Spot) or a neutral density filter to reduce the glare of the Moon.

Not being parfocal means that when you swap this eyepiece with another from the Omni series (or another brand), you will need to significantly readjust the telescope's focus to get a sharp image. Parfocal eyepieces require little to no refocusing when changed.