Saturday, June 2, 2018

Focus on Mars

No new images this week, I'm afraid.  Clouds and rain for the entire week.

Don't worry, though.  I'm continuing the focus on Mars anyway!

One of the frustrating things about using a telescope at high magnification is getting perfect focus on an object.

Touch the focus knob to give it a twist and you'll impart vibrations on what you are seeing through the eyepiece or the imager . . .  Wait a few seconds for the image to stop shaking, evaluate the focus and try again.  And again.  

Change magnification by swapping the eyepiece or the imager?  You'll need to adjust focus again.  And again.

I've mitigated this with a DIY project to add electronic control of the stock focuser that came with my SkyWatcher.

Here is a photo of the focus controller's display showing the temperature, focus tube position, and bar graph of relative position.  The display is covered with dew, hence the streakiness of the photo -- pointing out the importance of water resistance in astro gear. 🙂

Focuser OLED Display

The display will stay illuminated while the focus tube is moving and will timeout after a few seconds of the tube stopping.

Focus on Mars ahead!

The OLED display actually can show 65k of colors, but I chose a pale orangish red to help preserve my night vision when looking at it.

Glorious Color!

To move the focus tube of the scope, I have attached two stepper motors, one on each side, for twice the amount of torque.  The motors are microstepped for 4095 steps per revolution and rotate in synchronicity with each other, but in opposite directions.  In the image below, a driveshaft coupler is visible.  The stepper motor shaft is 5mm, but the focus shaft is 3mm.

One of Two Focus Motors.  Thin Plywood Mount.

The wires from the stepper motors go to the output of a couple of ULN2003 motor drivers.  These drivers are ultimately controlled by firmware running on an ATMEGA328P processor.  

The user (me!) controls the focuser in two ways, over a Bluetooth link, and/or via the hand controller attached to the electronics box.

The Bluetooth link supports the Moonlight protocol so I can make use of existing ASCOM and INDI drivers on the PC.  This is important so that software such as SharpCap and FireCapture can manipulate focus position.

On the hand controller, press the yellow button to move the focus tube up, and the red to move the focus tube down.  Holding the green button while pressing one of the others prevents the acceleration algorithm from ramping the motor speed to anything above a super slow myopic snail's pace.  

Hand Controller 

Here is a short video clip showing the stepper motors in action.

Oftentimes, when I'm focusing on an object, I use the hand controller to adjust the tube up and down.  While peering through the eyepiece or watching the imager output, I can judge the focus by looking at the tightness of the stars.

But sometimes, it is easier to get the best focus by using other means.

This illustrates the importance of having third party support for the focuser.  It is really important in the early morning hours when you are trying to achieve optimum focus on an object and you are tired and grumpy from being up all night. 😏  

Take a look at how I used SharpCap to get Mars in focus:

SharpCap and AutoFocus, May 26th, 2018
The telescope was pointed at Mars and the ZWO ASI224MC imager was in place and attached to the laptop and plugged into a USB3 port.  SharpCap was connected to the focus controller via Bluetooth link.  It moved the focus tube outward and then inward while evaluating the contrastiness of the object in the green box (Mars.)  It then determined the point of optimum focus and subsequently moved the focus tube to that position.

Auto focus! On Mars! How cool is that?


  1. This was useful info that helps me understand more. Thanks for posting. "Contrastiness" haha.:-)

  2. "Contrastiness," non-technical descriptive handwaving. ;-)

    SharpCap has more focus assistance techniques built in. Descriptions can be found here:

    SharpCap Focus Assist


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