Image Link and Closed-Loop-Slew with Temma Mounts (on OSX) Ken Sturrock November 8, 2015

Transcription

1 Background Image Link and Closed-Loop-Slew with Temma Mounts (on OSX) Ken Sturrock November 8, 2015 Plate Solving is a generic term for a technique that compares the stars in an image (a film plate in old terminology) to a known set of star patterns and, from that comparison, determines where the image is located in the night sky. Plate solving technology is used in numerous places. For example, if you look at an image on the WWW site AstroBin: you can pass your cursor over the image and the site will show you an annotated version. You can also try to upload your own images to astrometry.net and let that site plate solve an image. This is useful because it not only determines where the image was taken and what objects are in the image, it will also tell you the positional angle of the picture as well as the image scale (the number of arc-seconds per pixel). Plate Solving is also built into the image-processing program PixInsight. In PixInsight, you can find it under Script/Image Analysis/Image Solver. Image Link Software Bisque s implementation of Plate Solving, which is built into The SkyX Professional, is called Image Link. Image Link comes in two forms: Traditional Image Link uses coordinates embedded in the image s FITS header, as well as image scale and mirror information supplied by you, to compare your picture to the built-in star catalogs. All Sky Image Link (aka blind image link ) does not require any outside information it relies entirely on the pattern of stars. All Sky Image Link does require that you download and install a large database from the Bisque WWW site and, as you should expect, runs more slowly. The All Sky Image Link Database and installation instructions can be found here:

2 You can search through the Bisque forum and find many posts, often by Tom Bisque, describing how to configure and use both versions of Image Link. I m going to just step you through how to use the All Sky version with a Temma. Once you have the allsky database installed in the Astrometry sub-folder of your SkyX Professional settings folder (~/Library/Application\ Support/Software\ Bisque/TheSkyX\ Professional\ Edition/Astrometry/) then you should be able to perform an all-sky Image Link. The first step is to take a ten second exposure of something through your camera. The exposure has to be long enough to get a decent number of stars but not so long that the image is saturated. Ensure that the Automatically Save Photos option is checked in the camera control window (go ahead and set up the Autosave options so that they make sense to you and name things logically). Sony Cameras If you are using a Sony CCD camera, you may have to adjust a setting under preferences/advanced. If a camera has a lot of pixels then The SkyX assumes that the chip must be physically large and that this will give you a wide view of the sky with many many stars. All those stars will slow down the blind search algorithm, so if there are a lot of pixels, the SkyX will automatically crop the image to cover a smaller part of the sky. The problem is that Sony chips often have small pixels so there are a lot of pixels but the chip is also small. The cropped image now will not show enough stars. If you find that blind searches fail with your Sony chipped camera, scroll to the bottom of the advanced preferences dialog and adjust the values so that it doesn t crop nearly so much. In my case, I d rather put up with a lower search in the name of reliability. When the image you have taken comes up in the viewer, click on the Photo button and then choose To Image Link. Now, click on the All Sky tab. Make sure that the Blind radio button is selected and go ahead and check both of the Use All Sky Image Link options. Now press the All Sky Search button.

3 Hopefully, it worked.

4 At this point, you now have the diagnostic output from the Image Link run, and the SkyX has also placed a square representing your image on the sky chart. You can close the Image Link window. Go to the Find window, you should be able to click on the rectangle representing the image and, as long as you didn t click on a star, the SkyX will identify the center of the image and select it in the lower left pane of the window. Synchronizing on an Image Now, the center of the image is the selected object in SkyX. This means that you can click on the Telescope control tab/window and synchronize your mount on that location (if you use T-Point then don t do this). If you are using a Takahashi Temma mount and your mount was not previously synchronized, then it s easiest to aim the telescope east and take a picture to the east. When you synchronize on that image, then everything should be set.

5 Now, you can give Closed-Loop-Slew a try. Simply click on a star on the sky chart and, instead of pressing the Slew button, press the Closed Loop Slew button instead. The telescope will slew to the object, take a ten second picture, display the picture, re-center the mount and take another picture. In typical SkyX fashion, you ll only get a dialog box or explicit warning if something went wrong. If you are using a Temma and the mount went slewing the wrong way, then you probably don t have the mount pointed the right way when you synchronized it. Disconnect from the mount and camera, turn the mount off, point it east with the camera on the west side of the meridian, turn the mount on and try again. If you can use Closed-Loop-Slew by hand then the script should be able to use it too.