Re: SEMI-APO???

Semi-APO = cost more than an Achromat, less than a true APO, with color control greater than an Achromat and less than a true APO.

This is a marketing term for scopes that do not make APO standards, but they would like to get more for them than the cheaper Achromats they sell.

Some have color crossings that tend to bring in the blue and allow the red to stray, which fools the eye into thinking there is less color as the eye is less sensitive to red than blue.

If you want an APO, you have to specify that and purchase that specification.
My $.02 8)

Floyd Blue said:

Semi-APO = cost more than an Achromat, less than a true APO, with color control greater than an Achromat and less than a true APO.

This is a marketing term for scopes that do not make APO standards, but they would like to get more for them than the cheaper Achromats they sell.

Some have color crossings that tend to bring in the blue and allow the red to stray, which fools the eye into thinking there is less color as the eye is less sensitive to red than blue.

If you want an APO, you have to specify that and purchase that specification.
My $.02 8)

Yup. I have a WO 66SD Semi Apo (Petzval design) I picked up from ATWB a few years ago. In visual use it is extremely good, but not perfect. On bright objects there is some slight color fringing, not as obvious as an achro...but still there and possibly an annoyance for some. In imaging, there is a very noticeable violet fringing to brighter stars, this can be processed out to a degree in my experience, but overall the image suffers. I really like this scope, but given the amount of $ I am willing to spend or am allowed to spend (CFO ) Semi Apo works well enough for imaging. But for visual, not so much as the aperature is lacking...Just my $.02 ((plus Floyds $.02 means you are ahead $.04 )