Acceptable amount of movement in an AP Lens Cell

I recently purchased an Astro-Physics 180EDT f9 scope and noticed that the lens is somewhat loose in the cell. I have a few other AP scopes have not noticed this issue. At least not to the same degree.

I have talked with other owners and this seems to be a common situation but I would like to understand just how loose the lens should be.

I have another worry relating to this issue and that concerns vibration during transportation. I would think that a loose lens would suffer to some degree from the slight vibrations its subject to during transport.

Can anyone enlighten me.

The lens is actually contained, even though it can float loosely inside the aluminum cell. The glass is resting on cork pads in the back, and these may have compressed a bit with time. The front retainer cell is not actually in contact with the glass, and should not be. It is there primarily to prevent the lens from falling out when the scope is held upside down. The reason why the lens is not tightly held is because the glass contracts slower than the aluminum cell. If it were the same size as the inside diameter of the cell, and the temperature dropped by 20 degrees, the glass would be severely pinched.

In a normal airspaced lens, the front retainer must put some pressure on the glass to keep the airspaces from shifting. This 180 lens is oil spaced, and it cannot have any pressure on the glass, either from the side or from the front retainer. It must only rest on the cork pads on the back. In this respect it is quite the same as a Newtonian mirror, which also cannot have any pressure from side or from the clips in front. Pressure from the retainer ring will cause the lens to distort with resultant astigmatic images.

There is no worry that a shifting lens set will cause any kind of optical problem. The shift is very minute compared to the diameter, and consists of perhaps .01 to .02 inches at most. Under use in the sky the lens will settle nicely on the lowest diameter and the cork pads on the back. The three elements cannot move relative to each other since they are oiled together, and even a team of horses could not pull them apart. Air pressure at 14lb/sq inch sees to that.

When those lenses were made originally, we were content to let them float freely in their lens cells (same as your normal Newt mirror). Today a typical oiled lens is glued into the cell in several places with a very soft rubber cement. Can it be done with this 180 lens? Yes, but that will entail quite a bit of work and time for me, and I suspect quite a bit of money for you.

Roland Christen