Dew heater

My SCT fogs up most evenings. I’m a little reticent to use a hair dryer and not sufficiently rich to buy a proper dew heater, so I played around to make my own.

I dug out some nichrome wire, cut it to the length of the diameter of my scope (about 1m) and played around with voltages until I had a small increase in temperature (comfortably warm enough to hold onto with my fingers). It must only be a few degrees. Just be warned its VERY easy to fry you fingers so start with low voltages and work up.

I had a Jaycar kit MOSFET power supply hanging around, slight overkill I know. I dug up a relay I’d used for something else, added a DHT22, an LCD display along with an Arduino and behold!


The power supply is turned on by driving the relay. The Arduino code reads the DHT22 every 5 seconds, calculates the dew point temp, displays the current temp, the dew point temp and relative humidity. If the current temp is within 4 degrees of the dew point it turns on the power supply.

BTW the knob is only used to control the contrast on the LCD as it gets to bright when the scope is taking pictures.

Note the insulator on the nichrome as well. My scope conducts!


An now turned on


More experiments with collimation – an alternate approach

Can anyone tell me why this shouldn’t work? I brought a Hotech SCT collimator a week or so ago, I was sick of not being able to accurately collimate using the out of focus star method. The Hotech was quite easy to use but my final collimation was worse than before, just poor operator I suspect.

I started looking at 60 second shots and the stars were clearly badly collimated, the longer the exposure the worse the star looked, and it’s not the tracking. Out of sheer frustration I started taking 1 second exposures continuously and began twiddling the secondary mirror knobs. The star began to get smaller, and smaller and smaller and more evenly shaped. It ended up with a perfect circle at 400%. The star was out of focus, but not so much that it looked like a doughnut.

So next step run the autofocus routine, the star (it was mag 5) shrunk still more and was still perfectly shaped. Collimation fixed. Now when I’m on on either side of focus or in focus the star is a nice circle. Why can’t we do collimation this way all the time? It was nice and easy and produced a great result.