What’s more is that people can even shoot deep space objects (DSOs) using this same afocal method, only adding a shutter release cable and taking shots that use the longest possible shutter times that these cameras allow. This is a sensible way to try your hand at long exposure stuff since you are likely to already own such a camera, right?
With the affordability of new digital SLRs, it is no longer necessary to shoot through an eyepiece to connect a camera to a telescope. In fact, because the lenses of these cameras can be removed, the camera can mount to a telescope using a traditional T-Ring and T-adapter. This is a method known as “prime focus.”
In this configuration, the scope actually becomes the camera lens. The specification of this lens becomes that of the scope to which it is attached. For example, if the camera is connected directly to a 10” Newtonian scope with a 1250mm focal length and f/5 focal ratio, then that scope becomes a 1250mm, f/5 fixed telephoto lens. The ability to connect a camera directly to a telescope using the prime focus method has many advantages, but the best way to show these advantages is to talk about the DISADVANTAGES of the afocal method:
1.) Vignetting - Using prime focus gives you the largest light cone, allowing the most uniform coverage of the 35mm film/CCD plane. When you add an EP to the drive frame, that coverage becomes severely limited.
2.) Lower contrast - Shooting through more glass is never a good thing if it can be avoided. Shooting through an EP does lower the contrast of the object you are shooting.
3.) Star sharpness - The quality of the star images are only as good as the eyepiece you are using. Not many EPs, especially those commonly used in EP projection or afocal techniques, give pinpoint stars to the edge of the field of view.
4.) Field curvature - A "flat field" refers to the "warping" of the proportions of the image. While the stars may be flat, the proportions are normally off, or bent. This means that the distances between stars are not as uniform as they'd normally be. This is a problem with prime focus photography as well (since very few optical systems are perfect in this regard...some apos and RC are the exceptions), but not to the degree of an added EP, especially one of inferior quality.
5.) Inconsistency - Because these techniques use a variety of EP focal lengths, adapters, and camera lenses, it's hard to know exactly what EFFECTIVE focal lengths and speeds you are shooting at. While you can go wide and fast in certain configurations, it's hard to know the extent of this with any amount ofconsistency. That makes it hard to learn this hobby. There are enough variables to worry about...adding more of them doesn't help.
6.) Lens quality - The quality of the lenses in many point and shoot cameras are sub-par themselves. Unless you have a nice lens on your point and shoot camera, it is more likely to degrade the image rather than help it. No matter how great your scope's optics are, if you are shooting through a typical point and shoot digital camera lens...well… you get the idea.
As you can see, this is most certainly NOT the best method for connecting your scope. Remember, it’s merely a “work around,” allowing people to use their existing point and shoot cameras with their telescopes.
One of the purposes for writing on this subject is to convey that the afocal method should no longer be used once you’ve actually purchased a digital camera with a removable lens.
I get this question all the time: Does Scopetronix make an adapter to connect my new digital SLR afocally? Well, yes they do….but why would you want it? A simple T-ring and T-adapter is less expensive and will allow you to take advantage of a MAJOR strength of these new digital SLRs.
The only disadvantage is that you lose is some flexibility with the prime focus method. After all, the focal length of the system is now FIXED. However, if you need more power for planets and the moon, the addition of a teleconverter or TV Powermate can give you the close up views you need with very little degradation to the image.
As far as exposure times, etc., the digital SLRs are normally fully equipped, with more advanced functions such as longer bulb exposure times, better noise characteristics, higher ISOs, etc. That, coupled with the ability to add nice quality lenses for piggyback work, makes these cameras highly desirable. Using the afocal method does give you a wide variety of focal lengths and speeds, but those are never consistent and they come at a cost where quality is concerned.
But the real proof is in the results. I have yet to see any truly GREAT images of DSOs taken with afocal techniques. Good results, maybe...but not truly GREAT.
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