Digital Camera Astrophotography
Astrophotography with "prosumer" digicams may be easier than you
Imaging Resource article by: Bret McKee
(Original posting date November 20, 1999)
The night sky has been a source of wonderment for as long as there have been people to gaze up at it. The ancients believed that there was magic in the celestial bodies and even named them for their deities and heroes. It is not surprising that people love to photograph those objects and that viewers are invariably impressed with almost any decent astrophotograph.
While taking spectacular photos of deep space objects like nebula and galaxies requires special equipment and lots of practice, it is fairly straightforward to take good photographs of the relatively bright objects in our solar system. It is quite easy to take photographs of the moon, and while it is more challenging to take photographs of the planets it might be possible for anyone with access to a telescope.
The remainder of this article will focus on using a digital camera to take photographs of the moon. Because digital cameras provide instantaneous results and bad shots cost nothing, they are an excellent way to experiment with astrophotography.
The most important thing that is needed to take astrophotographs of the moon is some sort of magnifier. While a telescope will provide the most magnification, it is possible to use binoculars or a spotting scope as well. For the remainder of this document it will be assumed that a telescope will be used. A tripod is necessary for the telescope to minimize the vibration inherent to the high magnifications used.
It is also necessary to couple the camera to the telescope so that the image can be recorded. There are two common methods of coupling cameras to the telescope: a T-mount and afocal coupling.
Afocal coupling is the simplest way to take photos through a telescope, and works with virtually any camera. Basically this method involves pointing the lens of the camera into the eyepiece of the telescope. The photo to the left shows how to afocally couple a camera and telescope. The best results are obtained by first focusing the telescope at the eyepiece and then fixing the cameras focus at infinity. A trick to improve the focus accuracy is to look through some sort of optical device (binocular lens, spotting scope, etc) while focusing the telescope. This trick is necessary because there is no way to disable the powerful auto-focus built into the human vision system, which can enable you to see a sharp image even if the telescope is not focused exactly at infinity. When afocal coupling is used, a second tripod for the camera will help to ensure that everything remains vibration free for sharp photographs.
A T-mount is a two piece adapter system designed to hook cameras to telescopes and spotting scopes. The first piece is called a T-ring and attaches to the camera. For most removable lens film cameras the T-ring attaches to the camera in place of a lens, but for cameras with non-removable lens, which is virtually all amateur level digital cameras, the T- ring attaches in front of the lens where a filter would go. For Nikon 900/900s/950, CKC Power has a custom aluminum available.
On the telescope side, there are two different adapters available, but only one will work for digital camera use, and it requires some modifications. The problem is that all of the adapters are designed for use without a lens on the camera, which dramatically changes the focusing necessary. The type of adapter that can be modified to fit is a tele-extender. This is a device that is usually used to focus the telescope directly on the film through the eyepiece, which results in extremely high magnifications because it is several inches from the eyepiece to the film plane. Unfortunately for digital camera users it is not possible to do this. A tele-extender is made of two tubes, one of which fits inside of the other. One of the tubes is threaded to fit the telescope, and the other is threaded to fit a T-ring. For this use, both pieces of the tele-extender need to be shortened so that the camera can be placed about 1/2 to 1 inches from the eyepiece. In this configuration it is functioning as a mounting bracket for afocal coupling. The advantage is that the camera is rigidly mounted to the telescope, eliminating the problems associated with keeping the two devices aligned.
|Telescope to camera adapter parts laid out in the same way they fit together. From left to right they are telescope visual back and eyepiece, tele-extender inner sleeve, tele-extender outer sleeve, T-ring. Both tele-extender pieces where shorted by cutting them on the inside edge.
|Visual back and eyepice installed|
|Tele-extender inner sleeve threaded to visual back|
|Tele-extender outer sleeve and T-ring over inner sleeve|
It is important to realize that magnification is a double-edged sword. While high magnification allows more details to be visible, it also decreases the effective speed of the lens, which requires longer exposure times (increasing the problems with vibration and CCD noise), and it magnifies any vibrations in the telescope or camera.
Computing Focal Length
To determine the 35mm equivalent focal length of the system being used, three simple calculations are necessary. First the magnification of the telescope must be determined. For binoculars and spotting scopes, this is usually given, and for telescopes this is simply the focal length of the telescope divided by the focal length of the eyepiece being used. For the examples here, the telescope has a focal length of 1250mm and the eyepiece is 25mm. This means the magnification is 1250/25 = 50. Once the magnification is obtained, simply multiply the focal length of the camera lens by it. For the first sample here, the focal length was 8mm, yielding a focal length of 8x50=400mm. This is NOT 35mm equivalent, however. To find the 35mm equivalent, it is necessary to know how the size of the CCD compares to a 35mm frame. The easiest way to determine this is to use the 35mm equivalent focal length as provided by the manufacturer. For a Nikon E950, Nikon indicates that that focal length is 7-21mm, and they also indicate that this is 38-115 35mm equivalent. To find the conversion, simply divide either of the second numbers by the corresponding first number (i.e. 115/21~5.5). Then multiply the computed focal length by this conversion factor. For the example photograph here, the 35mm equivalent focal length is 400x5.5=2200mm.
When most people think about astrophotography they think immediately of very long exposures. This is quite natural since astrophotographs are taken in the dark, but for the moon it is simply not true. A quote which was attributed to Ansel Adams was that "the moon is a sunlit scene", which is a much more accurate way of thinking about photographing the moon. The moon is actually a very bright object if it is magnified enough to fill a significant portion of the frame and even with the extreme magnifications required (and the associated loss of reduction in image brightness) the shutter speeds were high enough to allow hand holding of the camera.
The photographs of the moon that accompany this article were taken with a Nikon E950 set to M-Rec mode. The focus was fixed at infinity and the photos were taken with afocal coupling (as described above). The exposure mode was set to Program with spot metering. For the first image, the exposure was 1/68 sec @ F2.7, Focal length ~ 2200mm (35mm equiv.), and for the second image the exposure was 1/81 sec @ F3.4, Focal Length ~ 3800mm (35mm equiv.). If the images had not been sharp enough, the camera could be tripod mounted, and the self-timer could be used to activate the shutter. These two things can drastically increase the sharpness so be certain to use them if your pictures are not as sharp as you would like.
It is important to consider the phase of the moon when photographing it. When the moon is full light striking it is reflected essentially straight back at the earth, making it very bright and low contrast. Much more pleasing results can be obtained by taking the photographs anytime except for the three or four days surrounding the full moon.
The moon is essentially a black and white object with fairly high contrast. The appearance of lunar photographs can be enhanced (sometimes dramatically) by sharpening the image and possibly by adjusting the brightness of the image. Both of the lunar photos included with this article were sharpened in Photoshop with unsharp mask (set at 200%, radius 1.0, threshold 0).
Here is an example of using unsharp mask. Unsharp mask was repeatedly applied to the same image. The original image is too soft, and the rightmost is too sharp. Filtering can help but like everything, too much of a good thing is not a good thing.
|1X Unsharp mask 100,1,0|
|2X Unsharp mask 100,1,0|
|3X Unsharp mask 100,1,0|
There are a number of places to go for information on astrophotography. A search at just about any web search engine for astrophotography will find lots of web sites. If you want a book I highly recommend Astrophotography for the Amateur by Michael A. Covington. It contains lots of really valuable information, including exposure tables, formulas to compute focal length and aperture numbers.