Dobsonian Fan

[Builder]

I’ve noticed on some Dobsonian Telescopes there is a FAN mounted to the bottom of the mirror. I don’t understand why you would put one there. On mine I made a 12V Warmer that runs off the power supply for the frost problem I was having in the winter air at night with the humidity. 

A fan would be used to Cool the glass, but I’ve not experienced a scenario where I’d need one. But al would also say that after all these years of looking up. I caught my first Daylight moon today. 

[MarsG76]

I find that when I put my 3 fans behind the mirror on my 14" dob, the slight air current blown up stops the dew from settling on the primary, no need for a dew heater.... a dew heater is still needed for my secondary tho.

[Paul73]

One thing that you don’t want is thermal air currents in your dob these can be caused by having a big chunk of warm glass at the bottom of the tube. This temperature difference can be large if the scope is kept indoors and then taken straight out into a cold night. The fans cool the mirror faster.

I too, just heat just the secondary. But don’t use fans. The primary mirror is tucked away at the end of the tube so rarely suffers from dew.

Paul

[Paz]

The fans speed up the cool down time of the primary mirror so you get better views sooner. They can also break up tube currents and pockets of air in front of the primary whilst your scope is cooling and this can also help to improve the views whilst still cooling.

[John]

I have a fan fitted behind the mirror of my 12" dob but I don't find that I need to use it. I have tried it once or twice but the scope cools quite quickly without it on and it makes no difference to the views once the scope is cooled so I just don't bother with it now.

I have read that combined with a suitable baffle fitted in the scope just above the primary mirror, a fan, or fans, blowing in a certain direction can speed up the dispersal of the boundary layer of warm air that forms across the mirror surface but I've not explored that as yet.

[Uplooker]

I have seen the effect that having a fan running behind the primary an have. AN Other member on here used it with a 12” Newt that they used for imaging. The image seen on screen of the Airey disk was improved significantly when the fan was running, as opposed to when it was not running.

As such I fitted an old brushless PC fan to my SW 12” PDS, as well as a Kendrick secondary dew heater. 

[Stu]

Fans can be useful if they don't add any vibration. I remember the first look I had through my 16" Sumerian, I was horrified to see diagonal lines instead of stars! I thought there was something horribly wrong until I turned the fan off and all was well! I only ever used it before observing after that.

[Mark at Beaufort]

7 minutes ago, Uplooker said:

I have seen the effect that having a fan running behind the primary an have. AN Other member on here used it with a 12” Newt that they used for imaging. The image seen on screen of the Airey disk was improved significantly when the fan was running, as opposed to when it was not running.

As such I fitted an old brushless PC fan to my SW 12” PDS, as well as a Kendrick secondary dew heater. 

That is interesting Ian. I have a fan built into the 12" Dob but have never used it. Perhaps I should experiment.

[alanjgreen]

22 minutes ago, Mark at Beaufort said:

That is interesting Ian. I have a fan built into the 12" Dob but have never used it. Perhaps I should experiment.

I prefer the "star test" in my big dob with the fan on too!

I always observe with the fan ON.

- In summer I get the fan on an hour before I start, in winter I dont bother, just switch on when I get outside. But my mirror is always outside so reasonably close to outside temp.

- Once the roof opens, then you feel the temp drop. So, I feel the fan is merited.

[niallk]

I always use the rear fan to reduce cooling time on my 2" thick 15" mirror - it helps. I switch off to observe. In fact I'm going to make a clamping bracket for the bottom of one of the truss poles to allow me to also use a fan aimed at the face of the mirror - should further help cooling.

I never used a fan on my 10" 250px mirror- just gave it ~45 mins outside if chasing planets.

[Timebandit]

 

Hi

I have never used the fan on the 14" Dob to which has a fan fitted as standard.

Also the 8" I have recently received I see also has a fan fitted as standard.

Personally if the scope is stored outside in a unheated Obs/Shed then I think the benefit of connection up and starting the fan is very very limited. The main purpose of the fan is to speed up the acclimate the lump of glass, the primary mirror. Obviously the primary performs best when it is at ambient temperature. Therefore any scope left outside in a unheated environment  should be very close to ambient temperature. Obviously if you store your scope inside in a heated environment then the picture is different. The fan then would have benefit in use in putting air flow over the mirror ect to bring to ambient temperature quicker.

 

 

 

[scarp15]

As above 14" and 8" solid tube dobs with fans, had almost forgotten that they were there as they have been unused for quite a long time. My scopes do live indoors in a centrally heated room (turned down a little) yet at the opposite, window, end and well away from direct heat. They get taken to dark sky locations (opportunity to cool down a little in the car) and achieve ambient temperature quite quickly levelled horizontal, whilst I'm setting up organising everything else. I tend to avoid using mid power for the start of and high power e.p's for the early period of the session. 

[Miguel1983]

Hi,

Maybe a stupid question, but do you need the fan to blow at the mirror or rather pull air through the tube ?

Seems unlikely because its mounted so close by the glass, but you would be creating an under-pressure so...

[alanjgreen]

1 hour ago, Miguel1983 said:

Hi,

Maybe a stupid question, but do you need the fan to blow at the mirror or rather pull air through the tube ?

Seems unlikely because its mounted so close by the glass, but you would be creating an under-pressure so...

Fan should suck air in through the bottom of the scope and blow it up the tube.

[tuckstar]

Here's a link to the build thread for when I fitted a cooling fan to my 250.

https://www.eastmidlandsstargazers.org.uk/topic/9817-cooling-fan-mod/

[estwing]

Got a fan on my Dob..never used it. Cooling a dob mirror begins the minute I set up in Elan for the weekend and I don't do planetary so that doesn't matter anyway. 

I think in all the observing sessions I've had the primary has dewed up twice?

[laowhoo]

On 1/24/2018 at 05:44, Uplooker said:

I have seen the effect that having a fan running behind the primary an have. AN Other member on here used it with a 12” Newt that they used for imaging. The image seen on screen of the Airey disk was improved significantly when the fan was running, as opposed to when it was not running.

As such I fitted an old brushless PC fan to my SW 12” PDS, as well as a Kendrick secondary dew heater. 

 

Exactly our setup, and keep them going to brush the boundary layer. We're also on concrete wh/ is significantly warmer than the air. But we're also trussed, not tubed.