Cooling fan usage during imaging on RC telescopes: vibration concerns

Hi Paul,

I have three fans on my PWI CDK 14. When switched on, the Hfr reduces significantly, since tube seeing is reduced dramatically. Unfortunately I have to switch them off at 22h o’clock due to the noise bothering the neighbors. Otherwise I would keep them switched on. But I would also advice to make a comparison test, since it also depends on the fans and setup:

1. Air volume ventilated
2. Bearings of the fans and resulting vibration
3. Potential resonances and resulting self amplifying vibrations

CS Rüdiger

Rainer Ehlert · Jan 27, 2026, 08:30 PM

Ruediger · Jan 27, 2026, 08:08 PM

Hi Paul,

I have three fans on my PWI CDK 14. When switch on, the Hfr reduces significantly, since tube seeing is reduced dramatically. Unfortunately have to switch them off at 22h o’clock due to the noise bothering the neighbors. Otherwise I would keep them switched on. But I would also advice to make a comparison test, since it also depends on the fans and setup:

1. Air volume ventilated
2. Bearings of the fans and resulting vibration
3. Potential resonances and resulting self amplifying vibrations

CS Rüdiger

OFF topic question

¿do they blow air into the tube or do they suck the air out of the tube?

¿and what about the dust collection?

Thanks

They blow air into the tube, exactly to avoid dust. You have a slight air over pressure in the tube, like in a clean room. It blows polls and dust out from the scope. At the air intake of the fans there are filters installed. Fortunately not too fine, but good enough. This could be improved.

BTW: Big scope have very often fans on the side of M1 blowing a continuous air stream parallel to the surface of the mirror. This avoids static stacked layers of air with different refraction over the surface. Quite effective.

For the last 13 years, I have used a 10” RC with three fans behind the primary that are always on, and I have not seen any impact from them.

They certainty help keep the dew at bay and cool the tube early in the evening.

I am on my second RC and only use the fans to cool down the primary to ambient temperature, then turn them off.

In my opinion keeping them on worsens the tube currents which does not help with the star profiles. The fans do not help against dew either.

After 30-45min cooldown time with the fans on, I switch them off and turn on my dew heater strap on low power which is wrapped around between the focuser and the rear of the primary mirror housing. This definetly helps against dew on the primary. For the secondary mirror I use a DeepSkyDad secondary mirror heater.

Everything else I tried (Dew shield, fans on, hopes & prayers) never helped against dew here in southern Germany.

Just my personal experience.

Clear skies.

I have to run the fans on my RC (an ASA600) to get good imaging. I recently saw FWHM values below 0.8” so vibration does not appear to be an issue. Of course the ASA600 is a considerably more massive scope than your 12” but if you have a solid mount and good fans, you shouldn’t have a problem.

John

Rainer Ehlert · Jan 29, 2026, 02:33 AM

I have a Mewlon 250 and this one has 8 round holes on the back end. Does it make sense to add some fans?

Thanks Rainer

OK, I will run a test and just finished Turbocharging 🤣 my Mewlon 250S bought in 2006.

😵‍💫😵‍💫😵‍💫

I had all materials here and only needed to purchase 4 air filters for 50mm fans.

I assume that if there are vibrations this will sum out as I guess the vibrations will not synchronise like the tests with Metronomes as seen in several you tube videos or will the fans synchronise ¿?

https://www.youtube.com/shorts/RYeNu159Sgc?feature=share

Mewlon_Turbo_0.jpeg

Mewlon_Turbo_1.jpeg

Mewlon_Turbo_2.jpeg

I am a newtonian builder so wider FOV means it isn’t completely apples to apples but a lot of this can still apply

If you can, get the largest diameter fan possible. They can push more air at a lower rpm then a smaller fan. Not always possible on a pre-built scope. In the case, be sure to use a high quality fan. I feel like a lot of commercial scopes use cheap commodity fans. Noctua is an excellent choice.

The cool thing (pun!) is that while there is plenty of conjecture and theory floating out there, it’s very easy to see the results for yourself. I use sharpcap but any capture software that can do high fps live view will work. Do whatever you need to do get the highest fps possible, usually 8 bit + a windowed FOV. Center a bright star and slightly defocus it as if you were doing a star test. Now on/off the fans… if there are any issues induced by their vibration, it will become obvious.

As previously stated it’s very beneficial to leave the fans running if they do not cause any vibrations. Dew mitigation and mirror boundary layer disruption are all big wins. And if it’s a closed tube or well covered truss, air moving up the tube will keep the secondary from dewing as well.

In fact, with this same test you can watch the internal scope thermals do their thing. Try it out with fans off and fans on. Very telling. I’ll have to dig up the videos I made of the process and post them at some point.

If noise is a concern, you don’t always need the fans running at full blast. I’ve experimented with running them off the dew heater outlets on my powerbox, turning the pwm output to 90% reduces the noise quite a bit. If you have a powerbox with multi-voltage outputs a 12v fan at 9v to get lower rpm and noise.