Inside Focus

Inside Focus

Outside Focus

Outside Focus

Eyepiece: 6 mm Zeiss Abbe ortho EP (540x magnification)

“This is a triplet airspaced Fluorite APO, where the Fluorite is used as the center element. It is my
personal telescope, which I got as a handselected sample in the early 90'ths, after my first
sample did not pass my expections. The startest shows a better quality than the Zygo testreport. Probably because the lens does not have the best uniformity and shows in this report some smaller zones, which are not detectable in the above startest. The colorcorrection of this lens is far better as we see on the above images, maybe the CCD camera or the ortho eyepiece used for eyepieceprojection gets in some trouble with such a fast system. The lens is smooth and shows an overal good spherical correction. On some starparties in a side by side comparision when observing Planets and Double-Stars many users have confirmed, that this lens shows the same razorsharp images as a handpicked Zeiss APQ 150 and might even beat some other famous 6" ED Apos. To see such small differences again very high powers are necessary. At the time when I bought this scope there was nothing better on the market,  today the choice is larger and one could obtain the same quality for less money”

Markus Ludes (APM-Telescopes)

Note: Obstruction
This 6"  refractor shows an artificial obstruction image due to the secondary of the Maksutov Newtonian collimator. The obstruction is about 37% in diameter.
 

Although the leftmost image of this refactor was made a large defocus-setting and the artifical central obstruction clearly is not centered I decided to try and simulate this telescope. A central obstruction of 38% was used in the simulation. The large amount of defocus (>30 waves) clearly makes simulation more difficult. The simulated image therefore shows traces of undersampling (distorted edges). If we forget the amount of coma that is present and simply use the LSA/HSA mix as an estimator of the wavefront the total wavefront is 0.274 PV and 0.078 RMS (at 550nm). These estimates are not as fine as they can be the coma effect (look especially at the simulated image at the 3o clock and 9o clock features. At 3 o clock (90 degrees) 4 distinct inner rings are visible and at 9 o clock 5 inner rings are visible.

Cor Berrevoets