lostbit I might be able to also try another
idea is to increase exposure to 4 seconds to see if I spot any star way out of focus.
That usually works, but if you are really out for focus, you will need to max out the gain of the camera and use much longer exposures.
Here is another thing you can try if you have very dark skies.
Point the scope towards a bright star (Capella, Sirius,...) or a bright planet. Just get it pointed enough so that the star is within the frame of the sensor.
Increase the camera gain until you see a Max ADU value that is perhaps a quarter of a saturated frame (sensor saturation is usually around 65535 [16 bit number], but it can be different for different image capture programs).
Move the focus and take another picture.
If the Max ADU has gone up, you are focusing in the correct direction. If the Max ADU goes down, you are going in the wrong direction.
Rinse and repeat... and adjusting the exposure time or gain at each step so that you don't saturate the ADU... at some point, you should be able to see the points of light visually.
The reason this works for rough focus is that the amount of photons from a star that reaches the camera sensor is a constant (unless the star's disk is larger than the camera sensor). When you are out of focus, all those photons are shared among many pixels, and thus the max ADU (that comes from the brightest star in the view) will also be low. When you bring the optics into focus, the total photons is spread among fewer pixels, so the Max ADU (the ADU of the central part of the brightest star in the view) will increase.
I use this "blind focusing" quite often when I have absolutely no idea where the focus is (typical when you have compound optics, like reducers, etc). The sky has to be dark enough so that the sky background is dimmer than a defocused star.
For simple optics, like the cheap ZWO guidescope, simple measurements should be sufficient to get you close enough to see star images.
Chen
