Sunday, May 7, 2017

Cheshire Eyepiece

The Cheshire eyepiece is the simplest telescope collimation device. There are plenty of makeshift solutions, like the film canister with the hole in the cap, but the best guide for making a decent one along with all the underlying theory can be found here. One day I've decided to shim my Astroscan front glass to improve the collimation and 3D printed my own Cheshire.

The construction looks very simple, but there are several important features making the collimation and the printing easier.

The eye hole has a conical depression which makes the edge of the hole sharp and short. That also helps to avoid small hole deformation during 3D printing and makes it easier to shape with a knife later in case of imperfections printed in the light path. It's 2mm in diameter. The depth is equal to the height of the stop flange, so the resulting diaphragm is placed exactly at the level of the focuser's drawtube rim.

The front side has a smaller hole calculated to match the Astroscan's focal ratio 1:4.2, so when you look through the hole, the main mirror's edge is close to the hole edge.

Inside of the front ring I've made 8 holes for precise insertion and fixation of crosshairs' wires (see the 3D model below). I have a spool of thin tungsten wire which is the best material for the task.

Inside of my Cheshire, I have several concentric steps around the center hole, which are helping with other elements of the image alignment. The steps are necessary to improve the contrast of the concentric structure in the side light.

I've used the bright semi-transparent PLA material so I don't need any side holes for lighting the hole for visibility of its reflections. It works in the daylight or with the red flashlight shining at it from the side.

Тhe heavy duty packing tape (first image) is used for the tight fit in the slightly wider than a standard 1.25" Astronscan's EP draw tube. So this Cheshire can be used in a normal draw tube as well.

Printed it out of PLA with 0.2mm layer height after one test print to adjust 1.25" EP diameters

Full screen interactive 3D Preview.

No comments:

Post a Comment