Posted 13 February 2005 - 12:52 PM
Posted 13 February 2005 - 12:57 PM
Posted 13 February 2005 - 01:10 PM
Posted 13 February 2005 - 02:02 PM
Notice carefully that the fans are on the bottom side of the box pushing air across the surface of the mirror and out the top side. This example is BAD. I have serious problems about Alan Adler's Sky&Telescope article regarding fan placements. I understand he had no choice in the matter because his mirror was sealed at the back and thus, he had to put the fans on the side, but he made a major error in which I'm going to explain here.
Take in mind that I respect Alan, but the truth is the truth and it's as simple as that. I've personally conducted numerous tests which have proven this. Bryan Greer is a leading expert and has legitimate concerns regarding vibrations but hold tight before you judge and I'll explain something here. This photograph is an example of what NOT to do in my first experiment, and is also how many people are making the mistake of placing them. Remember that not all fan placements are the same and it depends on the design of the telescope.
I tested this design in two ways. One, I racked the out-of-focus star test to observe how the boundary layer behaved with the fans running. In Adler's review, he claims that the fans sweep air across the face of the mirror and out the opposite side during observation. My tests concluded beyond a doubt that this does not happen and instead, the air will ricochet. If you observer the star test, this is clearly evident. Even after hours of cooling, the images are poor with the side fans kept on this way. The air does NOT just simply scim off the surface and flow out the opposite side. The second test was conducted during a smoke test. In this case I used the configeration depicted in this photo. I placed a piece of paper at the bottom, without the mirror of course and lit a match. As the smoke filled the box, I turned the fans on and all the smoke did was stir around the box. This did not eliminate the gremlins!!!! during visual use but instead, simply breaks up the boundary layer to increase cooling efficiency. I'd like a shoot-out with these guys who claim that they can leave the fans on during observation with a fan that wrongly pushes the air across. Take in mind that all we are trying to accomplish is to simply get the mirror cooled, that's all. Side fans do increase the speed of cooling but pushing air does NOT magically makes images snappy. Nothing beats having no boundary layer at all. Bryan is my mentor on the suject and I have nothing but the highest regard for him. I've spoken with him on numerous occasions to discuss matters.
Bryan explains that rubber bands are the simple solution to reduce vibration. I totally agree with this. Remember though that tube are far more susceptible to this vibration, especially metal tubes, because they resinate. In a Truss however, using wood and truss poles, the vibration is less likely to make it to the focuser and has proved to be so during my tests. The next picture I will show is how it should be done.
Posted 13 February 2005 - 02:15 PM
Posted 13 February 2005 - 02:20 PM
Posted 13 February 2005 - 02:26 PM
Your second pic din't come through completely. So, as
I understand it, you only have suction fans on the top now ...
pulling air out only. No fans pushing?
Do you have inlet vents on the bottom side for air to be sucked in to the mirror box?
Posted 13 February 2005 - 06:06 PM
Posted 13 February 2005 - 07:16 PM
Posted 13 February 2005 - 07:28 PM
Posted 14 February 2005 - 10:48 AM
Posted 14 February 2005 - 11:35 AM
I think you're right about using fans to pull at the desired point of exit. The "push" arrangement does not guarantee movement of the warmed air once it is past the fan. My scope has a round tube, so side mount was not attractive. Instead, I went rear mount pulling over the mirror and out the back. I also used low density foam around the edge of the cell to prevent air from moving other than how I intended.
A good test for effectiveness of your fan arrangement is to center on a bright star and rack the eyepiece in so that the star image fills the primary. Before the fan comes on, you can see heat plumes in the tube, it looks very much like Bryan Greer's schlerien (sp?) images. (BTW you can also see the effect of body heat by placing your hand at the top of the tube - wow!) Turn on the fan, plumes go away in seconds. Now if I could only do something about the atmospheric seeing ...
