Home > Consumer Reviews > Orion 40mm Sirius Plossl 1.25" Eyepiece

The reason to buy a 40 mm is to go to LOWER magnification to have a brighter view of deep sky object. This eyepice will give you (40/32) square (e.g. 1.5 times) brighter images of galaxies.
As galaxies are big enough not to require much magnification (most M list are >20 ' - compare with suns 32' diameter) lower magnification is actually better. Field of view is not that important if you can't see it because it is not bright enough - is it? I mean is better to see smaller galaxy then none.
NB. Depending of the F ratio you may not be able to use a 40mm though - see books - can't remember right now the rule for that.

The widespread existence of 40mm Plossl eyepieces with 1.25" barrels in most companies' product lines is a mystery.

Most people get eyepieces in this focal length to get the widest possible field of view with a 1.25" barrel eyepiece - yet a 40mm Plossl witha 43 degree apparent field of view shows no more sky than a 32mm Plossl with a 52 degree apparent field of view! (In my scope, they both show about 1.35 degrees of actual sky). But the 32mm will have a wider apparent field of view and somewhat higher magnification. There's really no reason to buy the 40mm Plossl.

But I did, a while back. Now I've learned. This eyepeice is actually of decent quality, although there is some minor astigmatism of the image towards the edge when I used it with my 6inch f/8 telescope (not seen in the shorter focal length Sirius Plossls). All the Sirius Plossls are good values, but get the 32mm version instead.