How does magnification with a CCD camera work?

[MauriceN]

Last night was a clear sky here in lower saxony, Germany and I took my telescope, placed it on the garage and the first thing I unintentionally locked onto was Saturn. This beeing my first time seeing Saturn in person made me sceptic so I grabbed my 2x barlow and 10mm plössl to take a better look at it. First thing I realized was that the magnification was not as strong as I was hoping for. Second thing I noticed was that I really just found Saturn and it was an awesome experience! Now I'm asking myself, how is it possible to take nice "close-ups" so to say of Saturn or other planets even more far away/smaller with a CCD if you don't even have an eye piece in front of it? Maybe this question is extremely stupid but this is bugging me the whole day already. Some say you should use barlows for planets like this, but still, how is a 2x or 3x magnification going to help me if I couldn't see it clearly with a 5mm?

Thanks for any enlightening words!

[newbie alert]

Your image scale with camera's depends on the size of the chip on the sensor..

In your case with the barlow it deteriorated the target because of light scatter..the red blue and green within white light gets diffracted in the atmosphere and all at different points so you get a blurred image..with a smaller image scale it gets scattered less so you get a sharper image..look at Jupiter with a 25mm ep,then try a 12 and do the same with a 6..the sharpest brightest image will be with the 25mm,and the dimmer but biggest with the 6..

[MauriceN]

1 hour ago, newbie alert said:

Your image scale with camera's depends on the size of the chip on the sensor..

In your case with the barlow it deteriorated the target because of light scatter..the red blue and green within white light gets diffracted in the atmosphere and all at different points so you get a blurred image..with a smaller image scale it gets scattered less so you get a sharper image..look at Jupiter with a 25mm ep,then try a 12 and do the same with a 6..the sharpest brightest image will be with the 25mm,and the dimmer but biggest with the 6..

So how do people get big images that are sharp and bright?

[Merlin66]

Forget the eyepiece view.....

when using a camera it’s the pixel size of the camera and the focal length which come into play.

A rule of thumb is that the “optimum” focal ratio for imaging would be  x5 the pixel size in micron.

so, for a 5 micron pixel size a f25 would be a good start. If your scope is a basic f5 this means a x4 or x5 barlow.

Hope this helps.

 

[JohnSadlerAstro]

Hi,

Magnification with a CCD or similar camera works essentially like a DSLR with a lens. The telescope acts as a big, telephoto lens. Hence no need for either an eyepiece lens or a camera lens in the system. The way that the focal length affects the magnification is a bit weird and complicated, but basically more focal length = more magnification. By putting in a Barlow, you effectively increase the scopes focal length. A 1000mm f-length scope with a 3x Barlow in use will effectively have 3000mm f-length. You probably will be using one telescope for imaging the planets, so you'll have a fixed focal length apart for when you're using Barlows. This is a fundamental number you wont be able to change. However, the amount of magnification also depends on these 2 things:

1. The size of the sensor, not in pixels, but in actual size. 

2. The size of the pixels.

So, a tiny, tiny sensor with 1mp of pixels will give more magnification than a larger sensor with 1mp of larger pixels. The sensor is basically seeing a section of the image produced by the telescope; the size of that section depends on the size of that sensor. 

For an example, my 130P-DS has a nice large secondary mirror and a 2in focuser. The image produced by the telescope is just large enough to cover the sensor of my 1000d, although because the telescope is obviously limited in its field of view, and as the scope has a focal length of 650mm, the end result is a nice "zoomed in" view. You could look at it this way--the sensor of the 1000d is about this ________________ wide, so it sees this much of the scopes image.  If the sensor is even smaller, though like say this, the sensor size of my ASI120 ____ , it will have looked at a smaller section of the telescope's image. You can add a Barlow to double (2x) or triple (3x) the magnification.

However, you will eventually reach the point where adding more magnification will make the picture blurrier. Essentially a telescope has a limiting resolution. You could think of this as producing a certain number of pixels of image. If you continue to zoom in, you will reach the point at which the "pixels" or smallest details the scope can see are larger than the pixels of your camera. 

Try this: get a low-res photo from google images or somewhere. Take a screenshot and look at it at 100% zoom in paint or photos. It wont look grainy, though it will look small. Then, zoom in to 500% and take the screenshot again. Chances are it will look awful. The screenshot still has the same number of pixels, but the actual object being imaged is stretched so that its pixels are now waaay larger than those of your screen. We call this "oversampling". Oversampling causes issues when processing or sharpening images, so although the temptation is to magnify magnify magnify, you will reach a point at which "pixels" of your scope are the same size as the pixels of your camera. Leave it there--that's where you'll get the best images.

I'm hardly an authority on this kind of thing, perhaps someone with more knowledge will be able to explain in a more scientific way.

John

[newbie alert]

5 hours ago, MauriceN said:

So how do people get big images that are sharp and bright?

Focal length and aperture..more focal length the bigger the image scale..more aperture the brighter the image.