This tutorial covers the methods I use for astrophotography with my Canon XSi digital SLR and Celestron CGEM. It is similar to the tutorial I wrote in December of 2007 for digital SLR astrophotography with my Canon 300D. With the exception of the Orion ED80 refractor, I am using all new equipment. I am also using different software for processing the RAW images into a detailed composite photo. Note that there are different ways of going about astrophotograpy with a digital SLR. In fact, there are probably better ways. This tutorial depicts the methods I used for capturing the example images below. I hope you find my new tutorial useful and please feel free to ask questions or add comments in the “Leave a Reply” section at the end of the tutorial.
The images below are provided as an example of what can be achieved with an equipment setup similar to mine and the methods provided in this DSLR astrophotography tutorial.
Here is an overview of the equipment I use for DSLR astrophotography. My setup is not considered a high-end system by any means, but nice results can be obtained. First, the telescope is a Celestron CGEM Mount with the Orion ED80 Telescope as the imaging scope. Piggybacked to the ED80 is the Orion Awesome Autoguider Package. Included in this system is the ShortTube 80mm refractor and StarShoot Autoguiding camera. I endorse this autoguider in various articles on my website because it is a complete, low-cost system that gets the job done. Autoguiding has made a big difference in the quality of my images since I wrote the DSLR astrophotography tutorial in 2007. And finally, I use the Canon XSi (450D) as my imaging camera.An important note here is that my ED80 refractor and Canon XSi DSLR are a good match optically. That is, the pixel size of the DSLR sensor matches the resolution of the telescope. This is explained further in my article- DSLR Astrophotography Calculator.
Also worth mentioning, I use an extension cable for my NexStar hand controller since the original cable was too short. The extension cable add-on gives me an extra seven feet of cord length so I can move freely around the telescope. Extension cables for the NexStar hand controller are available here on AstroPhotography Tonight.
Here are the links to my equipment reviews here on Astrophotography Tonight: Orion Starshoot Autoguider Review, Celestron CGEM Review Part 1, Celestron CGEM Review Part 2, Celestron CGEM Review Part 3, Celestron CGEM Review Part 4, CGEM Astrophotography- The Next Phase, NexRemote Telescope Control for CGEM
Before attaching the DSLR, I perform a decent polar alignment and 2-star alignment for the CGEM’s goto functionality. For polar alignment, I normally use the drift alignment technique as described in my CGEM Drift Alignment Procedure. Alternatively, the CGEM mount has a polar alignment routine called “All-Star” that works pretty well for fast polar alignment. The main point is that a good polar alignment is important for long exposure astrophotography. This is true even with autoguiding since you will need to keep the object still in the field of view while setting up the autoguider.
Goto functionality is not necessary, but I have found it to be VERY helpful for getting the object into the digital SLR field of view without the finderscope. The CGEM’s goto capability is very accurate in my experience. Plus, when it’s time to move on to another object, it’s a snap!
Attaching the Digital SLR to the Telescope
Once the telescope alignments are complete, I then slew to the target that I want to image. At this point, the telescope reticule eyepiece is still inserted from the alignment process. If the object is bright enough, I will center the object in the field of view if necessary. Otherwise, I do this later with short exposures and hand controller adjustments. More on this in a bit. Once the telescope is pointed at the target, it is time to remove the eyepiece and insert the camera.
Connecting the digital SLR to the telescope requires a T-ring and T-adapter as shown below. The following steps show how to attach the DSLR to the telescope.
Step 1: T-adapter and T-ring are separate components. Note that the T-ring must be specific to the type of digital SLR used.
Step 2: The T-adapter and T-ring are threaded. The two components are combined.
Step 3: Attach to digital SLR. The T-adapter/T-ring assembly inserts into the DSLR just like the camera lenses do.
Step 4: Attach digital SLR to telescope. The DSLR with T-adapter/T-ring assembly is inserted into the telescope eyepiece holder directly (i.e., no eyepieces, Barlows, etc are used). This is considered “prime focus” astrophotography. The telescope optical tube serves as the camera lens.
Important tips: tighten the telescope thumbscrew to the T-adapter securely. Also, be sure to attach some type of DSLR strap to the telescope in case the camera falls.
