View Full Version : Wedge v. Stacking Software?

2008-Mar-16, 05:25 AM
OK a newbie question here. I've had a good Celestron CPC telescope for about a year now and was interested in getting into Astrophotography this summer. I just got a digital SLR Camera (Canon) and already have a connector for the scope.

I was about to buy myself a wedge but then I thought I don't need that if I do ~30 second shots and stack the images.

Is this difficult to do? What software do I need?

Or should I just buy the Wedge?



2008-Mar-16, 06:40 AM
There are a few software options and you can get decent results using this method (I dont have a wedge at present - so that is how I image. See my website link in my sig)

Check out the following:





For the spectacular deep sky images you see on this forum and around the web - you will eventually need a wedge. The stacking option can still be used in conjunction with a wedge - but obviously the wedge gives you the option of longer exposure times.

2008-Mar-16, 08:27 AM
A couple of things to consider and a couple of questions. What is it that you plan to shoot initially and, not being familiar with your telescope, what is aperature and focal length. These will all have an influence on whether or not you need a wedge at present. If you are intending guided long exposure shots of dim DSOs you will need a wedge to compensate for field rotation during each exposure. The software will compensate between, but not during shots and in order to achieve a good S/N ratio for dim objects, long exposures are needed. If you are planning planetary work. The software should manage the field rotation for you but I will defer to others with more experience on this. I think that in the end, as a GEM user, a wedge for your at/az will provide more versatility at a realatively low cost.

Kind regards

2008-Mar-16, 04:38 PM

The Earth (i.e. the sky) rotates at the rate of 15 arcsec/sec. If you do a 30 second exposures, the sky motion is 15*30 = 450 arcsec. Your Canon, assuming it is a 350D or equivalent) has 6 micron pixels. The kit lens has 18 mm focal length, i.e. the pixel scale is about 6/18000 = 68 arcsec/pixel. Without a tracking tripod your 30 second exposure will smear each star over almost seven pixels. This is too much for sharp star pictures and too little to call it star trails. For openers, forget the telescope and mount your DSLR on a solid fixed tripod. If you expose at f/3.5 and iso400 for about 10 seconds, the stars will be elongated by only 2 pixels, which is good enough to learn. Don't forget to use the selftimer. Even a single exposure from a light polluted sight will get nice constellation pictures. Stacking a dozen using software will be the next step. Then go to longer focal length and the need for a tracking tripod (not just a wedge, but a wedge with a RA drve motor) will be obvious.


2008-Mar-16, 05:44 PM
For starters stacking a zillion short exposures will work. After a while you will certainly long for something better. I lasted about 2 months before I gave up on this as just not providing the results I wanted.

Each time you read out an image you add read noise to the frame. Each frame, if you ignore read noise reduces the total noise in the image by the square root of the number of frames. So going from 4 to 16 frames cuts noise by a factor of 2. Going to 64 cuts it again by 2. You see the problem. Next you need 256 frames then 1024. Obviously there's a point of no return here. All this time read noise is being added to the result so the gain is really far less than 2.

But since read noise is constant each read by reducing the number of reads you reduce the noise level. So one 5 minute shot will have less than one third the read noise of 10 30 second ones.

The time you can go without rotation of the field being a problem depends on where in the sky you image. By taking the object near the meridian you get the longest exposure with the least rotation. By imaging at declinations closer to the pole you again increase the exposure time you can use. How long it will be depends on the image scale of the CCD/telescope combination. You'll have to exeriment and see how long your can go.

Also with longer exposures possible with a wedge you need a way to guide these. Most mounts can't go more than a minute or two without errors creeping in no matter how accurately you are polar aligned unless the image scale is very small. That adds to the cost and likely will delay the switch between methods.