To answer that we need to know exactly what you are doing to paste hyperlinks on the site. I am no expert, but am happy to help you with computer stuff, as I am a very raw beginner with astro stuff.
Thanks Andrew, I do struggle sometimes with the computer stuff.
What I have been doing is copying the url for a web page, highlighting the word in the text I want to link the web page to, click the "Insert link" icon, and then paste the url in the box.
I'm sure that's how I always did it on another forum with no problems. Here as you can see it just shows the url.
There is a button in the thread creation page, and also in the replies that says insert link. Alternatively the code is [*URL="your website here"] the text you want linked [/URL] without the star inside the first brackets.
Interesting reading, it appears there's not much advantage in stacking more than 15. I remember something similar when I used to to use a Gstar to take deep sky pictures , there was little diifference between a stack of 1000 x2.5sec exposures (integrations) and a stack of 200x2.5 sec exposures. However , if this is true for long exposures why do so many expert imagers stack 100's of hours of exposures . I find it difficult to get several hours of exposure time !
Philip
I believe the 'experts' are producing long signal totals through the use of multiple sessions over many weeks, mostly using capture automation to tend to the scope, camera, mount, etc so it can be done unattended to some degree (albeit they are probably in the house monitoring the computer or checking the run from time to time). Plate solving will allow matching of exposures over many nights I believe. There certainly seems to be some sort of pride in the acquisition of many hours, days, etc worth of signal data - all of which has to be processed. To the amateur working with a dslr it seems a very different world.
I recently ran ten, five minute subs of M104, out at Bretti (so fifty minutes of signal data) - which is my longest run to date. Even after stacking in DSS, with darks, flats, offset bas frames, the resultant image taken into Photoshop really didn't look too much better than one single sub.
I suspect my equipment maybe a limitation, one shot colour from a un-modded dslr but still I am happy with what I can produce. If your not interested in the 'competition' imaging world just do enough to satisfy your quality standards.
Now that my observatory is complete and my mount is on a pier I hope to be able to run longer subs then I can make some comparisons
there was little diifference between a stack of 1000 x2.5sec exposures (integrations) and a stack of 200x2.5 sec exposures
Well, there will be a difference because the former has 5 times the integration time. When comparing many short vs few long subs, it all comes down to the read noise of the camera. With no read noise, 1000 x 1 sec is identical to 1 x 1000 sec or 10 x 100 sec, in terms of signal-to-noise ratio (where noise in this case is shot noise). But with read noise to account for, the longer subs are better. But if you want statistical removal of artifacts, you'll typically want 10+ subs - and as you use more, the 'return on investment' diminishes with the square root of the number of subs.
There are a lot of armchair experts on this topic but many of them rely on folklore or limited empirical experience rather than physics. If you're really interested I'd recommend reading something authored by a person with a clue (i.e. a basic understanding of the science and mathematics behind imaging.) The articles by Craig Stark on signal to noise are a good start:
Rick is correct that there are a lot of myths out there. For instance, I recently read the book "Astrophotography with Mark Thompson". It's a nice introduction and generally provides excellent information, but you can tell that the author really doesn't understand the science of image calibration. For instance, he writes that bias and dark calibration are used to subtract noise from images. It might be simply a bad choice of words, but as written this is incorrect - these actually inject noise into the calibrated images, but they are effective at subtracting offset/current. It is actually impossible to subtract noise via image calibration, because white noise is, by definition, random (otherwise it would be signal). One can only to minimise it compared to signal.
That's a bit sad, Barry! One book I do recommend is the Handbook of Astronomical Image Processing by Berry and Burnell. It is thorough but also quite easy to read and understand.
There are a lot of armchair experts on this topic but many of them rely on folklore or limited empirical experience rather than physics. If you're really interested I'd recommend reading something authored by a person with a clue (i.e. a basic understanding of the science and mathematics behind imaging.) The articles by Craig Stark on signal to noise are a good start:
mmm, I looked at those utubes, I find Craig Starks presentations a bit smarmy and obvious. Yes of course hes righteously scientific, but skips out of left field aspects of imaging learnt by suck it and see experience that dont fit his expert view, especially megadata. The whole "crap in crap out" thing is all very sensible and heroic, but not quite how it can pan out in practice with imagination and non-scientific tools..
