Why does this large Newtonian telescope's front cover have two or three holes in it? The Michael Bernardo video How to use an Equatorial Mount for Beginners shows a large Newtonian telescope on an equatorial mount.
The cover of the telescope's large aperture shows what looks like three holes each with its own removable sub-cover, but I might be misunderstanding what I'm looking at, perhaps the &quot;central object&quot; is just a handle to rotate the cover when the other two holes are open.
But that raises the question &quot;why two holes&quot; rather than just one. I'm pretty sure that these are not used for this kind of thing but it would be great to know how they are used!
What are these?
These are used to reduce the aperture of the instrument. While you see three in this image, likely only one is removable.
The instrument appears to be a Newtonian reflector. Which means it would have a secondary mirror near the front (and center). So the thing that looks like a knob in the center of the telescope cover is probably just for grip (and not likely removable).
One of the off-axis caps is probably actually a cap. That can be removed.  The other thing that looks like a cap is typically non-removable. It is a holder for the cap on the opposite side (the one that can be removed).
Purpose
If you observe a bright object. Such as the Moon. You can use this to reduce the total amount of light.
Solar filters are large aperture telescopes will often employ the same idea. The Sun is plenty bright. Collecting &quot;enough&quot; light isn't the problem. By using a solid cap with an off-axis (off-axis only because it is necessary to avoid the central obstruction in the telescope) can help limit the total energy entering the telescope to avoid over-heating internal components and damaging the instrument.