qianbo I want to use this filter as an ERF.

This is what I get qualitatively for a couple of "IR-UV cut" filters.

(I have never done a more precise measurement with my spectrometer, since it is a hassle because I don't own a stable calibrated light source -- I have to use the Sun's Planck curve as my calibrated source :-).

I had used two cheap LED light sources. One at 465 nm and another at 495 nm.

In the table "no" means it is not cutting that wavelength at all. "Partial" means I see some attenuation, but not that much. "Most" means that the beam is significantly attenuated. "Yes" means that I cannot see much of the beam at all.

Notice that I do not see any cut at all with the Astronomik UV-IR Block filter. It may block UV, but at much shorter wavelengths. Probably a poor performer as an ERF.

The ZWO IR Cut filter, cuts a little of the UV at 465 nm, but did not appear to have any effect at 495 nm.

The IDAS HEUIB-II cuts most of 465 nm, but only partially at a longer wavelength of 495 nm. The HEUIB has an interesting characteristics; it is pretty agressive at the UV end. However, so that you can use it mounted on a camera in daylight, it also cuts a small amount of the red/green region (so that red/ green/blue maintains the same ratios).

The Baader "L" filter seems to be most like the IDAS HEUIB at the UV end, but does not have the notch in the red/green region).

The Baader Semi-APO is even more aggressive at the UV end, hence the name Semi-APO -- it attempts to make cheap optics look visually like an APO (no blue fringe around stars). It also uses Neodymium impregnated glass, which is also good at absorbing IR -- that might help you as an ERF. Albeit, absorbing too much IR (instead of reflecting it) might cause the glass to crack.

On the other hand, the Baader Skyglow does not knock much UV. It is kinda like the ZWO IR Cut filter.

The group of 3 narrowband filters (Optolong CLS-CCD, IDAS NBZ, and Radian Quad) completely knocks off UV. And they will still pass Hydrogen line for your intended use as an ERF. The Radian Quad is silly expensive for your use, though.

This set does not include numbers for the IR side, which you also need to take care of (while still passing Hydrogen Alpha).

Other than the narrowband filters, the Baader Semi-APO is the only filter in the above list to signifinatly knock 465 nm (I use them for my guide cameras when I do not use near-IR guiding).

A Baader UV/IR cut might be a good compromise, if you don't want to spend the money for a Baader Semi-APO, and assuming the Semi-APO does not crack from absorbing too much heat.

I did not have to make this decision when I bought my SolarScope SF-50 many years ago -- it came with a matched ERF.

Chen

I just tested the Baader H-alpha filter, and no UV leaks that I could see. Might make a good ERF.

Mine is 7nm wide (many years old), so it should not interfere whatsoever with your Etalon. No Neodymium to absorb heat either.

Baader now makes 6.5nm and 3.5nm versions of their H-alpha, but I don't think it makes any measurable dent to use even narrower H-alpha filters as an ERF, and the 3.5nm one is quite a bit more expensive.

Chen