Stacking with different exposure taken days or more apart

[Star Gazer]

Looking around the interweb I see lots of gorgeous images taken by some very talemted people. Many of the images are composeed of subs taken at significantly differing exposures in order to increase dynamic range. How are these images stacked? I have played briefly with DSS, but it wants all the subs to be the same exposure; what software is being used in this situation?

Also some astronomers are saying that they might improve on their current image by going back next year to grab some more subs. Doesn't this present all sorts of problems when aligning the images? What about taking subs with different optics? Is it possible to warp and stretch images so they all stack nicely?

Lastly, making a mosaic and getting all the subs to align must be nothing short of a miracle.

I'd be interested to know some of these stacking secrets.

[ollypenrice]

Oh, you are right. We are so talented!! However, for high dynamic range this is the best I know;

Compositing 2 Different Exposures via Layer Masks

I always use this method and think it gives the most natural results.

Mosaics are getting easier. Microsoft ICE (free) does well but manual calibrating and adjusting first is usually needed in astro imaging.

Registar by Auriga Imaging is a killer, especially for resizing images at different FLs for combining.

Then there's PixInsight but for that you need Harry; http://www.harrysastroshed.com/Mosaic.html

Take a step at a time. It isn't fundamentally difficult.

Olly

http://ollypenrice.smugmug.com/

[Big Jim Slade]

You can get it to work with DSS too. I've had trouble using groups like some people, but you can also stack (say) three tifs from three different nights, just make sure you tell it to stack 100% of the images.

[ollypenrice]

Just to add one thing; good flat fielding is essential or gradients will play havoc with mosaics.

Olly

[Jacko]

H.

I'm a novice at this but find it very interesting, I've asked this question before but didn't get a reply, so here goes again.

Excuse my ignorance, but as I said I'm a total novice,

What is stacking and how do you achieve it.

What is DSS.

What are subs.

Thanks.

Jacko.

[Sp@ce_d]

H.

I'm a novice at this but find it very interesting, I've asked this question before but didn't get a reply, so here goes again.

Excuse my ignorance, but as I said I'm a total novice,

What is stacking and how do you achieve it.

What is DSS.

What are subs.

Thanks.

Jacko.

Subs are single subframes (exposures). It's common to take multiple subs and combine them.

Stacking is a method of combining subframes (subs or single exposures) so as to increase the signal to noise ratio.

DSS is Deep Sky Stacker DeepSkyStacker - Free one of the applications available that is used for stacking.

[ollypenrice]

The point about 'stacking' multiple exposures is that the noise in each image is random so tends to cancel itself out but the signal is consistent so builds in strength.

Many astro pragrammes offer a range of mathematical options for stacking, from simple averaging to more complex ones in which extreme differences between one sub and the others are rejected. So if one sub has a satellite trail going through it, it disappears in the stacked image. Great!!

Olly

ollypenrice's Photos

[Jacko]

Hi,

That sounds like a clever method. So am I right in thinking that by stacking multiple exposures it gives more depth and definition to the image as well as taking out the imperfections?

Jacko.

[ollypenrice]

Hi,

That sounds like a clever method. So am I right in thinking that by stacking multiple exposures it gives more depth and definition to the image as well as taking out the imperfections?

Jacko.

Yes. When you have a very deep stack you have more signal and less noise. As well as looking better straight out, this allows you to improve the image in various ways but the main ones are,

- Stretching the faint parts. You brighten them, in effect, while holding the background sky at the normal level. Hidden objects then emerge, but only ones that really are there!

- Digital sharpening of of fine grained detail. Here there is some risk of creating atefacts and you need to be judicious.

Olly