Thursday, August 20, 2009

Pimp my Meade Telescope

As another project I bought an used Meade Telescope, a LXD55 8" Smith Newton with Autostar. This is a GoTo Mount, meaning once you did setup the scope it moves to the star or planet you picked by name and keeps the object centered. It was a nice learning experience however what I liked the most was enhancing it.

1. A Laser visible in the sky as a "finder".
2. The cross-hair occular should be illuminated by the main power, not a battery, and it should blink.
3. Same with the pole finder.
4. Dew shield with heater should be temperature controlled.
5. A simple light.
6. Lithium Ion Battery saver.
7. An electric focuser motor controlled by the Meade Autostar computer

Let's start with the Laser:

The problem for me was usually, what star am I looking at? During the initialization of the scope, it moves to the appoximate position of a star and your task is to center it. When you look at the sky you can see the star in question as the brightest object in that region. When you look through the main scope you can see tens of stars with similar brightness but certainly not the one you are looking for. And in the finder - a second attached but smaller scope with a bigger angle of view - there are three equally bright stars. So you have to move the scope for quite some time until you can be sure you identified the constellation and are pointing to the correct star. With a Laser it's a piece of cake.

ATTENTION: Only use Class 2 Laserpointers with <1mW power. They are strong enough to be visible in the sky given average moist weather conditions. And only these are not dangerous to humans or animals. And not dangerous does not mean completely harmless either. So never point at somebody else, neither direct nor indirect (reflections). Above image was shot with an exposure time of 8 seconds using a 1.4 aperture setting.

You can clearly see where the scope points to, you do not have to make any artistic exercises for multiple minutes when looking through the finder. The Laser is a stripped Laserpointer, the green ones work best as the human eye is most sensitive for this color.

It is placed at the telescope clamp onto a bar where I screwed in two bushings with three adjustment screws, similar to how a viewfinder is adjusted.

Not beautiful yet, but when I find the time for a CAD plan, I will ask a friend to mill that out of alloy.

One design question I asked myself was if that system should be completely independent from the telescope or plugged in to the Autostar. Given the fact that I keep forgetting to turn off the polfinder LED and hence the battery is always empty next time I need it, I certainly want to use the Autostar power. So I need to have a DC-DC converter from 12V to 3V between the AUX port of the mount and the Laser. Oh, and a switch somewhere on the mount, not the tube to avoid shaking it.
And a safety mechanism should be implemented as well to warn people the Laser is about to engage. And.....and I need a microcontroller.

I designed a PCB that
1. is connected to the Aux-Port to draw the power and listen on the Autostar internal bus
2. has rocker switches for the different functionality
3. a RJ-45 connector to connect to a daugther PCB, a tiny one distributing the signals required at the tube (Laser, Temp Sensor, Finderscope LED)
4. a RJ-11 connector for the I²C bus of the microcontroller as a future expansion connector
5. a few mini-usb plugs for the functionalities required at the mount (Polefinder LED, Lamp)
6. a connector for the dew shield heater

So when I switch on Laser, first the LED lamp starts blinking for a few seconds and then the laser engages.
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