Dobsonian Fan Experiment II

Hello all,
I performed an experiment with our Apertura AD10 (Carl) to see if using a stock rear-mounted fan improved visual clarity at high magnification. This is my first experiment attempting to highlight a phenomenon known as a "Boundary Layer" and to see if a stock rear-mounted fan disrupts it at all.
The Hypothesis:
Even an acclimated 10" telescope mirror will release heat at a slower rate than the ambient temperature drop throughout the night. As the night cools, the 10" mirror will not cool at the same rate and the slowly escaping heat from the mirror's surface will create a "Boundary Layer" above it.
As light travels into the telescope it must pass through this boundary layer, causing a distortion to the incoming light. This is due to the differences in air density within the telescope. Light passes through empty air faster than dense air. Thus, incoming light will not reflect evenly off the mirror and will make details at high magnification "dance" for the observer.
Supposedly, using a fan to assist with cooling and creating an outward flow of air for the heat to escape should cause the boundary layer to be disrupted and even blown away. This should make details remain more steady, so long as the fan doesn't also cause vibration.
My first goal was to identify that a boundary layer indeed exists after a telescope has been left outside to acclimate. To prove to myself that my mirror really is radiating heat as the night cools. And to see if using the fan made any difference to the distortions at all.
The Camera Experiment:
On January 10th, 2023, I let my Apertura AD10 acclimate for 10 hours in a shaded area starting at 12:00 with the Sun setting at 16:39. Before use the telescope was collimated and all the accessories were also acclimated for 45 minutes.
The accessories used were the stock AD10 RACI Finder and a ZWO ASI 120MC Planetary Imaging Camera. The reason I used this camera is twofold: One is because it is a cheap and popular camera used by many amateurs. My hope is that if one wished to repeat this experiment, it wouldn't be overly expensive to use the exact same equipment.
The other reason I chose the 120MC is because it has a small sensor and when used with the Apertura AD10 the field of view is similar to 250x with an eyepiece. This is considered "high power" or "high magnification" according to this article: www.telescope.com/Telescope-P...
After experimenting with Polaris on a patch of grass, I moved the telescope to nearby asphalt and aimed at Mars. Since Mars moves across the field of view much faster than Polaris, I edited the videos using PIPP to crop and keep Mars centered so we can better focus on the distortion patterns. To better analyze the distortions, I also de-focused Mars until I saw the shadow of the telescope's secondary mirror and its four supporting beams. With Mars de-focused, the slow moving waves of heat within the telescope were revealed to me.
I recorded two videos of the distortion patterns using the ZWO ASI120MC, both with the fan on and off, roughly two minutes apart from each other.
With the fan off I noticed waves of heat slowly moving across the mirror.
With the fan on the distortions moved much quicker, were more broken up, and seemed to move in a circular motion. This at least concluded (for me) that a stock-rear mounted fan does make a difference in the movement of thermal distortions. That the fan is actually doing something!
Any insight into what we are actually seeing in these videos would be helpful. I am not certain if the waves of heat I see are actually coming off the mirror, the secondary mirror or coming from the asphalt the telescope was set on. All I see is that the fan changes the directional flow and structure of the thermal distortions within the telescope.
To improve this experiment I think we should use a thermometer on the mirror itself to prove the mirror is still acclimating to the outside temperature. This will be implemented in future experiments.
Sources: skyandtelescope.org/astronomy...

[@jesusislukeskywalker4294]

looks a little bit like caustic waves. i recently discovered the phenomenon from the sun shining through my new venetian blinds. 😳 commonly found under bridges in venice 🤠 apparently 👍🏻

[@spacetimewithrobert4438]

Hello Jesus,
After looking up the definition of caustic waves, it seems that this is exactly what we are seeing. I really appreciate the clarification!! I can also imagine the reflection of water surfaces on the underside of a bridge just as you describe and indeed it looks similar to me. Thank you for expanding my vocabulary!
- Robert

[@kmagnussen1052]

I have a 10" home made Newtonian. The mirror was originally going to be a .Schmidt-Cassegrain thus a hole. But the price was right. I use a fan for 30 minutes just at full darkness. I turn the fan off unless the temp drops a few more degrees. I am not sure you can run a fan with any significant air movement while using the scope. The air moving even if it is the same temperature may cause distortion. But I want to see what you develop.

[@spacetimewithrobert4438]

Hello K Magnussen,
That sounds like such a challenging telescope to put together. I'd love to see photos of your homemade 10" with the hole in the mirror. Do you collimate it like an SCT, utilizing the shadow method to make a concentric ring? I am very curious how it performs!!
I like to follow @astro_mp and @astro_zane on Instagram. They make their own scopes and their creativity is mind blowing to me. If you're looking for others who dabble in home-made optics, they are fun to follow.
I think it would be a great to capture a video of what you're describing! How does this sound: First, record a video of the acclimated telescope pointing at a de-focused bright object after sunset. Then, use a fan for 30 minutes and turn it off again. Finally, record a second video with the fan off and compare the two. I will try this at a time when the temperature gradient won't change drastically in 30 minutes so both videos are recorded near the same ambient-air temperature.
Does this sound like a good experiment and if so, what would your prediction would be?
Thanks for helping!!!
- Robert
Edit: a word