7 hour subs look like 2 hour subs

[smr]

Just wondering what the point is really. Am I better off just taking 2 hour subs on everything? My M42 stack didn't really look much different between 2 and 4 hours. And last night I got 3 hours more on the Rosette to make it 7 hours in total, and it doesn't have anymore data in it than the 2 hour stack. A bit confused, is this because my Camera isn't modified?

[Z3roCool]

I am no expert but I think with Ha targets such as Rosette you may struggle to get a lot of data with an un-modded camera. M42 is very bright so you get a lot of the data fairly easily so reach a point of diminished returns after a short time (Unlike less bright targets). You may get less noise in the shots but I think sometimes, depending on your individual subs, this may not help (or Very little). Again, I am no expert so take what I say with a pinch of salt....still learning myself

[Ags]

Subs refers to "subexposures" - so i presume you didn't keep the shutter open for 7 hours?

Signal to noise is reduced by longer exposures, but the law of diminishing returns applies. Going from one hour of exposure to two hours will give a certain benefit, but then you have to go up to 4 hours to get a similar return, and then 8 hours, 16 hours etc for each step up. At some point you have to look at other options to improve signal to noise. The obvious one is to boost your Ha signal by a factor of 3 by modding your camera, or to reduce the background signal by getting to a dark site.

[The-MathMog]

1 hour ago, smr said:

Just wondering what the point is really. Am I better off just taking 2 hour subs on everything? My M42 stack didn't really look much different between 2 and 4 hours. And last night I got 3 hours more on the Rosette to make it 7 hours in total, and it doesn't have anymore data in it than the 2 hour stack. A bit confused, is this because my Camera isn't modified?

A few things, when you say "it has no more data", are you then looking at the image straight out of DSS (or manual stack), or are you looking at a fully stretched and processed image? I couldn't see much difference in my 45 minute stack and 4 hour stack, but once processing it, I saw the difference in detail I was able to pull out immediately.

And are you recording your images in RAW and not JPEG? The JPEG compression makes the image loose a lot of the very fine details that stacking lots of subs would normally bring out.

And someone please correct me if I got it wrong. When you take more subs and stack then, what you do is increase the signal to noise ratio, and for that ratio to double, you need to quadruble the amount of subs you stack. So if you have an image with a total exposure time of 2 hours, then to double the signal to noise ratio, you would need 8 hours of data (correct me here, forgot if it was quadruple or double).

So the longer you've been exposing a target, the more time, exponentially, you will have to put into it, to see further significant results. So you need to figure out at what point the time is simply not worth it for you. But more is of course always better

Lastly, if the subs you add to a image, is actually of lesser quality than the ones you used previously, then they will actually degrade the image. Lesser quality could be that the moon was out. That there was more moisture in the air. Turbulence in the upper atmosphere. Small tracking errors. Dew on the mirror/lens, etc. Always go through your images, to sort out ones of bad quality

Personally I wouldn't go far beyond 10 hours on a project, if I even ever get close to that

 

[wxsatuser]

17 hours ago, smr said:

Just wondering what the point is really. Am I better off just taking 2 hour subs on everything? My M42 stack didn't really look much different between 2 and 4 hours. And last night I got 3 hours more on the Rosette to make it 7 hours in total, and it doesn't have anymore data in it than the 2 hour stack. A bit confused, is this because my Camera isn't modified?

I have seen your Rosette and IMO you don't need a modded camera, you have good colour coming through.

[Gerry Casa Christiana]

Can I just ask what were the exposure times for the subs on each. I don't see it above. The reason I ask is that there is a point where your subs need to be going over 2 minutes to see a much improved signal to noise ratio. So the experts tell me at any rate. Love to see your pictures are they already posted elsewhere? 

Cheers 

Gerry