How can stacking improve astrophotography?

[zanazzi78]

Just finished this...

I thought i`d attempt a mosaic and the moon looked rather seductive a couple of weeks ago low in the westen sky so i htought i`d take a few picks.

 

[russ_watters]

Nice - what equipment are you using?

And a few critiques, if you don't mind:

-The photo appears slightly out of focus. I use Meade's DSI and with its low resolution, I actually hold the computer about a foot from my face while focusing to be able to see minute changes. Also, making very minor adjustments, then taking your hand off the focus knob while the picture stops shaking helps get a sharper focus.

-I'm not sure how you assembled the mosaic, but the borders between frames are very noticeable, implying you just laid them over top of each other. Photoshop has plugins for stitching and I actually use a program called "Image Assembler", which aids both in aligning and in blending the images.

 

[zanazzi78]

Nice - what equipment are you using?
And a few critiques, if you don't mind:
-The photo appears slightly out of focus. I use Meade's DSI and with its low resolution, I actually hold the computer about a foot from my face while focusing to be able to see minute changes. Also, making very minor adjustments, then taking your hand off the focus knob while the picture stops shaking helps get a sharper focus.
-I'm not sure how you assembled the mosaic, but the borders between frames are very noticeable, implying you just laid them over top of each other. Photoshop has plugins for stitching and I actually use a program called "Image Assembler", which aids both in aligning and in blending the images.

thanx for the pionters, Russ, I do find focusing difficult. I use a SLR with film, (no digital photography for me, yet!), the viewfinder is only an inch or so across so can be very difficult to see some times. It also makes the image appear fainter than when viewed through the scope, which in a pain. Normally i take multiple shots of the same object finly addjusting the focus in one direction for each shot, this normally garantees at least one very sharp image.

These shots were done as a test to use to attempt a mosaic so i wasn`t too concerned with the image quality. I`ve never done one before so i`m still learning and yes i did just layer the pictures. I`m not that good with photoshop so hints tips would be fantastic.

More info on "image assembler" would be great also. (i`m of to goggle it now!)

P.s. I really liked your M31 pics i might have a go in the next couple of weeks (if it ever stops raining Bah!)

edit: You mentioned in your post the Cass pic is a stack of 5x15 sec shots.
I`ve never stacked my shots. What are the benefits of stacking? How do you do it properly?

 

[russ_watters]

I do find focusing difficult. I use a SLR with film, (no digital photography for me, yet!), the viewfinder is only an inch or so across so can be very difficult to see some times. It also makes the image appear fainter than when viewed through the scope, which in a pain.

That's tough. I use a decent generic digital camera for the wide angle shots and have the same problem. I assume you're shooting through a telescope, though, so it may help to use a focusing aid such as a http://rao.150m.com/Focusaid.html" it is. I've been using it for years to make scenery panoramas.

edit: You mentioned in your post the Cass pic is a stack of 5x15 sec shots.
I`ve never stacked my shots. What are the benefits of stacking? How do you do it properly?

Stacking works via averaging (there are actually several methods, but this is the simplest to explain ). By taking multiple pictures and calculating the average color of a certain pixel, errors are canceled out, increasing the signal-to-noise ratio. A black, but speckled sky turns even gray (which then can easily be darkened without losing what you were photgraphing) and details in the object you are imaging are enhanced by building signal and reducing the noise.

There are a number of benefits:

1. It makes images brighter. This allows shorter exposures, making tracking less important. They don't add exactly, though, ie 2x15s=30s. I think it's an inverse square relationship or something: 2x15s=23s.

2. It vastly reduced all types of noise, be it atmospheric noise or camera background noise (the digital equivalent of graininess in film).

3. It increases the effective resolution of your equipment. Ie, when pixels overlap each other, the computer can split them.

The end result is that the stacked pictures are many, many times the quality of what you can see with your eyes or the raw images.

Attached are two pictures of Saturn I happened to have on this computer at work. They are my first half-decent one using a webcam, first posted way back in post 17 of this thread. The pic that says "frame cap" is a raw image, doubled in size to see the level of detail. This is what you would see on your screen when taking the pictures and is also about what you'd see looking through the eyepiece (though you would look at it at lower magnification). Stack 300 of them together and do some minor enhancements and you get the second pic.

Also, check out Jupiter in posts 50 and 51 of this thread. Europa's shadow on Jupiter is almost exactly at the theoretical resolution limit of my telescope. Looking through the scope or at the raw pics, I could only pick out the shadow about half the time. In the stacks, the shadow stands out nicely.

http://registax.astronomy.net/" is probably the most popular software - it is easy to use, and better yet, it's freeware.