Lagoon Nebula last night

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bloodhound31

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Did this one last night.<br /><br />10 x 30 second light frames at ISO 1600, 10 x darks removed, stacked and processed in deep sky stacker.<br /><br />Celestron Nexstar 11, equatorial wedge alignment, Canon 400D, prime focus on a waxing gibbous moonlit night. Also used one of those ambient light filters on the camera adapter. The resulting shot was a little darker, but way more detailed.<br /><br />http://www.aussiepeople.com.au/asignobservatory/Astrophotography.aspx<br /><br />Removing the darks seems to have made all the difference.<br /><br />Conversely, I opened the shutter on the same target and just let the telescope go with it's own tracking for fifteen minutes. (Fell asleep) Have a go at what it did! (Second image) I have no idea how to refine this tracking. I'm thinking when I get the ED80 on top, I will have to guide manually.<br /><br />Baz.
 
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billslugg

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bloodhound31<br />I'll take a shot at it.<br /><br />Calibrate the image.<br />On my screen, the image of the blurry stars is 256 mm x 153 mm. The aspect ratio is 1.67<br />The Canon 400D has a sensor of 22.2 mm x 14.8 mm. Aspect ratio is 1.50<br />Since the screen image is longer and narrower than the sensor, then the narrow dimension must have been cut off.<br />Therefore it is more accurate to calibrate to the long dimension.<br />The long dimension of the sensor is 22.2 mm. <br />The focal length of a Celestron Nexstar 11 is 2800 mm.<br />If you take the length of the sensor and divide by the focal length you get the arc tan of the image width in degrees. <br />Image width = arc tan (22.2/2800) = .454 degrees<br />On my screen, the image width is 256 mm. Therefore:<br /><i> 1 mm = .454/256 = .00177 degree</i><br /><br />Determine the error magnitude:<br />The stars are stretched out to 16 mm.<br />This is an error of 101 arc seconds in 15 minutes.<br />In 15 minutes a star will move 15/(60x24) x 360 degrees, or 1.25 degrees<br />1.25 degrees is 4500 arc seconds.<br />1.25 degrees on my screen is 706 mm.<br />The telescope skewed 706 mm plus/minus 16 mm.<br />This is an error of 2.3 %.<br /><br />The specifications on the Nexstar series quotes an encoder resolution of .26 arc second.<br /><br />The tracking error here is many orders of magnitude larger than the capability of the telescope. <br /><br />How about polar alignment? One possibility.<br /><br />Did you ever replace the worn gear that you show on your website?<br /><br />There is another possibility I am not real familiar with. On an Alt-Az mount, how do you account for field rotation?<br /><br />Also, the star trails are moving at 90 degrees to the major tracking error. Was it windy when the photos were taken? Can the wind get into your dome?<br /><br />Also, after about 3 minutes, it appears that the error went backwards for a minute or <div class="Discussion_UserSignature"> <p> </p><p> </p> </div>
 
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bloodhound31

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Wow! Bill, you sound like you are leagues ahead of me! I will seriously take a look at what you have said here.<br /><br />Not sure I understand some of it, but I am just perfecting drift alignment tonight, now that I FINALLY understand it. Clouds are in though...<br /><br />There are many functions on the Celestron for anti-backlash, calibrating motors and such, but I do not understand the data entry on the controller and the instructions are a little vague. I guess I just have to play with it and see. I think drift polar aligning will fix a lot though.<br /><br />Oh...No to wind. On close inspection of the worn teeth, they werent too bad, I think I caught it in time before any real bad damage was done. All teeth are still square edged. I think field rotation is countered by putting the alt az fork mount up on the wedge and correct polar alignment.<br /><br />Don't know about the backwards thing either.<br /><br />Would like to hear more from you Bill, you sound like you are pretty switched on mate.<br /><br />Baz.
 
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