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Canon EOS 300D/350D/400D/450D (Digital Rebel) Astrophotography

CAUTION! This page describes my first attempts with DSLR astrophotography back in 2004 and is not up to date. Please see other parts of my web site, and also Jerry Lodriguss's excellent site, for up-to-date information.

Introduction

This is a page of brief notes about camera settings, workflow, and practices that I have found most successful.

Yes! This material also applies to the Digital Rebel XT (350D), XTi (400D), and related cameras such as the EOS 10D and 20D. On cameras that have mirror lockup, you can use it to reduce vibration at the telescope. On cameras that have dark frame subtraction (noise reduction), you can use it instead of subtracting dark frames later on your PC. Those are the only significant differences. These instructions even apply to Nikon and Pentax DSLRs although you will have to work out a few details for yourself. If you follow the instructions here, you will get good results even if your camera isn't exactly the same model as mine.

I assume you have read the camera instruction manual and have taken and processed a few terrestrial pictures with it, and that you are familiar with the basics of piggyback astrophotography (putting your camera on the back of a telescope to track the stars).

For basic astrophotography information, see also Canon's DSLR astrophotography site. This page will be more specific than that one.

How complicated shall we make it? Some people want to take pictures as quickly and easily as possible; others will spare no effort to make them better. There are 3 main ways to use the Digital Rebel for astrophotography: Simplest: Just treat the camera as a film SLR and take exposures in the normal JPEG mode. You can use a cable release for long exposures, or just use the camera's shutter (with delayed release) for 30-second exposures, which are enough to show many deep-sky objects. Exposures longer than 5 seconds will show some spots from hot pixels, but most of these will disappear if you resample the picture to 25% of its original size. Intermediate (what this page is mostly about): Take long exposures and dark frames in RAW mode. Subtract out the dark frames to eliminate hot pixels. Do most or all of your processing with Canon File Viewer and Photoshop Elements, which come with the camera. Advanced: Use specialized astronomy software and take the computer into the field along with the camera. In this case the camera works very much like an astronomical CCD camera. See "computer control" below. See also my tutorial on image processing with MaxDSLR and MaxIm DL.

Advantages of the Digital Rebel

• Low price

• Low noise in long exposures

• Straightforward controls

• Can be completely computer-controlled

• Works well in moonlight and city lights (like astronomical CCDs)

• Can take Nikon, M42 (Pentax screw mount), and T-mount lenses via adapters

• Light meter and auto-exposure mode ("Av") work with all lenses and telescopes

Disadvantages of the Digital Rebel

• Small, dim viewfinder image

• No automatic dark frame subtraction (the 20D has it)

• No mirror lock (can be remedied by installing the Russian firmware hack, but even then, you still have shutter vibration)

Computer Control?

An increasing number of advanced astronomy software packages are going to support digital SLRs just the way they support astronomical CCDs. The leader of the pack is presently MaxDSLR, which I recommend.

Camera Settings

Settings you will probably set once and leave alone

Parameters: Parameter 2. This reduces the amount of in-camera sharpening and color enhancement, so that dark frame subtraction and subsequent computer processing will yield better results. It applies to modes P, Tv, Av, M, and A-Dep but not to the other modes (designated by icons). It is not absolutely necessary. Contrary to what you might expect, this setting apparently does affect raw files when they are converted to TIFF or JPG by Canon File Viewer 1.3. It is preserved in the EXIF data attached to the generated file.

Auto rotate: Off. Dark frame subtraction is unduly difficult if the camera has rotated some of your images to match what it thinks is vertical orientation. I find it easier to keep everything oriented the way the camera is oriented. Contrary to what you might expect, this setting does affect raw files when converted to TIFF or JPEG by Canon File Viewer 1.3.

Settings you will make before taking a set of astronomical photographs

Quality: Raw. You will be converting the images to TIFF with the Canon File Viewer Utility or Adobe Photoshop. Even though computer geeks often write it in all capitals, RAW is not an abbreviation. It means "uncooked."

ISO: 200. This is essentially the gain (amplification) of the analog-to-digital converter reading the signal from the CMOS sensor. Canon's published data indicate that long exposures at a low ISO speed come out with less noise than proportionally shorter exposures at a high ISO speed. We don't know for certain about the Digital Rebel, but a published review of the EOS 20D (in the British Journal of Photography) showed maximum dynamic range at the ISO 200 setting. Some astrophotographers recommend a considerably higher setting (such as 800) when attempting to photograph faint objects in long exposures.

Mode: M. Manual mode, of course.

