Fan Vibration Discussion

From: "Ron Wodaski" <ronw@n...>
Date: Wed Jun 26, 2002 5:23 pm
Subject: RE: [SBIG] Re: Elongation of Star Images at Low Elevation

Another possibility to think about is vibration, such as from a fan. I would
expect the orientation of the vibration to change with the rotation of the
telescope as you move to different parts of the sky. Like atmospheric
elongation, this is a small effect seen at very large image scales.

Ron Wodaski
author of The New CCD Astronomy
http://www.newastro.com

From: "Doug Wheeland" <wheeland@p...>
Date: Wed Jun 26, 2002 10:30 pm
Subject: Re: [SBIG] Re: Elongation of Star Images at Low Elevation

I was thinking fan vibration too, having dealt with that problem last
winter. I rotated the camera just a few deg. once I had a position in the
sky where the elongation was obvious. The elongation rotated with the
camera so I knew the fan would likely be the problem.

Doug W.

From: Philip Perkins <philip@a...>
Date: Thu Jun 27, 2002 5:37 am
Subject: Re: [SBIG] Re: Elongation of Star Images at Low Elevation

Doug, Ron,

I worried about the fan when I first used the camera. It was quite noisy
and could have been causing some measurable star image effects. I
contacted SBIG and they sent me a new fan right away. The replacement fan
is very easy to fit which may be worth noting if you have a suspect
fan. Apparently there have been odd batches of bad fans. The new fan is
very smooth and quiet (almost inaudible unless close to the camera).

Are there likely to be measurable star image effects from the
fan? (assuming the fan to be good). I expect that SBIG would have
conducted tests on this at some stage, i.e. on an optical bench, which
would remove the effects of atmospheric disturbance. Would SBIG have any
comment on this?

The experiment mentioned previously will determine whether it is the fan or
not. i.e. repeat the (2 second) exposure with the red filter in place and
if the "trailing" disappears then it is not the fan.

I have to say that I think a _highly_ unusual sort of fan would be required
to cause some 2 arc seconds of image elongation, in a very specific
direction, that is always perpendicular to the horizon, no matter what the
orientation of the camera. Any fan that caused anything remotely close to
2 arc seconds of image distortion would be making its presence known very
audibly, I would have thought.

I'm sort of 'smiling inwardly' as I write, because these sort of
suggestions, although very generously given, and indeed very welcome, are
an example of the sort of things that kept throwing me off the real cause,
for more than a year <g> In the meantime there is atmospheric refraction,
which is known to cause some 2 arc seconds (or more?) of star elongation,
and which of course is omnipresent.

Thanks
--Philip

From: "Ron Wodaski" <ronw@n...>
Date: Thu Jun 27, 2002 8:34 am
Subject: RE: [SBIG] Re: Elongation of Star Images at Low Elevation

In my experience, elongation from vibrations caused by the fan is more a
factor of the resonance of the setup than the roughness of the fan. I had a
perfectly good ST-7E a few years back that only causes elongation on a
Mewlon 210. The elongation was very noticeable, about 3:1! But it was only
visible at about 3500mm focal length or above, when using a Barlow. The
elongation disappeared if I imaged with the fan off.

In other words, the fan mostly is not a problem in this area. But you will
run into physical configurations that have a resonance period, and a certain
freedom to vibrate must be present, too. Cassegrains seem to be the most
prone to this (classical Cass or RCs, which have no corrector/meniscus).

Ron Wodaski
author of The New CCD Astronomy
http://www.newastro.com

From: "Ron Wodaski" <ronw@n...>
Date: Thu Jun 27, 2002 8:34 am
Subject: RE: [SBIG] Re: Elongation of Star Images at Low Elevation

From: "stan_ccd" <stanleymm@e...>
Date: Thu Jun 27, 2002 10:28 am
Subject: Re: Elongation of Star Images at Low Elevation

The 1st thing I do when I get a new camera is unplug the fan…

Stan

From: john gleason<dvj@e...>
Date: Thu Jun 27, 2002 10:52 am
Subject: Re: [SBIG] Re: Elongation of Star Images at Low Elevation

The camera fan can definitely induce vibrations, depending on
ephemeral qualities of the scope (I've seen it in Tak CN-212 f/12.4
and RC f/9). I have only seen it when doing hi-res work (FL>100",
FWHM<2.0 arcsec) but it is unmistakable and easy to diagnose – just
jam a twig (toothpick, matchstick, etc.) into the fan blades.

