WHAT SIZE DO I ORDER? The application table below lists sizes by clear aperture (filter size) and outer cell wall inside diameter (I.D.) The I.D. of the cell slips over and around the O.D. of the telescope tube or lens holder. If you do not see your brand and model, simply choose the closest outer cell I.D. size larger than the outside diameter measurement of your instrument. Example: part # 4250 (4 1/4") would fit O.D. measurements from 4" to 4 3/16". Do not choose a size the same as your O.D. since it will be too tight and not allow room for a felt lining. Ample felt is included for custom fitting.

When attaching the filter to your scope, it must be able to stay attached when the telescope is pointing downward. If not, you can use the supplied felt liners to ensure a snug fit.

ARE THE FILTERS COMPLETELY SAFE? All of our 2+ & Black Polymer filters are completely safe for unlimited visual use. If handled with care and common sense, all will last a minimum of 20 years to life.

CAN THE TELESCOPE OPTICS GET HOT OR DAMAGED? No, all the heat and intense light is blocked before it can enter the telescope. Your telescope may get warm from sunlight shining on it, not from the small amount of visible light transmitted through the filter.

WHAT DETAIL IS VISIBLE? Sunspots and surface granulation are the two main features. The only detail that cannot be seen with a standard filter (commonly called white light filter) are prominence and flares which require our highly specialized H-Alpha filter system.

FULL APERTURE VERSUS OFF-AXIS (REDUCED APERTURE? For telescopes up to 5" aperture, off-axis is not an advantage. Larger apertures suffer more from daytime atmospheric turbulence. This turbulence is magnified by the aperture. Perfect daytime "seeing" only occurs about 3% of the time but having a full aperture filter during such rare events can reallly make a difference to how much you can observe. But for most of the time, "off-axis" reduces the aperture and increases the focal length thus reducing the turbulent effects. We recommend and only offer off-axis for 10" and larger telescopes. The solar image is not darker with reduced aperture as we allow for this in the coating density. When viewing through the eyepiece, the field of view is not reduced; it looks the same as full aperture. If full aperture for 6" to 8" is still preferred, it can be stopped down using a simple mask as conditions dictate.

Options for Off-Axis solar aperture filters are listed below: Delivery may be from 1 to 2-weeks. Please select the model you require. Freight is approx $12.00 - Simply email us here and enter the model number you require in the message.