Can you believe all images from my ONTC telescope were manually focused? This is mainly thanks to the excellent construction of the carbon tube. Carbon fiber tubes have a couple advantages compare to standard steel tubes: first, they are extremely stiff and lightweight. Both properties are highly desirable for a large telescope, you don’t want any flex in the tube as this can screw up the collimation of the mirrors. Secondly, carbon has a much a lower thermal expansion coefficient than steel. As outside temperatures drop over the course of the night, the telescope tube will contract and the focal plane will change. Most setups therefore require frequent refocusing during the night. Focus on the carbon ONTC is rock solid and I often only need to focus once for an entire night of imaging.
It does take time to focus manually and I have to rely on bright stars. Autofocus would be a nice convenience feature to have, it enables full in-sequence focus and temperature monitoring. More automation means more sleep while the gear is outside 🙂
AUTOFOCUS
Moonlite sells (expensive) high resolution stepper motor kits for their focusers. I never bothered with these as I already had an unused Pegasus Astro FocusCube2 laying around from my TS 80mm apo an figured I could re-use that kit on the Moonlite focuser.
Pegasus includes a universal mounting bracket with the FocusCube2 and installation should be pretty easy on most focusers. Except, it did not fit directly on my Moonlite CRL 2.5″ large format Crayford Newtonian focuser. I had to cut a simple spacer block from a piece of scrap wood to fill the gap between the bracket and focuser. Some holes were drilled to fit all the adjustment screws. The original focuser lock screw was removed and I replaced it with a longer 1/4″ UNC bolt to thread the black motor bracket into the red focuser body.
The photo also shows a small metal rod and cable zip-tied to the bracket. This is a temperature probe, and SGP can be programmed to re-focus automatically when the outside temperature changes too much.
The accuracy of my diy motor mount had to be tested under the stars. After some adjustments to the focus step size, SGP showed very nice focus V-curves. I use an exposure time of 10 seconds for autofocus with 3nm narrow band filters.
DIY MIRROR COOLING FAN
Optics should be close to the ambient temperature for optimal performance, especially on larger optics such as my 20 cm mirror. Differences of only a couple degrees will create turbulent air layers above the mirror and inside the tube, and this distorts the incoming light rays. Imagine a candle flame: when you look through the warm air above the flame, your vision will be shaky and blurred. The same effect occurs in a telescope without adequate temperature acclimatization. The optics will magnify the blur effect.
Cooling the 20 cm mirror down to ambient temperature can take a couple of hours, so I built a simple system to increase the cooling rate of my telescope. It consists of a computer fan, shroud, a suspension mechanism to reduce vibrations, and a speed controller.
The actual cooling fan is a Noctua 120mm NF-F12 PWM computer fan. Noctua fans are known for their low vibrations, so they seemed ideal for my project.
I added a flat shroud to make the fan more efficient, it prevents the air from circulating directly back into the fan. The shroud was cut from a sheet of 5mm plexi glass and attached to the backside of the telescope with 3 elastic bands.
I figured the motor might cause too much vibrations at full speed so I added a cheap PWM motor speed controller to the 12V supply. This board was mounted on a piece of foam with electrical tape, not very elegant but it works to isolate the electrical contacts. Initial tests show that I can keep the fan running at max speed without any noticeable effect on image sharpness (star shape or star size) so the elastic bands seem to be very effective at isolating the fan vibrations.
Both the fan and speed controller can be easily attached or removed from the telescope in a couple of seconds. My solution greatly reduced the amount of thermal ‘plumes’ or ‘flares’ on stars at the start of my imaging sessions.
Once the mirror is cooled, I turn off the fan and cover the back of the telescope with a black fabric cloth to block stray light. I have lots of ambient light at night and my flats work much better with the black fabric in place.
Astro Virus 