Deep Sky Stacker - summing vs averaging

helioz (Erwin) · #1 05-09-2008, 11:37 PM

I'm very new to Deep Sky Stacker (and Astro imaging in general).
My question on DSS is this:
Which stacking method produces a sum of the images rather than an average?
I've been stacking 50 wide angle images of the milky way each taken at 30" exposure - total exposure of 25 min. Although the resulting image (see below) provides a lot of detail, it appears that the images are only averaged and not summed.
I would expect the summed image to produce a much brighter milky way than what I get.

Any help or other comments would be greatly appreciated.

Many Thanks

Erwin

Tandum (Robin) · #2 05-09-2008, 11:57 PM

I know of no sum option in any program. DSS lets you select medium or average as default. The whole idea of multiple images is to reduce the noise. A single exposure of the right lenght will show you the object but multiple images averages out the noise to make the image cleaner.

You will need to post process this image in photoshop to see the detail. I had a look at your jpg and the data is there, although it's badly distorted here due to size reduction, you just can't see it.

Here is a quick guide from an expert :- http://www.ezystyles.com.au/photoshop.html

Alchemy (Clive) · #3 06-09-2008, 07:19 AM

almost correct, deep sky stacker has no sum option , but maxim dl does.

however pretty much all images need a work over in photoshop or similar, so as tandum suggested try that first.

helioz (Erwin) · #4 06-09-2008, 08:35 AM

Thanks - I'll give PS a try for further post processing.

Does anyone use RGB and Luminance adjustment in DSS? Or do you just no worry about it and use PS for all adjustments?
I found the DSS RGB/Luminance adjustments a bit cumbersome to use.

Matty P (Matt) · #5 06-09-2008, 12:47 PM

Quote:

Originally Posted by helioz

Thanks - I'll give PS a try for further post processing.

Does anyone use RGB and Luminance adjustment in DSS? Or do you just no worry about it and use PS for all adjustments?
I found the DSS RGB/Luminance adjustments a bit cumbersome to use.

Don't use DSS to post process your stacked image as it is quite hard and confusing to use (from my experience). Stick with PS for your post processing.

Tandum (Robin) · #6 06-09-2008, 12:58 PM

Quote:

Originally Posted by Alchemy

almost correct, deep sky stacker has no sum option , but maxim dl does.

I've only just noticed that. Why would you want to sum? Wouldn't that just burn out parts of the image?

helioz (Erwin) · #7 06-09-2008, 05:15 PM

Quote:

Originally Posted by Tandum

I've only just noticed that. Why would you want to sum? Wouldn't that just burn out parts of the image?

I would expect that 5 images with 10' exposure would be stacked into a single image equal to 50' exposure by "adding" the pixels together.
If the image contains an object of high brightness so that 50' would lead to over-exposure then that's what I would expect to see in the stacked image if it was "added".

As I mentioned before, I'm very new to this game, so please let me know if what I say above is incorrect.

Many Thanks for your help so far.

Terry B · #8 06-09-2008, 05:44 PM

Quote:

Originally Posted by Tandum

I've only just noticed that. Why would you want to sum? Wouldn't that just burn out parts of the image?

Summing also reduces the noise.
Depends what you are trying to achieve.
If you are imaging a dim galaxy or supernova or star and you can only reasonably use 5 min exposures due to tracking etc then you are limited to the magnitude that your setup can detect in a 5 min exposure. If you double the exposure you will approximately be able to see 1 mag deeper. You need to add the exposures to achieve this not average them.
Added exposures reduce the noise by the square root of the number of exposures. ie 4 exposures added will 1/2 the noise, 9 added will 1/3 the noise etc.
You are correct that you can burn out bright parts of the image but it also allows you to see very dim parts that are only just above the noise.
I use adding when doing photometry to increase the S/N ration of dim stars letting me measure the mag of stars at mag ~16.5 with a reasonable S/N ratio. It doesn't matter that some of the stars will be burnt out.

Alchemy (Clive) · #9 06-09-2008, 09:04 PM

Quote:

Originally Posted by Tandum

I've only just noticed that. Why would you want to sum? Wouldn't that just burn out parts of the image?

yes possibly the stars depending on your images. however a bit of sneaky processing can get around that.... ie get the burnt out version and blend it with the unburnt out version and the value will always be lower than 255 (white) there are a huge number of variations on this theme,

helioz (Erwin) · #10 06-09-2008, 11:47 PM

Quote:

Originally Posted by Terry B

Summing also reduces the noise.
... I use adding when doing photometry to increase the S/N ration of dim stars letting me measure the mag of stars at mag ~16.5 with a reasonable S/N ratio. It doesn't matter that some of the stars will be burnt out.

Terry B,
what setting in what processing software allows you to select "adding"?

Tandum (Robin) · #11 07-09-2008, 12:11 AM

Cheers Terry/Clive, I have a number of sculptor galaxy images coming off the machine tonight. So I can do an add on the images and blend that result with an averaged result. Worth a try.

Erwin, IRIS is a free package that will do add, Maxim DL is not free and it has a sum option.

Terry B · #12 07-09-2008, 03:50 PM

Quote:

Originally Posted by helioz

Terry B,
what setting in what processing software allows you to select "adding"?

As Tandem said, Iris at
http://www.astrosurf.com/buil/us/iris/iris.htm

helioz (Erwin) · #13 09-09-2008, 02:35 PM

Thanks to everyone who's helped me out on this one.

For all the beginners (like me), this is what I found:
I consulted a very learned Astro Photographer about this question and he has actually tried and compared both (add + average) stacking methods. The resulting images contains the same information but require different adjustments in Photo Shop.

The moral of the story is, it doesn't matter if you sum or average, the resulting image contains the same information.

I hope this helps someone else to get their head around this question.

For a detailed explanation click on this link:
http://keithwiley.com/astroPhotograp...Stacking.shtml

Erwin

Terry B · #14 09-09-2008, 04:42 PM

Quote:

Originally Posted by helioz

Thanks to everyone who's helped me out on this one.

For all the beginners (like me), this is what I found:
I consulted a very learned Astro Photographer about this question and he has actually tried and compared both (add + average) stacking methods. The resulting images contains the same information but require different adjustments in Photo Shop.

The moral of the story is, it doesn't matter if you sum or average, the resulting image contains the same information.

I hope this helps someone else to get their head around this question.

For a detailed explanation click on this link:
http://keithwiley.com/astroPhotograp...Stacking.shtml

Erwin

Interesting link. He is correct but has made an assumption that when you add an image you then will normalise the value to prevent the brightest part from being saturated. This is essentially the same as averaging. If however you don't care about the brightest part of an image being saturated then adding is different.
I remeber seeing a deep stack done with amateur equipment that was a ~day of exposures total added together. It was a time series of images looking at a variable star. Each individual frame showed stars down to about mag 16 but the sum of 24hrs of exposure displayed stars down to about mag 22.
This level is limited by the background sky glow. Averaged frames just don't have enough difference in the pixels to display these stars.