I have acquired a cemented 80mm achro objective lens in a mount - it came from an old spotting scope.
I would like to use the lens to make a rich-field astro scope but I'd like to know it's focal length as accurately as possible - is there any straightforward way of working this out ?. I would guess it to be somewhere between f5 / f6 but I guess I'll need to know more accurately than that.
Thanks.
John,
England
I would like to use the lens to make a rich-field astro scope but I'd like to know it's focal length as accurately as possible - is there any straightforward way of working this out ?. I would guess it to be somewhere between f5 / f6 but I guess I'll need to know more accurately than that.
Thanks.
John,
England
