which filter to choose for astrophotography

I am Bortle 7 (was Bortle 6).   For your 533 mono camera and the FRA400 (I have the FRA300), a HSO set is ideal and also LRGB for bright, wide filed objects.  For the L filter, the type depends on the type of LP in your area.  If you have the new, brighter LED lights that all of the cities are installing now (which I have to contend with), don't bother with any of the traditional LP filters as they won't help much, if at all.  Just get a regular 1.25" LRGB set with IR blocking so you don't need a separate IR cut filter.  As for the brands, I have both Chroma and Antlia.  Don't buy Chroma- they are way overpriced these days.  Antlia is good and Astronomik (since you mentioned them) are ok.  I have never tried ZWO filters, so I can't speak to those.

I use Astronomik MaxFr 6nm Ha & SIi.  I use a Chroma fast F3 for OIII.  They are all excellent. I just purchased  use Astronomik MaxFR 6nm OIIi for better parfocal focusing.  I am confident it will perform well.  The main consideration of the 6nm as opposed to the 3nm in OIII is light pollution gradients and Signal/Noise.  The 3nm OII will show higher contrast with a darker background. BUT, I live in Bortal 7.

Even in Bortal 7,  6nm is fine for Ha. 

The main concern of lower cost filters is consistentancy in manufacturing.  Thay may claim  90% transparency, but that can be substantially lower batch to bacth. And, there is no way you will know, unless you have extensive testing equipment. Quality control in manufacturing cost.  One imager to the next claiming excellent results is to subjective and based on limited equipment use. I have seen extensive testing on YouTube of filters claiming to be in the high percentage,  but testing showed the wave spike of Ha to be so for off center that the filter was only getting 25% of the Ha.  You would never know and assume the object faint, poor seeing or some other issue.

The other issue you see in lower cost filters is star halos. This shows up worst with lower F ratios. OIII is especially prone to halos around bright stars.

LRGB filters are much cheaper.  You might want to consider a set for star color data. SHO creates poor star color. HOO is a little better, but stars color can be captured with 30sec subs and as few as 20 per channel can be adequate. 

Be aware that all this demands a confident knowledge in software post processing.

Lynn K.
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A few other pointers. Pick your filter’s with bandpass (measured in nanometers like 12nm, 6nm, 3nm, etc.) for narrowband (NB) targets (Ha, [OIII], and [SII]) based on your LP and mount’s guiding capes.

If you have a lot of LP (e.g. your Bortle 6) you will want narrower (e.g. 3nm-6nm) bandpass to suppress the LP but the lower the bandpass the less signal you get (but better contrast) - you will have to take longer exposures for (5 minutes or more) which then requires better guiding. If you have a lesser capable mount then you should probably lean towards shorter exposures (2-3 mins) but stack more subs to get more signal. If you were in Bortle 4 or better then 12nm would work.

The other big consideration/luxury would be getting filters that are parfocal. If your filters are parfocal you will be able to set focus for the Ha (filter you will typically use the most) then generally speaking you can leave your focus  when you go to the next filter (usually [OIII]). If your filters are not parfocal then you will need to read up about “filter offsets.” They are not difficult but can be frustrating when you are starting out and new to NB processing. 

If I had to do it again I would go with a quality Ha filter, shoot a lot of Ha targets and get used to processing the subs (lights, flats & dark flats for the 533) first. Then I’d pick up a quality parfocal [OIII] that matches the quality the other filter when you can. I would pass on the [SII] for now - there’s not a lot of targets that use it, at least when you’re new to NB. 

I would wait on the LRGB unless you are shooting the Moon. We are rapidly approaching nebula season and the Ha is the most useful since it’s the most common element/metal in space and you can shoot targets even when the Moon is up unless the target is close to the Moon.

You can combine Ha in Red channel and [OIII] in the Green and Blue channel - aka HOO images. Once you learn how to do HOO you’ll be able to save for the [SII] then go the next level and get collect more data for the previous targets for SHO, HOS, Fornaxx, etc. images.

CS & GB!

kip

I have taken note of your guidance. My current choice is :
Antlia OIII 3 nm Pro 1.25" - due to a limited budget, first purchase
Antlia H-alpha 3 nm Pro 1.25" - due to limited budget, second purchase
second option :
Astronomik Narrowband-Emissionline OIII with 6nm FWHM and MFR coating
Astronomic Photographic Narrowband-Emissionline H-alpha filter with 6nm FWHM and MFR coating
 Astronomik Narrowband-Emissionline SII with 6nm FWHM and MFR coating
or purchase a set of HSO
Astronomik HSO 6nm 1.25'' (M28.5) filter set, 3 filters
https://www.astronomik.com/en/fotografische-emissionslinienfilter/schmalbandfilter-sii-6nm/astronomik-hso-6nm-filtersatz-1-25-m28-5.html

Is this a good choice ?

What is your budget?  I have gotten by for years with optolong which were not too expensive, just see my pics back when i had the 70mm.  The OIII was a tiny bit disappointing if a bright star is in the field but I would do the same decision over again due to price point.  I am looking to finally upgrade the OIII this year to something like Chroma.  1.25" helps too for price.  I would choose that over ZWO but not sure your budget can deal?  If you have to split maybe start with HSO first if you want to shoot nebulas, and LRGB if you want more broadband targets.  Now that I see you are around 70mm telescope I almost entirely shot HSO for first year with the exception of Andromeda/Bode galaxy if you want to break up the purchase.

I prefer the short bandpass Antlia’s - 3nm Ha and 3.5nm [OIII] - since my backyard Obsy is at a  Bortle 4-5 location.

Again, go Ha first - almost ALL nebulae have Ha. You may have to wait longer to buy but you will use it on every NB target. [OIII] is not as common, pure [OIII] targets are rare. 

Can your mount support longer exposures? Do you have good (<1.0 admin) guiding/tracking?

If yes go with the 3-3.5nm. If no go with the 6nm. Both are otherwise good choices given your budget.

A solid mount makes everything easier.

CS & GB!

kip

I decided on Astronomik. I'm buying an HSO and will be learning on these filters for the time being.
The rest of my equipment is AM3, ASIAIR PLUS, EAF , EFW so it's not the worst for guiding.