Three Fathoms Observatory
Stewart Yeung

 

 

 

 

 

 

 

                                                                                                                                                                                     Current Special Feature

Special Feature in August 2008

Image Quality Series No.2

 

Further investigation yields the following results :   

1.    Rotating the focal reducer/camera assembly 180°,  the star elongation shifts from

the 4th quadrant to the 2nd quadrant.

2.    Taking out the focal reducer and re-collimating the native F/10 system, the star elongation mitigates noticeably.

3. The issue of skewed optical path was addressed by adjusting the optical tubing assembly

until flat-field images look concentric. The elongated stars at corners remain there.

4.     In the native F/10 system, shifting the focal plate of the CCD further away from the back plate also seems to help.

The original focus is around 13000 count in Robofocus (relative to the farthest OUT position count of 25000).

Adding an extension tube increases the back-focus to a less skewed count of around 15000.

5. Collimation and thus image quality deteriorates after significant telescope swings.

    

 

Preliminary Conclusions :

1.    As evidenced by the result of rotating the camera assembly, the image distortion is correlated to the position relative to the

primary mirror/corrector assembly.

2.    The star elongation is off-axis distortion in nature so it is most prominent at corners which are farthest away from the

center of the optical path.

3. Optical path perpendicularity is not the major cause of the off-axis distortion.

4.    This off-axis distortion is the result of the interaction of the camera/focal reducer and the primary mirror/corrector assemblies.

Mis-collimation in the latter is accentuated by the former.

5.    Precise collimation is disturbed by primary mirror shift in the SCT system.

 

 

MY SOLUTIONS TO THE IMAGE QUALITY PROBLEM

 

1.    A SHORTENED NOSEPIECE HAS BEEN ORDERED TO DECREASE THE FOCAL REDUCTION RATIO TO AROUND 0.7X (INSTEAD OF 0.58X)

2.    RE-COLLIMATION AFTER EACH CAMERA ASSEMBLY MODIFICATION.

3.   TO OPTIMIZE THE SCT OFF-AXIS OPTICAL PERFORMANCE

 BY SHIFTING THE BACK-FOCUS OUT THUS DECREASING THE SEPARATION BETWEEN THE PRIMARY MIRROR AND THE CORRECTOR.

4.    ULTIMATE SOLUTION TO MIS-COLLIMATION ALSO ENTAILS FIXING THE MIRROR SHIFT PROBLEM.

MORE INVESTIGATION TO BE UNDERTAKEN IN THIS ASPECT SOON.

 

 Astrophotos comparing the image quality in the F/5.8 and Native F/10 (Re-collimated) systems will be posted below soon.