Shutter speed: Bulb. This means "keep the shutter open as long as the switch is closed." You will of course be using an electrical cable release, which can be homemade. If you do not have a cable release, you can still take 30-second exposures by setting the shutter to 30 seconds (30") and using the self-timer (delayed release) to start the exposure without shaking the camera. The term "Bulb" (B) goes back more than 100 years and originally referred to a setting where the photographer used a rubber bulb on the end of a hose to hold the shutter open with air pressure. It was distinguished from "Time" (T), which meant, "Press the button once to open the shutter and again to close it." T mode still exists on a few cameras, such as the Nikon F3.

• Turn off "Review" so the LCD doesn't automatically light up after each picture is taken. This will save battery power as well as preserving your friends' night vision. You can still use the playback button to review your pictures whenever you want.

• Disable "Auto Power Off" so the camera won't shut itself down when neglected for a couple of minutes between pictures.

Noise reduction?

If this feature is turned on, then every time you take an exposure longer than 1 second, the camera will immediately take a second exposure of the same length with the shutter closed, and subtract it from the first, to eliminate noise from "hot pixels" (points on the sensor that give nonzero readings in the dark).

This is a quick way to get good astrophotos. You can do your dark frame subtraction in the camera, without needing a PC at all.

The problem is that it's time-consuming. If you want to take three 5-minute exposures, noise reduction will require you to take three 5-minute dark frames as well. If you turn noise reduction off and do your dark frame subtraction on your computer, then one dark frame would serve for all three. Or you could take two or three dark frames, average them to eliminate non-reproducible variation, and then use the combined dark frame, which is more accurate than a single dark frame would be.

So in what follows, we'll assume that if you have noise reduction, it's turned off.

Focusing

You must focus manually.

Unfortunately, the Digital Rebel viewfinder is not designed for manual focusing. It gives a small, dim image. To use it effectively:

• Set the eyepiece diopter (see picture at right) so that you can see the focusing screen with perfect clarity. (Practice this during daylight, preferably in dim light, wearing whatever glasses you will wear when doing astrophotography.)

• Focus on the brightest star you can find, in the same general area of the sky (so that nothing flexes as you move to the final target).

  When photographing through a Meade digital telescopes, use "high precision" mode to go to a nearby star before each exposure.

• Check the focus electronically. As soon as you think you've focused, take a short exposure (maybe 30 seconds) and view it, enlarged, on the camera's LCD screen. Refine your focus as appropriate.

  Remember that the LCD screen will magnify 10× (which is twice what you'd use viewing a 35-mm slide with a loupe), so you can focus very accurately this way. In my experience, this is as good as any other method of focusing.

  You can get continuous electronic focus assistance from DSLR Focus or ImagesPlus software if you have a computer with you at the telescope.

• To make viewfinder focusing easier, add some magnification, such as a Canon Angle Finder C (about $200). (I have also used an Olympus Varimagni finder, which fits but does not give a very bright image. Various Pentax and Minolta angle finders may also fit.)

  Note that to add an angle finder, you must remove the rubber eyepiece frame, which lifts up (see picture at right).

• Try adding diffraction by putting a piece of window screen in front of the lens. It will produce bright spikes on all the stars, and you can focus the stars so that the spikes are brightest.

• Try a Hartmann mask (a lenscap with two big holes in it) in front of the lens while focusing. It will make out-of-focus stars appear double. The holes need to be big to admit enough light.

• You may be able to get some help from focus confirmation if you are using a Canon lens that is f/6 or faster. This is a way of getting the autofocus mechanism of the camera to tell you whether it thinks the picture is in focus.

  To try it, bring a bright star into the very center of the frame (the autofocus box). Hold the shutter button halfway down and adjust the focus. There will be a red blink, just like the autofocus signal, when the focus is correct.

  Instead of pushing the button halfway down, you can use the equivalent button on your cable release; or if your cable release doesn't have one, simply unplug it and plug a mono (not stereo) 2.5-mm phone plug in place of it. The plug doesn't need to be connected to anything. It will short together the ring and sleeve connections and make the camera think the button is halfway down.

Taking a Picture and Dark Frame

Shortcut #2: If you have a Digital Rebel XT or EOS 20D, use the "noise reduction" option when taking the picture. This will perform dark frame subtraction inside the camera. It will also double the amount of time required for each picture.

The normal way to take an astronomical photograph with any kind of electronic camera involves both a time exposure of the subject, and a dark frame taken with the lenscap on, to replicate the hot pixels so they can be subtracted out.

Hot pixels are spots on the image sensor that leak electrons, so they act as if they've been exposed to light when they haven't.

Hot pixels are more numerous, and hotter, at higher temperatures. You will find that your Digital Rebel works best on cold winter nights. Let the camera get good and cold, but carry a spare battery in your pocket, and put the warm battery into the camera before going to work.

