In this document I’ll describe my current image acquisition process. Some of it will be generic in nature. However, much will necessarily be specific to the DSLR and other hardware/software that I use.
For a description of tripod mounted DSLR photometry please see Des Loughney’s article in the February 2009 VSS Newsletter, pages 17-20 “Digital SLR Differential Photometry”.
I use a Canon 450d DSLR with a Nikkor 180mm f2.8 lens (set to f4 for better images) piggybacked on an 8” Celestron telescope on a CGEM mount. A laptop computer controls image acquisition and telescope pointing via a USB to serial adapter, USB hub and a single USB cable to the computer.
Celestron NexRemote software (supplied with the telescope) and Sky Charts planetarium software (freeware from Cartes du Ciel) combine to control telescope pointing. I’m not going into details of using these programs here but would be happy to answer questions via email.
Computer time is updated frequently via the Dimension 4 application mentioned in [cblink=”tom_richards”]Tom Richards[/cblink]’ article “How Good Is Your Timing?” in the August 2010 VSS Newsletter.
Canon EOS Utilities software (supplied with the camera) automates image capture. Many camera parameters can be set through this program, but they could also be set on the camera itself. On the camera set the mode to “Manual” and through menu functions set both long exposure noise reduction and high ISO noise reduction off.
After the telescope mount is turned on and control established via the computer, the next step is to slew to a magnitude 2 or 3 star near the target star to check alignment.
With the USB cable connected, turning the DSLR on automatically launches EOS Utility. Click on the Camera Setting/Remote Shooting button. In the software set the save location to a newly created folder on the desktop. Set the following EOS Utilities settings:
- Image format RAW only
- Save images to computer hard drive only
- ISO 200 (or other if necessary)
- Daylight white balance
- Use computer time for image time stamp (in preference section)
Click on the Live View button in EOS Utilities to bring up a live video stream of the image through the camera. If focus is reasonably close the star image will be visible. If nothing is visible check focus and the lens cap.
When you have an image use the telescope slew controls to centre it (use the magnify and 200% options for a magnified view). Now go back to the Sky Charts window and synchronise the camera pointing with the correct star on the planetarium display.
While looking at the Live View window on the computer adjust focus until a slightly blurry star image is attained. We don’t want a perfectly focused image for photometry. Close the Live View window. To avoid focus creep I use a guide scope ring to lightly clamp the lens barrel. In the past I’ve used masking tape or the like to accomplish the same effect.
Now slew to the target star, take a test image and use the Star Image Tool in AIP4WIN to check FWHM and maximum ADU value for each target and comparison star. With my camera and lens FWHM of ~5 to 7 pixels is suitable.
Exposure is adjusted appropriately and a new image acquired and checked. This process is repeated until maxADU is ~12,000 (out of a maximum of 16,384 for the 14 bit analogue to digital converter).
For standalone cameras (i.e. no computer) you would record a series of exposures of various lengths, download to your computer and use your photometry software to examine them. The appropriate exposure will be the one with acceptable trailing and maximum ADU value in the relevant star images without any over exposed pixels. Be careful not to change the focus setting or you’ll have to repeat the process.
When focus and exposure are set the next step is to delete all the test images and prepare to acquire the actual images to be measured. In EOS Utility set the exposure to BULB and click on the little clock icon to bring up the sequence dialog window.
I usually set the delay time to 1 second. Set the correct exposure, as determined above. Set the interval between images to at least 5 seconds longer than the exposure time to allow enough time for the previous image to be downloaded. Set the number of images to be recorded, I usually record 10 for a normal magnitude measurement. However, a time series dataset may have as many images as you want. The most I’ve recorded is 500 in one acquisition session. At about 12MB per image that’s a lot of data.
Then click the start button to initiate image capture.
You will need to record dark frames to be used later in the image calibration process. Simply put the lens cap on and record a series of images (I use 16 dark frames) with the same exposure as your light frames.
When finished slew to the next target and repeat the process. If you use a different exposure or ISO setting you will need another set of dark frames.
Flat and flat-dark frames will also need to be recorded. I use a home built light box based on a design in The Handbook of Astronomical Image Processing by Berry and Burnell. There are lots of DIY versions on the web and some commercially available light boxes as well.
I record 16 light box images (flat frames) with an exposure sufficient to reach about two thirds of the maximum ADU value. The lens cap is then put on and 16 more images of the same exposure are recorded (flat-dark frames). Flats only need to be recorded every few weeks unless your imaging setup is changed.
AIP4WIN is used to create a master flat frame and a master dark frame for calibrating the target images.
That’s all there is to collecting the images. You will need to modify this procedure for your own camera, mounting system, etc.