Reflectors and Thermal Mismanagement

By Daniel Mounsey

Over the past several years, a lot has been said about reflectors and their thermal behavior. Many of us are aware that cooling fans, mechanical design, careful collimation, a good mirror cell, good quality optics, our surroundings and good seeing are just some of the crucial elements to accomplishing memorable views in modern truss dobsonians. For example, look at the countless threads regarding collimation here on CN, all designed to give us that last percentage of purity we strive to achieve. Why else do we go to so much trouble? Yet many veteran observers are still being eluded by a few serious problems, either ignored or not even understood by the ATM’s who build their own truss dobsonians. In fact some ATM’s even go as far to say that focuser placement has no effect on image quality. I have decided to share some videos that really deserve more careful attention and I will discuss each in an attempt for us to see the bigger picture. 

During a recent comparison I conducted between my high quality 6” refractor and high quality 14.5” reflector, I invited a couple of friends over. While doing the comparison, we encountered some serious issues with the reflectors thermal behavior, in fact Dee Frey of Cloudynights and my friend John Curry of Astromart were with me during the comparison. It is my opinion that large, open truss reflectors may not even be capable of achieving proper thermal equilibrium at all! This is partly due to the nature of their open design and the people who surround them at star parties, not to mention the heat that rises off the pavement and crawls into the optical path. After conducting countless tests, I am convinced that large aperture reflectors require special attention if they are to perform remarkably well or more efficiently. 

Truss reflectors can experience moments where they perform incredibly well on Jupiter, Saturn the Moon and multiple stars, but by no means consistently well. While we scrutinize these particular targets, we are more often reminded that something just isn’t right, something is still out of place. Why can’t we just get that last 10 or 15% of the image so everything just snaps right into place and just holds there, suspended like a pinball?

Sure, the aperture is incredible on certain deep sky targets but truss dobs in general are almost always hindered by this subtle noise or static that annoys us at times. The tips that follow are still no guarantee fixing this, but there is one thing these tips certainly will do. They will help us reduce certain thermal factors that plague the truss design, the telescopes that treat us to so few, good planetary nights, but those once in a while life time memories do remain vivid in our minds. 

THE FIRST BUT SHORTER LIVED CULPRIT

I’ve decided to share the first common over-site that still remains today. Did you know that the secondary central obstruction has pretty much been ruled as a low priority culprit to images being degraded, in fact tests by very reputable and highly respected observers have even verified this by suspending obstructions in front of refractors and were still getting incredible image quality, however these tests are still not conclusive for reasons you will see in this video, courtesy of Alan Garcia and Bryan Greer. Creating your own obstructions for experiments isn’t quite the same as a large secondary holder with a mirror in it, since they don’t store heat the same way. There are a few issues taking place in this very short video, so lets discuss them. Pay close attention to what’s going on here. In the back ground you will notice the jet stream, flowing from right to left across the entire field. This can be checked via CRWS http://squall.sfsu.e…eam_init_00.gif but it’s not the problem I’m discussing. 

We will ignore the boundary layer over the surface of the primary optic and look very carefully at the secondary support housing. Although short lived compared to the primary optics, secondary thermals can still exist and bleed off like solar eruptions seen in this video. Look directly at the edges of the secondary housing. Imagine in your mind how much improved the image would be were it not for the support system resting in the optical path. Regardless of what many of us say or think, central obstructions can easily degrade image quality, more so because of their thermal behavior in these larger 15” and 30” reflectors. Many of us resort to math to try an dispel the central obstruction myth but that math we are reading does not account for thermal behavior. http://www.fpi-proto…vids/clip03.mov

THE SECOND, MOST PROFOUND AND LONG LIVED CULPRIT IS “YOU”

Now the next two videos will reveal one of the biggest problems open truss tubes face. I know some observers may laugh when I ask them to please step away from the telescope while I’m scrutinizing certain objects, but I’m really not exaggerating when I say that. Those who laugh are not even close to being aware of the problems they are causing. Pause for a moment and ask yourself why we spend thousands of dollars on high quality mirrors, fans and expensive collimating tools, yet we completely forget that we ourselves can be many times more problematic than the thermal on the mirrors themselves. Since we understand that a primary optic in a reflector just 2 deg F. above ambient temperature can have a boundary layer, then take a good look at YOU. At least a mirror has the potential to acclimate, but YOU do not. So, are you going to attach several fans to yourself and freeze? No, but there is something you can do. 

During my tests I measured and noted several temperature readings of myself, the ground and my reflector. All measurements taken at 7:30pm

1.) Ambient air temperature at 61 deg. F 
2.) My 10” dob has a thin mirror just 1” thick. It too was also 61 deg. F and settled down fast.
3.) My own body heat and head 72 deg. F 
4.) Pavement was 65 deg. F 
5.) The back of my hand 89 deg. F
6.) Inside of my hand 93 deg. F

By 11:15 pm the main differences were as follows.

1.) Pavement 61 deg. F
2.) 10” primary optics 55 deg. F
3.) Ambient air 55 deg. F

Now that we have some numbers in place, let’s discuss what we are doing wrong here. If a mirror that’s just 2 deg. F above ambient temperature can allow a boundary layer to creep into the image, then how can we expect our own body heat not to be a major source of the problem too, especially when our head and hands are 30 to 40 deg. F above ambient temperature? Not to mention that they don’t even acclimate like a mirror can? When we move our dobs, we sometimes even place our hands over the upper cage ring and we keep them there while we view, not realizing the full impact. Thermals that are caused by our own body heat are PROFOUNDLY destructive to image quality! There is no way we can expect to stand right next to a huge open optical path or focuser with our hands on the focuser, right next to the entrance of the optical path and not have this occur. And in case you are wondering the full impact, this is exactly what it looks like.

http://www.fpi-proto…ids/rdemolo.mpg

Notice how the currents here are drifting from right to left. It only takes a very small amount of air to make the currents drift in one direction. Also know that gloves and thick down jackets also do not stop these currents from pouring out and across the field of view. There isn’t even any current light shroud that helps prevent these currents either. They float right through them just like you see in the videos.

http://www.fpi-proto…r/vids/rslo.mpg


HOW CAN WE SOLVE THIS PROBLEM?

There are several possibilities. The first thing is for each observer to learn which way the air moves when their seeing is optimal. In my case, it’s an onshore breeze that moves from west to east. Since the planets rise from the east and transit towards the south and set towards the west, then it makes perfect sense to have my focuser placed on the left hand side of the upper cage assembly so my body is now placed, strategically downwind. If your seeing conditions never experience this, then it probably doesn’t matter since you are not going to get stable air flow or seeing anyway. 

For years I often wondered why every single ATM places there focusers on the right hand side of their upper cages, so I conducted a survey of my own and learned that almost every observer is left eye dominant see here. 
http://www.cloudynig…ll/fpart/1/vc/1

This probably explains one of the reasons ATM’s continually do this. ATM’s should always offer the option to decide this if customers expect their moneys worth. My point is that there are a number of observers who are adamant that aperture rules. Well, that depends on your definition of what excellent views are really all about.