| article: The Effects of active Tube Ventilation on High Resolution WebCam imaging with a Newtonian Telescope |
Active Ventilation
In the past the amateur astronomers relied on the passive ventilation of their tubes.
The tube was designed fairly larger than the primary mirror and the mirror cell was constructed as “open” as possible.
After 30 to 60 minutes of cooling down to the ambient temperature the Newtonian was usually regarded to be “operational”.
Any image degradation still visible was regarded to be caused by external seeing.
During the first tests of my homebuilt 12 inch Newtonian I sometimes found egg-shaped stars in my images.
Usually – but not always - that happened at lower angles in the sky.
Read more details about that problem and view example images.
After discussions on various newsgroups it became obvious that these caustics might be caused by tube currents.
So I built an active ventilation system with a little PC cooling fan of 50 mm diameter.
This fan is pulling the air from the center of the primary mirror cell.
This is the most primitive setup and easy to install.
Simple test setup for the fan
High Resolution WebCam imaging
I used a Phillips ToUCam Pro in combination with a 5x Televue Powermate Barlow lens.
The effective magnification of this setup is 6x, giving a resolution of 0.165 seconds of arc per pixel.
With the Nyquist theorem for two dimensions (diagonal of the pixel, equals 2.8 times) this is just giving the critical sample rate according to Rayleigh which is 0.45 seconds of arc for this telescopes.
Click here to read more about the resolution of telescopes.
The high readout rate of the WebCam (five frames per second, uncompressed) is providing a lot of frames in a short period.
So I hoped to be able to image the actual effect on the image of a star with and without the active ventilation.
Testing Conditions
The design of my 12 inch is already providing a reasonable good passive ventilation.
Also the telescope is always store outside an hence close to the ambient temperature anyway.
Therfore it was quite difficult to image any improvement of the active ventilation.
The following conditions must coincide to give a noticeable improvement:
- Rapid change of the ambient temperature in the order of 5 degrees per hour or more
- External seeing conditions must be better than appr. one second of arc
Several times there was no difference at all in imaging Jupiter or brighter stars with or without the fan switched on.
This time an approaching thunder storm dropped the ambient temperature very fast.
The storm passed by and the sky was clear again.
But the dropped temperature was leading to a hazy, not very transparent sky which is very often a sign for good seeing.
Test Results
In fact I could achieve high resolution images of the improvement caused by the active ventilation.
I picked Arcturus (Alpha Bootes), a reddish star of magnitude -0.04, fairly high in the sky at that time.
The following images are the mean of 1000 single raw frames.
Out of Focus Image without the Fan running
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Out of Focus Image with the Fan running |
Please note that the shape of the star pattern is not round without the fan running.
In the recorded video there were occasional strong caustics almost looking like flares on the sun.
The extremes were averaged out by image combination.
Focused Image without the Fan running
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Focused Image with the Fan running |
With the fan running the star is looking almost perfectly round.
Without fan the star is elongated to the upper left.
Also the star’s size is bigger by about 20 percent.
Results for short Exposure long term Observation
The following example is Arcturus again.
The observation was made during twilight, hence the blueish sky.
The image is made from an average of 1500 carefully aligned rawframes.
Each rawframe was 1/500 of a second of exposure.
This is short enough to freeze the seeing in each raw frame.
The total integration time was 300 seconds, hence 5 minutes.
Fan not running, typical Raw Frame
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Fan running, typical Raw Frame |
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Fan not running, Mean of 1500 Frames
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Fan running, Mean of 1500 Frames |
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Fan not running, Profile
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Fan running, Profile |
As you can see in the profile the peak value increased from 200 to 250.
Thw FWHM of the star improved accordingly.
With the fan running there is still a slight imperfection in the image (probably coma, still under investigation).
Conclusion
The positive effect is “reversible”.
That means if you switch of the fan it takes only about 10 seconds and the image quality is getting worse again.
It took appr. 30 minutes with the fan turned on until the mirror lost most of his thermal energy and was close enough to the ambient temperature.
From that moment on there was no noticeable difference in image qualitiy anymore.
For me it is obvious now that the fan can be helpful for high resolution as well as for deep sky imaging.
This is true whenever the ambient temperature is changing with relativle high rate.
If you want to have a look at the (heavily compressed)
raw frame videos of 2 MB click here.
They are nicely showing a Speckle-like effect in realtime.
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