Even though I have them both for quite some time, today was the first time ever I did a direct comparison. The Moon was visible from within my city (Cologne), today it's 10th day after New Moon, so I started with the Vixen. As the seeing was surprisingly steady, I then decided to also set up the C5 next to it.
Vixen SP-90M (white) from around 1985-87, all original.
Celestron C5 Classic (white) from around 1993-99, all original.
Baader Eudiascopic 20mm + 7.5mm, Vixen Plössl 10mm, volcano top Ortho 7mm. I keep a Celestron 1 1/4" prism diagonal for both of these scopes.
20mm – Vixen: 50x; C5: 63x
10mm – Vixen: 100x; C5: 125x
7.5mm – Vixen: 133x; C5: 167x
7mm – Vixen: 143x; C5: 178x
In both scopes I changed back and forth mainly between the 10mm and 7.5mm and went along the terminator. I eventually concentrated on the crater Gassendi which was exactly on the terminator. The central mountains were barely lit by sunlight; one of the mountains – the one closest to the terminator – was just a tiny bright dot. This crater is known for all the ridges and rilles on the crater floor; however all that was still pitch black tonight.
The 20mm I only used to find and center the Moon. However it presented a nice full view of the Moon in both scopes. I really liked how the Moon filled out most of the field of view especially in the C5.
Comparing the 90M and C5:
- The slight difference in f/l ratio was readily visible at the eyepiece. The 10mm in the C5 (exit pupil = 1mm) was a bit brighter and mouches volantes even further reduced, compared to the 90M (exit pupil = 0.9mm). Same goes for the 7.5mm. To my eyes, it was a bit easier in the C5.
- Actually I was surprised at how well the C5 kept up. It was in fact much more enjoyable to watch the moon at 125x with the 10mm (1mm exit pupil) in the C5 than in the Vixen at 133x with the 7.5mm (0.7mm exit pupil).
- When I then placed the 7.5mm (or 7mm) in the C5, I first thought that I'd have come close to its limits – but in fact it was just the seeing; in those moments of best seeing the lunar details presented themselves with absolute clarity and sharpness at 167x–178x. So, both scopes could very easily manage all the magnifications. There was no real difference in perceived sharpness. Both scopes were able to show similar details in the central mountains and along the rims of Gassendi; but obviously aperture wins. The sun-lit peak of the central mountain closest to the terminator was a strikingly bright and crisp light point especially in the C5. There's also a long ridge that starts at the crater walls and then spreads around Mare Humorum. The short part of that ridge that was lit by the sun tonight also seemed brighter and kind of "crisper" in the C5 than in the 90M. In the end I'd have to give the nod to the C5 when it comes to the absolute amount of details visible.
- Speaking of tube/local seeing, the C5 was more affected and views in the 90M more steady, but the difference was small. But at 13°C (55°F) local temperature it wasn't a really cold evening (two days ago it was much different!). Maybe under different circumstances, the C5 would have more trouble with that.
- False color: The 90M showed a blueish/purple rim around the Moon, and – depending on fluctuations in seeing, your eye position etc. – also some dark purple haze in deep shadows next to bright, sun-lit details at the terminator. The C5 didn't show any false color; only when I moved my eye well out of the optimal viewing position some was visible which is caused by the eyepieces (I guess).
Overall, I would say today the C5 beat the 90M on the Moon tonight. It's bigger aperture totally outweighs any inherent disadvantages of the SCT design. It also reminded me why I wanted a C5 in the first place – it's still small enough to deliver decent "medium-high" power views without the sometimes endless cooling times of the C8 or also several Maksutov scopes.
I am sure that with according eyepieces the 90M would have even handled 200x or more pretty well, maybe even better than the C5 would – but as I dislike these small exit pupils so much, I don't bother anymore to even try that out.
When using both scopes and their mounts, they are very different to operate, but both are just a delight to handle. The only problem is that the focuser of my C5 has become very rough recently. It feels more like a cogwheel than a nice fluid motion. But no mirror shifting whatsoever that I could notice. I have to restore that somehow. But the simple AC motor of my C5 works, whereas the DD-1 drive of my Vixen doesn't. Well that's the joys of classic ownership, always something left to do