Three Fathoms Observatory
Stewart Yeung

 Current Special Feature



Artifacts in Flat Image

after adding Pyxis to the image train (with the focal plane shifted outward)

Note the arc-shaped bright areas at the top. This bright arc changes in position as the rotator is at different position angles.



The Previous flat image before adding the Pyxis :

The problem was diagnosed as internal reflection inside the tube of the image train, compounded by the skewed alignment of the mechanical axis after adding the rotator.

The problem has been solved by two remedies :

1. applying flocking paper to the inside of the image train tube to mitigate internal reflection

2. centering the mechanical axis by adding shim to the attachment between the rotator and the image train, in a trial and error fashion.

The result :

As before, illumination is still uneven, but it can be flatted out completely.


Thanks for advice and tips from Terry Tuggle andWolfgang Renz.




Flat image at PA180deg as light frame reduced with Flat at PA240deg

Note that the differences (dusts shadow and illumination patterns) are obvious, not subtle.