Telescope Reviews: Hyperstar rocks!! Some recent fun (M42/M45/M31etc)

Both absolute aperture (diameter or sq in) and focal ratio count, of course, but in different ways. To make a decent astro image you need a very long integration time. The integration time and the absolute aperture (sq in of photon-bucket area) determine how many photons you have captured from your target. But these are quite useless if you cannot extract them from the skyfog noise. In general, the skyfog, even at the darkest locations on Earth or in near Earth orbit, is brighter than the dimmest parts of the nebula that you are trying to image.

Now the detector comes in. There is a limit in electron-well depth and most detectors' electron wells will be saturated and overflow if you shoot a single 3-hour exposure with most OTAs (just ignore the thermal noise from the detector itself, controllable by chilling the CCD). So we have to make up the total integration time by taking shorter sub-exposures. But these subs cannot be infinitely short, otherwise we cannot process out the skyfog properly. That skyfog has to fall (on the histogram) on a region where the detector behaves linearly and is unencumbered by Read Noise of the sensor. A fast Focal Ratio enables the subs to be short (30sec at f2, 1min at f2.8, 2min at f4, etc) even if you are at the darkest site on Earth.

So, in brief, the shortest subs you can get away with is determined by your Focal Ratio. The dimmest part of the nebula that you can image at a given Signal to Noise Ratio is determined by Integration Time and how many sq.inches your photon bucket has. The Hyperstar excels on both counts, fast focal ratio (f1.9 to f2) and large bucket area. APOs fail on both counts, slow focal ratio (typically f5 to f7 meaning subs no shorter than 3 to 6minutes each) and a bucket area at one quarter to one sixth that of a C11 or C14.

Longer and ever longer subs with a Hyperstar; will they do much good? The Hyperstars, while they have a very flat (not curved) image field, show severe vignetting (easily corrected by flats). Consequently the skyfog peak on the histogram is very broad. The corners of the frame need much longer subs than the middle. 30sec subs are ample for the darkest sites for the middle of the frames. For the corners, at a very dark site, one needs closer to 60sec frames. Going to 180sec frames may start chopping off the dynamic range (for the middle of the frames) unduly. For narrowband I found 90sec frames at my heavily light polluted home adequate, but I presume one could benefit from, say, 3 minute subs shooting narrowband at a dark site, perhaps even slightly longer if the filter is very narrow. Of course, in general, we do not care that much about stuff at the corners of the frames. All we want is a sky background that looks reasonably noise-free. So in practice there is not much going in favour of shooting subs much longer than, say, 60sec each when imaging in white light.

This is how severe the vignetting is:

My review of a C14 Hyperstar is available here. and how short you can go with subs on any OTA I discuss here.

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