How do I determine the size of filter to order?
- The filter slips over the sky end of your optical tube. You must measure the outside diameter of your optical tube. To measure the diameter use a ruler, tape measurer, or caliper. You want to measure the widest distance across the end of your telescope. Measure from the outside of the casing on one side, across the center, to the outside of the casing on the other side.
- Order the solar filter size that is just larger than your measurement. You are comparing your measurement with the solar filter inside diameter (ID).
For example: A measurement of 4" (102mm) across the end of the optical tube would require a solar filter with an inside diameter of 4.25” (114mm) because it is the size just larger than your measurement. Therefore you would order a solar filter with the part number of SF425.