Aperture for Camera.
All those years ago, I thought you had to use a telescope for astrophotography. But really all you need to get started is an off-the-shelf DSLR camera, some quality lenses, and the ability to have them track the sky. In fact, this avenue of astrophotography is so compelling to me that, even with high-quality astrographs, astronomical CCD cameras, and exotic filters at my disposal, I still frequently return to my DSLR and its lenses to capture certain targets.
When it comes to wide-field astrophotography, well-corrected astrographs with focal lengths less than about 400 mm are extremely rare and generally astronomically expensive. But one of the enduring appeals of the DSLR is the wide range of interchangeable lenses available. It's much easier to find quality DSLR lenses ranging from 400 mm all the way down to 8-mm fisheye lenses covering a 180[degrees] field of view.
There is a reason for this, of course. The shorter the focal length (and wider the field of view), the more difficult it is to control optical aberrations, especially away from the center of the image. The camera-lens folks have got this figured out, and highly sophisticated lens designs using multiple elements abound. And due to the immense size of the photography market, they can be had relatively cheaply. These lenses make excellent optics for astrophotography as well!
There is a caveat or two when using a camera lens for the night sky. Stars, being point sources of light, are an acid test for lens quality and sharpness. While stars in the center of the field of view might be round and sharp, those near the edges or corners of the frame often appear as elongated streaks, distorted into "seagull" shapes, or simply out-of-focus, multi-colored blobs. It takes a critical eye to spot this in a daylight image, but in a star field the aberrations stand out like a sore thumb.
The typical mitigation for this is to stop the lens down. It's well known in the photography crowd that, say, a 200-mm f/2.8 lens doesn't perform at its peak sharpness until you stop it down to f/4 or f/5.6. The exact adjustment varies from lens to lens, and there are websites aplenty with test charts for various lenses from different manufacturers.
Increasing the focal ratio ("stopping down the lens") decreases the opening through which the light enters the camera and restricts the amount of light that's focused onto the image sensor. It does so by blocking the edges of the lens using only the central region of the lens where the sharpness and aberration control are at their best.
Typically the light gets reduced using a 6- or 8-bladed diaphragm within the lens assembly, though some high-end lenses use more. Usually the blades have straight edges, which create spikes or rays (one for each blade) radiating from a point source of light. This diffraction from a stopped-down lens is glaringly apparent in an image filled with bright stars. In some high-quality lenses, the blades' edges are curved to reduce this effect.
Diffraction spikes can sometimes add "sparkle" to what would otherwise be a lackluster image. When I first started astrophotography through a camera lens, I liked this effect. But over time I found that spikes tend to get in the way of the real subjects shot through my 200- or 300-mm lens, cutting across subtle nebulae and detracting from an otherwise fine image. When you see these spikes, it's a dead giveaway that the image was taken through a camera lens. Most of the time I prefer to see the four diffraction spikes typically seen in images captured through a Newtonian reflector--or none at all, as you'd get with a refractor.
My Simple Solution
The cause of these spikes is well known, and I'm certainly not the first to find them objectionable or to want to eliminate them. One easy way to do this is to purchase a set of step-down rings that thread onto the front of your camera lens to allow the use of smaller lens filters. I looked at this at first but quickly found that the cost can really add up when buying specific-sized rings for each lens I own, as well as needing several additional sizes to achieve the f/ratios I desire. I then figured out a more economical alternative.
Replacement plastic lens caps are both cheap and abundantly available online. It's a simple matter to purchase several lens caps and then bore them out to the correct aperture with a drill press and a few hole-saw bits (like the ones used to cut into a wooden door to install a knob or lock). I ordered several caps to experiment with and a 50-mm hole-saw bit. Note: When ordering lens caps, try to find ones that use small plastic clips, rather than a clip system that uses springs. The spring system takes up most of the area that needs to be bored out and won't work for this project.
While cutting a lens cap is relatively straightforward, remember that the hole has to be right in the center of the cap. Finding the center is pretty easy--my son R. Stephen, who is much more mechanically inclined than I am, used a set of calipers to scratch three circles of the same diameter as the lens cap at about 120[degrees] intervals, using the outer edge of the cap as the center point of the circles. The scratched lines naturally intersect at the approximate center of the cap. This is particularly helpful if you're using a hole-saw bit that uses a central pilot bit.