Digital SLR Computer Connection
One thing really nice about my Canon XSi is that I do not need a special shutter control cable like I did with the Canon Digital Rebel (300D). The XSi uses a standard USB cable that works in conjunction with the EOS Utility that came with the camera. I actually have a remote connection to the XSi and telescope using buried Cat. 5e cable that runs in Schedule 40 conduit from my telescope to control room in my garage. On both ends I use simple USB to Ethernet adapters. If there is enough interest, I can document this remote setup. Just leave a comment in the “Leave a Reply” section at the end of the tutorial. Not to worry, my tutorial works the same if you are using a laptop at the telescope. My remote setup is nothing more than extensions of the USB cables. Most of my work is done indoors where it is comfortable!
At this point, the telescope is pointed at the object that I want to image and the DSLR is inserted in the telescope. With the USB cable already connected to the computer and XSi running on AC power (with optional adapter), I turn on the camera. The camera mode dial should be set to manual as shown below:
If everything is connected correctly, an autoplay pop-up will appear on the computer screen:
Just cancel this pop-up by clicking the red “X” in the upper right corner. Open the EOS Utility that came with the DSLR.
EOS Utility Instructions
1. Upon launching the EOS Utility, the camera control screen will open as shown below. Click on “Camera settings/Remote shooting”.
2. This will bring up the main interface for remote operation of the XSi. Note that it is already set to manual from the mode dial setting earlier. In order to take long exposures, the DSLR must be set to “bulb”. In the screenshot below, it is already set to bulb. By default, it may be on another exposure setting. To change to bulb, simply hover your curser over the icon (where bulb is shown in the screenshot) and right-click (or double-click). Now click on the left double-arrow button and it will go to the bulb setting. Now just move the cursor away.
Do the same for the other settings in this area. The screenshot above depicts the settings I used last time I imaged.
Shooting mode: bulb
White balance: daylight (popular setting)
ISO speed: 1600 (this varies depending on the object)
Metering mode: evaluative metering (which was default)
Image recording quality: RAW (maximum data)
Image save location: computer (images automatically upload to computer after each exposure is complete)
DSLR Test Shot
At this point, it is time for a test shot to see how well centered the object is and how much focusing is required. To take a a shot, press the shutter button at the top right hand corner of the EOS Utility interface.
For bright objects, I only take about a 1 minute exposure but perhaps 2 minutes or more for faint objects. The shot duration will be displayed in the window to the left of the shutter button. Be sure to click the shutter button again once the desired amount of time has elapsed. This will stop the exposure and the image will automatically transfer to your computer and will display in the quick preview window.
If the object is not in the center of the field of view, I simply make fine adjustments with the telescope hand controller. First, I turn the slew rate down to about five and press one of the directional buttons on the hand controller for about 3 seconds. Then I take another test shot to see how the object moved in the field of view in comparison to the initial test shot. I make note on paper of the movement that the directional button caused. This process is repeated a few times with other directional buttons and documenting the effect. Once I know what direction the object moves for all of the directional buttons, I can center the target nicely in the field of view. This process actually goes fairly quickly and is much easier than trying to center the object through the camera viewfinder! Here is and example of the notes I took during a recent astrophotography session:
|Directional Button on Hand Controller||Direction that the Object Moves|
|Left||Up and Right|
|Right||Down and Left|
|Up||Right and Down|
|Down||Left and Up|
After the object is centered, I will start the focusing process. Note that many times I will actually focus while I’m centering the object since it minimizes the number of trips to the telescope (remember I’m imaging remotely). For focusing, I use the popular Bahtinov focusing mask. This thing makes focusing very simple and quick! The mask just lays over the end of the telescope tube. Then I take trial shots with the digital SLR and adjust focus until the diffraction spike pattern is symmetrical. The video below demonstrates this process on a SCT rather than the ED80 that I use. It is the same process though. Note that many times I further fine-tune the focus after my autoguider is going (discussed in the next section). This makes for a cleaner image for determining the symmetry of the diffraction spikes. Make sure that you remove the Bahtinov mask when the focusing process is complete!
After the initial focusing is complete, it’s time to fire up the autoguider. I am using the low-cost Orion Awesome Autoguider Package. The Orion system is composed of the following items:
- Orion ShortTube 80 refractor telescope
- StarShoot AutoGuider
- 1.25″ Extension tube (for camera focus)
- Guide scope rings
- Guide scope ring mounting bar
- 10′ USB cable
- 6′ autoguide interface cable
The autoguider is piggyback mounted to the top of my ED80 imaging tube with adapters from ADM Accessories. Note that the Orion Autoguider is “ST-4″ compatible so it works with most equatorial mounts with an autoguide port.
