Astrophotography - DSLR? Webcam? Motor?

[NeilFawcett]

I think your understanding is pretty much there

Thanks! I can understand the workings now so it makes far far more sense, and better still understand the hurdles and implications etc!

So if you don't mind, another pair of questions? (Tell me if I'm wearing out my welcome!)

webcam planets - You mentioned capturing Mars with thousands of webcam stills. Do you feed all/most of those images into a stacker which then processes them? Or did you manually have to select the better captures? Also if the telescope has moved slightly between image 1 and image 6000, I assume the stacker can see that and automatically aligns/adjusts the images (ie: if the telescopes tracknig has wondered off a bit)?

fast/slow - If we compare two 150mm diameter telescopes, one with a focal length of say 600mm and another with 1500mm. The 600mm is considered "fast" and better for deep sky, while the 1500mm is "slow" and aimed at planetary observations? I'm really really confused by this difference as I really can't see the difference between obvserving say Saturn and a nebula. Infact, logic would tell me that the nebula is far far fainter so you'd want to be dedicating as much FOV to it as possible, which would be the "slow" (planetary) telescope. Ie: The "fast" (deep sky) telescope has a wider FOV so less magnification, so it's wasting more of its light capture to other areas of the sky? Surely - contrary to fact? - the "slow" (planetary) telescope will be dedicating more of its mirror to the nebula (with its narrower FOV) so will capture more light from it?

[JamesF]

For planetary imaging I feed all the stills (as an avi) into Registax. If I don't have too many frames I might check the mall, but where I have thousands I skip through them first checking to see which ones are best focused and then which have a decent level of detail. If I find one I'm happy with in the first five hundred or so I'll go with that as my "reference" frame. After that Registax can align all the images to cope with the planet having moved about the frame and pick what it thinks are the best ones for stacking.

The fast and slow scopes thing took me a while to get my head around, too.

To deal with planetary imaging first. Planets, even big ones, occupy a tiny amount of sky. As I said in post #18, Jupiter at its biggest is only about 1/72nd of a degree wide. Mars, despite being far closer to us than Jupiter is never more than half that. To get a good image with lots of detail in therefore, you want the plate scale to be as small as your scope and camera can cope with. That is, you want as small an area of sky as possible to be covered by one pixel. Plate scale is inversely proportional to focal length, so to make plate scale small you need to make the focal length big. So, as long as you can collect enough light and you have enough aperture to get the resolution, you want as long a focal length as you can cope with for planetary imaging.

For the sake of argument, let's say you have a scope that you're using for planetary imaging and it gives an image of Jupiter on the camera 0.4mm in diameter, which is probably about right for my 127 Mak without a barlow (it's a 127mm aperture 1500mm focal length scope). If you wanted to use it to image M13, which is 1/3 of a degree wide, the image would be almost 10mm wide on the camera sensor. You might get away with trying to image that using the Mak as long as you used a DSLR. M13 isn't large for a DSO though. When it comes to nebulae, the North America Nebulae would be 80mm wide on the camera sensor. M31, the Andromeda Galaxy is much the same. They just won't fit and you need to increase the image scale to get them all in the frame. The only way to do that is to reduce the focal length, and by a fair bit. I'd suggest that about 600mm is the maximum you'd want for a crop sensor DSLR. If you did that and kept the same aperture you'd have an f/4.7-ish scope, which is really quite fast.

Sadly large, well-corrected fast optics aren't cheap. so people tend to compromise a bit and use smaller size optics (which slows the focal ratio again) and then for larger targets add a reducer (to reduce the focal length again). Another down-side to really fast scopes is that the range of focuser travel in which the image might appear in focus is as thin as a razor blade. Slower scopes have a bit more leeway.

And what about needing to collect more light to image faint objects? Well, because the way the camera works is that it effectively "remembers" how many photons have arrived at each pixel, within certain limits it doesn't matter if they arrive in one big rush, or over quite a long period of time. With a really fast scope, f/4, say, your photons will arrive in a big rush, but if you can't afford lots of aperture and go for something at f/6 instead, you just have to wait longer, which is why you'll see people posting exposure times of anything up to 30 minutes for a single frame.

There are other issues such as accuracy of tracking, too. That needs to be much better as the scope gets slower.

So, to summarise, I guess, you can potentially image some DSOs with a long focal length scope if you want to, but many of them just won't fit on the camera sensor at the image scales you get by using long focal lengths and that means you have to use a shorter focal length/faster telescope. If you don't want to be restricted in what you can image and you're only going to buy one imaging system then regardless of tracking issues and so on, you really have to think about faster scopes.

James