Greetings all, some time ago I read an article (don't recall the reference) that addressed the use of diffraction spikes as a means to achieve a higher level of focus versus when astroimaging versus using a Hartman mask. The approach was touted as being easier and much more accurate. String or some other thin material, positioned to bisect the objective lens, creates the spike effect. When the width of the spikes has been minimized - excellent focus has been achieved. Has anyone used this technique? Is a single "string" used, or two at 90° apart? And, what is the thinness/thickness of material to use? Guidance will be much appreciated. Thanks, Bob Christ -- See message header for info on list archives or unsubscribing, and please send personal replies to the author, not the list.