Posted 14 February 2005 - 05:53 PM
Yea, I've done those tests and it's amazing how much heat bleeds off the hands. My fans move very smoothly and not to fast.
Posted 25 May 2005 - 06:04 PM
Posted 26 May 2005 - 06:07 PM
Posted 26 May 2005 - 10:23 PM
Posted 29 May 2005 - 10:32 PM
Since then however, and through a simple statement made by someone in another thread, I have come to the conclusion that it is to a large degree a moot point! Even if one reduces the bl to almost zero - if the mirror is STILL say 10 degrees hotter than the ambient (measure it - you'll be surprised at how hot it really is!) even if it is just 4 degrees above you have taken ANY figure mirror and made it a much worse one due to uneven warping. Trying to remember the numbers I believe it is around 3.5 deg where your at a 1 wave mirror. Thus getting the mirror temp down to the ambient is, I believe, the ultimate solution - when this happens you will not only have the mirror in the shape it was intended but there will BE no boundary layer!
OK... that all said... sinse I could easily be missing something else and thus be wrong... did you try the ultimate test - did you turn the fan off... view - on - view... back and forth? This would be the true determinor. You could have, for example, had a mirror that just happened to be cooled (thus no bl), or you could have had a mirror that was overcorrected (some mirror makers at one time were doing this intentionally - the thinking being the mirror will ALWAYS be hotter than the ambient - a truer statement then most realize)... or ?
thanks for the comments.
Posted 29 May 2005 - 10:49 PM
Were you at RTMC with Plettstone when we saw the boundary layer in the 15" F-5? Yes, the on off test was conclusive. It was also especially conclusive with Ed Grissom and John Pons tubes as well, however, it only takes about 1.5 hours to completely get the boundary layer out of the way on my 12.5" Starmaster when set up at my home near the ocean. For that reason, I intentionally left the mirror in a warm room all day to let it bake, then installed it to prove the test to myself. Once it cooles completely, I shut all fans down. If I'm in a constant ambient dropping temperature, then the fans stay on longer.
Posted 29 May 2005 - 11:59 PM
Yes I was there and I was on the side (talking to someone else) when someone commented about seeing the boundary layer - that was you?
I am not sure that you have answered my question however - were you able to see a difference with the fans on and then off... or are you indeed answering it by saying that your only using it to cool the mirror and thus after this period of time (you mentioned 1.5 hours) then it is doing better?
thanks for the comments. By the way - you've done a fantastic job on the install - it looks great!
[I AM convinced that better thermal management is much more important than currently viewed by the scope manufacturers. I believe we have some exciting times ahead of us in this regard!]
Posted 30 May 2005 - 12:30 AM
Posted 30 May 2005 - 01:50 PM
Yes that does make sense... here in SE Arizona our temp drops very rapidly and quite a bit as well. My 30" never - well almost never catches it. The other night it finally was within a degree of ambient when the Sun was rising in the east - that bad. Most of the time it is 10 degrees warmer than the ambient. I may have mentioned to you one of our projects here is to come up with an active cooling system for larger mirrors - not practical (from an install standpoint) below about 18" but for larger mirrors it would be great to have.
I have been promoting better cooling here in our club - one member after seeing my setup installed a thermometer on his mirror and tried a test out one night - at dusk the mirror was the same temp as the ambient... an hour later it was 9 degrees WARMER than the ambient with no cooling on the mirror - then he put a 20" box fan blowing on it - 90 minutes later it was STILL 7 degrees warmer than the ambient. This was not a huge mirror - not a 28 or 30 ... rather it was an 18"!
Posted 30 May 2005 - 04:21 PM
My scope has a round tube, so side mount was not attractive. Instead, I went rear mount pulling over the mirror and out the back.
Why not use a curved focuser adapter. You can buy them in whatever size tube diameter you want. Come to think of it, I've got an extra Moonlite adapter around. Maybe I'll try this. Hmmmm.