Barlow test

The most common problem with cheap glues is the actual crooked glue itself, which gives a traffic light output (transverse chromatism). The problem can be seen by both the (artificial) star and the displaced Newton rings. The ring test is faster and should be done this way: take a green laser pointer with a diffuser (matte plastic or a couple of cloths) and by tilting the pointer, find the position where the rings are visible. It is advisable to do this in a dark room because the rings are quite faint. Normal barlow will have the rings in the center. On the left the normal, on the right the curved glue:

However, you can try to use the latter as an ADC by turning it to a position where it reduces atmospheric dispersion.