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.