A new toy

Firstly many thanks for the kindness shown to yesterday's sunrise shot. It was quite popular.

Apologies for this image, i'm feeling decidedly peaky as a gall stone passes slowly through the bile duct. Kept me awake most of last night and despite going to work I have been little use to anyone today.

I've acquired a new toy for photography, which requires careful alignment to work properly. This is a view through the alignment tool. I know to some this will be very familiar but how about others, any ideas what it will be used for?