The connection of the Starshoot autoguide camera is very simple. There are two cables- an autoguide interface cable and a USB cable. The autoguide interface cable plugs into the back of the autoguiding camera and plugs into the autoguide port of the telescope mount. The USB cable connects to the back of the camera and into USB port of the computer.
The Orion autoguider runs off of the software program PHD Guiding which was developed by Stark Labs(as a freeware program) to be a simple way to guide your telescope. Once the autoguider is plugged in, I launch the PHD Guiding program on my computer and begin the setup. Here is my review of the Orion Autoguiding System. Note that toward the end of the review I have provided a quick start procedure with corresponding video. This is the basic procedure for setting up the autoguider and locking on to a star. Once I am guiding, I normally do another fine tuning of the focus with the Bahtinov mask still in place. When I have achieved perfect focus (at least as good as possible), the mask is removed and it is time for collecting some frames!
Collecting Light Frames
The EOS Utility remains open throughout the process of setting up the autoguider. At this point, I take a decent-length trial exposure of the object. I will use M31 The Andromeda Galaxy as an example for this part of the tutorial. A 5-minute exposure of M31 is a good starting point. Before going too far, I should make mention of file uploading from the DSLR to computer. When each exposure is complete, it automatically uploads the image to a specified directory on your computer. It’s easy to select a directory in the EOS Utility:
- Click the “Preferences” button at the bottom left of the utility.
- This brings up the Preferences box. Click on the “Destination Folder” tab.
- To the right of the Destination Folder field, click the Browse button.
- Set up a new folder in the location of your choice and save.
A typical directory structure for me goes like this:
Astrophotography>11-28-10>M31>5min lights ISO1600
So the trial 5-minute exposure would go into this directory. If it looks good, it will become part of my collection of light frames. Assuming that everything looks good with the trial photo, it’s time to setup the interval timer for the first set of light frames. The EOS Utility has a pretty nice automatic shutter control program. To set it up, click on the interval timer button:
This brings up the Timer Shooting Settings box as follows:
Delay time: the amount of time (in seconds) before the “first” exposure starts.
Interval timer shooting: the amount of time (in seconds) until the next shot will be taken. This provides time for the previous image to upload to your computer. This time has to be longer than the exposure time.
Shots (minimum 2): the total number of exposures to be taken.
Exposure time: the total length of time for the shutter to be open taking the image.
Once the Start button is clicked, the DSLR automatically starts the shutter control routine based on the timer shooting settings. This is a nice time to take a break and go get coffee!
It is normal in my experience for the RAW images to be washed-out and have a reddish-cast to them (probably from light pollution). This is especially true with long exposure times. Here is a single 10-minute frame of M31 taken at ISO 1600. It’s no problem though, the reddish-cast will be eliminated through the stacking process!
Various exposure lengths ISO speeds are used in DSLR astrophotography. My image of M31 at the beginning of this tutorial was taken over the course of several nights. I collected data at various exposure times and ISO settings. Here are the settings I used for the composite photo (all taken at ISO 1600):
- 30 frames at 5 minutes each
- 3 frames at 10 minutes each
- 4 frames at 7 minutes each
Important tips: make sure you have achieved the best focus possible. Take lots of light frames! The more frames that you stack, the better the signal-to-noise ratio (more signal, less noise).
Collecting Dark Frames
The purpose of taking dark frames is to subtract them from the light frames later in the process. This helps to eliminate thermal noise in the image caused by the build-up of heat on the imaging chip. Dark frames are taken with the same exposure time and ISO settings. The process is simple:
- Cover the end of the imaging telescope with the dust cap. Note that it must be a tight fit so no light will reach the telescope optics. The DSLR must take exposures of total darkness.
- Follow the same procedure for collecting light frames above. I always create a dark frames directory for the exposures to upload to. Here is an example: Astrophotography>11-28-10>M31>5min darks ISO1600.
My preference on dark frames is to take a set after each set of light frames for each ISO/exposure time setting. I don’t always take as many dark frames as I do light frames though. So the imaging session for my M31 example above looks like this (in order of operation):
- 30 light frames at 5 minutes each at ISO 1600
- 5 dark frames at 5 minutes each at ISO 1600
- 3 light frames at 10 minutes each at ISO 1600
- 3 dark frames at 10 minutes each at ISO 1600
- 4 light frames at 7 minutes each at ISO 1600
- 4 dark frames at 7 minutes each at ISO 1600
After collecting all the light frames and dark frames, it’s time to start the processing phase.
