Use scopes of at least 8 inches (200 mm) and focal lengths of less than a 1000 mm for faint, larger nebulae. Some are also very large–many degrees across. On the other hand, faint nebulae demand larger aperture just to capture enough light to see them. Use apertures of 5-8 inches (125 to 200 mm) with focal lengths of 1000 mm or less. (This is because they will usually have longer focal lengths, resulting in higher magnifications and smaller fields of view with typical eyepieces.) For bright nebulae, choose medium aperture scopes with relatively short focal lengths to capture the entire gas-dust cloud in the field of view. The only sacrifice going to larger scopes is the loss of a wide field of view encompassing the entire nebula. ![]() Larger scopes targeting this nebula and other objects of its class will reveal details that cannot be seen in smaller scopes. The bright, larger ones like the Great Nebula in Orion is best seen in relatively small scopes. Thanks to Michael Clark for reminding me that with an even number of blades, the number of rays will be the same as the number of blades, instead of twice that, since the diffraction spikes from opposite blades end up on top of each other).Nebulae: Clouds of gas and dust vary enormously in size and brightness. (Most of the faster Nikon 50mm lenses need to be stopped down for AP - so while the faster versions can be handy for terrestrial photography, there's little point in paying the substantial extra cost if you only want one for AP).Īlso, note that best infinity focus may not be exactly at the end stop, and that when stopped down, diffraction spikes from the aperture blades may give you a bit of a starburst filter effect around bright stars (there will be twice as many rays as the number of aperture blades (for odd numbers of blades) - each blade causes a diffraction spike either side of the star. Some lenses work well wide open for astrophotography (Like the Nikon 180/2.8 ED), others may need to be stopped down several stops for acceptable quality. ![]() (Different folk use different numbers to 400 - it depends how picky you are, and whether you're using a crop or full frame sensor, but 400 should be in the right ballpark (it also depends on where in the sky you're aiming)). So at 25mm, you should be able to go to around 400/25 = 16 seconds. To avoid star trailing due to the earths rotation, aim for an exposure of around 400 / focal length or less. ![]() (Note that the effective light collecting area is NOT the same as the size of the front element - at 16mm f4, the effective light collecting area is only a 4mm diameter circle, while your 85mm at f4 has an effective light gathering area of a 21.25mm diameter circle - so it catches about 25x as much light. For point sources, like stars, the light collecting area (effectively the focal length divided by f-ratio, squared) determines how bright the stars are, since you always end up with a point image regardless of the focal length. What was actually at work here to limit the capture of the stars, from existing in the image to not existing at all?įor extended objects like the northern lights, the f-ratio determines the brightness, as for normal photography.
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