Three Fathoms Observatory
Stewart Yeung

 Current Special Feature



Placement of Lepus Reducer in the Image Train

As the reducer is designed for a backfocus of about 105mm, tailor-made adapters are deployed to overcome the problem of a long image train.

Instead of the usual placement inside the Pyxis rotator, the reducer is inserted between AO-8 and Pyxis Rotator 2", using tailor-made adaptors.


The Resultant Long Image Train

From Left to Right : ST-10XME > CFW-10 > AO-8 > Lepus Reducer >Pyxis Rotator >Moonlite Microfoucser




The first successful astrophotos after installing the reducer : M33



Though the FOV of 20"x13" is not yet able to cover the whole galaxy, the spiral arms would not be discernable if not for the reducer.


Another successful image : M1




Quality of image examined under CCDInspector






The Trade-Off

This setup is in a precarious state, requiring meticulous re-centering of the light cone after each mounting/remounting.

Every time I re-mount my image train, there is misalignment of the image cone :

The Flat-Field


The vignetting CANNOT be elimated even after applying the flat-field.

This vignetting arc rotates with the rotation of the field of view, implying that the scope side adapter is misaligned,
and/or there is a tilt of the image train due to the weight of the instruments.

Due to sagging and misalignment at the various attachment points
in the long image train, the light cone missed the center of the CCD.
Since the light cone behind the reducer is just about the size of the CCD,
a slight deflection would give rise to obvious vignetting at the edges.

It’s not just slack in one particular component, but all components in the whole long, heavy image train contributing to the misalignment.

The remedy is to re-center the light cone on the CCD by carefully adjusting the
tilt/alignment screws of the MoonLite focuser. This is a time-consuming tial-and-error process.



Terry Tuggle's advice has helped me in solving the vignetting problem.