Cooling fan usage during imaging on RC telescopes: vibration concerns - Forums

Topic starter

9 hours ago

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

Frequent contributor

9 hours ago

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

8 hours ago

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

Frequent contributor

8 hours ago

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

7 hours ago

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

Frequent contributor

7 hours ago

Danke und Grüße Rainer

Behyar Bakhshandeh

2 hours ago

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.

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Rainer Ehlert

Frequent contributor

2 hours ago

Ruediger · Jan 27, 2026, 09:28 PM
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.

I asked because I am evaluating adding some fans to the rear of my Mewlon 250S. This one has 6 holes with 63mm diameter.

Tobiasz

48 minutes ago

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.

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Gary Seven

31 minutes ago

I have the iOptron 10” variant and it is LOUD.

Safe to say that noise level is a proxy for vibration.

Back in my telescope making days I replaced Radio Shack fans on my DOB with Silenx fans and it made a noticeable difference in high power diffraction rings.

Silenx makes a wide assortment of fans, when time permits changing out my iOptron fans will get done.

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