SharpCap Forums

I can successfully get SharpCap to control my Starizona autofocuser, but I can't quite figure out how to use the Focus Assistant. I made several tries last night using FHWM, but even though the focus scan was "successful", clicking "Go to best position" did not improve the focus over what it was before I started the Focus Assistant, even though I could hear the focus motor operating so I knew that SharpCap was indeed making adjustments.

What would help me the most is a detailed description of the steps to use the FHWM focus assistant with an autofocuser, and what to expect at each step, and also a description of the options. Are these the general steps: Choose Scan Options, click Scan In or Scan Out, and then wait until SharpCap displays "Focus Scan successful", then choose GoTo Best Position or GoTo Best Score?

What are the best settings for the Scan Options? I tried several different settings, but without a real understanding of what they mean I found it difficult to know which settings were best. What shape of graph should I see in order to know that I've chosen enough iterations?

Since I have an SCT, after selecting some Scan Options, I chose "Scan Out" since I think this is the direction that moves the focus point out, thus moving the mirror "in"; is that correct? On my autofocuser, there are Out and In buttons, and I know to use the "Out" button as the last focus adjustment in order to move the mirror "in" to prevent backlash, so I am assuming it is the same with the Scan Out and Scan In options.

Also, GoTo "Best Position" and "Best Score" each offer a couple of options (positive and negative, if I recall correctly); a description would really help for those options. Thanks!

Ah, a very good question

Sadly I haven't got there yet with documentation for 3.0/3.1 (Dave who helped write the docs for 2.9 has been busy on other projects).

First I'll suggest that it's a good idea to play with this functionality which you can do in the 'High Speed' test camera. Set the ASCOM focuser option to 'None' before opening this camera and you will find that the camera has a 'focus offset' control that varies between -10 and +10 that can be used as if it were a focuser.

Secondly, it's important to choose the right focus measurement. For star fields I'd suggest multi-star FWHM, for lunar/solar/planetar I'd suggest 'Contrast (Edge) Detection'. Also note that good focus scores are the lowest ones for FWHM and the highest for contrast detection.

The idea of the focus scan is to let a scan run so that it starts on one side of the best focus point, goes through the best focus point and then ends up on the other side. When this is achieved you should get a graph that looks like a hill with a maximum at the best focus point (contrast detection) or a valley with a minimum at the best focus point (FWHM). The various options for the scan are

* How big is each movement step in the scan - this depends on your focuser and will take a little experimentation to find the best value. For some focusers a change of +/- 25 is a big change for others it is hardly noticeable. To begin with set this to maybe the size of step you use when manually performing coarse focus adjustments and then tweak from there.

* How many focus score measurements to take at each step. Note that the focus measurements are also affected by the averaging settings (left hand tabs), so for instance if the measurements are set to average over 5 frames and you choose 3 samples per step, SharpCap will wait for 15 frames at each step. Bigger numbers give less error in the measurements but a slower scan.

* How many steps to run during the scan - start with 10 to 15 - you can always scan again in the same direction if you don't go far enough.

Once you have a graph showing a hill or valley you will see that SharpCap draws a curve that roughly follows the points (best fit curve). Once that appears you can use the goto 'best position' or 'best score' options and the fun really starts...

The reason that the last part is fun is because many focusers suffer from backlash, a lack of reproducability (not always going back to the same position for the same value) or both... If you are lucky enough to have a perfect focuser then the best best is to choose 'goto best position' - this will return the focuser directly to the position of the hill or valley in your plot. If your focuser suffers badly from lack of reproducability, 'goto best score' is a better option - this will re-run the scan but will aim to stop when a focus score matching the best score in the previous scan is found.

Note that in both cases, SharpCap will attempt to avoid problems with backlash when you choose 'goto best xxxx' by moving the focuser well past the point of best focus and then approaching it again in the direction that the scan came from. If you have a very large backlash in your focuser (this could be the case for an SCT if you are driving the focus knob as they have dreadful backlash) then SharpCap may not be moving far enough past the point of best focus to make this work correctly. In this case it may be helpful to move the focuser well back past the point of best focus manually (you can right click on the graph) and then return to the point of best focus.

Note that in the graph, points recorded while the focuser is moving in the -ve direction are shown as red, points recorded while moving in the +ve direction are shown green. If your focuser has backlash and you scan in both directions you will find that the two hills (or two valleys) are offset from each other due to the backlash - the distance between the two gives you an idea of the amount of backlash present in the focuser.

Hope this helps

cheers,

Robin

This shows the peak in a -ve moving scan captured using the test camera with the best focus position indicated. You can also see the trace of a previous +ve moving partial scan in green.

cheers,

Robin