Astrophotos with a scope

Started 6 months ago | Discussions thread
Bob in Baltimore
Bob in Baltimore Senior Member • Posts: 1,068
Re: Astrophotos with a scope

Soul Collector wrote:


As far as magnification goes, I read the camera acts like the eyepiece, and the scope acts like the lens, so M = FL of scope / diagonal of sensor. My FX sensor is 43mm diagonally, so mag with Orion scope would be 600/43 = about 14X

With my camera and 200-500, max mag would be 500/43 = 11.6X. So, not much difference between the scope and lens.

Finally, how does one go about achieving the high mags needed to shoot the small DSOs? It seems a smaller sensor is needed, since as the diagonal distance decreases, mag increases. Is this where the dedicated CCD, CMOS, etc astro imagers come into play?

I think that the camera/eyepiece analogy can lead you astray. Magnification has no real useful meaning here.

The magnification a simple lens is

M = f/(f-d(Obj) )


f is the focal length of the lens and

d(obj) is the distance of the object.

Since the object in our case is at infinity, the magnification is the focal length divided by - infinity! Which is to say, zero.

You can find other formulas for more complex optical systems, but the fact remains that you are going to divide by infinity, and get zero magnification.

The more important concept is plate scale, the number of arc seconds per millimeter at the focal plane. The formula is

p=( 206265/f ) * (pixel size)

For a 300 mm lens on a D850 (which has a pixel size of 4.3 microns) this turns out to be roughly 3 arc sec per pixel.

Longer focal length gives more pixels on your object of interest. (This is sort of an analog to magnification.) The Moon is 1/2 degree in diameter. 1/2 degree = 1,800 arc seconds, equals 600 pixels across the Moon with a 300 mm lens on your D850.

Want more detail, then you want smaller pixels, not a smaller camera as you were speculating. BUT, seeing is probably 2 seconds or so, so smaller pixels don't buy you much most of the time. (Nor does a longer focal length if seeing remains constant.)

Also, it takes 9 pixels to get you color information due to the Bayer filter on your sensor. All in all, starting at 300 mm to 500 mm max on your D850 is a great place to start. You will want to use shorter focal lengths to capture wider fields of view.

All this is not to say that you don't gain with longer focal lengths, but it comes at a cost and requires experience. My best Moon shots come from using my friend's 600 mm to 900mm telescopes. Part of that comes from them having better optics than does my 70-300 zoom, especially at 300 mm. And some comes from capturing those better seeing moments.

And if you want planets, then you need the focal length and sophisticated stacking techniques. The upcoming Mars opposition will yield a whopping 22 arc second image. 7 pixels across at 300 mm on your D850.

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Bob in Baltimore

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