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Focusing properly is the number one challenge an astronomer faces after attaching a camera to a telescope. It is also the most critical. Regardless of the quality of optics and other equipment, all goes for naught if the image caught by the camera is not sharp to begin with. |
Parfocal eyepieces have been around ever since astronomers realized that focusing a camera through the normal camera viewfinder was nigh impossible even under dark skies and with acute eyesight. Parfocal eyepieces are regular eyepieces that have a focal length identical to the camera's film-plane or backfocus distance. In many cases, these can be homemade by using a regular eyepiece with an approximate focal length, and then adjusting the focusing distance by attaching a removable parfocal ring. Manufactured parfocal eyepieces are also readily available with the parfocal ring permanently attached or glued to the eyepiece barrel.
How well do they work?? Performance depends on a number of factors. First, the eyepiece lens itself should be of high quality and not add any aberrations of its own. The manufactured units available are fairly disappointing in this respect since low-quality, inexpensive kellner and huygens lenses are employed. Heck, we even include a nice plossl with the Series IV which is really overkill. My own opinion is that if you want to take this approach to focusing, you are far better off making your own. Second, the film-plane distance or backfocus distance calibrated must be extremely precise in order to avoid bloated images. Unfortunately, setting the proper distance with a parfocal ring is usually a hit-or-miss operation, and parfocal rings are always susceptible to slippage. But for about $8, using a parfocal ring and an existing eyepiece is the most economical way to do a little better focusing. Yet, even with a good design, proper calibration, and skill, all parfocal eyepieces will fail to perform properly in the end. The reason is very simple: Your eye automatically compensates for images that are out of focus by up to 3%. So, even though you think you have attained a pinpoint image through a parfocal eyepiece, this still may not correspond to the proper film-plane or backfocus distance necessary. You do not need to do a lot to prove this hypothesis: just count the number of times you are hitting the electric focuser button when you are viewing a planet. It seems that the image you are seeing is never good enough or stabilized. And your eye itself changes from night to night. This is not due to changes in the optical chain, rather the automatic compensations your eye continually makes to satisfy the brain's interpretation, misinterpretation, and reinterpretation of the image. Eyes are wonderful technical instruments for normal wide-field viewing, but generally lousy for focusing on specific points. Some anthropologists believe this is an evolutionary leftover from our hunter-gatherer days. Instead of time making improvements in our vision, it preferred to make improvements in our brains. One of those improvements was the ability to easily distinguish patterns. The only way to achieve proper focus in astrophotography is to bypass the above problem altogether. And this is exactly what all focusing devices manufactured by STI do. Instead of relying on attaining a pinpoint image for your eye, you are presented with a source of light that has a highly visible bar pattern running across it. Seeing changes in the pattern while focusing is very easy and not subject to reinterpretation of the brain or dependent on the acuity of your eyesight. While this is not a "what-you-see-is-what-you-get" approach like other methods, it is a fool-proof "what-you-need-is-what-you-get" solution to focusing. For those who do insist on a WYSIWYG approach, check out our new CVF 'Spotlight' Series focusers which reduces your eye error to only .2% . But that's about as good as it will ever get! |
Focusing Screens & BrightScreens
One would think that changing the focusing screen on a 35mm camera could solve the problem. Yet even the best focusing screen available, the Beattie BrightScreen, only provides you with another f-stop in brightness. For these to be effective partial solutions to focusing on stars, one would like the image to be brightened at least ten times over. Unfortunately, no focusing screens can accomplish this. Various
screens are available for the old Nikon and Olympus cameras to replace the originals, and at $25-$50 a pop it may be worth trying a few out. In the end, you will see that focusing is apparently improved, but that your images are still not pinpoint sharp. In short, enhanced focusing screens will not provide enough light, plus you still have to contend with the problem of your eyes tricking you. The worst part is that the star images in viewfinders are still much too small to be usable since the pentaprisms used in camera viewfinders tend to de-magnify them. Check out our CVF Series 'Spotlight' focusers with proprietary focusing screens which provide low-light loss and easy focusing.
One of the better ones, the DW-2 and DW-4 units for Nikon cameras, provide 6x. That's somewhat better, but there are still better ways to spend $200. Other industrious individuals have developed tall smokestack magnifiers to overcome the problem. Even with this, there are still other disadvantages that magnifiers bring: 
The mask is placed directly in front of the main telescope to block out all light except the light entering through the circles. You focus until only one circle appears instead of many. With low f-ratio scopes in particular (f4 and under) , masks may provide an effective solution to focusing. Scope optics at these ratios usually have high field curvatures. The Hartmann mask is one way to minimize the bloated images at the periphery of the field by averaging out the focusing across the entire field. Still, in doing this one is sacrificing pinpoint imaging in the center of the field. On the downside, I've found myself perpetually squinting when using a mask since there never seems to be enough light. But that is understandable since most of the light is being blocked in the first place by the mask. On the upside, the Hartmann mask approach to focusing is scientifically similar to what STI does: it provides you with a simple pattern (circles) that you simply make converge. If you haven't tried a Hartmann mask, you should. Check out Tom 
focus for astrophotography purposes, it would need to be a complete, and much more sensitive, replacement of the entire autofocusing electronics in the camera. In essence, one would actually need a better camera inside of the existing one specific to focusing to overcome the original camera's limitations. 
This can result in perfection - but at a cost: the focus is only good for one picture! To focus again for a subsequent shot, the camera has to be opened, the film removed, and the knife-edge reinstalled. Needless to say, this results in a lot of wasted film. In addition, focusing in this manner is just plain uncomfortable, especially if you are focusing anywhere near the zenith. But, the major problem with knife-edge focusing is that very few people know what to look for in the first place and repeatedly miss the focus point. See our 
A three to five second image is taken, and then one refocuses with software repeatedly until the four diffraction spikes generated by the interference are distinct and bright. In a broad sense, the procedure is very similar to using a Hartmann mask except for the pattern. 
A variant on the same theme is the Bahtinov focusing mask. Success in using it depends on having a very bright star available to focus on. In addition, the size of the interference pattern generated is specific to the f/ratio of the telescope being used. Multiple masks are needed when the f/ratio is changed, as well as the physical aperture of the telescope objective. Success in using this mask relies on how well you can center the main diffraction spike in the middle of the pattern. Whereas this depends on keen judgment of position, the Stiletto relies on the total absence of an element (ronchi bar) which is easier for the eye to determine. The same Stiletto also works with any scope of any f/ratio. If you own multiple scopes or thinking of upgrading to a new one, the choice is clear.
As just about everyone realizes, it’s more comfortable and convenient to focus a camera without wearing glasses. A person who has perfect distance vision gets good focus just with the telescope’s focuser.
