Fans in cold weather - part 1

I live in Nova Scotia, but travel 2000 miles to Alberta a few times each year. I recently purchased an 8” f/8 Discovery to leave at the Alberta location so I could have a decent scope to use when I am visiting the area. I did a number of common modifications to the telescope which included installing three small fans on a board, which is mounted just below the primary mirror cell. The fans suck air down the tube and exhaust out the bottom of the tube. The fans are operated off a small 12V gel cell with a rheostat to control their speed. I have calculated that with the fans operating at their lowest setting, I am getting two tube changes of air every minute.

Over my Christmas visit, I spent about five nights observing with the scope. The temperature varied, but was always below zero (Celsius) which meant there was a significant amount of ice crystals in the air making the transparency quite poor for this location (NELM from ~4.9 to 5.8). I found the little gel cell did not operate well in the cold weather and I didn’t always use the fans. I was normally viewing planets using a 7mm Pentax (230x) and when conditions allowed, I used a 5.2 mm Pentax (310x). The views at 310x were usually sharp only for perhaps 60% - 70% of the time as the sky steadied for a few seconds.

During the evening of December 27, the clear sky clock predicted good seeing (5 out of 6). My brother and I set up to do some planetary observing just before midnight. The temperature was -21° C (-5° F) and never changed through our observing session. I did install the battery and I turned the fans on to their lowest setting. Over the next 15 or 20 minutes, we observed Saturn and slowly increased the power as conditions allowed. As during the previous sessions, we began at 230x and moved to 310x after about 10 minutes. After another 10 minutes, the image was very steady and I decided to try more power. I placed a 9mm Nagler (T6) in a 2.5x Powermate for 450x. Much to my surprise, the image was steady and very crisp. Over the next 20 minutes we traded views and marvelled at the wondrous image of Saturn.

At some point while my brother was viewing, I leaned around and switched the fans off. A few minutes later, he reported that he couldn’t get the image to focus. I told him that I had shut the fans down about 5 minutes earlier. We tried to turn the fans on again, but the little gel cell would not power them. We decided to take a break and go in for tea and I placed the gel cell on a trickle charger in the garage.

Gary Weber

Continued from part 1...

After an hour we came out and hooked up the battery and it powered the fans at their low setting just fine. Once again, it took about 15 minutes or so to get to 450x, but it worked and the image was very steady. After about 20 minutes the battery gave out and the fans quit. This was noticeable as the image deteriorated and at 450x was not sharp at all. We commented on this and placed the battery on charge again and went in for more tea.

About an hour later, my brother went to bed and I went out for more observing. Jupiter was high enough to try viewing at this point. The pattern repeated itself – about 15 minutes to get to high power while running the fans. Saturn was rock steady at 450x and Jupiter was steady only about 25% of the time. In the first 5 minutes of this session, Jupiter was not sharp at all at 310x. I could only focus a sharp image at 450x after running the fans for 15 minutes or so. After viewing both planets for about 20 minutes, I was chilled so I packed up and went to bed.

My preliminary conclusions are that running the fans indeed makes quite a difference in the ability to use high power. What would normally be considered limits due to seeing conditions can be mitigated by running fans in this configuration. Without the fans, 310x was effective part of the time. After running the fans for 15 minutes, 450x was rock steady on Saturn and moderately effective on Jupiter. The part that still confuses me is that there was no change in the ambient temperature during our observing session – it may have dropped a degree C during the two hours prior to our observing. The telescope had been outside all day and was fully cooled when we began observing.

Although this is not a comprehensive test, it certainly has sold me on the value of operating fans to get the most out of high power viewing.

Gary Weber

I *always* run a variable speed 120mm side mounted fan and an 80mm rear mounted one in my 16" f/4.5. I have an insulated temperature probe affixed to the side of the mirror and another probe to monitor air temperature, but in practice I don't usually bother turning the fans off even when the temperature differential is within 2 degrees. I used a sandwich of open and closed cell foam to mount each fan, which seems to have done a good enough job isolating vibrations that I consistently observed Mars @ 600X during the recent opposition whenever the seeing permitted.

This last Sunday we had truly exceptional seeing, and just for fun I decided to see how high I could push the magnification. I was able to get good views of Saturn @900X and decent ones at 1200X. I am convinced that this would not have been possible without these fans in place.

Get yourself a bigger battery so you don't have to worry about losing power during an observing session.
Jim