The dark frame should be taken under the same conditions as the actual exposures, preferably very soon afterward, and should be the same length.

My usual procedure is to take several pictures of different deep-sky objects, all the same duration (e.g., 5 minutes), and then take one dark frame of the same duration. More sophisticated astrophotographers take several dark frames and average them together, to make sure they contain only reproducible artifacts, not random fluctuations.

More advanced cameras (Digital Rebel XT, EOS 20D, Nikon D70) will, at your option, perform dark frame subtraction automatically, in the camera. This is handy but can be time consuming if a lot of pictures are to be taken.

Processing Pictures on your PC

Converting Raw Files to Linear TIFF

The methods described here are obsolete. It is much easier to work with Canon raw (.CRW, .CR2) files with DeepSkyStacker or other astronomy-specific software.

By "linear" we mean that the brightness values in the TIFF file will be exactly proportional to those recorded by the sensor in the camera, without any correction for monitor gamma. This is essential so that hot pixels can be subtracted out. (If a pixel has a value of 10 from actual exposure and 100 from hot-pixel leakage, it will read 110. We want to be able to turn it back into 10 by subtracting 100 from it. That will only work if the numbers haven't been monkeyed with.)

Then open your file and convert it to a 16-bit TIFF. When you're generating them with Canon File Viewer, only 16-bit TIFFs are linear; the "linear" setting is ignored if you're generating 8-bit TIFFs or JPEGs.

Do this to both the picture(s) and the dark frame, making sure exactly the same settings are used.

Hint: Canon File Viewer does not use any of the lossless compression that is permitted in TIFF files. These files are going to be big! I recommend ZIPping them if you want to keep them. Or just keep the .CRW files and regenerate the TIFFs as needed.

File Viewer versions There have been reports of problems doing the TIFF conversion with Canon File Viewer versions earlier than 1.3 (September 2003 release). You can download an update here (under Drivers/Software). With Canon File Fiewer, only 16-bit TIFFs are linear. In the other modes (8-bit TIFF, JPEG), checking "linear" has no effect.

Canon File Viewer is being superseded by Canon Digital Photo Professional, formerly supplied only with higher-end EOS cameras. You can download it from here (among EOS Digital Rebel applications) and although called an "updater," it is actually the complete program. You must already have Canon File Viewer installed. The new program lets you choose a linear curve whenever you convert a raw file to JPEG or TIFF. Open the raw image and hit Ctrl-T. Don't be alarmed that the "linear" curve seems to sag deeply, like this: The horizontal scale on the graph is logarithmic but the vertical scale is not, and that's why a linear function appears as a very bent line. For more about this, click here.

Subtracting the Dark Frame

The methods described here are obsolete. It is much easier to work with Canon raw (.CRW, .CR2) files with DeepSkyStacker or other astronomy-specific software.

If you'd rather do the subtraction with Photoshop or Photoshop Elements, proceed as follows. Open both of the 16-bit TIFFs...

If you're using Elements, it will insist on converting to 8-bit color depth. That's OK, though not ideal. (It's still linear.) The full version of Photoshop will work with the full information content of your 16-bit TIFFs.

Don't panic - your picture will look quite dark! If it doesn't contain any overexposed areas, it may look black. To see that there's something in it, do Auto Levels, then undo it. We want to work with the unaltered TIFFs here.

Click on the title bar of the dark frame image.

Make sure the Marquee tool is selected, then right-click on the dark frame image itself and choose Select All.

At the top, choose Edit, Copy.

Click on the title bar of the picture image. Then, at the top, choose Edit, Paste. Your picture goes black, but again, don't panic!

At the top, choose Window, Show Layers, and make sure the Layers subwindow is visible. Set Layer 1 (the dark frame you've just pasted) to Difference (instead of Normal), leaving its opacity at 100%.

Now the dark frame has been subtracted.

Last, choose Layers, Flatten Image, and save under a new file name.

Working with JPEGs If you want to do dark frame subtraction with files that were recorded by the camera as JPEG (JPG), you'll find that Opacity=100% is not the best setting. Opacity near 50% will probably produce better-looking results because of the nonlinearity of the response curve. You will not get perfect subtractions; hot pixels will leave small lumps or doughnut-shapes behind.

Adjusting Levels

This tells you the picture does not span the available brightness range from black to white. Slide the black and white pointers closer to the black part of the histogram, and then move the gray pointer to the left or right to adjust the midtones:

→

Resampling and Sharpening

Images for display on the Web should generally be something like 300×300 to 700×1000 pixels. Images for sharp 5×7 prints can be downsampled to half the Digital Rebel's initial resolution (i.e., 50%, which will make your 2000×3000-pixel image into 1000×1500 pixels and get rid of a good bit of noise and speckles).