Fitting Cooling Fan to Sky-Watcher 250PD-S

Fitting Cooling Fan to Sky-Watcher 250PD-S

  1. Why it’s cool to cool. The mirror of a large Newtonian reflector is a large block of glass, a material which dissipates heat slowly but also has a significant thermal capacity. While the mirror cools down, due to variations in density, air currents are set up which can spoil the image. The larger the mirror, the longer a mirror takes to cool and as temperature may be falling continuously for many hours while observing, it may never reach a stable temperature…without a fan! Mirror mass increases with the cube of its diameter, so a 10” mirror is twice the mass of an 8” (1000/512) and 10” seems to be the size when fans are sometimes included in the standard telescope design.
  2. Research the internet. ( shows only a small 12volt fan is needed, preferably with a high speed for initial cooldown followed by slow speed while the temperature falls during observing. This Youtube video ( was also useful, even though it’s quite slow and a bit long. Do not mount the fan directly to the mirror cell even though my telescope had tapped holes for this. Mount it on a baffle plate to stop air re-circulating from discharge to suction and it also reduces vibration. Although I used the resources above I added the following: an alternative 5.2v USB supply (for ultra low speed), an illuminated on/off switch to avoid leaving fan switched on and flattening the battery and I used the 3 mirror cell locking screws to secure the baffle instead of Velcro tape.
  3. Equipment List.

fan: ex computer 12v 102mm with integrated 3 speed control – RS Potts, Babington Lane Derby – £4,

baffle plate – 3mm black Perspex/acrylic machined to outside diameter to match recess in mirror end and with hole to match fan duct size – – £22. Alternatively, use old 12” vinyl record, more cutting but much cheaper.

12v plugs and sockets: Discount Store Swadlincote High St. – £1.20 each

12v/5.2v USB converter – ebay – ? already had one

small illuminated switch – ebay – £2.40

small canister for switch – Discount Store Swadlincote High St. – 80p

12v re-chargeable battery – RAG member Bob Williams – contribution to Observatory Fund

various M4 and M5 screws, washes, nuts, low power cable, black adhesive tape – Discount Store, Swadlincote.

5. Procedure I’ll let the photos speak for themselves. Be cautious cutting acrylic because it splinters easily. Always support on rear side of cutting tool and use fast speed and minimum force. Peel off clear protective film only after all shaping is complete. To get the approximate positions in the baffle plate for the 3 locking and 3 collimation screws, I made a cardboard template to transfer the hole positions to the acrylic baffle plate, first attaching masking tape to the surface. Drill holes in the acrylic with a sharp 5mm drill and a 20mm hole borer for the locking and collimation screws, respectively. Route wiring so it does not shorten as telescope is moved and so switch and speed controls are handy. Fix to tube with black fabric adhesive tape. A bit crude, but could not think of a better way.

6. How does it perform? On low speed  range (5.2v) its silent so unlikely to be any detectable vibration. Will only use high speed for initial cooldown, then swith to low speed while observing. Since fitting the skies have not been clear so will report back as soon as it’s been tested.

cardboard templatefan with 12v and 5.2v adaptor

trial fit on telescope

transfer positions from template to acrylic baffle

drill holes for locking and collimtion screws

drill holes for M4 fan screws

fit fan to a shiny baffle plate with nuts on the outside

trial fit and trial run

route cable and add the illuminated switch mounted in small plastic canister next to speed control, fix with black adhesive tape.

finished arrangement, red light on switch is brighter than it appears.


1 Response

  1. Looks like you’ve done a great job – and I know that someone will come across this post in the future and be really grateful that you’ve posted it – they will find it ideal to tell them what to do!

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