CN FORUMS ARCHIVE Telescope Reviews: Refractor vs. Reflector ?

Let's all go through the fans one more time since this thread has gotten so long and some may have understandably lost track. I'm sorry I said "contend". What I meant was that the rear fan "deals" with the thermal issues wonderfully. When I said "contend", that implied "struggling", but was not what I meant.

There are two fan placements for two different designs. One is the truss, one is the tube. Each design will need a different fan placement in my opinion.

There are reasons why I prefer the tube over the truss. First, it prevents any body heat from crossing or entering the optical path, with the exception of the front if one is not careful. Second, the tube also acts as a funnel for the fans to flush out any thermals coming off the secondary system as the fan's air flows up the tube. In order to deal will this, Grissom and Pons use a rear fan pushing up the tube. The reasons I'm sold on this is because the field tests proved beyond a doubt, that the placements were correct based on the consistant images that were seen at the eyepiece. It was also proven by turning the fans on and off. If you guys recall, I said the fans must stay on at all times at regulated speeds. The boundary layer also gets sucked off the face of the primary and flushed out the front in this manner.

In the truss, it's harder to deal with. A rear fan can not be fully relied on, because there is no tube to allow the air to flush through and clear out the secondary thermals or optical path in any way. A truss may not have tube currents like a sono tube, but it does have mirror box currents and is susceptable to body currents crossing and entering the optical path. These facts can be proven and demonstrated by conducting an out of focus star test.

If you were to just rely on a rear fan with a truss, the air would NOT flow smoothly up the optical path because there are lips around the top of the box, not to mention no tube to carry the air flow up to the secondary to flush out any currents in the optical path as well. The truss is also not as solid as a tube and is more likely to flex and degrade collimation, especially in fast scopes.

For this reason, a side fan has to be utilized with the truss instead of just a rear fan because it will not flush the boundary layer out of the system without hitting something. In a tube, especially with a thin mirror, I do not believe a side fan is needed and our tests showed this. Also, the truss and its entire secondary system is left bone dry on it's owns. Bryan Greer who is an expert on these issues has proven that secondary thermals can be existant for a period of time. It's not as bad as the primary of course, but everything has an effect on the image.

The Starmaster telescope I have was not without flaw. There were numerous problems with its design in my opinion, which is the reason I made all the changes to it. I've also asked Bryan Greer to help me design a 10" F-6 for serious Jupiter observations with a different approach. I strongly suggest people read his S&T articles and get up to speed. These are Sep 2000, May 2004 and June 2004.

On p.123 of the June issue, there is a picture by Gary Seronik's 6" Newtonian with a side fan and holes at the opposite side. Although I do not agree with this method, that is in no way meant to imply that Bryan Greer did this. I personally look up to Bryan and he is my mentor on the topic. I strongly suggest people call him and just spend one hour of their time learning and listening to him.

--------------------