Hello everyone,
I recently got the Sky Rover 28mm UWA, which is basically the same as the Astro-Tech 28mm UWA, and I have seen prominent chromatic aberration at the edges when viewing Venus and the Moon, I know this eyepiece is not meant for such objects but I used it for testing. In the center the views are perfect, it is just around the edges.
Can any users share their experiences with this type of eyepieces, is it normal for wide fov EP to show some chromatic aberration at the edges? Or is my EP defective?
I am using the EP with an 8” f/5.9 reflector.
Thank you.
There are 3 main chromatic aberration problems in the outer fields of widefield eyepieces:
1) the very thin line of color right at the field stop. This is usually blue, but can be other colors. It is usually seen only when something bright leaves the field. This is not unique to ultrawides, and can be found in eyepieces of many apparent fields, but it is in ultrawides, so I mention it.
2) lateral CA, or prismatic smear. This is caused by a slight offset in the position of focus for each color. It is similar to the bending of light by the Earth's atmosphere, which makes the sun red as it sets.
You see it primarily on brighter objects, where one side is bluish and the other side reddish or orange. In well-corrected eyepieces, this is a minor issue unless you enjoy looking at bright stars right at the edge.
3) Chromatic aberration of the exit pupil, or CAEP. This is when the ultrawide eyepiece has a strong tint to the outer, say, 10° of field. This is sometimes referred to as the "Ring of Fire" because it is usually yellow, orange, or red-orange.
It is not usually seen at night and the low power ultrawides that have it are usually not used on bright objects that show it, but it makes the eyepieces unusable for daylight viewing. As I understand it, in order to keep the low power ultrawides from being too large and too heavy, the number of lenses is reduced. Additional lenses could correct the issue, but are not used due to weight, size, and cost. This affects eyepieces from Tele Vue, Explore Scientific, Pentax, Astrotech/Sky Rover/Stellarvue (and all the other labels this eyepiece is sold under), and a few others, all ultrawides.
None of these are indications the eyepieces are defective. They merely represent the compromises present in different designs. There is one widefield 30mm eyepiece with virtually zero of these, but it has 9 elements. Had the eyepiece you bought had 9 elements, it would have been twice the price and probably twice the weight. Use it as intended--for wide fields at low power--and not for planets and moon, and you'll enjoy the eyepiece.
Edited by Starman1, 25 April 2025 - 08:50 AM.
