Equipment – FotoSharp


Fotosharp adapter

Fotosharp with a generic T-ring for comparison

The Fotosharp “adapter” is actually a T-ring which attaches to any lens which is compatible with a T-adapter to allow the lens to work with focus confirmation on a Canon EOS body. Some lenses are directly compatible with T-rings and need no adapter, but for a telescope the typical T-adapter is a threaded two-inch diameter tube which mounts to the T-ring and allows the camera to be attached to a 2-inch focuser. Unfortunately since I received the FotoSharp I have been recovering from minor surgery and have only had one chance to test it — and the New Equipment Gods noticed, and sent clouds in.

In brief, it works, though there are certain techniques and limitations to bear in mind.

In order for the Fotosharp to work, you must place a star on one of the AF points of the camera, adjust your focus until the camera beeps or until the focus indicator in the viewfinder either turns on or at least flickers with some regularity. The camera beeps only if the focus indicator stays on steadily, but you will need steady skies for that. I used the center AF point and placed Vega on that point. The sky was fairly good for my location – but that is on the northern city limit of Toronto, in the middle of a large metropolitan area. My sky quality meter gave a reading of 18.07 magnitudes/arc.second — and that was before moonrise — and I could see down to about magnitude 3. My best guess is that Vega’s light was reduced by two or three magnitudes from what it would have been under a pristine sky.

After getting no more than an erratic ‘in focus’ reading off Vega (twinkle, twinkle, little star, is my focus off too far?), I adjusted to what I thought was the steadiest flicker and moved over to M57. I used Nebulosity to average 11 35-second exposures at ISO 1600 starting with the JPEG versions of the images, which were saved in both RAW and JPEG. When I have more time I will go back and see if I can do any better with the RAW images.

This is a 128×128 crop of the central portion of the image, and you can click on this thumbnail to see a 1024×768 crop at the same scale. I haven’t posted the full image because no darks or flats were used, and the larger the image the more defects in technique show up. However, I consider the focus to be good — particularly considering that I took no images to confirm focus but instead started banging away with the camera.

M57 image crop at 100%

Some processing details: The images were captured using an unmodified Canon 20D at the prime focus of a William Optics FLT-110 (f.l. is 715 mm, f/ratio 6.5). 20 frames were captured under the control of a Canon TC80N3, but based on a quality estimate made by the Nebulosity program only 11 were used to create this final image. The remainder show guiding problems rather than focus problems. No darks or flats were used, and there are several ‘dust bunnies’ in the full images. In addition, the image train would benefit from a field-flattener as the outer edges of the frame show distortions. The mount is a Losmany G-11 with a Gemini controller, and is align to no better than a degree of the pole (the scope was taken to Starfest in August and has not been realigned since then – “I bin sick!”). The Gemini sky model was based upon the three bright stars of the Summer Triangle, but was good enough to place M57 at the center of the camera frame. The image was processed in Nebulosity to reduce sky glow and neutralise a color cast (also due to sky glow), and was passed through Noise Ninja in Photoshop CS2 to reduce background noise.

Additional note: Finding the AF point The FotoSharp needs a star in an AF box in order to work, and placing the star in that position can be a little tricky if you can’t see the boxes. Luckily Canon has provided a way to turn on the AF display LEDs in the viewfinder which makes it easy. If you have your manual, read the section on Manual AF point selection.

For the 20D, the process is to press the metering mode selection button (which is the rightmost front button), then press the AF point/enlarge button (which is the right most button on the top rear of the camera). This toggles the AF point display LED. The main dial can be used to move through the available AF points or the rear multi-controller can be used to move directly to a specific AF point. The centre AF point is best for focusing on a star.

Once an AF point is selected, move the star in the viewfinder so that it lies within the AF point box. It may be necessary to toggle the display a couple of times for visibility – the star is sometimes overwhelmed by the bright red LED. The display may turn off while you are doing this, but you can repeat the process as necessary.

In correspondence with Dr. Brady Johnson of K-W Telescopes I found that scopes up to f/11 have been used successfully with the FotoSharp, and stars down to magnitude 4. I expect the critical factor is the brightness of the focus star on the chip.

Cameras modified for H-alpha sensitivity should work if you can still focus in daylight using a normal lens. Note that some camera modifications leave the camera unable to focus with a normal lens without further adjustments to the camera. This is because the modification consists of removing a blocking filter. If no replacement filter is inserted or if the replacement filter is the wrong thickness, focus problems can result.

[The Canon 20D has been replaced with a Canon 40D, which has “live focus.” This feature reduces or even eliminates the need for the FotoSharp.]

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