|
I would have to disagree with several of the points here.
Most scopes that have fans have 3 or 4 not one. This then spreads the flow more evenly. I have not ever noticed distortion in my images from the fans on or off. Have you seen this effect with high powered eyepieces?
My Planewave CDK17 for example, will not come to tight focus if the temp difference between the primary and ambient is more than about .7C or 1C. You see it quite clearly.
The fans speed up the equalisation process and the images become noticeably sharper once the temp difference is around .3C or less.I leave the fans on all night. I almost never see dew on that scope.
This is an open truss system so the idea you don't need fans with an open truss system is not correct. It makes a huge improvement. It speeds the vital thermal equalisation process and I am not sure about the cool spot and distortion theory. Perhaps in a poorly designed fan system or one with only one fan (rare).
I have not trialled with and without fans on successive subexposures to see if there is a noticeable difference to sharpness but my educated guess it there would be none and if anything slightly better with the fans on as it keeps the temp difference in the case of my CDK to around .1 to .2C which is minor.
With my AP Riccardi Honders, it has 2 fans and 2 backplates that remove allowing circulation with the primary. The tube is wider by about 50mm than the primary so air can get around the primary. It also have thermal blankets which look like wet suit type material with velco strips to fasten them. They seem to work well. On 3 nights, one without the fan going, I did not get dew but there was a small amount first thing in the morning that must have formed at the end of the night. The 2nd time without fans (I thought they were on but they weren't) and no dew except again for first thing in the morning. The 3rd time no dewshield (it was windy) and fans going but rear plates on (I was worried about windborne dust).
Some very slight dew starting to form. So the fans, the backplates and the dewshield help a lot. The inside of the scope and dewshield is flocked.
Roland recommends to never use a hair drier. I know many use them on SCTs but thermal shock is a possible way to crack your corrector plate if done too hard. I also have a hair drier in the past and found it to be a very temporary solution and dew forms again relatively quickly.
Some Planewave CDK models now have 2 fans that blow across the primary. I think that is a great feature.
Greg.
|