This digital SLR astrophotography tutorial provides my method for obtaining the deep space images found in my deep space image gallery. I hope it will be useful for beginners in astrophotography with a digital camera; specifically a digital SLR (DSLR). Below are a few of my favorite astrophotos that I have taken using my telescope and Canon Digital Rebel. Click on thumbnails to view full size photo.
Beautiful images can be produced with today's digital SLR cameras and software. I hope to demonstrate how this is done on a basic level in my DSLR astrophotography instructions below. First though, I will start with the setup I use for astrophotography of deep space objects.
One note before I get started, most of my imaging time is spent in the comfort of my control room inside the house. Thus, my tutorial provides a lot of insight to imaging remotely! :-)
Special Recognition- I would like to thank my good friend, mentor, and exceptional astro-imager Ed Henry of Clearview Farm Observatory for all the guidance and inspiration he has provided to make this a most interesting and exciting pastime!
2. My Astrophotography Equipment
The telescope I use is an 80mm Orion ED80 APO refractor piggybacked to a 12" Meade LX200. Here's the details:
Orion ED80 Aperture: 80mm
Orion ED80 Focal Ratio: f/7.5
Orion ED80 Focal Length: 600mm
LX200 Mount: Heavy Fork
Accessories include a Telrad viewfinder.
Also see my Webcam Astrophotography Tutorial for Planets!
I control the Canon Digital Rebel with ImagesPlus software. This software provides a number of essential tools such as focusing, custom shutter control, image calibration, alignment & stacking, digital processing etc. The ImagesPlus software is available here:http://www.mlunsold.com. Much more detail will be provided later in my DSLR astrophotography tutorial.
4. First Things First- Drift Align
A good Polar alignment is very critical for taking long exposures. My simple drift align method can be found here. Note that this is based on my Meade LX200, but the concept is the same for any equatorial mounted telescope.
I operate my Canon Digital Rebel remotely through the ImagesPlus software using a custom shutter cable and USB cable. Although I built my own custom shutter cable, pre-made cables can be purchased from Hap Griffin Astro Cables: http://www.hapg.org/astrocables.htm. Basically, the custom shutter cable combined with ImagesPlus software allows me to override the Canon's 30 second exposure limitation. Here is a brief description of both types of cables. Note that they are specific to my setup with the Canon Digital Rebel (300D).
Custom Shutter Cable- The camera end of the cable has a 2.5 mm, 3-conductor connector. It plugs into the remote control terminal on the side of the Canon Digital Rebel. This is normally where the Remote Switch RS-60E3 plugs in (not used in this application though). ImagesPlus software operates the custom shutter control through this cable.
2.5 mm, 3-conductor connector
The other end of the custom shutter cable is a parallel port connector with a special pinout for operating the shutter control. Hap Griffin can provide custom shutter cables with parallel port connectors, serial port connectors, USB, etc. for various computer ports and DSLR's.
USB Cable- this is the standard USB cable that comes with the DSLR camera for uploading photos. However, since I'm imaging remotely (i.e., from my garage), I have extended it with Cat. 5 UTP and a USB Extender. Before this, I just hooked together a couple of 16 foot USB booster cables (like this one: Iogear GUE216 USB 2.0 Booster Extension Cable) and strung them across the yard from my garage. One end plugged into the computer USB port and the other end attached to the standard camera USB cable. Then the standard cable plugged into the USB port on the DSLR camera. Everything runs underground now though.
Connection of the two types of cables to the digital SLR shown below. These connections are necessary to operate the ImagesPlus software as explained later in my tutorial.
Attaching the DSLR to the telescope requires a t-ring and t-adapter as shown below.
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.
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 utilized). Important tips: tighten the telescope thumbscrew to the t-adapter securely. Also, be sure to attach the DSLR strap to the telescope in case the camera falls. I have actually had this happen a few times in the past, but my camera was spared since I strapped it to the telescope! See DSLR strapped here.
Center a bright star in the DSLR field of view. Manually adjust the telescope focus as close as possible through the camera viewfinder. Then use the ImagesPlus focusing routine. Reference screenshot below. Here is the procedure:
Click on the Focus tab in ImagesPlus.
Click the Release button. A short exposure will be taken by the digital SLR camera. Note that a prompt may appear indicating that the camera needs to be changed from bulb and RAW format for the focus routine. If so, click ok. Another prompt will appear later to change back to bulb and RAW format.
Draw a small frame around the bright star.
Select the star by clicking on the center of it when prompted.
Click the Release button to obtain the initial focus reading.
Make a small adjustment to the focus. Be sure to note the direction that the focus knob is turned.