Processing in DeepSkyStacker
DeepSkyStacker is a popular freeware program that allows you to register and stack all of your frames including lights and darks. DeepSkyStacker can be downloaded here.
For my astro-photos that this tutorial is based on, I only used DeepSkyStacker at it’s most basic level. Thus, I loaded the light frames and dark frames then processed them with default settings. Here is the basic procedure:
- Launch the DeepSkyStacker program.
- In the upper left menu (under Registering and Stacking), click on “Open picture files…”. Now select your first set of light frames. To select multiple files, just hold the shift key down and click on the first file in the set and then click on the last file in the set. This will select the entire set of files. Click the Open button when all files are selected. Repeat this step for other sets of light frames (i.e., if you had other sets with different settings in other folders). Be sure that only good quality light frames are loaded. It’s a good idea to review all the images prior to this step to eliminate any that are of poor quality.
- Click on “Dark files…” in the same section. Repeat the step above but select the dark frames that were captured. Include all dark frames for each set of light frames.
- Across the bottom of the DeepSkyStacker screen will be a list of all frames that were loaded. Here you will see the details for each frame that is loaded. Note that the column of boxes on the left side of the list are not checked. Just go back to the menu on the left side of the screen and click “Check all”. This will automatically check every box in the list for processing.
- Once all of the frames are selected, click “Register checked pictures…”. The Register Settings box will pop up:
I just leave the default settings as shown above and click the OK button.
- Now the Stacking Steps box appears. Again, I leave everything default and click the OK button.
- Now DeepSkyStacker begins the automatic processing phase. This part can take several minutes depending on how many frames are involved. Basically, the program creates a master dark frame, does registration, subtracts the master dark frame, computes offsets, and stacks the frames together for a final composite image.
- Click “Save As” and save as a 16-bit TIFF image.
Here is the DeepSkyStacker output (cropped and resized). There is still much work to be done, but the composite image from DeepSkyStacker is loaded with information that will be drawn out in the next steps.
ImagesPlus Digital Development
This next step is performed in the program ImagesPlus. In the past, I used ImagesPlus for DSLR control and image set processing. However, with the new Canon XSi, I use the EOS Utility for camera control and DeepSkyStacker from most of the image set processing (as discussed above). However, I still like the Digital Development tool in ImagesPlus for enhancing the composite photo. I hope to eventually provide an alternate method in PhotoShop in case you do not have ImagesPlus.
- Launch the ImagesPlus program.
- Open the image file that was saved in DeepSkyStacker. Go to the Color menu, Brightness Levels and Curves, and select Digital Development. Reference screenshot below.
- Pull the Break-Point slider to the left until the desired amount of detail obtained.
- Bump the Backgd Wt. slider slightly to the right to darken the background a bit.
- Click the Done button when finished.
- Save the file as either a 16-bit Uncompressed TIFF for further processing in PhotoShop (next section).
Here is the ImagesPlus output. As you can see, the Digital Development tool really brought the image to life! Now just a few tweaks are needed in Photoshop.
Further Processing- Adobe Photoshop
I won’t spend too much time on this part. Basically, the following tools in Adobe Photoshop are used to clean up the photo for the final version:
- Crop (to center image)
- Rotate canvas (for proper orientation)
- Levels (tonal range and color balance)
- Unsharp Mask (sharpens image but introduces noise!)
- Color Balance
- Hue & Saturation
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Tips & Tricks
- Balance the optical tube in the area of the sky that you will be imaging in. Make sure it is loaded with all the equipment you will be imaging with. The main purpose for this is so the gear mesh or backlash is minimized so that the telescope doesn’t have to take up slop and the autoguider has to take longer to track on the star due to this slop. It helps minimize strain on the gears and motor so that the telescope can track smoothly.
- After focusing the DSLR for the first time, use a fine permanent marker and draw a short line on the telescope draw tube where it enters the telescope. This will make it much easier the next time to get the focus close for the DSLR.
- It may be helpful to focus on a bright star before slewing the telescope to the target object. The diffraction spikes (from the Bahtinov focusing mask) may be clearer and more distinct with a bright star making it easier to determine thier symmetry.