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Spectrum 40.5 mm Threaded Optical Solar Film Filter - ST-40.5MM

SKU: SPR-ST-40.5MM
To ensure that the solar filter you buy will fit properly, please make sure your camera lens will accept a 40.5mm threaded filter. Thanks.

Spectrum 40.5 mm Threaded Optical Solar Film Filter


This Solar Filter from Spectrum Telescope is designed to thread into any 40.5 mm camera lens. It contains a high quality ultra thin (.002") black plastic polymer film in the aperture of the filter. This film is extremely durable and will last for many years provided the included directions regarding care and use are accurately followed. A single layer of absorptive filtering material dispersed throughout the substrate is used in the construction of the ST-40.5MM Solar Filter. This filter is completely safe for direct viewing of the Sun as well as solar photography since it is a Neutral Density 5.0 filter and blocks 99.999% of incoming sunlight.

This filter's full aperture allows for less atmospheric turbulence, thereby enhancing daytime viewing. The clear aperture has been hand selected and meticulously scrutinized to make sure that only the finest, high quality filter is put on the market. Whether used in conjunction with a telescope or for taking photographs, the 40.5 mm Thread-On Solar Film Filter presents the Sun in its organic yellowish-orange form. The Spectrum ST-40.5MM Filter arrives packed in a foam-lined box placed within a plastic box to safeguard it during transit. The filter box can be used for storing your filter between uses.

What About Your Finderscope?

Adding this solar filter to your telescope will adequately filter the light coming through your focuser, but have you considered what you’ll do about your finder scope?

Here are some suggestions for dealing with your finder that will assure you do not damage your eyes (or anyone else’s) by looking at the Sun through your telescope’s unfiltered finder:

  • Remove Your Finder. Ditching your finderscope when solar viewing will remove the possibility of someone accidentally looking through it to spot the Sun. Of course, not having a finder makes centering the Sun in your eyepiece more difficult, but with practice, it can be done. Set the mount down so the telescope is pointing in the direction of the Sun. Put the main solar filter on so you can check your progress, and then move the telescope around until it casts a shadow that produces a nice, sharp silhouette of the scope. The OTA will look circular. Now move your telescope up or down with your hand controller, slow motion controls, or very carefully by hand if required while looking through the eyepiece. You will not get a warning when you are close, but you should be able to align with the Sun using this method. Practice ahead of time before any big event, like an eclipse, to make sure you’ve got the process down.
  • Filter Your Optical Finder. This solution is not recommended for a reflex or red dot finder since it is too easy to accidentally look around the window. If you have one of these finders we recommend you either remove it or replace it with a dedicated solar finder (see below).

    To make your optical finderscope safe for solar viewing you’ll need to buy a piece of Baader Solar Film for Visual Use. This film comes in different size sheets and cuts easily with scissors, allowing you to make your own filter. You can go super low-tech and use a rubber band to hold a piece of the film firmly around the finder or you can get fancy and build your own slip-on solar filter. However you attach the solar film, you need to make sure there are no light leaks at all and that it doesn’t accidentally fall off when you move your telescope around. Any unused film can store flat between two pieces of cardboard and will keep for years. It is nice to have around, just in case of a solar emergency :-)

  • Buy a Dedicated Solar Finder. Check out the Tele Vue Sol-Searcher Solar Finder. This special finder can only be used when observing the Sun, but it works very well. The Sol-Searcher is reasonably priced and can be attached to your telescope with Velcro or with #10-32 screws (user supplied).