Click the Release button again. If the focus was improved, then the "Current Frame" will have a lower pixel value than the "Current Best". But if the focus did not improve, then the "Current Frame" will have a higher pixel value than the "Current Best".
This procedure is continued until the "Current Best" pixel value cannot be improved upon. It is helpful to check the graph after every adjustment to see which way the green line (which represents the Current Frame) slopes. If it slopes upward after the last adjustment, then the focus got worse. And of course, if the line slopes downward, the focus improved. Tip: an electric micro-focuser would be very nice to have here! It would save time going back and forth to the telescope and it would provide very fine adjustments to the focus.
Before obtaining goto capability, this is how I located and centered an object in the camera field of view. It's very similar to finding an object through the telescope viewfinder, except the DSLR viewfinder is used.
Target the astrophotography subject using the telescope viewfinder first. I highly recommend a Telrad viewfinder since it provides a large, unmagnified view of the sky.
If the object is bright (e.g., The Orion Nebula), it may be possible to see it through the DSLR viewfinder. If so, center the object within the field of view through the camera's viewfinder.
If the object is not bright enough to detect in the DSLR viewfinder, then position the object in the FOV by identifying a nearby star. A good star chart will be helpful here.
Some tweaking to the telescope's aim may be necessary. This will be determined when trial exposures are taken later.
This method utilizes TheSky software program (version 5) with the telescope's goto. It has made my job a LOT easier!
Center a familiar bright star (like the one used in the focus stage above) in the DSLR camera viewfinder field of view.
Establish a telescope link in TheSky program (this step assumes that the telescope has been set up in the software program).
Now find the star in TheSky map and click on it.
Click the Synchronize button.
Now find the astrophotography target in TheSky's map and click on it.
Click the telescope slew button.
If everything has been set up correctly, then the object should be fairly well centered. This will be determined when trial exposures are taken later.
10. Setting up Custom Shutter Control in ImagesPlus
With the DSLR off, open "ImagesPlus Camera Control" program.
Go to camera menu and select Canon DSLR then select camera control for 300D/Rebel.
Select the Bulb Capture tab in the Canon DSLR Control window.
Control Type is 0x378 x Parallel Port, Shutter=DO=P2AF=D1=P3.
Click the Reset button. The default Shutter Sequence Parameters should appear as depicted in the screenshot below:
With cables attached, turn on the camera. Be sure that the DSLR is set according to Section 7 above (Digital SLR Settings)
A Windows pop-up will appear because the camera is connected (prompting you to choose a photo upload program). Cancel this feature. The photos will be uploaded through ImagesPlus.
Select the Connect tab in the Canon DSLR Control window.
Click the Connect button. The EOS Kiss REBEL 300D will show up as the available camera.
Click on the Transfer tab in the Canon DSLR Control window. Click the Select button at the bottom of the Canon DSLR Control window. Select a directory on computer hard drive to store the images.
Click the Bulb Capture tab in the Canon DSLR Control window.
Click the Edit button to change the default Shutter Sequence Parameters
In this example, I set the Interval to 10, Exposure time (seconds) to 300, Total Exposures to 5, ISO to 100, and added the prefix "M42" which will show up in the file name.
Click the Save button.
At this point, another shutter sequence could be added if other settings (e.g., ISO, exposure time, more frames, etc.) are desired. For this tutorial, I'm keeping it simple with just one set of frames.
Everything is set to start taking the light frames. With the digital SLR still connected, move on to the next section
Click the Release button and the camera will follow through with the custom shutter settings. Note the exposure duration (i.e., time elapsed) and the number of released exposures.
Wait for the shutter routine to complete. The images will be uploaded automatically to the previously selected directory on the computer ("Image Transfer Progress" pop-up will appear when the images are being uploaded to computer).
Leave DSLR connected and move to the next section to take the dark frames.
Tip: Before taking a lot of exposures, experiment with trial settings first. I always take a single short exposure (e.g., 60 seconds) first to make sure my subject is centered in the field of view.
I also vary the ISO settings and exposure lengths on single exposures to see what works best. Once I'm satisfied, then I proceed to taking a large set of frames.
A note about the ISO setting- the higher the ISO setting, the brighter your image, but the more noise that is introduced. I rarely image at the maximum ISO setting of 1600. The images are just too grainy. I chose ISO 100 for my M42 example because I did not want to burn out the center of the nebula. Ideally I would probably take several frames at ISO 100 and several frames at a higher level like ISO 400 and combined them. But for my tutorial, I'm keeping it simple! :-)
12. Taking the Dark Frames
At this point, it's time to take some dark frames. These frames will be used later during the calibration stage in ImagesPlus. Basically, the dark frames will be subtracted from the light frames to remove much of the hot pixels and other artifacts. It helps to polish up the final photo a bit!
