You may already own a digital camera, and it may occur to you that you ought to be able to use it with your scope.
You can, but note that consumer digital cameras (digicams) cannot inherently take the long exposures needed to shoot dim deep-sky objects (click on the Long Exposures button above for an explanation about why this is so). You can get around this problem by taking a series of short exposures and “stacking” them (adding them together) with software designed to do this, such as Adobe Photoshop (or purchase a really large-aperture scope <grin>). Amateur astronomers who have been the most successful using digicams for astrophotography, use digicams that provide a good set of exposure controls (as a good 35-mm film camera does) rather than a digicam that is a simple “point and shoot” camera designed to do all the thinking for you. (This is an issue completely unrelated to the digicam’s resolution - the “megapixel” capability of its CCD chip.) Many of the Nikon Coolpix digicams provide good exposure controls and are commonly used by those astrophotographers successfully using digicams.
- For more information the technique of stacking images, do an Advanced Search in Google on “stacking images”.
- For some examples of astrophotos using a digicam and stacked images, see Forrest Egan’s Digital Astro web site.
Since you can’t remove the lens on a digicam (at least, any digicam currently selling for less than $2,000) you will be doing afocal -- sometimes called eyepiece-projection -- photography. Here, you mount an eyepiece on the scope and place the digicam so that its lens looks at the eyepiece. For this to work, five things must be true:
- The optical axis of the digicam’s lens must be perfectly aligned with the scope’s eyepiece optical axis and they must be exactly parallel. If the digicam’s lens is off-center to the eyepiece
you will get severe vignetting, and if the digicam is tilted relative to the eyepiece you won’t be able to achieve focus across the whole image.
- The front of the digicam’s lens must be extremely close - almost touching - the scope’s eyepiece. Otherwise (and possibly anyway - see below) you again get vignetting.
- The diameter of the eyepiece’s uppermost glass lens must be at least as large, or (better) larger, than the diameter of the digicam’s lens. You can probably intuitively see that if you aim a digicam lens
at a smaller-diameter eyepiece essentially you’re looking through a “small hole” and you will get vignetting.
Even 2” wide field-of-view scope eyepieces aren’t all that large compared to camera lenses, so in general this means that the digicam’s lens needs to be no larger than 20mm or so.
long run this will evolve to be a real problem for using high-resolution digicams with telescope eyepieces.
Currently-available lower-cost digicams with lower pixel resolutions can use smaller-diameter lenses because the CCD chip they use is small. But the “holy grail” of digicam resolution is above 6 megapixels (and trying for 10) since that will provide an equivalent resolution to 35mm film. As digicam manufacturers strive for 6 to 10 megapixels their digicams will require a fairly large CCD chip which will necessitate a large-diameter lens; this will in turn introduce even more vignetting problems when coupled to telescope eyepieces. (Of course, there may be a technology solution available in the future that proves my worsening-vignetting prediction wrong.)
- The digicam must meter light readings through the lens (TTL metering).
Many digicams use a photocell on the front of the camera for light metering, and in this case when you try to take an exposure of the moon the camera will be taking exposure readings of the dark ground or the scope’s rear cell that it is aimed at, rather than the bright moon and you’ll be doing a lot of futzing around to get a good exposure or perhaps any exposure at all.
- The digicam’s zoom lens must be designed such that it moves towards the filter threads when zoomed to telephoto. If it moves away from the filter threads you will likely get vignetting. This is an artifact of the digicam’s zoom lens design and as one example it is a problem with the Kodak DC260/265 digicams - at telephoto the lens is retracted a lot.
To accomplish the optical alignment you must find a way to firmly mount the digicam on the scope. This is a real challenge since there are few digicams that provide a way to attach themselves to something else.
Most consumer digicams retract the lens into the camera body, and they usually do not provide threads to mount filters on the lens. So you can’t simply use an adapter and T-Mount system like you can with 35mm single-lens-reflex film cameras as described on the 35mm Camera and Film page here. There are three possible ways for you to mount a digicam on your scope:
The best option is a large-diameter wide-field eyepiece mechanically coupled to a digicam with a smaller-diameter lens. See the Direct Digicam-to-Eyepiece Adapters page for more information on this option. A second option is an adapter that holds your eyepiece inside of the adapter and has a threaded coupler that mounts to your digicam. See the Eyepiece Projection Adapters page for more information on these. Both of these options require that your digicam’s lens has filter threads, or that an adapter is available that can be mounted on your digicam that accepts threaded filters. Note that each of these options may require you to get a “step-up ring” that mates the filter thread size of your digicam to that thread size used by the adapter. If so, B&H Photo & Video has one of the best selections of step-up rings available anywhere.
The third option, for digicams that cannot accept threaded filters at all, is a “clamp-on” adapter - click on the Clamp-On Adapter link at the top of this page for more details on this adapter.
Note that whichever system you use, by adding a digicam you are hanging a lot of weight on the rear cell of your scope. You will need some heavy-duty counterweights to keep the scope in
balance. Purchase a Lens Shade (aka Dew Cap), which helps by providing weight on the front cell, and check the Balancing an SCT page for a discussion of balancing your scope.
In all cases, achieving good focus is just as difficult as it is with any other astrophotography setup. Be prepared to spend a lot of time gaining experience in focusing. This problem is mitigated a lot
if you purchase a flip-mirror system. Fortunately, digicams don’t require you to take film to be developed and examine your results a day or so later.
Be sure to use the digicam’s self-timer to trip the shutter. This provides time for the vibrations you caused by hitting the shutter release, to damp out before the shutter trips.
Scopetronix has concentrated on products that adapt digicams to telescopes for a number of years. Jordan Blessing, the owner of Scopetronix, has written a good tutorial on using digicams for astrophotography, and I recommend that you take a look at it.
used their digicams to take good astrophotos, and more and more of these photos are being published by Sky & Telescope magazine. You need to assemble the right equipment as described above. A while ago Jon Bent Kristoffersen e-mailed me with the results of his testing of digicams, and had good success with a Fuji MX2700. While this digicam’s lens is reasonably fast by digicam standards - f/3.2 - the lens is small enough to avoid vignetting. He has tested other digicams and notes that “My theory is that you need a camera with a very small front lens that can be put very close to the eyepiece... The trick thus seems to be to find a camera with a small lens, quality optics, and a high-res CCD.” Scopetronix recommends that the Nikon Coolpix digicams have the attributes necessary for astrophotography with a digicam (although there are others as well).
However, if you plan to purchase a digital camera only to be dedicated to astrophotography you may be better off getting an astronomical CCD camera as discussed on the CCD Camera page here. A small CCD camera doesn’t cost that much more than a good digicam and because it is cooled you can take longer exposures of
deep-sky objects. Although astrophotography with digicams has come a long way in the last year or so, in general the astrophotographs taken with CCD cameras are still better than those taken with digicams.
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