We then used the drill press with a few "C" clamps to hold the caps firmly in place with the cap centered and made two test aperture masks, one that turned my Canon 300-mm f/4 lens into a sharper f/6 system, and one to stop down my 200-mm f/2.8 into a sharper f/4 system. I hit the fresh cut with a little fine sandpaper to remove any burrs that could potentially add any unwanted diffraction spikes, and I was ready to test them out.
Do they work? You bet they do! Images shown at the bottom of this page were taken with the Canon 300-mm lens. The left photo was taken with the lens set to f/5.6 using the internal diaphragm, while the other was taken with the diaphragm fully open but with the lens stopped down to f/6 using the test aperture mask. It worked just as well as I'd hoped--the second photo appears to have been shot with a small refractor, rather than with a camera lens.
One important point to remember when using an external aperture mask is to make sure the camera's internal aperture diaphragm is open all the way; otherwise, the blades will still introduce diffraction spikes in the optical path even when you've put on the circular mask.
Another clear illustration of the effect of the lens's internal aperture blades can clearly be seen when focusing. The out-of-focus stars appear as octagons due to the lens's internal diaphragm, while the stars shot using the aperture mask show a much more "refractor-like" appearance. It's worth mentioning that a top-quality camera lens operating at full aperture won't exhibit diffraction spikes, but its overall sharpness is visibly reduced, and edge aberrations, often appearing as elongated or even V-shaped stars when operating "wide open," are still often compromised; in all forms of photography, there are tradeoffs to be made.
Amateurs with access to a machine shop (or a friend with one) can also purchase inexpensive aluminum lens caps and mill them out to the specific aperture sizes needed to achieve more precise focal ratios. Hole-saw drill bits come in a limited range of sizes, so you'll need to calculate the f/ratio produced by a particular bit simply by assuming the new hole is your clear aperture. For instance, with the aperture mask in place, my 200-mm camera lens now operates as if I am imaging through a good quality 50-mm f/4 refractor (as we typically denote telescopes by their clear aperture, whereas camera lenses are listed by their focal length). The tradeoff here is you'll need to take longer exposures to acquire the same signal-to-noise ratio, but if you have to stop down your lens to reduce edge aberrations, you're already making this compromise anyway.
The proof, they say, is in the pudding. I've since made several lens-cap aperture masks for all my camera lenses to use with my modified DSLR. This simple improvement is both easy and inexpensive. No more "starburst" diffraction spikes for me!
* RICHARD S. WRIGHT, JR. is a software developer for Software Bisque, and writes a monthly blog on imaging for Sky & Telescope readers at https://is.gd/3yZ02K.
Caption: INTERNAL STOPS The diaphragm in a camera lens is designed to reduce the amount of light entering the lens, but it can produce diffraction spikes around bright stars.
Caption: NO SPIKES Author Richard S. Wright, Jr. describes his novel approach to creating lens masks that eliminate the diffraction spikes caused by internal iris diaphragms. He used the Canon EOS Rebel 3Ti DSLR above with a 200-mm f/2.8 lens stopped down to f/4 using a modified lens cap aperture mask to shoot the colorful Rho Ophiuchl region in Scorpius on the facing page.
Caption: CENTERING The aperture should be precisely centered in the cap, or you may end up with distorted or flared stars in your images. The center was quickly determined using a set of calipers described in the article.
Caption: DIFFRACTION DISTRACTION Images that include bright stars or planets, such as this shot of IC 4592 with the interloping planet Mars at lower left, exhibit strong diffraction spikes because the 200-mm was stopped down from f/2.8 to f/4 using its internal diaphragm.
Caption: SAFETY FIRST Using a hole-saw drill bit and a drill press, it only takes seconds to cut through the plastic cap. Be sure to secure the cap in place.
Caption: ROUND STARS The improvement in star shapes using the lens cap masks is apparent even when focusing. Out-of-focus stars display an octagonal shape when using the internal Iris of the 300mm lens (top), while the same field above using the lens cap mask displays clean, round stars.
Caption: BEFORE AND AFTER Compare these shots of reddish IC 434 (which includes the Horsehead Nebula) and the Flame Nebula to its lower left. Both views were captured through the same 300-mm f/4 lens, but the shot at left used the lens's internal iris stopped down to f/5.6. The image at right used a lens-cap mask producing an f/ratio of 6. Note the distinct improvement in star quality, particularly the bright star Alnitak between the two nebulae.
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|Title Annotation:||ASTROPHOTO DIY|
|Author:||Wright, Richard S., Jr.|
|Publication:||Sky & Telescope|
|Date:||Mar 18, 2018|
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