The procedure is simple, just put the lens cap on the telescope and take at least 3 frames with the same settings as the light frames. I like to create a directory for the dark frames to keep them separate from the others. More on dark frames and calibration coming up in my tutorial.
13. Converting RAW Images to TIFF
Now that the subframes have been captured, it's time to convert them from a RAW image format to a 16-bit TIFF file. This task is easily performed in ImagesPlus. Here are the steps:
On the file menu, select Canon Raw File Development and Conversion.
Find the folder with the light frames. Note that all of the light frame files should have the extension ".CRW". This extension indicates that the file is in a RAW format. Example file: m42300SecISO100_000012.CRW. A nice feature of ImagesPlus is that the seconds and ISO setting is automatically built into the file name!
Select the light frame files to be converted. To select several at once, just click on the first file in the series, hold down the shift key, then select the last file in the series. The entire series should be highlighted.
Now the Canon Raw File Conversion box should be open. Leave the Conversion Parameters set to Canon White Balance Color.
Select "16-Bit TIFF" for the Output File Type. Note that the prefix defaults to "CVT_". This means that the output file (i.e., converted file) will have the extension".cvt".
Select an output directory for the converted image. I usually leave it at the default which is where the RAW files reside.
Click the Convert button. The conversion process may take awhile depending on the number of light frames, speed of computer, file size, etc. This is a good time to get a cup of coffee!
Click the Done button when the conversion is complete.
Repeat the conversion process above for the dark frames as well.
Automatic processing is utilized to calibrate the light frames. Click File then Automatic Image Set Processing. a) Under the Process tab, select Color (probably the default selection). b) Under the Light Frames tab, click the "Select Light Frames" button. Now select all of the converted Tiff files. Example file name: CVT_m42300SecISO100_000012.tif. c) Under the Dark Frames tab, click the "Select Dark Frames" button. Now select all of the converted dark frames. Example file name: CVT_m42dards300SecISO100_000017.tif d) This tutorial does not cover Flat Frames or Bias Frames so these tabs are skipped. If these frames were obtained however, this is where they would be loaded for calibration. e) Go back to the Process tab and click the "Process" button. f) Do not click Done yet. Move on to the next section for a Translate, Scale, and Rotate of the calibrated frames.
The Automatic Image Set Processing box should still be open. Click on the Align TSR button.
The following box should open:
Select Centroid as the Alignment Method.
Under Alignment Type, select Translate + Rotate.
Under Alignment Selection, select Common COR Point, Reference Image, and Auto AdvanceAfter Select.
Now minimize the Align TSR box and the Automatic Image Set Processing box.
The first frame of the calibrated Tiff's should be displayed. Holding the shift key, click on a reference star in the image. The frame will automatically advance to the next frame. Click on the same star in the second frame. This process is continued until a reference star has been selected for each frame. Eventually the starting frame will be reached. This is indicated by the red box around the reference point as shown below:
Now maximize the Align TSR box again. In the Alignment Selection area, select Common Angle Defining Point or Star.
Repeat step 7 above on a different star. When all of the frames have been marked and the starting frame appears, a blue box will surround the second reference point (Common Angle Defining Point or Star) as shown below:
Now maximize the Align TSR box again and click the File Set Control tab. Select a folder where the calibrated & aligned image will be saved.
Click the Align Methods tab again and click Align. This process may take awhile depending on the number of frames, speed of computer, file size, etc.
After the alignment is complete, click on the File Set Control tab in the Align TSR box. Now click the Aligned button under Combine. Choose Average under Combination Methods.
Now click the File Set Control tab. Select an output directory and check Auto Display.
Click the Standard Methods tab again and click the Combine button. Again, this process may take awhile to complete.
When the files have been combined, click the Done button. The calibrated, aligned, and combined file will be displayed. The file name should be CombinedFilesAvg.
With the CombinedFilesAvg file still open, 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 an 8-bit or 16-bit Uncompressed TIFF depending on the program to be used for further processing (next section). Alternatively, I could finish processing through ImagesPlus which has a number of tools for the job.
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Final note: the image above could have been improved by taking more light frames and combining them. I only captured 5 exposures for this example, but 10 or 20 would be better. Also, exposure length is important for gathering as much light as possible from those faint deep space objects. The longer the exposure, the more light gathered. Just keep in mind that longer exposures can introduce noise, star drift, etc. With experience, you will figure out the optimal exposure time for your setup.