Cooling Fan for 250 Flextube Dob

[SeedyF]

It's been a while since I did a DIY project but I've been convinced that my dob needs a fan!

Inspired by various threads on here I have ordered a silent PC case fan, a Zalman Fanmate 2 speed regulator, and a 9v Battery clip to hook up some power. Hopefully they will arrive by Saturday so I can get busy

Meanwhile I've made a cardboard template to cut a piece of MDF to a size/shape that I can attach to the end of the scope and still leave the collimation bolts accessible.

I'll post back full details as it progresses! Lets hope Mrs F doesn't have any great plans for the bank holiday weekend

[kirkster501]

How long will a 9v battery run the fan for mate?

[SeedyF]

I dont really know. I have a converter i can plug into the power tank if it proves too juicy. But i dont really want another trailing wire. I am copying other 9v battery solutions so will just suck it and see to start with.

[SeedyF]

Well I was delayed by a week as the fan didn't arrive but today I have finished the job!

Here is the kit -

• Zalman Fanmate 2 - fan speed controller - Amazon £3.75
  
• Zalman 120mm quiet fan - Amazon -£5.94
  
• 9v switched battery box - Maplins - £2.09
  
• Sticky back Velcro - Wilkinsons - leftover from earlier project!
  
• 4 x M4 40mm bolts - Wilkinsons £1.10
  
• White paint - the garage!
  

The fanmate 2 comes with one connector that will hook up to the fan and a second that should connect to a PC motherboard. Ultimately I had no use for the connectors - I just focussed on the colour of the cables matching reds and blacks and sealing off the white and yellows. The fan connector included a white cable and the motherboard (the power source) cable included a yellow cable. Neither of these were required and so were capped off. As I understand it these are temperature or voltage sensor cables, but I had no need for them. I tested that the fan worked - and which way round it needed to be attached to send the air the right way down the tube. I did this by sticking the 9v battery box wires into the fan connector and the fan ran perfectly, then connecting the fanmate to the fan and sticking the 9v battery cables into the connector for power to the fanmate. Everything worked fine. Interestingly the fanmate at full power spun the fan slightly slower than just putting the power straight to the fan. Time will tell if this is anything to worry about.

Now on to the assembly. I started by making a cardboard template for a base plate to fit the bottom of the scope - the plan was to get the right width and leave space to access the collimation bolts. In the end I realised collimation will need to be done without the fan base plate fitted - more of that later! Using the template I cut a piece of 4mm MDF to the right dimensions and cut out a centre circle to match the inside dimensions of the fan blade.

Once I was happy with the piece of MDF that would be the base plate to hold the fan I stuck velcro on the bottom end of the scope on to the metal supports that hold the primary mirror. I then stuck corresponding velcro to my MDF base plate.

Next I positioned the fan on the MDF and drilled holes to mount it. The fan was mounted using 40mm bolts and nuts. The holes in the mdf for the bolts were deliberately a very snug fit to minimise vibration.

The fanmate 2 has two connectors - one for power and one for the fan. There was a generous length of cable so I trimmed them all back to a length that minimised the excess of cable to keep the finished look as tidy as possible. I connected the fanmate to the fan cables and the fanmate to the battery box cables. I took a short cut and twisted the cables together and covered them with insulating tape - at some point I will solder the connections. Again I tested that everything worked before going any further.

Once I had everything connected and was happy with the arrangement - it worked just fine - I dismantled it all and painted the base plate with some white emulsion I had in the garage. Once the paint was dry I screwed the fan into place, attached the battery box to the base plate with velcro, attached the fanmate to the base plate with velcro and stuck the baseplate to the scope. I must admit I am really pleased with how neat and tidy it all looks.

The fan creates a good flow of air up the tube - you can feel it easily at full power in the open tube. If you put the tube cover on and remove the off axis cap it is a sturdy flow of air. At full power it's very distinct and at minimum you can still feel it on your hand. It's pouring with rain so I will have to wait for a field test to see how effectively it cools down and if there is any vibration if I keep the fan running at low power. And ultimately see if keeping the fan running helps with imaging planets! I understand that the velcro should help reduce any vibration being transferred to the OTA but I need to be convinced .

I think I may have over cooked things with a 120mm fan, a 92mm was probable enough. I guess at least it will speed up the cooling down. All in all I am really pleased with the results and it's pretty neat and tidy

Initially I thought I would be able to leave this baseplate arrangement in place and still use the collimation bolts. However the bolts don't stick out far enough over the additional baseplate to be able to use them. I could replace the bolts with longer ones but for now I will remove the fan base plate assembly, collimate then put the fan in place. As the whole baseplate is attached using velcro this shouldn't be an issue, its removed and replaced in seconds.

One additional benefit - the extra weight at the end of the OTA will be useful as I have to always have to add a counterweight when I attach the webcam and more weight when I put on the DSLR. So at least I will need less counter weights. I have no idea how long the fan will run from the battey. If the fan drains the 9v battery too quickly I plan to add a connector to take power from the powertank instead. Lets see how it goes for now.

[knobby]

Nice job... I've thought about this myself, but not got round to it!

[jimmyjamjoejoe]

Looks good. I run a 120mm from 2xAA batteries, keeps it slow enough to avoid vibration but still has decent flow (3v is a little low though and some fans wouldnt run on that, i was lucky). The batteries are 1200mah rechargables i think, and give a good 8hrs or more per charge.

[SeedyF]

Looks good. I run a 120mm from 2xAA batteries, keeps it slow enough to avoid vibration but still has decent flow (3v is a little low though and some fans wouldnt run on that, i was lucky). The batteries are 1200mah rechargables i think, and give a good 8hrs or more per charge.

That's useful to know thanks. Zalman quote their silent mode for this fan as being 900 rpm using 7.5v. The speed controller actually takes the output down to 6v so I'm fairly confident that it will be vibration free.

[Astrokev]

A really neat job there. Well done.