|
The ota does not need to be cool itself, but the air sealed within the OTA **NEEEDS*** to be at the same or very close to the temperature of the primary mirror. Otherwise you suffer the same thermal distortions we saw in your C14 the other night, at a lesser scale.
Essentially, you're aiming to get all the air inside the tube and primary to within 3 or 4 °c of ambient temps... The closer you can get to matching the optics to ambient, the less internal tube currents you have to deal with.. Then you are only at the mercy of local seeing conditions, and not the conditions set by the thermal currents happining inside your OTA as a result of the mirror being cooler that the OTA internal temperature, or vice versa, the mirror being warmer than the air inside the OTA and ambient temp causing a warm boundary layer on the mirror....
This warm boundary layer of air on the primary is what we saw the other night when observing Jupiter earlier in the evening.. Obviously we were battling the seeing when Jupiter was at a low altitude, but once it was up at say 60° altitude and the OTA tube had cooled to a reasonable level, the seeing was still a problem but the views of Jupiter was markedly improved..
Pauls cooling setup addresses every problem that there is essentially. If I were to change anything, I would have added a low power internal dew heater directly behind the corrector plate [READ: touching the glass) as no matter what you do with those external heater bands of yours, you will still suffer dew problems on a humid night... We are after all talking about a piece of glass less that 0.5mm thick... Lots of surface area, but not much thermal mass... It will dew up...
Paul informed me back when I had my C11 that regardless of anything else he'd tried (dew shields, heater bands, keeping everything at equal temps etc.) the best, without out fail, way to keep dew off a 14" corrector plate is to hit it every 3 or 4 minutes with a hair drier.
Lets think about it logically though. 90% of the time, you will be using the C14 @ F/2 in hyper star config. At this focal length, the thermal currents you will introduce by having an internal dew heater DIRECTLY touching the interior side of the corrector is going to make SFA difference to the overall image quality.. For planetary imaging, heater bands kill the image quality because you're looking at imaging at 14m focal lengths... with hyperstar, much like with standard wide field imaging... Thermal currents and seeing conditions (atmospheric or dew heater induced seeing issues) mean very little.... and you have the 12.5" F/20 DK for planetary imaging and observing... I would go to every length to make sure the C14 hyperstar setup is as fool proof as possible at F/2. This includes cooling the mirror to ambient temp to avoid internal currents, and doing whatever (within reason) to avoid the ingress of dew onto the corrector.....
Its been a big day for me and I've rambled here, but I think I've conveyed a few worth while things to consider...
Decide first what the primary use of the scope is... full on pelt cooling may not be necessary if the primary use of the scope is going to be wide field imaging using the hyperstar.... Keep planetary imaging to the DK, visual observation with the DK... and the occasional F/7 imaging through the C14 that is going to require some pretty serious attention. The person to ask about that is 'Spearo' Frank produces amazing images with his C14 setup... and I don't believe he has done much if any modification to the OTA to permit this...
Pauls setup is exceptional, but he demands exceptional things from the optics... Imaging at 14m FL as he does for planetary work is a far cry from 2.5m @ F/7 and a whole different ball game than 710mm @ F/2
To be honest with you mate.. I'd say lets leave it as is for the time being until we can assess what you want to do, and what major issues your plans are going po present before we go any further......
Thats enough out of me for now.. its late and technically.. im supposed to be working... but yeah... Hopefully somewhere in all this rambling I've been somewhat helpful..
In anycase, you know I'm with you every step of the way, no matter what you decide to do, I'll be there to give you a hand and an extra half a brain to think through options.
Regards,
Alex.
|