CYSI ‘13 – Astrophoto Like a Pro! Basics and Ideas

We’ll be Using Cabrillo’s Cameras This week – But…. If you have, or will have some day, a digital SLR camera, then the ideas to follow will be gold! We’re going to talk about basic ideas in photography and how a camera works

Basic Controls… First Rule - Learn your Camera! Spend a lot of time just playing with your camera. Go through all the menus, change settings, see what they do, make sure you change them back before you forget (often cameras are pre-set to control settings that are good for most situations) For starters, look for a button labelled “menu” on the back of your camera Practice, practice practice!

Basic Controls: F-stops f/ stops…. The f-number is the ratio between the distance to the image plane and the aperture of the lens. There’s an internal set of blades which close down the light after it enters the lens so you don’t use all the light hitting the outside of the lens. Why would you throw away light? Because if the aperature is narrower (higher f-number) then ther will be a larger “depth of field” (range of distances that will be in focus For pure sky photos, you don’t care about depth of field because EVERYthing’s at essentially infinite distance! But for shots with foreground that you want in focus, you will care. Still, that’s an argument for keeping your foreground objects as FAR in the distance as you can, so they’ll be reasonably in focus even on your widest lens opening In astronomy in general, things we photo are FAINT so we want the f-stop to be as SMALL as possible to let in as much LIGHT as possible The lens is usually a little “softer” (focus not quite as sharp) at the widest (smallest f-number) setting, but this is often a compromise worth making

Basic Controls: Exposure Exposure: how long the light hits the chip More is usually better as you pick up more stars and more of the dim night time scene But if you go too long, you’ll have your stars trail. You may not want this… General rule: you can go up to 20 seconds of exposure on a 33 mm lens for shots near the celestial equator before stars trail noticably, for a cropped Nikon camera like our D40 or D7000, or most cameras out there. Expen$ive full frame cameras have chips the same size as 35mm film frames; then the general rule is 20 sec for a 50mm lens. For longer focal length lenses, you must go shorter or you’ll get trails. For example, double the lens focal length means cut the allowable exposure in half.

Basic Controls: White Balance Modern cameras will automatically process your images in the camera to account for what the light source is that is lighting your scene. Your brain does the same thing, once a few seconds of accomodation has passed. To get “natural” colors, if you’re shooting daylight but in shade, you’re illuminated by blue sky rather than direct sunlight, so the camera will “redden” your photo to compensate. Less so if it’s a cloudy sky. If you’re indoors shooting under an incandescent light, which is very reddish light, the camera will blue your picture. These settings are shown on your camera usually with little icons. For astronomy photos, I’d recommend using the “sunlit” setting. If, for artistic reasons, you want to change the color before it emerges from the camera, you would use the White Balance setting to do that.

Use Software to Merge and/or Make Fisheye Pix Wide Angle Lens vs. Fisheye Lens – you can get the same effect as a Fisheye if you shoot 4 frames which overlap in a box-like fashion, and then use Hugin software (free download) to create the fisheye effect on the total image.free download Use Photoshop (CS2 and up) to Photomerge pix into a single image, and then…Photomerge …Use Photoshop (PS 7 and above) to make a fisheye out of a single picture (how to)how to Resolution will vary across the frame because of the distortion, but a small price to pay for being able to use a rectilinear wide angle lens to do fisheye imaging!

A Real Fisheye Image: Shot with Samyung 8mm Fisheye