Under The Knife

Ventilating a C-14

 

The goal of this project was to improve the thermal characteristics o the C-14. With so much glass, and virtually no air flow through the tube, it can literally take all night for the C-14 to come to ambient temperature. Making things worse, the temperature usually drops through the night, so the mirror is likely to remain somewhat above ambient as its thermal mass prevents it from ever being exactly at temperature.

This is a hotly discussed topic on the various SCT discussion sites. It's also something that the high-end scopes have dealt with by mounting cooling fans behind the mirror, blowing air across the back surface of the mirror to quickly bring the temperature to ambient.

Commercial Solution:

One company, Lymax, (www.lymax.com) makes a highly regarded product called the SCT Cooler. It's basically a fan mounted in such a way that it blows outside air up through a tube which is inserted in the focuser. The tube has outlets which point the air back towards the mirror. The air then exits through cutouts which let air blow out between the focuser and the air tube.

While a nice, off the shelf solution, I had three concerns:

1) I don't think that this is the most efficient manner of cooling. The air will get blown around the outer edges of the mirror and back out the focuser. Since the mirror is thinnest at that point, one really wants to supply lots of air directly to the central portion of the mirror where it's thick.

2) It blows air in front of the mirror. Any dust or debris would tend to land on the mirror as the air circulates around.

3) Price. It's priced reasonably, but I figured I could do the job for about $20. Call me cheap...

Home-made Mirror Cooler:

Since I had recently dismantled the C-14 to grease up the mirror support tube in hopes of reducing mirror-flop, I had a good idea of what the internals of the scope looked like. It's actually very simple.

The mirror is glued to a metal carrier. This carrier has a fine screw thread which is used for focusing. It is guided by a tube with a thin layer of grease providing lubrication and a bearing surface.

Here you see the scope "exposed". The main tube has been removed, but the rest is intact.

Note the retaining ring. This fits in a groove on the support tube and prevents the mirror carrier from falling off.

Also note the cork and mirror lock-ring. The cork prevents direct metal-to-mirror pressure and probably gives a bit to absorb pressures from expansion/contraction of the parts. This has to be adjusted tight enough to keep the mirror from flopping around, but loose enough to prevent mirror distortion.

The back of the mirror. Note the solid build with the three-lobed mirror carrier. I understand that the smaller SCTs only have one lobe.

These lobes can be used to lock the mirror in place without putting pressure on the mirror itself.

Note you can also see the focusing screw near the '1' on the bottom lobe.

Also note the markings on the mirror. Quite a bit of discussion revolves around properly aligning the primary and secondary mirrors. I was a disbeliever until I realigned the mirrors according to the marks and started getting vastly better images!

 

Once the mirror is removed from the rear part of the scope, you have a simple shell. Not much to damage except the main mirror carrier tube

Next, several holes were bored in the back of the scope. Note their positioning, exactly in between the lobes on the mirror support. This provides maximal space for the internal fan.

Also note that the holes are not very large. This was done to provide some resistance to outflow so that the air would flow around the back of the mirror before exiting the tube.

The multiple hole hole is the air intake for the fan.

The tube was painted after the holes were bored. Probably could have used even flatter paint, however since this is all behind the mirror, I wasn't too concerned.

The cooling fan is wired to the power supply and a switch.

And a view from the back. This was painted in a glossy black to match the original paint job. A few fingerprints will remind me to always let paint dry before touching.

The final view of the reassembled scope after surgery!

Post Surgery Comments:

The fan appears to work like a charm. Though weather hasn't cooperated, in quick tests, use of the fan appears to greatly reduce mirror cooling issues. One note however, running the fan while the scope is NOT in use is best. Air currents can be readily seen when the fan is turned on. I'm not sure if this is because of heat from the motor, or something else.

More comments to come...