How to Collimate a Refractor that doesn’t have a Collimatable Lens Cell

[Jim L]

My AT72EDII 72mm refractor took a bit of a tumble when I knocked my setup over jumping up to face what I thought might have been a mountain lion but which was actually probably a small mouse foraging in the dry grass. The fall didn’t misalign the doublet lens within the cell, but the threaded on cell was no longer square with the optical tube.

By loosening the cell and rocking it with my hand I was able to achieve proper collimation. Using a set of feeler gauges I found that a 0.35mm gauge placed at the right location would level the cell when the cell was tightened. I marked the opposite high side with painters tape and went to work lowering the marked side using sand paper taped to a piece of glass plate.

I sanded a bit, reassembled the scope, checked my progress using a Cheshire eyepiece, and repeated the sequence as necessary. Five easy minutes later collimation was perfect.

[wookie1965]

If it has a retaining ring you could loosen that and with the flat of your hand smack the outside of the cell all the way around this may centre the glass. 

[Jim L]

8 hours ago, wookie1965 said:

If it has a retaining ring you could loosen that and with the flat of your hand smack the outside of the cell all the way around this may centre the glass. 

Thanks, Wookiee. Most of my achromats work that way, and I use that method often, but this was a different situation.

The doublet glass in this particular scope is held in alignment within its cell with eight grub screws (four each element), and was correctly aligned within the cell. What appears happened to my scope is that the mating surface between the lens cell and optical tube was very slightly compressed on one side due to the sideways impact of the scope on the pavement. I came to this conclusion by observing that collimation went wonky in the instant the threaded in cell seated itself against the tube, and not before.

Back to inexpensive achromats for a bit, misalignment between the focuser, lens cell, and telescope tube is a common problem when the cell and/or focuser are secured to the tube with Philips head screws, like my ST80. The contact surfaces between the tube, lens cell, and focuser often aren’t sufficiently precise, and collimation suffers as a result. Most recommendations are to put a bit of a torque on the misaligned part to bring it to collimation and hope it stays that way when the affixing screws are tightened. I’ve had mixed results using that method and even if collimation is achieved it’s very susceptible to being lost with the slightest bump because the parts aren’t fully seated and in support of each other.

Correcting the mating surface between elements so that the fully seated elements are in correct collimation makes that collimation robust, permanent, and repeatable. It’s not all difficult to do and requires nothing more that what’s shown in the photograph and a Cheshire eyepiece.