Meade lx65 8" (ACF) vs Celestron 8se

[HappySkeptic]

The AVX mount was delivered yesterday. Now if Explore Scientific would just ship the damn telescope.

Question about filters. What light pollution filter for viewing, and where does it go?

Nice looking mount!  Yeah, I hope you don't have to wait too long for the refractor.  Covid is messing up Chinese manufacturing.

As was mentioned, filters screw into the barrel of the eyepiece.  For 1 1/4" eyepieces, you need that size filter.  For 2" eyepieces (if your focuser and diagonal are of that size), you need a 2" filter. 

The exception is if you use a 2" focuser and diagonal, and then use a 2" to 1.25" adapter before your 1.25" eyepiece.  In that case you have the option of using either size filter, as the adapter is usually threaded for 2" filters (though you have to check that - not all are), or a 1.25" filter could go on the eyepiece.

As for which filter -- get a good one, or don't use one at all. Good ones are from Astronomik, Televue and Lumicon (get their new models. Televue's models from pre-2018 were not good, and old Lumicon were "mostly" good, but a few years they had a bad manufacturer). There may be one or two other good ones.

As for which type, go for narrowband UHC (Ultra High Contrast) filters. These pass the OIII and H-Beta lines in the green. They are often 22nm to 27nm of band-pass. I just bought the Televue one because it doesn't include any pass-band in the Red (which can give red halos on stars). I have an old Lumicon one, but I didn't like the red halos. NOTE: Don't bother using filters for galaxies or star clusters. These are for nebula only.

There is also the OIII (Oxygen-3) filter, which is amazing for certain supernova remnants like the Veil Nebula. The problem with them is that they dim the field a lot, making them better for larger aperture scopes. Yes, you can use them on yours, and it will make the Veil Nebula awesome, but you will want to use it only at low power. The OIII filters are recommended to be used at 3.5mm exit pupils or larger, which on a 4" scope would be 29 power or less. You might use a higher power on some small planetary nebula, however.

Overall I use an OIII the most on my 16" telescope, as it has the light grasp for it. But most people recommend the UHC for a refractor of your size.