josefran I have the sky-watcher mak 150, the oag ZWO and the 178 mm
Can you focus visually using an eyepiece?
If so, try to focus visually on something in the daytime.
Then measure the optical distance from the metal back of your OTA to the focus location of the eyepiece (most good eyepieces will provide that focus location in the data sheet).
That focal location is exactly where you want to place a camera sensor. If you have the pancake version of the ASI178, the flange to sensor distance is 12.5mm. So, you need to use enough spacer to position that focus location 12.5mm inside the camera.
Most beginners are surprised by how long the spacer needs to be when you use a camera, but you need to compensate for eyepiece users who use diagonals.
For infinity focus, you use the thin lens equation to compensate for the amount you need to reduce the spacer length for night time use.
https://en.wikipedia.org/wiki/Thin_lens
For example, if the object you are using in the daytime is 100m away, and in your case, the focal length of the OTA is 1.8m, the night time focus will need the spacer to be about 32mm shorter than what you used in the daytime. If the daytime object is 1 km away, the night time focus needs a spacer that is 3.2mm shorter.
Chen
