The Auto Calibration command will automatically calibrate the mount by taking images of a large number of points across the sky. The key to this feature is the use of the PinPoint LE Engine. PinPoint is an astrometric analysis product, which identifies stars in an image, finds matching stars in a catalog, and uses this information to determine the exact position of the telescope.
The full version of PinPoint is available from DC-3 Dreams, SP. The full version includes Visual PinPoint, which can be used to perform astrometric analysis, discover minor planets and supernovae, generate MPC reports, and much more.
Auto calibration also requires MaxIm DL imaging software from Diffraction Limited. MaxIm DL is a powerful, full-featured camera control package with extensive image processing and analysis capabilities. It supports a wide variety of astronomical imaging cameras and DSLRs. In this case, MaxIm DL is used to control the camera during imaging.
The process is as follows: MaxPoint slews the telescope to a position on the sky. These positions are selected to be approximately evenly spaced across the sky. MaxPoint then commands MaxIm DL to take an image. MaxPoint knows the approximate position of the image; it uses the PinPoint LE Engine to calculate the exact position, using known stars from a catalog as a reference. Once this is known, a point is added to the observation list. The telescope moves to the next object while these calculations are proceeding. If a match cannot be found for some reason, the point is skipped and the process continues.
First, a number of settings need to be made, as follows:
Note that the camera settings can be adjusted as the calibration run is proceeding.
If your camera has a filter wheel attached and controlled through MaxIm DL, a list of the available Filter slots is provided. Select a suitable filter; for the shortest exposures, usually the clear or "no filter" slot is selected.
The Exposure (sec) time depends on your imaging equipment. The idea is to use a long enough exposure to guarantee that you have a significant number of stars in each image, without making the whole process take too long. A good starting point is 10 seconds.
You may wish to use Binning to speed up the observations, by decreasing the size of the camera readout and increasing the camera sensitivity on short exposures. Please see the MaxIm DL manual for more information on binning.
Note that the PinPoint settings can be adjusted while the auto calibration run is proceeding.
PinPoint needs a number of settings to be made. The most important is to set the path to the star catalog. You can click the Browse... button or type in the path directly.
You must also select the Catalog Type. The most commonly used catalog is the Guide Star Catalog (GSC), which goes down to approximately 15th magnitude. GSC-ACT uses the GSC catalog with a modification table to make the astrometry more accurate. Tycho2 is of very good accuracy but goes only to 12th magnitude. The USNO A2.0 goes extremely deep, but is recommended only for very narrow-field imaging.
The Guide Star Catalog, which is perfect for most situations, is included on the MaxIm DL program disk, or can be downloaded from http://gsc.dc3.com. Note: if you use Internet Explorer to download the files, be aware that it may add a .txt extension on some of the files, which will make the catalog non-functional; you must remove the files. Also, you need to have both the GSC and TABLES folders. The path to the catalog should be pointed to the folder above these two folders (e.g. if they are both in C:, then set the path to C:).
You may wish to use the larger A2.0 catalog if (and only if) you have an extremely narrow field of view.
The Projection should be set to TAN (tangent plane) for the vast majority of optical systems. Some specialized optical systems need the ARC (Azimuthal Equidistant) projection. If in doubt use TAN.
Sigma is a measure of how good a signal-to-noise ratio the star must be before it is used. This is usually set in the range 3 (allow very faint stars) to 6 (strong SNR only). The default is 4.
Min Bright is the minimum allowed brightness of the star. This depends on your camera; set it high enough to ignore noise spikes such as hot pixels, but low enough not to eliminate the fainter stars. The default is 200.
Min Star Dia. is the minimum diameter of star image that will be detected. Set this small enough to not reject valid stars, but large enough to skip hot pixels and cosmic ray hits. The default setting is 2.
Cat Max Mag is the maximum brightness to use from the star catalog. If your camera covers a very large area, this will limit the number of stars used. Having too many stars slows down the processing unnecessarily. The default setting is 20.
Cat Expand determines how far beyond the edges of the image to look. MaxPoint will supply PinPoint with the approximate center of the image; but if the telescope is too far off, it may not be able to see enough stars to get a match. By increasing this number, a larger area of catalog stars are included. More stars slow down the processing slightly, but increase the probability of a match if the telescope is significantly off target. The range for this number is 0.3 to 0.8; the default is 0.3.
Max Time is a limit on how long PinPoint is allowed to take to reduce the observation. If for whatever reason PinPoint cannot find a match, it may sometimes take a long period of time to give up. Setting a limit on the processing time prevents wasting an excessive amount of time on a bad image. The default is 60 seconds.
Max Stars limits the number of stars used for the match. This in turn limits the processing time used to calculate the match. The default is 500.
Unbinned Pixel Size (Arc Sec) is the resolution of the telescope/camera combination in arc-seconds per pixel, with the camera unbinned. PinPoint needs to have an idea of what the plate scale is, in order to match up the stars with the catalog. If you are not sure how to determine this number, click the Pixel Calculator button.
The resolution is calculated as arc-seconds per pixel = 206 * pixel size (microns) / telescope focal length (mm). For the vast majority of camera models, MaxIm DL already knows what the pixel scale is. Click Get sizes to retrieve this information. Then enter the focal length of the telescope in millimeters; this information should be available in the telescope manual; in some cases it is printed on the instrument itself. If you use a focal reducer, this number should be scaled appropriately. Click Calculate to determine the pixel size in arc seconds, and then click Close.
Select the number of alignment points to use. The number is approximate, and is based on an average horizon of 30 degrees. If you raise the horizons, or some points fail to solve correctly, you will get fewer points. If you lower the horizons you will get more points.
Click Start to begin the calibration run. Click STOP to stop the run. If you click STOP, you can resume the calibration run by clicking Resume, or clear everything and start over by clicking Start again.
There are two status fields and a progress bar. The left-hand status field tells you what is happening with the auto sequence. The right-hand status field tells you what is happening with the PinPoint engine.
Before trying auto calibration for the first time, try doing a manual calibration so you can get the feel for how MaxPoint works.
You will want to turn on the Map display so you can watch the telescope move and add observations across the sky.
If you use the Set Calibration command in MaxIm DL and set it up to work with the exposures taken with the camera, then MaxPoint will automatically apply that calibration to each exposure. Make sure to set the CCD temperature in MaxIm DL.
Start the telescope around 45 degrees above the horizon. For German Equatorial mounts, start well away from the meridian. This ensures that the auto calibration can find a set of initial calibration points without wandering too far from the starting point. Once an initial set of observations have been made, MaxPoint will start improving the pointing of the telescope, making later observations more on-target. This minimizes the chance of PinPoint not finding a match.
On German Equatorial mounts, MaxPoint will work on one side, and then switch to the other side to continue. It will force a pier flip by moving the telescope far enough across the meridian to ensure that the mount will flip.
If you are using a fork mount, make very sure you have the Dec Limit set properly on the Telescope Setup page. Otherwise you may run the risk of the camera impacting on the mount.
After the calibration cycle has been performed, you should save the observations using the Calibrations Observations dialog.
You can add manual points to the automatic set by turning on Add to current cal on the MaxPoint Control Panel, and then doing manual calibrations in the usual fashion.