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:

Bob in Baltimore wrote:

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.

This sounds a lot the idea of using a DX sensor (or smaller) when focal length limited. For instance, if I'm shooting an animal fro 100 ft away, and I can't get any closer, a DX sensor will yield a better image because it has a higher pixel density than my FX sensor e.g. it puts more pixels on the target.

Thinking strictly in  terms of pixels on the target. this is correct. But the difference is not as large as you might think. The Nikon D500 has a pixel size of 4.2 microns. not a big advantage over the D850's 4.8 microns.  And my guess is that the D850 seniors is better overall (quieter, etc) than the D500-, but i could be wrong.

Using you Mars example, it seems that my current setup would be mostly useless. If Mars would only be 7 pixels across on my D850, that's extremely small, since my sensor is 8256 pixels wide.

Yes, your system is not good for Mars. The longer focal length of your Celestron might be an advantage here, if the optics are actually sharp. But the real trick for Mars is in taking thousands of images. People use the movie mode to capture enough frames.

Even a small 80mm refractor with a 600mm FL doesn't seem like it'd do much better. So I'm wondering, its it even possible to image very small DSOs with a DSLR, or does one need a specialized camera with much smaller pixels?

Take the Ring Nebula, for example. It's angular size is about 1.5 x 1'. How much focal length would be needed to image this and get a reasonable size representation?

I guess I am used to visual observing, so all this is a new way of thinking. If I want more mag (and a larger image), I just use a smaller eyepiece.

Yep, but there is a limit to how much you can get out of a given telescope. And when you are doing the visual thing, you are looking for those few good glimpses of steady seeing. A different game.

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Bob in Baltimore

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