Those perfect evenings come along only a couple of times a year. You should be able to get a couple of weeks of above average seeing during the year with the rest of the year being ordinary to poor. Of course this will depend on where you live, local topography, prevailing weather conditions yadda yadda.
Longest run of poor conditions. I've been able to image only about 8-10 times since last sept (Lostock not included). Plenty of times I've setup and then packed it in 3 hours later. And in that time I had one night that I would have considered good conditions. But even prior to that seeing had been terrible for ages, but I put alot of that down to my specific location and the prevailing microclimatic conditions (sea breezes early followed Katabatic winds later at night during winter)
Robert, Striker and Dennis are in Brisbane about 100 km away from me and they seem to get less cloud cover than I get, or they are more persistent and stay up longer. But when I do get a good night I have less light pollution than them to worry about.
Now I gather you're talking about imaging with the NexImager, and not a DSLR? Conditions need to be much better for planetary imaging than say for wide field to semi-wide field DSO imaging. Another thing to consider it how long are you leaving your sct to cool down before imaging?
If you've not already done this, as an experiment, set your newt out at the same time as your sct, late in the arvo/early evening (protect from sun to prevent heating up
). Hopefully is is a fairly good night. If it turns out not to be, try this again another night. Check that collimation is as good as you can get it.
Let them sit for an hour and check the seeing through each, using eyepieces to give the same or similar magnification (high mag) and the same star. Defocus to see the diffraction rings an notice any movement then focus to try to see the airy disc (check collimation again while you're at it) and again check for image disturbance. Do the same thing each hour and see if there is a difference in the patterns between sct and newt. The open tube newt should equalize with the ambient temp quicker and show less diffraction disturbance than the sct. Doing this I'll be surprised if your SCT mirror is close to ambient in 5 hours, particularly in Canberra. Remember to your trying to
equalize internal/external temps, not necessarily trying to lower your mirror temp. A mirror temp that is lower than ambient has it's own problems as well. Here in Bris and the surrounds we're fortunate to not have really larger day/night differentials most of the year. I reacon this is why bird likes to come up to Rockhampton in May. Most stable time of the year for us here.
Anyway, after trying this a few times you'll soon be able to tell the difference between poor seeing conditions and tube turbulance inside the SCT. Hint: instead of being irratic sharp scintillating movements it will look more like leasurely boiling movement (though the greater the differential the more active the boiling).
Hope this ramble has been of help. The best seeing in the world won't do any good if your scopes not settled.