Primary mirror cooling fan

[Flexie]

If i were to fit such a fan to rear of primary,
and the primary being adhered to an interior plate,
will the fan still cool the mirror for me?

Cheers,

Steve,..

[K9shep]

Excuse my lack of knowledge but what does a fan do for you? I live in Phoenix, AZ do I have a need for one on a Nexstar 8SE??

[KeithBC]

Excuse my lack of knowledge but what does a fan do for you? I live in Phoenix, AZ do I have a need for one on a Nexstar 8SE??

It ensures that the temperature of the mirror is equalized to the temperature of the surrounding air. This is important in the cooldown phase prior to starting observing, when the mirror is losing heat. It is also beneficial during the session, when the scope is trying to cool down below the air temperature and below the dewpoint, because it prevents the scope temperature from dropping further.

I have no idea if the climate in Arizona warrants one, but your scope is an SCT, so it is a moot point. There is nowhere to put a fan on one of those, and nowhere for the air to circulate. Mirror fans tend to be used on Newtonians only.

[skaven]

If i were to fit such a fan to rear of primary,
and the primary being adhered to an interior plate,
will the fan still cool the mirror for me?

Cheers,

Steve,..

I'm not 100% sure I follow what you're asking, but I'll take a stab at an answer and see if it helps. The important thing when mounting a fan to a Newtonian mirror is that air be allowed to flow around the glass -- so there must be a reasonably clear path from the intake to the exhaust. If the mirror is mounted to a solid plate with no perforations, blowing air on the metal plate will probably be of limited utility, especially if the mirror is actually held away from the metal plate with the adjustment screws. However, most Newtonian mirror cells these days are designed to be fairly open. So while you likely do not have a 100% unobstructed view of the back of the mirror, you are probably able to see a good 90% of it, with the mirror cell's support structure butted up against the glass in appropriate locations. If this is the case, just make sure you position the fan so that the air blows against the center of the back of the mirror, such that the air can flow easily radially out away from the fan, across the back of the mirror.

[shannong]

If i were to fit such a fan to rear of primary,
and the primary being adhered to an interior plate,
will the fan still cool the mirror for me?

Cheers,

Steve,..

The short answer is "yes". It will help. Read this link for the reasons why and a lot of other good info about cooling primary mirrors on Dobs. Telescope Optics Topics

The primary use for a cooling fan is to keep the primary mirror at the same temperature of the ambient air around it. When temperatures are cooling the mirror cools more slowly than the air around it because it has a higher heat capacity. This creates a temperature difference, and the mirror creates a warm blanket of air on it's surface. This warm blanket of air causes diffraction of light as it passes from the cooler air to the warmer air, which happens twice as it travels to and reflects from the mirror. This is exacerbated since there are turbulent air currents that are created between these two layers. This distorts your image that is reflected to your secondary and ultimately to your EP.

The cooling fan does this for you:
1. For those who keep their scope inside in the air condition, it is cooled more quickly to the ambient air temperature outside since the air blowing across the cell can absorb and carry away the heat held within more quickly than static radiated heat alone. Just like when you blow on your hot soup before eating it.
2. As temperatures outside continue to drop, it will help keep the temperature difference to a minimum through the same mechanism as 1 above.

Ultimately you need to cool the mirror surface rather than the entire cell. So having air blowing across the mirror face will cool it more quickly than cooling from the backside of the cell just like blowing on your soup spoon would help but not as quickly as blowing directly on the soup. However, cooling from the only the backside is still a significant improvement over not having a fan at all.

Another benefit of air moving across the surface is that it will "scramble" the hot and cold air layers on the surface of the mirror. This removes the discrete layers of air temperatures that causes the image distorting diffraction. This means you get immediate reduction in diffraction as soon as the fan turns on even before the cooling occurs.