1.25" filters are usually between 27 and 28mm clear aperture. They are also mounted.
Actually less. The filter thread is 1.125x42tpi, that is 28.57mm outer diameter and usually leaves about 26mm. The (unmounted) 31mm wheels usally have (also) a thread for the 1.25" filters, so that limits their clear aperture to 28mm.
The key thing to recall is that the sensor to filter distance on the 7 and 8 pos FW's by ZWO is 10mm, but that on the new ASI2600MM and new 7-pos custom-matched FW, it is longer (not sure how much).
Removing the tilt plate, with the bolt-on wheel you end up with
~22.5mm. Depends a bit on the filters.
Edit: Stupid me . Also there it's the cutout of the wheel that limits the aperture, so the relevant distance would be around 20mm....
Whatever you buy, if you can't afford Chroma or Astrodon, I recommend stretching for Astronomik if you can. In particular their Deep Sky RGB are superb.
+1 on the Astronomiks. I use the 31mm ones with my 1600. I had considered Antlias for the 2600, but found no reason not to take the Astronomiks again (they are only ordered though, don't have them yet).
For a refractor, I'd take Chroma over AD because of the higher blue cutoff.
And for the TO: Yes, I'd (strongly) suggest to go for the 36mm. We'll see how long you can withstand to buy the 2600 (I couldn't)
Edited by Der_Pit, 12 March 2021 - 06:20 AM.