tag:blogger.com,1999:blog-59377884801269937212011-04-21T15:01:32.757-07:00bjengnoreply@blogger.comBlogger11100tag:blogger.com,1999:blog-5937788480126993721.post-42771825366483071952008-07-06T06:30:00.000-07:002008-10-01T02:13:57.119-07:00<a href="http://1.bp.blogspot.com/_ApSANcpnMe0/SHDJL-QTpgI/AAAAAAAAA78/Ae-6Lw4M7xQ/s1600-h/vixencollimation1crop.jpg"></a><br /><div>a. f ratio = focal length/diameter (mirror or objective).</div><div>eg. A telescope of 600mm focal length with an objective of 80mm Dia will give an f ratio of 7.5</div><div></div><div>------------------------------</div><div></div><div>b. Magnification = focal length of the objective/focal length of the eyepiece, </div><div>so, our 600mm telescope with a 20mm eyepiece give 600/20 = 30 times. </div><div></div><div>A 2x barlow will double the focal length to 1200mm making the magnification 1200/20 = 60 times. </div><div>Focal reducers reduce the focal length, of course. The popular .63 FR will reduce the focal length to 378mm, so the 20mm eyepiece will give 378/20 = 19 times (18.9 actually)</div><div></div><div>------------------------------</div><div></div><div>c. In photography, the focal length controls the image size. </div><div>The formula is Pixel size (microns) x 206 /focal length(mm) = Field of View/pixel (arcsec). </div><div>This gives you the amount of sky that each pixel sees, </div><div>eg. For our 600 mm telescope and a digital camera with square 7.8 micron pixels, the formula will give us 7.8 x 206/600 = 2.678 arcsec/pixel.</div><div>Multiplying this by the number of pixels in the array gives the size of the camera’s Field Of View in arcsec.</div><div>So for a camera with an array of 3024 x 2016 pixels, </div><div>the result is 2.678 x 3024 = 2.25° and</div><div>2.678 x 2016 = 1.5°</div><div>Giving a Field of View of 2.25° x 1.5°</div><div></div><div>-------------------</div><div></div><div>d. Focusing </div><div>A critical job in astrophotography, but how close do you have to get to be "in focus"?</div><div>The formula is, </div><div>In-Focus Zone = Focal Ratio² x 2.2 in microns.</div><div>for our example telescope it is 7.5 x 7.5 x 2.2 = 124 microns,</div><div>and with 1000 microns in a millimetre, it means .124 of a mm, say 1/8th of a mm.</div><div>For a Meade SCT, it is 10 x 10 x 2.2 = .22 of a mm.</div><div>It's all in the photographic speed of the telescope. </div><div>A fast f/2 system will have to be within (2 x 2 x 2.2 = 8.8 microns) = less than a 10th of a mm! Not easy with a rack and pinion focuser </div><div class="blogger-post-footer"><img width='1' height='1' src='https://blogger.googleusercontent.com/tracker/5937788480126993721-4277182536648307195?l=dasformulaeofinterest.blogspot.com' alt='' /></div>bjengnoreply@blogger.com0