Facts about Focusing
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. Below is an overview of several traditional methods used to focus 35mm, DSLR, and CCD cameras. While hopefully the comments are informative, keep in mind that I am slightly biased in favor of our focusers and plug them subtly, but relentlessly, throughout. Despite what I have to say, you will find those who are successful with any of these approaches. ( Personally, I consider these people just plain lucky! ) - Richard Parfocal Eyepieces Focusing Screens Magnifiers Hartmann Masks Focusing Software Diffraction Focusers Knife-edge Focusers
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 reduce your eye error to only .2% !
To compensate for the smallness of star images through a viewfinder, various magnifying assemblies are available. These usually attach to the regular viewfinder, or replace the entire pentaprism assembly in the SLR. You should be aware that most of these magnifiers are relatively worthless as far as astrophotography goes. The main reason is that they do not provide enough magnification to be useful. When one adds a magnifier, its basic function is to reduce the error of the human eye described above by reducing the field of view and subsequent error. To be effective in this respect, a magnifier should provide upwards of 20x magnification. Those magnifiers commercially available for use by amateur astronomers usually provide no more than 3x magnification.
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:
All magnifiers reduce the brightness of the image you see. Period. There is always an inherent trade-off between magnification and brightness. (But, you already know this since you have seen the difference in brightness between a 25mm eyepiece and a 7mm eyepiece when viewing a sky object.) It is no different with magnifiers used on viewfinders. And this is the reason most people using the magnification approach also wind up getting a brighter focusing screen to go along with it. The combination of magnifier and brighter focusing screen definitely is better than nothing, but isn't the most cost effective or practical approach. Aside from this, every time you would like to use that nice 35mm camera for taking regular pictures afterwards, you have to go through the ritual of removing all of the add-ons and replacing them with the original components. In addition, all this effort can go for naught if the flip-up mirror or focusing screen in the camera are not positioned properly in the first place.
To bypass this problem, the Series IV was designed as a standalone focusing unit. No modifications to your own camera are ever necessary. You attach the Series IV to the same t-ring, focus it, take it off and replace it with your camera to take the actual picture. No muss, no fuss, no fiddling around cross-threading in the dark. For those who insist on a what-you-see-is-what-you-get image, check out the new CVF Series 'Spotlight' focusers which provide high magnifications and custom viewscreens for any applications.
Hartmann masks, either those home-made or manufactured really do work - for very bright objects, that is. A Hartmann mask consists of two or more identical circles which serve as the only apertures for light to enter the telescope.
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 Kendrick's Quikfocus for a starter. Then, come back here when you find yourself squinting too much.
Like the Hartmann mask, the Series IV 'Stiletto' has the ability to average out focusing for a particular scope. Since the 'Stiletto' helical focusing assembly can be fine tuned, the setting (film-plane or backfocus distance) can be tweaked by the user to average out field curvature, curvature in the film itself, or curvature induced by coma correctors. Our instruction manual shows you the step-by-step approach to minimize these negative effects.
There are various focusing software programs on the market for CCD cameras. And, they all will do a good job focusing - given enough time. And time is the word. My experience has been that it is easy to waste over a half hour trying to get a decent focus with software. Too many iterations are necessary to get a pinpoint image. During that time, temperature changes, mirror slippage, and tracking errors can creep in and make all the focusing work go down the drain quickly. And in the end, one would think that the pixel resolution would be better from software than from the analog approach we take. Not.
User experiences have shown that it takes only a few seconds for the Series IV to focus a CCD or 35mm DSLR camera and that the resulting resolution is better than software. I'm positive that sooner or later better focusing software will inevitably evolve. But for now, focusing with a Series IV is faster and more precise. The only software/hardware which comes close is the RoboFocus setup. But at over $800, the price is a little steep for most folks. And for 35mm DSLR owners, none of the computer laptop paraphenalia is even needed in the field !
You've already heard from others that knife-edge focusing is the very best way to achieve pinpoint focus. And they're right! Knife-edge focusers are easily made. In essence, a razor-sharp edge is placed at the exact film-plane distance of the camera being used at the center of the optical train. Many professional astrophotographers attach the knife-edge to the film rails of the camera they will actually use.
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 knife-edge tutorial for more information on this.
In the end, the Series IV 'Stiletto' has incorporated all of the good things about knife-edge focusing, and done away with all of the bad things. It is far easier and faster to use than a knife-edge, much more user friendly and ergonomic, and can be used for each frame on a roll of film without waste. Plus, it works for CCD and DSLR cameras too!
Diffraction focusing a CCD is a relatively new procedure. In this approach, the cross pattern of the vanes in the secondary holder of a reflector telescope are used for focusing. For other telescope designs without vanes, thin wooden rods are placed crosswise in front of the telescope to simulate them. In this procedure, a very bright star must be used.
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. On the very positive side, this system works very well and a cheaper solution would be difficult to find. On the negative end, the procedure can be enormously time-consuming taking repeated exposures and refocusing in between each time. Also, the proper width of the obstruction grating or grid itself is dependent on the focal ratio you are working at. Just making a cross out of a pair of sticks isn't the most scientific way to go about this. But if you are a CCD camera owner, you should definitely give diffraction focusing a try if you have the time.
If you do not have a lot of time or patience, however, then take a look at the STI Series IV 'Stiletto' or CVF 'Spotlight' Series focusers. They do put you in perfect focus in only seconds. You have our Clear-Sky Guarantee on it!
And, here's what one of the pros has to say:
"I purchased the Canon EOS 10D DSLR Focuser device from you a while ago, and let me say, as a Professional Astrophotographer with 2 decades of experience, with hundreds of images published Internationally, Front Cover of Time & National Geographic, Science, Astronomy & S&T included…………………that this is one of the Best and most Time Saving Focusing Products I have ever purchased. Believe me I’ve tried them all over the years. Honestly, if there is a better or easier to use focusing device out there, I certainly don’t know about it! The proof is in the pinpoint star images, and the sharpness of the images. Just check out the images at my website. I just tell it like it is!!!!" - John Chumack The Chumack Observatories http://www.galacticimages.com
screens are available for the
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.