Quote:
Originally Posted by ZeroID
Decided to move on from Orion, think I've done it to death. Last night was ok and I had a go at Thors Helmet (NGC 2359) as it was well positioned. Ran 104 exposures @ ISO 1600 for 30 secs ( 16 at 45secs) ) giving me nearly an hour of exposure on the 450D\Lunt102 f7 combo.
Couldn't see anything on the frames but pushing the processing hard revealed it with faint green swirls centred in the stacked image. Nothing like the detailed images I've seen posted.
So the question is just how much exposure is required for this one to get any real results ?
Descriptions of this object always call it 'bright and colourful' so I'm a little perplexed. If it requires a heap more exposure I'm going to have to radically modify my imaging regime in my LP'd zone.
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Brent, as Jen intimated, it is not so much the signal level that you need to address as it is the case that you need to reduce the noise.
There are a few ways to do this;
Reduce the intensity of the sky background - A 9nm narrowband filter for example will reduce it by a factor greater than 10x (over typical RGB colour separation) .... Bearing in mind that the relationship between signal/noise and exposure is a square (^2) function, a system that has 10x the noise will require 100x the exposure to achieve the sasme s/nr.
Reduce the readout noise of your camera- take fewer exposures of much longer duration.
Reduce the noise associated with accumulated thermal signal - ie) use a cooled camera.
This is a fairly simplified account. It is probably also worth understanding the way the total s/nr is influenced by discrete factors when you have several noise sources contributing to the whole.
You square the value of each source, add them all together and then take the square root of the whole. The significance of this is that if you have one noise source that is numerically much greater than (the) others, reducing the lower level noise contributers doesn't help much at all.
To elaborate further, take for example M42... the nebula is of such high intensity that the dominant source of noise in the image will be the statistical variation in the arrival time of the photons. Thor's helmet on the other hand is of much lower intensity, in which case, technique, equipment and sky conditions play a much larger role.
best
c