A Rough Guide to Autoguiding a Goto Mount.

Transcription

1 A Rough Guide to Autoguiding a Goto Mount. Definitions: ASCOM Platform A downloadable, free programme, allowing PC software such as Guidemaster to communicate with most commercially available GOTO Mounts. See and download the latest platform version. Autoguiding The ability for a telescope, mount and camera set up to track a star accurately for long periods compensating for object drift caused by Earth Rotation, poor mount polar alignment and mount drive errors, (Periodic Error). Goto Mount A telescope mount, fitted with motors and encoders capable of being aligned on several reference objects in the sky which will then point the telescope to user inputted stellar co ordinates, (usually this consists of the user selecting one of thousands of objects retained in the mount database, once the object is selected by the user the mount will slew the scope to the selected object and track it across the sky). Guidemaster A freeware autoguiding programme for the PC. Works with the ASCOM drivers to communicate with the GOTO mount and send correction instructions when used with a guide camera. See and download the latest version. Introduction To the beginner or uninitiated, autoguiding can seem like a black art only to be performed by the elite few with thousands of pounds worth of equipment and special powers. I used to think this was the case until last year when I was determined to get my gear working properly with long exposure imaging capability. The following description is a guide to how I approached the subject of getting MY gear to autoguide. This may not necessarily be the best method for others with different equipment but the description is intended as an illustration as to how easy autoguiding can be and provide a catalyst for those wondering if autoguiding is worth it. Equipment: Goto Mount. I have now successfully autoguided with 2 off-the-shelf readily available GOTO Mounts, the Celestron CGE and the Synta HEQ5. I see no reason why these methods would not work with other mounts such as the Synta HEQ6, Celestron ASGT, Meade LX & LX200, Celestron Nextar/CPC etc. In fact any of the scopes covered by the ASCOM Platform which the autoguiding software uses. IMHO if the mount is covered by ASCOM and there is a means of connecting a PC to the mount/handbox then the methods described here should work.

2 Telescopes: 2 telescopes must be used for this method; the primary imaging telescope and a guiding telescope. I have so far used an 80mm APO refractor at F6.9 as my primary imaging scope and a Celestron C11 or a Skywatcher 127 Maksutov for guiding. Other combinations may be used so long as the 2 scopes are mounted on the same mount and can be moved independent of one another. The best and most common method is to mount one scope in guide rings. I use ADM rings fitted on the secondary scope and the primary scope mounted within the rings. This configuration allows me to lock onto a guide star adjacent to the object I am imaging with the secondary scope and adjust the primary scope until the target for imaging is framed correctly in the imaging camera. I recommend that the secondary guide scope be a longer focal length than the primary imaging scope, this gives far more accurate guiding than using short focal length guide scopes. My C11 with ZS80 Mounted in ADM Guide Rings PC, Laptop Preferred for portability. I use a 1.5Ghz Celeron PC fitted with 512Mb Ram and 20Gb HDD Laptop PC for all my astro work. The model I use does not possess a serial port but does have 4 off USB 1.1 ports which I use to communicate to the mount etc via USB / Serial adapters. A PC with a serial port will work just as well, although configuration of Guidemaster will vary. Provided the PC is capable of running ASCOM and Guidemaster and has USB or Serial Ports then this will do. Guide Camera: I use a Mintron 12VEX black and white enhanced Astrovideo camera for guiding. This camera is very sensitive to low light and can be set to automatically stack up to 128 individual frames to enable the user to view very faint objects. I use the SVideo output direct into a simple USB 1.1 Video Capture card fitted to the USB port of the PC. The capture card uses the standard windows video drivers which Guidemaster then uses to capture an image of the guide star. Others use modified Web Cams for guiding with equal success. My Method: Fit the scopes to the mount and get the mount polar aligned as good as possible, the closer to the correct alignment the less Guidemaster has to do. Slight discrepancies will be compensated for by the software but aim to get as close as possible. Connect your guide camera to the PC and power up. I do this first because Guidemaster can be a little temperamental if it does not see a camera. Download ASCOM and Guidemaster and install them on your PC. Next I use a USB / Serial Adaptor and the serial lead supplied with the mount to set up communications

