How to Autofocus with ASIAir

 

V2.1 - 2/21/2023

Just an update that EAF firmware version 3.3.7 now adds support for a maximum step count of 600000.  This should help hyperstar users who switch back and forth between native mode and require 120k+ steps to reach focus.

V1.5.3

One recent feature that is still being improved is autofocus.  Using the ZWO EAF you can electronically focus your telescope and now it can be done automatically.  The EAF is attached to your telescope focuser either with the included bracket or with a custom bracket.  If you are handy you can even make one on your own.  Before AF can work efficiently you must ensure that the shafts are locked really good.  Most shafts including that on the EAF have a flat side that must have a set screw torqed properly against the flat to reduce backlash. As of this writing, ZWO has released a new EAF that runs off USB power.  The original version required 12V and a USB connection.  I have been told that the mechanics and gearing are the same so should be interchangeable.

Everyone is familiar with autofocus on their cameras or phones.  These work great with bright daytime scenes but don't work well or at all under low light levels.  With telescopes the current method utilizes multiple exposures while advancing the EAF motor in fixed steps to plot a vcurve similar to the one shown at the top of this page and is based on the size of stars in the field.  You first must start with an image that is in eyeball focus.  Then, a dozen or more shots are taken and are run through a formula to determine the curve.  The curve is an approximation, but the apex should represent the position with the best focus.  Lastly the routine will do a short validation test taking 3 or 4 shots near the apex to select the best possible position of the motor.  The curve will never be precise due to atmospheric seeing.  However, we are not shooting for precise.  We really want to know where the bottom of the vcurve is.  This point represents the EAF position that should result in the lowest HFD or smallest star size.  This is the red dot shown in the first image here.

So this explains how it functions.  So how do we use it?  In principle it is very easy.  The autofocus settings are located under Focuser Settings.  There are two main settings that affect how well AF works, AF Exposure and Step Size.  The AF Exposure setting here is the default AF exposure. The exposure used depends largely on what filter if any you are using.  Shorter exposures generally work best. However with narrowband or broadband filters exp of 1-3 seconds wont work very well.  In those cases you want to use 3-4X what you would use with a luminance or no filter.   Because each scope is different you have to try different exposures to see whats best.   If you use an EFW filter wheel, you will notice now that each filter has its own AF exposure time.  This is set under the filter wheel configuration screen.  This allows you to set custom exposures based on the current filter used. 

The next setting is Step Size.  This is the number of EAF motor steps that will advance between AF exposure tests.  This can be useful to tune the AF sequence when you have custom setups where the default of 30 steps is not correct.  Examples for changing this are when you have connected the EAF to your 10 to 1 fine focus shaft.  You could multiply the default by 10 to make it 500 steps.   Another example would be if you use a homegrown belt system to focus your Redcat or camera lenses.  Or maybe you just want to speed up the AF sequence and are ok with a little loss of precision.   Whatever the reason this setting can help you tune the AF for your particular setup.  My C11 Edge with a Feathertouch focuser uses 60 steps.

The other settings dictate when asiair should start an AF sequence.  These are pretty self explanatory so I wont detail them here.  Just note that these can be used in combinations so that any activated trigger will kick off an AF.  If a trigger is met, asiair will wait until the end of the current image before initiating the AF sequence.  

I will add a note here about temperature changes.  The EAF contains an internal temp sensor which can be used to monitor for environmental changes.  A remote temp sensor can be added for focusers that have a slot for such a sensor.  I have also been inclined to add a remote temp sensor into the rear cell of my SCT to monitor the change internally to the OTA.  In most cases you can just use the internal EAF sensor.  Also its important to note that these sensors are NOT calibrated.   They will not accurately measure the outside air temperature. All we are concerned about is the change in temp not the absolute value. 

You can initiate an AF routine manually by clicking the focuser icon on the left side of the screen and then clicking AF.  Clicking the play icon starts the routine. The process can be stopped by pressing the stop icon.  The settings described previously are only used when an Autorun sequence is running.  Otherwise you must conduct the AF sequence manually. 

The other EAF settings such as fine and course have no effect on autofocus. Currently the backlash setting is only used when manually focusing using the on screen controls or EAF hand paddle.  Exposure and step size are the only settings that control how AF works.  Its important to get them right.  This may change in future releases but ZWO currently wants to keep it simple. 

If you are getting error messages that the EAF moved less than 900 steps then you need to increase your step count by 5 or 10 at a time until you have success.   Smaller step sizes will generate a more precise curve but will take much longer to complete.  Conversely, using a step size too large will generate a curve quickly but will not be very accurate.  Use a step size that will take at least 12 sample points then compare a number of preview images. If your precision is still off then decrease the step size by 5 or 10 and try again.  Try to use as large of a step size as you can and still get consistent results.  Dont go overboard on the step size.  Increase it by 10 each time and see how it performs.   Every system will be different. 

The ASIAir autofocus is evolving and expect to see improvements in the function with each new release.