Ian Gillingham

Thanks for the helpful comments! I have tested with 2 stops down (f7.1) and sure enough, there is a noticable improvement. The stars in centre frame are circular, but still some coma showing heading out to the edges. I guess it's the best I can expect from this lens. I shall now go and look for second hand manual prime lenses, see if I can pick one up at a decent price. I've attached one of last night's subs to show the coma improvement. No metrics, so it's more perceptual at the moment. Cheers, Ian

I've caught the astrophotography bug and after several months of trying 1" exposures on a fixed tripod, I bought a HEQ5 Pro mount, on which to fix my Nikon D90 DSLR. Presently I'm using my 105-300mm Nikon zoom lens, which seems to do an 'alright' job and have captured a couple of 'not bad' images of M42 (the usual). My biggest issue at the moment is that, even with best focus using live-view and a bahtinov mask and a well aligned mount, the stars in my images all have a wierd conical shape. I'm wondering whether anyone might be able to diagnose the cause please? Could it be that the 300mm dslr zoom lens just isn't up to the job? Could it be spherical aberration? I don't want to have to buy another lens or astrograph *just* yet, if I can avoid it. I don't think it's a focus issue. I don't think it's a tracking issue either, because the conical shapes are the same at 5 second and 300 seconds. I have attached images of both scenarios, just to show the star shapes - simple OSC single frame, no processing at all, so ignore the sky glow in the 300 sec exposure. These are screen shots from "Raw Therapy", as my images are raw DNG files and I wanted to zoom in on the star shapes. Thanks, Ian