3 between the PC and Mount. Connect the PC to the Mount or Handbox using whichever method you have chosen or whichever is supported by your mount. Open Guidemaster, you now should get an image through your guide camera, if not select the camera drivers, Mintron = Windows Video drivers. Select telescope from the top menu and then select configure. If the ASCOM drivers are installed correctly you will see ASCOM listed, select this and then follow the menus and select your mount from the list. Select the port that the USB/Serial lead is connected to and then open comms with the scope. If you have selected correctly then the small ASCOM Hand Controls will be displayed over Guidemaster and a few more screen buttons will become active. Click on the Camera menu and then Options, now enter your camera details, pixel size in width and height. In the scope section add the focal length and aperture of the secondary guide scope. These details are very important as this determines Guidemaster accuracy. I now select the integration on the Mintron buttons to 32x to give a sensitivity which provides plenty of guidestars but doesn t take too long to build the image. I find larger integrations tend to throw Guidemaster. Now, at this stage we need a clear sky with some stars, the previous steps, (except polar alignment), may be done anytime, in fact it is probably better to get everything communicating and set up during daylight to avoid wasting valuable imaging time. Slew to a bright star and centre this in the secondary guide scope, now fit the guide camera to this scope as orthogonal as possible, e.g. top of camera level with top of scope etc. Focus the star in the Guidemaster window and adjust brightness/contrast until the star is sharply focused and other fainter stars can be seen in the background. Guiding with a 127 Maksutov side by side with the ZS80 on an HEQ5 mount Now comes the clever part, calibrating Guidemaster so it knows the orientation of the guide camera to RA and Declination. Click on the calibrate button, GM will now prompt you to select a star. Left click on the chosen bright star. GM now slews the mount and checks which way the star moves. Continue selecting the star when prompted by GM until GM displays the vector pop up when everything is OK or an error message if not. If the Vector window is displayed click on the save button to save the calibration. Do not move the guide camera in the guidescope for this session, if you do, you will have to recalibrate. If you had an error message then try swapping the RA or Dec directions in GM window one at a time and recalibrating. Now you are ready to image.

4 All you have to do is focus your imaging camera, slew to the object you wish image then choose a guide star in GM. Pick a reasonably bright star, not too bright as this will swamp adjacent pixels and not too dim as GM may have difficulty locking on to it. If necessary adjust the guide ring screws to find a suitable star in the guide scope, (do this only if your guide scope is mounted in the guide rings). Once you have a suitable star in the GM window click the Guide button. GM will display a pop up asking if you want to select the star or leave GM to automatically find one. I prefer to select my own star so I select manual and click on the guide star. GM will now display several graphs and targets. At the lower right there are 2 graphs, these will be constantly updating showing the movement of the guide star in relation to the original position, just above these is a check box, Dynamic Agressiveness. I recommend checking this box, this will then adjust the aggressiveness of the adjustment instructions that GM sends to the mount on-the-fly, which certainly calms things down with my set ups much more than using the manual aggressiveness sliders at the top right. 2 targets show how close each command brings the star to the original point which is most useful. Let GM settle until the 2 graphs are nice and flat with no big peaks then start imaging with your camera. As you get used to the autoguiding start to play with some of the GM settings to try and get the GM adjustments to the mount as small as possible, trial and error sometimes pays off. NOTE! Do not touch the scopes, cameras or mount during guiding as this really gives GM a headache and may result in strange slews as GM tries to regain control. Conclusion And there you are, simple eh? Don t worry it seems very complicated when explained all in one. Here are a few tips to bear in mind along the way: Do like I did, learn each bit in stages, don t try to do it all at once. To be honest it s taken me the better part of a year to get to a stage where its become easy to set up and operate. A year because of the lack of clear nights, my lack of PC skills and getting the kit right and most of all getting my kit just right so I can set it up easily and quickly. Know your equipment. Get to know how best to work each part of the kit in its own right, this is very important when trying to get equipment to communicate with each other. Get the PC talking to the mount and camera before hand, preferably in daylight when you have time and can see what you are doing. Do some dummy runs in daylight before hand so setting up becomes second nature, then when you do have a clear night you only need to concentrate on the Autoguiding part.

5 A permanently aligned mount is a real benefit especially one that is drift aligned but the beauty of autoguiding is that it will work with a reasonably well aligned portable set up also. Practice your alignment until it is second nature. Finally, this method works for me and my kit, it may not work for your set up, there are so many variables that I cannot cover every eventuality so please take this article as it is intended; a rough guide to autoguiding and not a step by step guaranteed method. Thanks for looking and clear skies Phil Jaworek