Image Acquisition and Processing

Image Acquisition

Most of my recent images have been acquired with the SBIG ST-10XME connected to a RC Optical Systems 10" on an AP mount or an SBIG STL-11000XM connected to a Takahashi FSQ refractor or Nikon 180mm camera lens on an Astro-Physic mount.

Preparing to Image

Here are the things I worry about when imaging:
  1. Telescope Balance
  2. Polar Alignment
  3. Focus
  4. Drive Calibration
  5. Check 1-4 again
  6. Take the Picture

Balancing your Mount

Optimally balancing the mount is important. I have found that to get good results, the RA axis of the mount should be slightly heavy to the east. Not heavy enough that the mount swings wildy when the clutches are loosened, but enough that the mount will move slowly on its own. Heavy to the east means that when the counterweight shaft is on the east side of the mount, the weights should be moved further down the shaft. When the scope is on the east side of the mount, the weights should be moved closer to the mount. Also, I have found that I get the best results when the declination axis is balanced so that it is neutral. I don't worry about this with the FSQ, as the mount is more than capable of handling mis-balance in Dec with that small weight.

One other thing to keep in mind is that the balance radically changes if you remove the CCD camera to use an eyepiece. Be careful, because when you release the clutches, if you have not made any attempt to change the balance, the scope will take off all on its own.

Polar Aligning your Mount

The Astro Physics mount has a polar alignment scope. It is a great aid to getting close alignment, but can be improved on by a few different methods. I used to use the drift method. There are lots of places that explain the drift method, so I won't go into that here. In my experience, if I drift for 7 minutes at 220 power with no noticeable drift, then my alignment is good enough (Famous Quote - "Perfection is the enemy of Good Enough").

Today, for the AP mount, I use TPoint from Software Bisque. Typically, I create a model with about 15 stars and adjust the mount based on the results from the model. Usually I can get within 1' in altitude and azimuth after just a couple of iterations before the sky is completely dark.

Focusing your Telescope

Patience, patience, patience. Keep muttering that to yourself as you attempt to achieve best focus. Focus is some ancient latin word that means "You were perfectly focused when you attached your camera, but you decided that you need to spend 30 minutes verifying that fact". If at all possible, use a remote focusing device, it really helps keep the wiggles down when you are trying to wring out the best focus. I use the RoboFocus with all my scopes and lenses. For the all telescopes, I use MaxIM CCD for acquisition and use the Inspect tab for focusing. MaxIm give you four items to look at: FWHM in X and Y, Peak Value, HFD,and a 3-D like graph showing how the light is distributed. I have found this method to be better than the automated routines like FocusMax. I cannot quantify this, but it always seems that I can get just a touch better focus by adjusting it myself.

To make sure focus is good, I need to re-focus the refractor if the temperature changes by more than 2 degrees F, which on some nights can happen in 20 minutes. The Nikon 180mm lens and the 10" RC, with its carbon fiber trusses, are pretty much immune to temperature changes.

Calibrating your Drive

After I have found the object I plan on imaging and placed it somewhere in the CCD, I am ready to calibrate the drive. The program TheSky is great for finding guide stars and framing the object. It allows you to set up a field of view indicator that has the CCD imaging chip and the guiding chip overlayed on the screen. You can use it to help figure out where to move the object to get a suitable guide star. For the FSQ scope, a guide star brighter than mag 11 works fine, although brighter is better.

Unless the object is close to the pole, calibrating at a 1X (which is also my guiding rate) for 5 seconds in each axis is enough determine the guiding rates. Close to the pole, you may have to increase the RA movement to 10 or 15 seconds. Also, do a calibration every time you move the telescope. Technically you do not have to, but it takes so little time and just about guarantees that you will track well.

Taking the Picture

Acquire lots of images, toss out the ones you don't like and combine the rest. That pretty much sums it all up. A typical object may require 10-20 exposures of 10-15 minutes. MaxIm allows you to take a sequence of exposures automatically. This is a great feature and allows you to actually look at the sky with your other telescope while the exposure is in progress. I use guide integration times of 1 to 5 seconds. After all this setup, I start the mount tracking and watch it for a minute or so to make sure all is well. One thing to remember is to set the delay between exposures. This allows the camera to re-acquire the guide star before beginning the next exposure. I set this number to 5 times the guiding rate. If I am guiding at 1.5 seconds, I will set the delay to at least 8 seconds. In summary the process is as follows:

  1. Set up your exposure times, guide rate, delay between exposures, the name and where you want to store your files
  2. Start tracking and watch it to make sure you actually are tracking
  3. Start the exposure
  4. Visit with your buddies or use your other scope
  5. Don't forget your dark frames

Image Processing

(Watch this space for a more full blown, step-by-step processing example)

For image processing I use a number of different programs: MaxIM, Mira, CCDStack and Photoshop.

After the images and darks have been acquired (and the multiple darks averaged to one), the dark subtraction takes place in MaxIm CCD. From there MIRA AP is the processing workhorse. In MIRA, I register and then combine the images, usually using a mean combine. After the combine, images are opened in MaxIm CCD for the DDP process. The DDP process results in slightly sharper images and smaller stars. To me, it is a more pleasing result. After processing, I bring the image into Photoshop for an occasional unsharp mask or other and then print it on my HP B9180 printer on HP Advanced Photo paper. To get the images ready for web use, they are saved as a .jpg using Photoshop, usually with medium quality to get them to around 80 kbytes. This does tend to butcher the images and make the stars look a little blotchy. If you want to see the real thing, send me an email and I will be happy to send you the full image.