If choosing between the C5 and a SkyWatcher 127mm Mak, keep in mind that the Mak has been widely reported on CN to actually only have 119mm aperture. In these small sizes millimeters matter.
This is misleading.
The total aperture diameter of the Synta MC127 is 127 mm whereas the total aperture diameter of the Celestron C5 is also 127 mm.
The difference is in the central obstruction. The MC127 has a central obstruction diameter of 39 mm whereas the C5 has a central obstruction diameter of 52 mm.
The clear aperture diameter (t-stop effective aperture diameter) for the MC127 is 121 mm whereas the clear aperture diameter of the C5 is only 116 mm.
So the MC127 has an additional 5 mm of clear aperture diameter as compared to the C5 due to the smaller central obstruction.
The contrast-adjusted aperture diameter for the MC127 is 88 mm whereas the contrast-adjusted aperture diameter of the C5 is only 75 mm.
So the MC127 has an additional 13 mm of contrast-adjusted aperture diameter as compared to the C5.
The relative central obstruction diameter of the MC127 is 31% whereas the relative central obstruction diameter of the C5 is 41%. What this means is that the MC127 will have 1.3 times better contrast than the C5. This is particularly important for planetary, Lunar, and Solar viewing.
The t-stop of the MC127 is about t/13 whereas the t-stop of the C5 is about t/11. The f-stop of the MC127 is about f/12 and the f-stop of the C5 is about f/10.
Overall, the MC127 is a much better optical system than the C5. It has a smaller central obstruction, a larger aperture, better contrast, and lets in more light. There is also the fact that the C5 suffers from comatic aberration whereas the MC127 is coma-corrected.
The only advantages of the C5 are its smaller size and weight, the slightly faster t-stop/f-stop, and it is easier to use focal reducers with.
The advantage in t-stop is minimal: exposures are approximately the same between the MC127 and the C5 (only a 1.4X increase in exposure time is needed). Between needing 1.4X longer exposure times versus not having coma correction and a 1.3X loss in contrast, it is better to have the coma-corrected optics with the improved contrast, especially for Terrestrial, Lunar, and Solar viewing. Coma correction is less important for planets or "faint fuzzies".
For Solar System observing, the MC127 is the better choice. However, if you are primarily looking for DSOs, then the C5 can be the better choice, especially when combined with a reducer-corrector.
Edited by Nicole Sharp, 05 October 2023 - 12:44 PM.