Milky way spreads itself ower the complete celestial hemisphere. This picture was taken from one of a few places in Croatia (cape Kamenjak near coastal town of Pula) where the sky is still reasonably pristine, even if LP of Pula is noticeable in the lower right corner of the picture. It is a 5 min. unguided exposure at F/4 and ISO 400.
Only a few are not moved by such an image. But, not many attempted to make such an image themselves. The problem is in (still) quite expensive lenses (the so called fish-eye lenses) that are able to create a full 180 degrees field of view on film. For DSLR the problem is that a fish-eye lens designed for film will produce too a large image. In other words, a specially made fish-eye lenses, tailored to the chip sizes of DSLRs in question are required. They start to apear, but usually cost more than the complete camera with two or three basic lenses.
Somewhere in the twenties (of the last, 20th century!) all-sky cameras that solve this problem by using a large curved mirror to image the whole celestial hemisphere appeared. They are simmilar to mirrors that are today often mounted at street crossings to help drivers see "behind the corner". At that time, the required mirrors have to be made at great cost. About 50 years later someone noticed that then popular car wheel decorations are actually a kind of convex mirrors, and cheap all-sky camera was born. Such whell decorations are history now, but there are other similary cheap solutions now.
The principle of a mirror all-sky camera with a convex mirror. Starlight is reflected from the mirror towards the camera, eqipped with a standard objective lens. The camera sees the image of the sky formed by the mirror, that fits well into the field of view of the camera lens. A concave mirror can be used too, but the field of view will be limited to about 170 degrees by the opposite mirror rim. Convex mirror does not have this problem, acually, it can cover almoust 360 degrees. If you do not belive, just look into a spherical christmass-tree decoration! A great advantage of this construction is that the optical quality of the mirror is not so important. This makes cheap mirrors or mirror-like objects quite usable. Also, the exact shape (sperical, parabolic, or something simmilar to that) is also of no big importance. Only the distortion near the image rand is changed by the exact shape of the mirror.
The convex mirror is put on a horizontal surface, and a normal photographic camera looks into it from above, actually taking pictures of sky image produced by the convex mirror. The neccesity to hold the camera above the mirrror in some way puts the image of the camera holder (usually a sort of tripod) and the camera itself in the sky image, producing a characteristic look of the image.
The convex miror produces a so called virtual image of the sky that lies a few centimeters behind the mirror itself, and is more or less curved the same way mirror is. This means that the camera should be focused to the distance of this virtual image, not to infinity. Trial and error are a rule here, but a good start is the separation between the mirror and the camera increased by about 10%. As the sky image is curved, the camera lens should have enough deepth of field at this distance. If not, part of the image will be blurred due to insufficent depth of field. To solve this, the camera lens has to be stopped down (this problem will appear below F/4 or less), or the separation between the camera and the mirror should be increased. If one has a simple camera, with one lens, the separation is actually fixed. One simply moves the camera away from the mirror untill the whole celestial hemisphere appears on the image, leaving a little spare place arround it, to compensate for possible small camera missalignments that can appear when transporting the all-sky mirror and repeatly mounting the camera on it.
I found a good all-sky mirror in the form of a stainless steel pot cover. It took for a while to find a cover that has approximatelly a spherical shape (as judged by the eye only, looking at its profile) and enough curvature to show complete hemisphere when looked at from a reasonable distance (50-70 cm is a good starting distance). A flat rim, if it exists, does not matter.
This is the picture of my final pot cover choice. It is 24 cm in diameter (slightly less than 10"). Quite important is that it has approximatelly spherical shape (see the image below), as many covers I saw are very flat in the middle with rapidly increased curvature near their rim. They will deform the sky image too much. After I found the model I liked, I carefully inspected several pieces for scratches and bumps, which were not to rare, and choose the one that looked the best to me. It costed me about 20 EUR (or $ if you preffer), of course in croatian currency. I found it in a big shopping mall, and I recomed to search in such shops, as specialized shops tend to have more expensive parts. As we will not cook, the quality (and thickness) of the steel the cover is made of, does not matter to us. Also, the central handle (avoid covers with two handles on the rim) does no harm, as it can be removed easily, and, even if it is left in place, it will mostly be coverd by the image of the camera itself.
To repeat: looking the cover in profile, we have to see approximately constant curvature (the yellow circle). Our eyes are quite good in judging this, even without any tools. Also the curvature should be deep enough (this one just does), for the "mirror" to be usable for us. Also, If the central handle is large, check that it is screwed on, not permanently fixed.
We have to hold our camera (film or digital) at the appropriate height above the mirror. There are many ways it can be done. The simplest is to use a standard photographic tripod and mount the camera below it. Many tripods have a provision to do that, as this is usefull for macrophotography.
With a little effort we can construct a dedicated holder, like the one pictured here:
The mirror itself lies on a circular piece of plywood, 10 mm thick in my case. Anything between 10 and 15 mm will do (3/8-5/8"). Tri wooden legs, made from scrap pieces of wood hold the camera platform (a circular plywood ring with a hole for the objective lens) above the mirror. When in use, the all-sky camera is put onto a horizontal surface (or a horizontal guiding platform, if you prefer guided images) the photographic camera is put onto the all-sky platform, with its lens passing through the central hole. Taking care that the camera is focussed proprely, we only have to make our exposure.
I later added a piece of angled steel plate that allows the camera to be firmly screwed to the all-sky construction. That was very usefull when I transported the camera a lot, for instance, when I worked on LP studies, and the whole team traveled all over the region and took pictures and measurements at several locations per night. At bottom plate a circular level is added to check the horizontality of the bottom plate. The hole in the middle of the bottom plate is used for fixing the camera to my german mount, when I want to make guided exposures.
A look from above onto the camera ring with the camera holder.
The simple way to determine the required separation between the camera and the mirror is to put the mirror to the ground, and look on it through the camera viewfinder. By moving up and down we seek the position in which we see the whole mirror, plus a small margin arround it, in the camera. After that, we only have to measure the distance between the camera and the mirror. If we have a zoom lens, we can determine a separation (my recomendation is about 70 cm), and simply adjust the image size by using the zoom.
If not quite sure, make your tripod 10 cm higher. It is much easier to shorten its legs than to prolong them, if the need appears!
A few last tips: if you use a digital camera, do not forget to switch it to manual focusing. Othervise, the camera will not be able to focus the sky image (it is too dim!) and will spoil the focusing you hopefully adjusted at day on clouds or simmilar...
Avoid direct reflections of streetlight at any cost. Pot cover is not a perfect mirror surface, and scattered light will spoil the image, like in the example below:
The polish of the mirror can be improved a little by additional polishing, but scratches will stil remain. I used the first stainless-steel polishing agent I found in the shop, and it worked OK. When the all-sky camera is put into shadow, results are much better:
At the end, a few data about my all-sky camera: The tripod height (between two plates) is 82 cm, pot cover diameter (curved part without the outher rim) is 22 cm. I also used a second mirror, 24 cm in diameter, this one without the flat rim. Both mirrors produced very similar images. With film camera a standard 50 mm lens provides the full size all-sky image, and the sharpness is OK at F/2.8, but to leave some tollerance, I mostly worked at F/4. If EOS is used, the required focal length is about 37 mm (or 35 mm, if you have a classic 35 mm wide angle) also at F/2.8 or F/4.