by David J. Watkins
Astrophotography Aquiring Images - Locating Your Target
Point and Shoot Cameras:
As discussed in my earlier pages, point and shoot cameras, and or cell phones cameras as of now, are quite limited when it comes to astrophotography. It seems that point and shoot cameras are being used less and less and cell phone cameras are evolving into the new point and shoot camera. There are mounts for attaching your cell phone camera to your telescope, but the image quality you will get from a phone camera is not near the same level as you will get from even the entry level DSLR cameras. The mounting is not solid where the phone is securely attached to the scope as an interchangeble lens on a DSLR. Currently cell phone cameras do not shoot RAW images to enable image stacking. Cell phone cameras do not have the capability yet for high frame rate video. Manual control of the sensor is also a limiting factor. You need to at least be able to control things like ISO and shutter speed.
If you could solidly mount your phone to your scope, had more manual control of the sensor, and be able to remove the plastic lens over the sensor, I could certainly see phone cameras used for decent quality planetary imaging. The pixel count and sensor size is currently ideal for planetary imaging.
For serious imaging of DSO's (Deep Space Objects) I think the phone cameras still have a very long way to go. The QE (Quantum Efficiency) of the sensors is not there yet, and for DSO images like galaxies and nebula, pixel sizes larger than 5 microns yeild better results.
There are some great images being captured with stock or off the shelf consumer grade DSLR cameras. Many astrophotographers refer to these DSLRs as "un-modified" cameras. The biggest limiting factor for DSLRs is the IR filter on the sensor that blocks out the higher end of the red wavelength spectrum. You can somewhat compensate by exposing for a much longer time period, but it will still not quite compare to the images from the modified DSLRs or Grayscale cameras.
DSLRs easily mount to telescopes with a T-adapter ring. There are T-adpater rings available for just about every camera manufacturer. With a DSLR you have complete control over the settings on your camera, ISO shutter speed, mirror lockup, shutter time delay, and aperture if you are using a lens. Most all DSLR cameras will allow you to shoot RAW images, and many will allow you to shoot in live view mode for focusing.
DSLR modified Cameras:
A "modified" DSLR camera is typically a stock DSLR where the IR filter has been removed and replaced with a specialized filter like a Baader filter. The specialized filters do not block out the higher wavelength red spectrum light. This enables you to capture much more detail in the various DSOs, and in shorter exposure time than on a stock or un-modified DSLR.
Canon offers a DSLR specifically designed for astrophotography with the IR filter replaced. It is their D60a model. There are also companies that will modify your DSLR by removing the IR filter and replacing it with a Baader filter specifically for astrophotography. Note that the Baader filter is not the same as a Bayer (RGGB) color filter. The modification can be performed for a few hundred dollars. The Canon D60a is about $1500. From what I have read and seen in comparisons, the modifications with the Baader filters tend to yield better results than the D60a. The D60a can be used to take daylight photographs in addition to astro images. Though you will have to play with the white balance in post processing to correct for the slight color imbalance. Once a DSLR is modified with a Baader filter, it becomes an astrophotography camera and is not well suited to daylight photography. The modification affects the focus plane, so autofocus will work correctly. But you will never use autofocus for astrophotography anyway. The Baader filter modified cameras also produce a heavy red color cast in daylight images.
Grayscale cameras will produce the best quality astro images and they open up a whole new world to astrophotography. There are several reasons for this. For one there is no IR filter blocking out the higher end red spectrum light. Since there is no color, there is no Bayer patter filter to block out additional photons from reaching the sensor.
A bayer filter consists of a red, blue, and two green filters over each 4 pixel array on the sensor of the camera. That means that not all of the pixels on a color digital camera will record all of the light. Each of the pixels in the 4 pixel array filters out all but the color it is intended for. So on the entire sensor only 1/4 of the pixels will record colors in the blue spectrum, 1/4 of the pixels will record red spectrum colors, and 1/2 of the pixels will record colors in the green spectrum. All color cameras contain a bayer pattern filter or a variation of it to record color.
All of the photons that reach the pixels will be recorded on a grayscale camera. The QE (Quantum Efficiencey) is typically much higher on a grayscale camera, that is the sensor is more sensitive to light. That means less time for an exposure is required. You can capture images in grayscale alone, but you can also capture color images with a grayscale camera. The color process is a bit more time consuming but yeilds much better results than with a DSLR color camera. To capture color images you take four separate sequences of images using Luminance (L), Red (R), Green (G), and Blue (B) filters. There are several filter wheels on the market so you do not have to remove the camera and attach each filter. You just rotate the filter wheel, and there are motorized wheels as well that can be controlled by your computer. The LRGB images are combined later with software to produce the final color image. The luminance (clear filter) image is grayscale, this image is exposed for the longest of the filtered sequences to capture the greatest detail and sharpness. The red, green, and blue filtered sequences are exposed for much shorter durations, as they are only combined to add the color to the image.
Another technique when imaging with a grayscale camera is to replace the luminance filter with a narrowband Ha (Hydrogen Alpha) filter, while still imaging with the RGB filters. An Ha filter will block all light except for that in the Ha or high end red light spectrum. This technique can produce stunning images of emission nebula.