For example, he says at one stage in his presentation "if you want a nice image, its gotta be smooth" (with emphasis), like this is a given, the holy grail. Well, really!, lately ive seen some serious amatures leave fine grain noise in very dim object images (retaining sharpness) and it oddly looks fine, even adds character.
mmm, I looked at those utubes, I find Craig Starks presentations a bit smarmy and obvious. Yes of course hes righteously scientific, but skips out of left field aspects of imaging learnt by suck it and see experience that dont fit his expert view, especially megadata. The whole "crap in crap out" thing is all very sensible and heroic, but not quite how it can pan out in practice with imagination and non-scientific tools..
For example, he says at one stage in his presentation "if you want a nice image, its gotta be smooth" (with emphasis), like this is a given, the holy grail. Well, really!, lately ive seen some serious amatures leave fine grain noise in very dim object images (retaining sharpness) and it oddly looks fine, even adds character.
Fred,
I wouldn't necessarily take artistic advice from Craig, however, I've not come across anything from suck it and see experience that isn't adequately explained by fairly simple science and maths. You can certainly get by without understanding the fundamentals and develop techniques that work, but applying a little bit of knowledge can make your image capture more efficient. In the absence of infinite clear skies that's got to be worthwhile
Craig is a neurobiologist who is trying to see details in people's functional MRI brain scans. I'm really glad people like him (which, rather narcissisticly, includes myself) aren't just each trying a "suck it an see" approach. I expect that if you ever get a tumour or develop epilepsy that you too will be glad that they do things "scientifically".
Craig is a neurobiologist who is trying to see details in people's functional MRI brain scans. I'm really glad people like him (which, rather narcissisticly, includes myself) aren't just each trying a "suck it an see" approach. I expect that if you ever get a tumour or develop epilepsy that you too will be glad that they do things "scientifically".
Funny you should mention this. My daughter had an Aneurysm in the neck 6mths ago, rare for a young person. It took 6 specialists, very many MRI scans (not good when your young), being constantly sent home with panadol before finally refered to an out of left field neurologist at north shore, the sort of specialist so hard to get you have to be near death (and it got that close) to apply a decidedly non scientific approach based entirely on years of experience to solve the problem, quickly.
Funny you should mention this. My daughter had an Aneurysm in the neck 6mths ago, rare for a young person. It took 6 specialists, very many MRI scans (not good when your young), being constantly sent home with panadol before finally refered to an out of left field neurologist at north shore, the sort of specialist so hard to get you have to be near death (and it got that close) to apply a decidedly non scientific approach based entirely on years of experience to solve the problem, quickly.
Medicine is one of those disciplines where its foundation is in science, but you cannot apply a purely scientific method to its practice in an individual. Much of medicine is about understanding the statistical likelihood of a diagnosis/treatment option and how that changes after a diagnostic test, but using common sense and judgement to vary from the usual treatment and diagnostic path. There is a common saying in medicine, that when medical students hear hooves, they think of Zebra's, never horses. But occasionally in medicine, they really are Zebra's - you just have to apply experience, judgement and common sense to know when thats more likely.
Astrophotography is similar in that there is a very solid scientific basis to the capture of images and the image quality, and what can be done to improve signal to noise ratios, but there is also a subjective element in the processing where individual preference in image colour and emphasis varies - a much more artistic framework.
As a comparison for us beginners would you mind processing say 20 subs of your data as you normally would in your images and post both images?
Guys??
Not going to happen??
No worries, thanks anyway.
I just thought a comparison between say 1 hr of exposure time compared to 10 hrs with the same data, processed the same way, would be very interesting.
I just thought a comparison between say 1 hr of exposure time compared to 10 hrs with the same data, processed the same way, would be very interesting.
Here's an example. NGC7497 imaged with Tak FSQ-106ED and QSI-683 camera (small crop from centre of field containing NGC7497 and some dark dust.). Subs were 15 mins each. The attached images are a simple integration and stretch of 5, 10, 20 and 40 subs = 75 mins, 150 mins, 300 mins and 600 mins.
According to PI's SubframeSelector script the noise measurements are: 3798, 3348, 2756 and 1912.
