With any booster, the key would be to have a simple attachment so you don't have to hold the booster by hand while viewing. If you do hold it by hand, the shake will negate any benefit from the higher power. I have not used the Swaro 2x booster, but have often used a Zeiss 3x12 Classic as a booster on scopes. It is much smaller and lighter, and consequently it is easier to fashion a rubber ring insert for it so you can simply plug it into the extended eyecup. 90x gives you about the maximum magnification you would ever be able to utilize during the day. Compared with a zoom at 60x or a fixed-power high-mag eyepiece, you lose some brightness and contrast (and a fair amount of field, but since you would use the booster only to check on something specific already in your viewfield, this last is not that relevant). Loss in resolution is only margninal, however, since at these powers it is just about always the limits of the objective's resolving power which you are running up against.
Kimmo