One more technique for grayscale cameras is narrowband imaging. Narrowband imaging is the technique that the Hubble telescope uses to image. Instead of the LRGB filters, a set of three narrow band filters Ha (Hydrogen Alpha), OIII (Oxygen III), and SII (Sulfur II) are used. You will often hear imaging this way referred to as "using the Hubble palate". These narrowband filters will block most light outside of their bandwidth. This also means that you can image from light polluted skies. Since most light is blocked, narrowband imaging requires many hours of exposure with each filter to capture enough light. The results can be spectacular. The three image sequences are combined later in software to produce the final image.
Narrowband filters are available in a few different bandwidths (nm or nanometers). Wider bandwidths are less expensive but allow more stray light to enter the camera. The narrower the bandwidth, the more expensive the filter, but the less stray light will enter the camera.
Grayscale cameras tend to be more expensive than stock DSLR cameras. They typically will include a cooling system to help reduce noise as the sensor warms up during the longer exposures. Another downside to grayscale cameras is that they require a computer or laptop and software to capture the images.
Most grayscale cameras use the same basic sensor chips made by either Sony or Kodak. The pixel size is typically larger, 4 microns or larger, similar to the size of the pixels on a DSLR. Though there are some grayscale cameras with 3 micron pixels. The larger pixel sensors are better for long focal length scopes, while the smaller pixel sensors are better for shorter focal length scopes or shooting with hyperstar or fastar. The most popular grayscale camera manuafacturers are Atik, SBIG, Starlight Express, and QHY. They typically offer the grayscale cameras in color versions as well. The color versions will obviously have the Bayer pattern color filter, but will offer a better QE than modified cameras. The QE of the color sensor will still not match the QE of the grayscale sensor because of the Bayer filter. The grayscale cameras still have an advantage over the color as you can image with the narrowband filters.
Filter brands include Baader, Astrodon, Astronomics, and Orion. They offer narrowband, LRGB, and other specialized filters for astrophotography and for viewing.
Web Cam video:
An inexpensive web camera can be used to capture planetary images. The web camera will typically have to be modified to adapt to a telescope or a barlow. There are many do-it-yourself instructions on YouTube or on other websites for adapting the camera to the scope. Web cams will normally shoot video at 15fps to 30fps. You can capture the video then use software like AutoStakkart! to pick out the sharpest frames, align them, then stack them for final image.
Autoguider, Planetary Imaging, Other Specialized Video Cameras:
Telescope GEM mounts are not perfect and even if you get a perfect polar alignment after two or three minutes your stars will begin to trail because of the slop in the gears or periodic error. That is where an autoguiding system comes in. For an autoguiding system, you will need a camera designed for autoguiding. One advantage of an autoguiding camera is that it can double as a planetary imaging camera as well! In fact an autoguider or planetary imaging camera will yield better results on planets and the sun than any DSLR, modified or unmodified, and even any large pixel grayscale camera. The small pixel size has a magnifying effect on the image. With planets, you need a lot of magnification. The other advantage is that the planetary imaging cameras will capture high frame rates or video, unlike most of the other grayscale cameras dedicated to DSO imaging. Autoguider and planetary imaging cameras come in both grayscale or color. Again the grayscale cameras seem to come away with the better images, though there are a few color sensors that are getting close to the grayscale.
The same camera manufacturers that make the dedicated grayscale and color cameras for DSOs also make guiding cameras and planetary imaging cameras. A few other companies like Celestron and Orion have a line of guiding and planetary imaging cameras. One new player in the field is ZWO, and they seem to be specializing in the high frame rate video above and beyond what the other manufacturers are offering. Another company, Point Grey has a large offering of high frame rate video cameras.
I currently have a QHY5L-II 1.2 megapixel grayscale guiding camera that I have used for planetary imaging. It will capture at 15fps at full resolution and will capture at a slightly faster rate with a smaller region of interest. I don't think I have been able to get more that 50 fps out of it.
I also have two ZWO cameras, an ASI174MM 2.3 megapixel grayscale camera designed for solar and lunar imaging, its pixel size is 5.86 microns, so it is not the best size for smaller planets. The problem with smaller pixel size sensors when imaging the sun or large objects is that they may magnify the object too much. Too much that you may not be able to fit the sun in the frame of your solar scope! When using the QHY at 3.75 micron pixels, I had to use a 0.5x focal reducer to be able to fit the sun without clipping the top and bottom off. The ASI with its 5.86 micron pixels, I have no problem fitting the entire disc in the frame. I have been very pleased with the results on solar imaging with it. It will capture 128fps at full resolution as long as you are capturing via a USB3 port. With a smaller region of interest, I have been able to get up to 300fps. The one thing about the high frame rate is you need a lot of disk space! I captured several 60 second videos of the sun and the files were 8 Gigabytes each!
My other ZWO camera is an ASI178MC 6.4 megapixel color camera with 2.4 micron pixels. The frame rate at full resolution is 30fps, but is capable of 100fps or more at a smaller region of interest. I chose this because of the results I was seeing on imaging smaller planetary nebula. This camera should also help imaging some of the smaller planets. At the time I purchased the camera there was no grayscale version available like there is now. I have not yet had the chance to use it, as we have not had any nights that were clear enough. ZWO has also just released cooled versions of the ASI174MM and the ASI178MC.