Bahtinov focus mask
The focus mask is used to facilitate accurate focusing when doing astrophotography.
How to attach the mask to the telescope: Each mask is supplied with a pair of bolts and fitting plastic covers. Just push the bolts through the holes in the mask and slide the covers onto the bolts from the other side. Now the mask can be simply hinged onto the dewcap or the telescope tube. This adaptation is very universal and fits for widely varying tube diameters.