The 1st thing I do when I get a new camera is unplug the fan…

Stan

From: "Doug Wheeland" <wheeland@p...>
Date: Thu Jul 11, 2002 4:58 am
Subject: [SBIG] Fix for camera/fan vibration

Two months ago I replaced the fan on my ST8E because it was vibrating and
causing elongated stars. The new fan reduced the elongation for a while, but
the problem returned with less amplitude to the vibration. The new fan seems
very quiet and smooth running so I got to thinking that something else on the
camera might be resonant with the fan. Indeed, the metal columns formed by the
cooling slots in the back plate were vibrating. I wove a piece of .25" plastic
tube through the cooling slots to dampen the vibration and that worked well.
Star elongation is gone and guiding is much better.

Regards, Doug W.

From: John Smith <johnzonie@c...>
Date: Thu Jul 11, 2002 8:07 am
Subject: RE: [SBIG] Fix for camera/fan vibration

Hi Doug,

Thanks for the info. When you first saw star elongation before you replaced
the fan, was the elongation always in the same direction with respect to the
camera and rotated with the camera? In other words, if the camera was
aligned with RA along the longer chip dimension, was the elongation aligned
with RA and if you rotated the camera by 90 degrees, did the elongation
change to the DEC direction? I'm trying to assess if this is an issue here.

Thanks,

John
www.hiddenloft.darkhorizons.org

From: "Doug Wheeland" <wheeland@p...>
Date: Thu Jul 11, 2002 10:33 am
Subject: Re: [SBIG] Fix for camera/fan vibration

Hi John... Yes, I did isolate the vibration to the camera by rotating the
camera and verifying that the elongation rotated too. In my case, pointing
at zenith is where the vibration was strongest, with both fans.

You can damp this type of vibration by inserting some small pieces of dense
foam in the cooling slots. It's a quick way to see if you have the problem.
The plastic tube is a cleaner, permanent solution but one needs to remove
the back plate to weave the tube.

Doug W.

From: "azgilrock" <gil@t...>
Date: Thu Jul 11, 2002 12:10 pm
Subject: Re: Fix for camera/fan vibration

Was there any noticeable change in the temperature you were able to
cool down to? It would seem to me that adding the plastic tube would
restrict the airflow and defeat the purpose of the cooling fins.
Although I would probably rather be a little warmer than have
vibration.

Gil

From: "Doug Wheeland" <wheeland@p...>
Date: Thu Jul 11, 2002 1:40 pm
Subject: Re: [SBIG] Re: Fix for camera/fan vibration

Did not notice any change... the area blocked is quite small as I left a
small space as I looped the tube through the slots. The tube touches only
the edges of the columns.

Regards, Doug W.

From: "Doug Wheeland" <wheeland@p...>
Date: Fri Jul 12, 2002 12:12 am
Subject: Re: [SBIG] Re: Fix for camera/fan vibration

Hi Frank. I put a photo on my web site...
www.pcisys.net/~wheeland/latest.htm

Regards, Doug W.

From: John Smith <johnzonie@c...>
Date: Mon Aug 5, 2002 4:45 pm
Subject: Fan vibration

There have been some comments in the past about fan vibration impacting
image resolution, with some folks using foam insulation to isolate the fan
from the camera. For something to do during the (never-ending, it seems)
monsoon season, I found something called a magnetic levitation fan that
apparently floats the blades on a magnetic field. Theoretically, this
should result in minimal transmitted vibration. I've ordered one of these
to experiment with but was wondering if anyone has ever tried one as a
solution to fan vibrations? (I ordered a ball bearing version as well as a
backup <g>)

I'll report any interesting results.

John
www.hiddenloft.darkhorizons.org

From: John Smith <johnzonie@c...>

Date: Thu Aug 8, 2002 3:39 pm
Subject: RE: Fan vibration - "results"

Today I received the below mentioned fans. I compared the vibration
somewhat subjectively, by holding the sides of the fan body to see what
difference there was, if any. I also compared them to the fan that came
with the ST-8E. The mag lev fan seemed to have the least amount of
vibration. The ball bearing had the worst. The SBIG fan was comparable to
the mag lev in some orientations but seemed to have other orientations where
it had considerably more vibration. The mag lev fan seemd to have the same
vibration level in all directions - I would estimate about 10% of the ball
bearing.

Without an accelerometer probe, I can't be more quantitative than that,
unfortunately. The changing level of vibration of the SBIG fan concerned
me. I would estimate in its worst orientation, it had around 2x the mag
lev. So, I decided to install the mag lev in place of the original. As
near as I can tell, it has approximately the same air flow.

I apologize for the highly qualitative nature of this note. If anyone else
is interested in doing these tests, the fan was obtained from
www.cyberguys.com and is their part number 148-0355 and goes for $11.

With a big "FWIW" <g>

John
www.hiddenloft.darkhorizons.org