just a thought

[onlyme]

a lot of people seem to fit a fan under the primary mirror to aid cooling and cut out on currents, i have seen a lot of pictures for this mod, yet i have never seen a fan fitted with a dust guard.

all you have to do is look inside a computer box to see how good fans are at gathering dust and depositing it in the direction of airflow.

all it would take to cut out on dust particles circulating in your tube while observing, and to stop the dust particles settling on your primary mirror when you switch off your fan, is to fit a fan filter that can be easily and cheaply bought from any outlet that sells the fans themselves.

alternatively you could use a fairly loosely weaved piece of cloth material (the type shirts are made of), cut it to size, and simply place it over the fan using bluetak or a rubber band.

[Gina]

Good thinking

[sbooder]

I have just been making a filter carrier for the fan in the last few days. I will post when fitted. I never needed one in the UK but it is a lot dustier in France when dry.

[Gina]

Dust is a lot worse indoors, to get into computers. It comes from carpets, dogs, cats and dead skin that we all shed and, if you have a solid fuel fire or boiler, there's dust from that, of course. However, good the cleaning, there is still plenty of dust about. Outdoors the UK is usually fairly free of airborne dust and even in an observatory there will be less dust than in the house due to lower occupancy and mostly open roof when you're in there.

[yesyes]

That brings me to another question that's been in my head for a while. Do people keep the fan running while observing? Or is it only used for cooling down the scope before observing / imaging?

[sbooder]

I run it while looking at planets and the moon, the airflow seems to calm the image and makes it sharper.

[NGC 1502]

all it would take to cut out on dust particles circulating in your tube while observing, and to stop the dust particles settling on your primary mirror when you switch off your fan, is to fit a fan filter that can be easily and cheaply bought from any outlet that sells the fans themselves.

A fan filter would reduce dust being blown towards the back of the mirror, but I suppose a downside could be less airflow, so the optics will take longer to cool.

I don't use a fan, I set up my scope, go back in to get extra layers, eyepiece box etc, and by the time I'm back out, the scope is well on it's way to being cooled down. But larger scopes often do need a fan (or two).

Regards, Ed.

[onlyme]

A fan filter would reduce dust being blown towards the back of the mirror, but I suppose a downside could be less airflow, so the optics will take longer to cool.

Regards, Ed.

that is very true, a filter would reduce airflow, but to a very minimal level.

fitting a 120mm fan instead of an 80mm fan would compensate for the lower airflow (plenty of 120 -80 adapters can be had off the shelf if holes are already drilled) , another advantage of using a 120mm fan is lower running noise and lower mechanical vibration.

speaking of vibration, surrounding the contact edges of the fan with a strip of bluetak works wonders

[sbooder]

speaking of vibration, surrounding the contact edges of the fan with a strip of bluetak works wonders

I snipped of the corners of an old square mouse mat (with the material stripped off) for the contact points and as far as I can tell I have zero vibration.

[Ericgraham]

Another way to cut down vibration is to mount the fan in a box on the ground and connect it to the scope with a flexible hose- I'm thinking of using a clothes dryer hose. Anyone tried this?

[singlespeedman]

I would have thought that this could be countered by altering the standard fan mod. I have been looking at fitting two small fans on each side of the rear as opposed to one in the centre. The idea being to have one as an intake and one as an exhaust. The airflow would be comparable but any particles would be pulled straight through instead of being able to settle.

Being an supposed IT professional I alway build case to create a wind tunnel where standard pcs have far more air blowing in or pulled though rather than fans pulling and blowing at the same air flow rate. I only need to clean my customers computers from dust every blue moon instead of every 6 months before applying this effect.

I do think the same would apply to the scope and I am in the process of rigging a dummy tube to test this theory

[Moonshane]

would there not be a danger of one fan just dragging out the air that the other pulls in?

I have never noticed any major dust build up but as Gina says, it's not too bad here. that said, I am using cloth caps on the end of my 16" now as they reduce light scatter and also act as a filter for the fan I am fitting.

an 80mm fan is generally plenty in terms of cooling for most mirror sizes as the glass can only rid itself of heat so fast.

[singlespeedman]

the process of blowing air onto a surface is not as efficient as having moving air flowing over it. All a fan does is blow air that is at a temperature from one place to another. The cooling effect comes from the movement of the air itself. Having a fan blow onto a surface would promote getting that surface down to the local ambient temperature faster than just waiting but a constant moving airflow would / should increase the cool down speed.

The only really problem I can think of is the noise of the fans. As the fans will be smaller they will make more noise to achieve a decent through put of air.

Right now I am playing with this idea purely to test my own theories.

I see air blowing onto the rear of a mirror as being prone to escaping round the sides of the mirror and therefore into the viewing zone. My plan is to have the cooling take place with the air not moving into the viewing part of the telescope and hopefully resulting in the ability to maintain a steady temperature without having any air movement inside the business part of the scope.

The question about dust just seemed to fit into my plans as well but was not my primary reason for starting this mods test stage.

[Moonshane]

interesting stuff.

will be good to hear of the results in due course.

my understanding of fans and mirrors is that they are needed to cool the mirror initially but then to remove /disturb the 'boundary layer' which is like a heat haze when the surface of the mirror is very slightly warmer than the ambient and thus acts like a weak lens.

I think the air blowing around the sides of the mirror in constant use would achieve this as would smaller fans blowing across the mirror.