Cooling fan usage during imaging on RC telescopes: vibration concerns

Dew Heater Reflectors Ritchey-Chrétien Acquisition Guiding Help Needed
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Paul MacAree avatar
Paul MacAree avatar

I’m looking for real-world experience with running cooling fans during imaging on RC telescopes.

I have a 12″ Ritchey–Chrétien (TPO/OPT, GSO-made) with the standard three rear-cell fans. Running the fans clearly helps to reduce dew issues over the course of the night, but I’m concerned about fan-induced vibration. A recent presentation on Astro Imaging Channel suggested that leaving fans on throughout the imaging session can be beneficial for dew mitigation, which prompted me to revisit this. I have found that running the fans continually does help with dew on the primary mirror and it has allowed me to reduce the heating duty cycle to the main mirror.

Since these fans are hard mounted to the rear cell rather than isolated or mounted near the dew shield, I’m curious how others handle this in practice. Do you run the fans only during initial cooldown, or leave them on while imaging? Have you seen any measurable impact on guiding, eccentricity, or FWHM with fans on versus off?

I’m particularly interested in RC owners’ direct experience rather than theory. Thanks in advance for sharing what’s worked for you.

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Rainer Ehlert avatar
Rainer Ehlert avatar

Hi,

I can not answer your question, but the easiest way to find it out is making a test.

Runa a series of images on the same night and turn the fans On and Off after each image and then compare the result of the On images and the Off images.

The question here is that not all fans on all RC’s are really the same. Some can be better and some can be worse.

Just an Idea

Ruediger avatar
Ruediger avatar

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

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Rainer Ehlert avatar
Rainer Ehlert avatar

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

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Ruediger avatar
Ruediger avatar

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.

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Rainer Ehlert avatar
Rainer Ehlert avatar

Danke und Grüße Rainer