Last night I observed with my vintage Celestron C 90 under the light polluted sky, located in the suburb of a great Kasakh city (where I spend my holidays with my girlfriend). Especially the southern and western regions of the sky were brightened. But the air wasn't too turbulent, so I decided to observe some double stars.
I used the original 0.96“ eyepieces and the Celestron 0.96“ 45 degree (amici-like) prism, but the performance on the stars wasn't very well... at magnification of 111x and more (with the 9mm and 6mm orthoscopic eyepieces) there was little rainbow-like colour, very little spikes and a faint halo around the stars - I don't think this was caused by the eyepieces, as I have used these Japan made eyepieces with other scopes and they have ever worked very well. So perhaps the prism could be the problem, or the little MAK itself. But the startest in direct focus (without any star diagonal, prism or mirror) wasn't so bad - perhaps very little miscollimation (but it is not easy to make this better on a C90), without a relevant coma. But because of more comfortable position when observing, I needed the Celestron prism.
But it took a lot of time to observe the following only four double stars cause of these problems with image and the Celestron ring focuser, that doesn't move as easy as needed (nevertheless I cleaned it from ugly clue-like China grease and put on Vaseline only a few days ago). And of course I wanted to make sure, that I've seen the image of split doubles indeed.
1) Castor (alpha Geminorum): not so difficult to split at magnification of 80x (12.5 mm Ortho), better to see at 111x (9mm orthoscopic eyepiece).
2) Algieba (gamma Leonis): more difficult to detect the double, magnification of 111x and above needed. And I had to focuse very properly, not so easy with the hard moving focuser.
3) Izar (Pulcherrima, eta Bootis): rather difficult, took a lot of time, cause it needed higher magnification of above 111x and I changed the eyepieces twice. I was able to see the double, but unfortunately I hadn't any impression of the star colours.
4) last but not least I took a look at pi Bootis, this double star was little easier to split. The 12.5mm orthoscopic eyepiece (80x) was sufficient, also nice to see at 111x, but the stars were then little faint.
Then I stopped observation because of worse air condition (atmospheric haze), but it was already one o'clock and temperature below 0 degree C.