The Baader UV / IR Cut 2 "filter is a very popular element for blocking infrared and ultraviolet radiation. It is even necessary for astrophotography with the use of lens telescopes, reducing errors in the correction of the lens for the IR and UV band. As a result of cutting off both ends of the spectrum, we obtain a much better sharpness of the photographed objects. The filter is made with the use of the latest technologies, has a coating reducing glare, can be combined with other filters as well as placed anywhere on the optical axis. The transmission of light in the transmission passes up to 98%. From the red side, a very sharp cut-off takes place at a wavelength of 690 nm. It is worth noting that the filter does not block the H-alpha line located at the red end of the spectrum. On the violet side, radiation of a wavelength less than 400 nm is cut off.
IR/UV cut filter is considered a must be filter in most refractors. It will counteract so called "star bloating", which is due to increasing chromatic aberrations on the fringes of the visible spectrum. Especially important for ED/APOs, but also recommended for reflectors utilizing dedicated correction lenses, like coma correctors etc.
In any case this inexpensive filter is an easy way to cover a camera sensor or even better a DSLR mirror and sensor. It makes cleaning easier and maintenance less required. Also, a typical DSLR in a reflector... is simply open to the outside world.
Many forums and posts on the internet will tell you that "you should use an IR cut filter whenever you are not using other narrowband or multi-band filters".
As an example a great comparison screenshot from the equally great ZWO ASI FB group, taken with an apochromatic refractor.
(credit: Carl Björk @ZWO ASI FB group)
Happy star-hunting and clear skies!