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Canon-Nebulosity Tutorial for Beginners - page 4 / 9





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(Caution: If you see “dancing” stars, you are looking at hot pixels. Remove the lens cap dummy!! I know because I’ve done it!) When the image with nice stars appears, the crosshairs will be off-centered to the right. There’s two ways to get it centered. You can either use the “-“ for zooming out located under the histogram or you can use the sliders on the right side and bottom of the images until the crosshairs are in the middle. Once you have the crosshair centered, click “Abort”. Now click “Fine Focus”. The screen will do nothing. Don’t just stare at it. Look in the lower left corner of the screen. There’s a message for you. “Click on a star.” Select a dim star and click on it. The screen will suddenly go blank and your star will appear in the upper left hand corner along with three letters with numbers. M, m and S. (Note: If the window is too small, use the zoom-out feature under the histogram) “M” is for maximum pixel intensity; “m” is the average of three maximum pixels; and “S” is for sharpness. You can use any or all of these numbers to get a fine focus as they all increase when focus is best. I like using “M”, the top number because it has a larger range thus more sensitive. Turn the focus knob of the imaging telescope until you have acquired the maximum number. The “M” maximum that you can get while focusing is 65535. That’s why you don’t want to use Sirius or other bright star for focusing because it will always be at 65,535 regardless of your focus. Remember that we’re using a relatively dim star. It may never reach 65535. The object is to reach the maximum possible number. You will also have a running graph of each letter that shows you when you’ve achieved the best focus. Remember that we said focusing requires a lot of time? Here’s why. Each time you touch the focuser, the telescope moves causing the numbers to be very erratic hence the reason why some folks like to use moto-focusers. Allow the telescope to settle down and give you an accurate reading. These numbers will also be erratic when the seeing conditions are bad. Once you have reached the maximum numbers, tighten focuser set screws and then click “Abort.”

Step #4: Previewing your image live on the computer screen. Slew the telescope to your target object. Click “Frame and Focus.” I like to zoom all the way out so that I can see the entire field of view. With your telescope hand controls move the telescope so that the target object is composed in the center or wherever you want it. Remember that on some telescope/camera combinations, the edges will have coma (caused by having a flat CMOS surface that doesn’t coincide with a curved optics) so it’s important to center your target as best as possible. Once you’re satisfied with the composition, click “Abort.”

Step #5: How long should my exposure be? To start, change the exposure duration to 30 seconds and click “Preview.” After each image, look at your histogram. As Kip Pauls says in his Nebulosity tutorial, “Is all of the data to the far left? If so, your entire image is faint and you should increase your exposure. Let’s say you are trying to image the very faint Rosette Nebula. At 30-seconds, the histogram will show almost all of the data on the left. It wasn’t until I increased the exposure to 180 seconds (3 minutes) that I finally got a decent histogram. Ideally, you want a maximum signal on the left and gradually decreasing to the right. Exposure time is something you must experiment with. Remember that the longer the exposure and/or the higher the ISO number, the more hot pixels, the more noise you will have in the chip.

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