Help for lens alignment/ Centration

Tran Le Quoc Nam wrote:

Hello friends,

I have a project to build a DIY telescope objective (A doublet) for astrophotography, but I encounter the problem with high precisely optical alignment of 2 lenses (An air-space doublet), therefore, I am writing for helping on aligning with a suitable budget (For any ideas, methods, tools, etc.)

I have used the mechanical alignment such as a metal barrel (Aligned based on the diameter of 2 lenses) and the result did not satisfy.

you might  need further adjustment flexibility, perhaps with adjustment screws, to allow further adjustment. individual lens centering. Compensation of slight tilt for each lens mount.

You could help basic observation of improvement in a setup with either a Siemens Star or a "point" light source being imaged while you adjust.

There are a bunch of things that you cannot correct with such adjustment. For example a misalignment of the two polished lens sides.

At best you have an achromatic lens pair, which you can align according to the design calculation for the objective.  Since a telescope is a long lens, you will probably notice that the achromatic level of correction leaves a significant amount  of longitudinal color aberration. I think you have no chance to arrive at a "really good" lens in this way. It is more an educational experiment. Lens design is a challenging task involving many parameters of a lens system involving 10 or more individual lenses in current commercially available tele lenses. And the glass properties, with primarily the refraction index and its dispersion, depends on the availability of glasses which significantly complicates the optimization (google Schott glass catalog or the like). The production of uniform optical glass, the precise polishing and anti-reflection coating of the lenses is another daunting task best left to experienced companies.

So assembling your own (refractor) telescope is mostly a fun educational enterprise.

Some people build mirror telescopes themselves. There may be clubs around helping with this: polishing a useful mirror. The glass oculars are usually bought complete by these people.

My personal viewpoint about astro-photography:  Most everybody on your side of the globe can basically see or photograph the same object. So there are always photographers around with more resources to photograph your object. You can see excellent examples of such photographs on the NASA website and on others. You can challenge yourself to see how far you get in photographing things that everybody can and with better equipment. Or you can move on to photograph personal things and people of your acquaintance.