It's pressure. It's a large black cylinder hanging off to the side. I just used my thumb as example of how I interact with it.
The double stack module is tilt however as it has the tiny brass gear off to the side. Haven't double stacked yet.
"all the way in" means just that. When you turn the pressure tuner, the large black cap will either move inward towards the scope, or outwards away from it. When you tighten it inward, less of the brass threads will be visible. It will become increasingly difficult to turn as you bring it in, and eventually it will bottom out. There is no risk with bottoming it out, and you'll know when you do because it will go from difficult to turn, to impossible to turn.
When you move the tuner "all the way out", you'll eventually remove the cap entirely. This "burping" of the etalon is actually a step that is called out in the manual since it exposes the cylinder to atmospheric pressure and is a way to "reset" the etalon. If there is a tiny leak, and you leave it screwed all the way in, you won't be able to build enough pressure to bring it on-band until you let more air in some how, and that is what "burping" the etalon accomplishes.
That said, every etalon is different. In my case, if I burp the etalon, it comes on-band literally 3-4 turns in. By time I even get halfway in, the etalon is completely offband and I'm able to see sunspots clearly.
If you are using a camera, one thing you can do is intentionally defocus the sun to remove plages and other bright features, and then tune the etalon while watching the live histogram. You should see the peak brightness of the histogram drop to the low end. Eventually as you continue to tune the etalon, the peak brightness will stop moving downwards and start moving back up. Right at the minimal point is where you have maximized the attenuation of the photosphere and your etalon should be on-band. Simply focus and enjoy the view.
Edited by jwestervelt, 07 May 2021 - 12:53 PM.