UV / IR Cut filter 1.25" - cutting infrared and ultraviolet.
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!