Fans for a large Newt

[DJT (David)]

I spent the last 3 days playing with a new to me 12 inch Newtonian and thoroughly enjoyed the experience. It's been a while since I used an eyepiece and certainly not on a scope like this so was blown away seeing my first Galaxy in Leo and the Horsehead neb amongst other must sees. Jupiter was great but a bit fuzzy.

i went to the jewel box and noted that whilst collimation was good, (gotta love the Cats Eye system) looks like I have significant tube currents even after 5 or 6 hours of cooling down.

I havnt managed to get the fan under the mirror working yet but largely because I am a Luddite when it comes to getting the right power cable sorted but I understand there are other mods I can do to try and clear tube currents above the mirror.

What's the experience of the forum trying to eliminate some of the guff from the tube?

Image of the scope attached. Ladder is out of shot...newt on a Gem? What was I thinking ( actually, I really like this scope..)

Hints on Fan Mods would be greatly appreciated.

[janoskiss (Steve H)]

Three things IME/IMO:

#1 Stick to one single fan. If you want more airflow, get a bigger one. That is, if you want to be able to keep the fan on while observing/imaging (one good reason to do so even when ambient temperature has settled is that it can help with dew). Once you have more than one fan they will run at slightly different speeds and will cause "beats": amplified low frequency vibrations due to slight mismatch in how fast they spin. Your scope tube/mount can easily resonate at those low frequencies as well.

#2 Install some basic speed control for the fan; 3 speeds and off is plenty good enough: Run it flat out for cooling off, at medium speed for maintaining temperature, at low speed or off for observing/imaging.

#3 Baffle your fan, i.e., mount it in a solid baffle: make sure it directs air from the back of the mirror up the tube with minimal leakage.

[Shiraz (Ray)]

I had a fabric "shower cap" made up that fits over the end of the scope and has a fan mounted centrally. The cap makes sure all of the air goes past the mirror and it also isolates the vibration. I run an 80mm fan flat out all of the time - it has enough airflow to break up tube currents and cool a 12 inch mirror reasonably quickly. As Steve says, it also helps with dew. The difference with the fan on can be quite startling - within a few seconds of switching it on, the image settles down and sharpens quite noticeably.

[DJT (David)]

Thanks Ray and Steve. Will stick to the one fan and look at speed control.