I am currently looking into ways how to get the maximum performance of my cheap "light-bucket" 5.9-inch Newton scope, with minimal resources. Unfortunately – as with all affordable Chinese-build scopes – it is
far from being "well-designed" and has several shortcomings, some of which should be addressed.
First of all, there are some points that might need addressing to get the maximum performance (the "last ten percent"), but which I will probably
not do:
- Increase the Fully Illuminated Field. The position and size of the secondary are such that only the central on-axis point is fully illuminated (and only just about). But then again the obstruction through the secondary is small (about 25%), so for visual use this should be fine.
- Wider Tube. At the moment the tube is just a whee bit larger then the mirror – the diameter of the tube is about 170mm (6.7 inch, haven't properly measured it yet), with the mirror diameter (hopefully) being 150mm (5.9 inch). A wider tube might aid the cooling, but that would mean a complete rebuild – but in that case I would probably try some sort of portable design or even some ultra-light Dobsonian design for that scope.
- Thinner Spider Vanes. These things are thick! Cast metal, baby! But probably the spikes won't bother me…
But if I wanted a perfect scope, I should start by getting a perfect mirror (and probably a larger one at that!), because my mirror is most certainly
not top notch. So as my time&energy&money are limited (and as I will not buy a better mirror), next are some points which should be reasonable, and which I
will therefore address:
- Focuser shifting. While I have addressed this already, the focuser is now too stiff and I need to readdress this problem once more.
- Baffling. I will have to add some felt in to focuser tube and opposite the focuser, and possibly baffle(s) in the focuser and at the mirror cell. Plus the secondary's holder is made from shiny plastic – I should baffle this as well.
- Cooling. This is at the moment the most obvious problem: Tube currents despite ample cool down time.
With regards to focuser shifting, I will have to see what I will come up with.
Mel Bartels has something to say about baffling:
Proper baffling ensures that no unwanted light enters the focuser. There is a baffle just below the focuser and a baffle opposite the diagonal. The focuser baffle is particularly important. Both are covered with Edmund Scientific black felt. The primary is also baffled, just in front of the glass, and totally enclosed in ultra flat black. It is an impressive demonstration to shine a powerful flashlight on any part of the scope, and discover that your observing buddy looking through the eyepiece with his eye cupped cannot tell you when you have the light on or off. The goal in baffling a minimalist ultra light is to block every ray of light not coming from the primary mirror. Extra baffling to absorb secondary reflections from the baffles is used in high performance refractors, but this is a luxury we can forego since the flashlight test is quite convincing when using light trapping felt or velvet.
With regards to cooling, the simplest way is to add a fan is at the rear. There are more elaborate ways of cooling (and the first order of business is keep the primary mirror as thin as possible), but these methods are overkill for my puny 6-incher: One could add a fan
in front of the primary, blowing on the primary. Or one could add a fan at the side, creating a laminar flow across the front face of the primary, and scrubbing that darned boundary layer of the primary. Best would be to actively cool the primary through some attached machinery (cooling pipes, peltier elements, and somesuch black magic).
Bah humbug, too complicated and not needed for my scope. (Though I would love for a bigger mirror to combine electronically controlled peltier elements at the rear of the primary, with a laminar flow across the front face of the primary – should make one heck of a good scope I'd reckon.)
Instead I will stick to putting a fan to the rear end of my scope, and call it a day.
But should it act as an "rear exhaust", sucking out warm air? Or should it be a "rear intake", blowing cold air at the back of the primary?
| Advantage | Disadvantage |
| "Rear Exhaust" | – Motor heat is kept away from the primary
– Better scrubs that boundary layer off the primary (maybe)
– Less dirt in the tube (maybe) | – Less efficient cooling of the primary
– Increases the dew-risk for the secondary (maybe)
– Works against the natural convection (possibly maybe more turbulent) |
| "Rear Intake" | – More efficient cooling of the entire rear face of the primary
– Keeps dew away from the secondary (maybe)
– Works with the natural convection, not against it (should be less turbulent, maybe) | – Motor warms the air
– Sucks in more dirt from the ground (maybe)
– Not so good in scrubbing the boundary layer off the primary (maybe) |
So I will go for the "rear intake" solution, and mount it on "some sort of circular mask" so the fan is decoupled from the scope, and air is forced in. With a 6-inch diameter primary I count on the glass being 2-inch or less, so it should be enough to cool it from the rear.
What I dread is adding batteries, cables, connectors and the like – it will complicate the setup, and I already hate the thought of it. And I need to take care of my powersource, change/charge the batteries – bah!
And to add a fan I have to take the scope apart, I guess…
