Focusing is one of the main worries of all astro-imagers.

For my LX200 I have made a Diffraction Spikes Mask, which - when aimed at a bright star - creates nice spikes at the star.
Those spikes should be single and have sharp points: when that is the case I am in focus.

I learned this when using my Newtonion telescope and implemented the same technique for my LX200 and William Optics Zenithstar 80 ED II APO

	Needle sharp diffraction spikes.
	I used a square piece of plywoord, 35x35 centimetres. After some experiments with other material I found that 3 millimetres thick laundry line generates nice spikes.

	This is the dew shield I made for my William Optics Zenithstar 80 ED II APO. Please note the metal pins in the dew shield: they create diffration spikes, which are great for focusing and also look nice in the image.
	Diffraction spikes mask with fishing line In the plumbing department of my local DIY shop I bought a plastic ring which nicely fits in the dew shield I already made for my ED80. To generate the diffraction spikes I used thin fishing line which gives more delicate spikes than the metal pins I used before.
	Diffraction spikes mask with fishing line The mask nicely fits in the dew shield and is easy to rotate to get upright spikes.
	Diffraction Spikes of the 'fishing line' mask [M15 - Globular Cluster] October 13, 2009 18:29 - 19:26 UT Imaging: William Optics Zenithstar 80 ED II APO, f/6.8 [focal length 545 mm], Astronomik Lum filter; ATK16IC b/w camera, K3CCDTools. Exposures: Lum 21x90s, unguided. RGB was not captured because of clouds.