Kendrick Part BM-6016-P. Fits tube ODs 258 mm to 342mm We are pleased to announce the addition of Kendrick's Bahtinov focusing aids to our range of imaging accessories. These plastic film masks are rollable, extremely lightweight and attractively priced. This innovative concept was invented by a Russian amatuer astronomer, Paul Bahtinov, and Kendrick is producing these masks with his permission. The Bahtinov mask allows astrophotographers (and visual observers) to get razor sharp focus easily in a matter of seconds. The difference in focuser position between the four pictures shown below is quite small, a few tenths of a mm. The method is very fast and incredibly sensitive. There is no guessing when you are focused - even during times of poor to moderate seeing. Simply put the mask on and it will become immediately apparent if you are in or out of focus. The position of the vertical spike even indicates how far away from focus you are. The masks have a series of slots oriented so they create a unique diffraction pattern that can be used to quickly bring your telescope to focus with an absolute minimum of fuss. These flexible plastic masks are constructed of an opaque black plastic film that is very tough yet very flexible. When rolled up and released, it will revert back to its natural flat state. This makes for very easy storage as they will fit into many accessory cases once rolled up. The 3 plastic tabs at the edge of the mask can be used as an aid in connecting the mask to your telescope per a method of your choice. If the tabs are deemed undesirable, they can be trimmed off.