© Sierra College Astronomy Department 1 Off-axis guiding and balancing (II-E)

© Sierra College Astronomy Department2 Lab II-E: Guiding Guiding is a necessity Even with a excellent equatorial mount and clock, exposures longer than ½ a minute begin to show signs of trailing Even with a excellent equatorial mount and clock, exposures longer than ½ a minute begin to show signs of trailing –Imperfect polar alignment –Periodic errors in the gear train –Fluctuation in the frequency of the AC power –Drive not set to proper rate –Flexure in telescope tube –Atmospheric refraction

© Sierra College Astronomy Department3 Lab II-E: Guiding Guiding This can be necessary even for piggyback photography (with a 50-mm lens) after 5 or 10 minutes even if polar alignment and the clock drive are OK. This can be necessary even for piggyback photography (with a 50-mm lens) after 5 or 10 minutes even if polar alignment and the clock drive are OK. With a telephoto lens of 300 mm, guiding may be necessary every minute or so. With a telephoto lens of 300 mm, guiding may be necessary every minute or so.

© Sierra College Astronomy Department4 Lab II-E: Guiding Guiding When prime focus or afocal photography is used, guiding must be continuous. When prime focus or afocal photography is used, guiding must be continuous. To guide you need a (high magnification) eyepiece with crosshairs To guide you need a (high magnification) eyepiece with crosshairs The guiding tolerance depends on the focal length of the camera or telescope (see Table 8.4), but 5 arc-seconds is often lower limit since atmospheric tubulence can blur stars to nearly that angular size. The guiding tolerance depends on the focal length of the camera or telescope (see Table 8.4), but 5 arc-seconds is often lower limit since atmospheric tubulence can blur stars to nearly that angular size.

© Sierra College Astronomy Department5 Lab II-E: Guiding Polar alignment accuracy How close to the pole must your telescope be? How close to the pole must your telescope be? –¼ deg from the pole should be good enough for most purposes. –If you’re well off, your exposure will have noticeable field rotation (see Figure 8.18) –This is inevitable if you are using an alt- azimuth mount (which is aligned to the zenith)

© Sierra College Astronomy Department6 Lab II-E: Guiding Clock drive imperfections No matter how good your clock drive is, it will suffer from periodic errors in the gears No matter how good your clock drive is, it will suffer from periodic errors in the gears The largest errors come from a worm gear which has a period of 4 to 8 minutes The largest errors come from a worm gear which has a period of 4 to 8 minutes Some telescopes drives can “learn” about these gear errors and make periodic error corrections (PEC) Some telescopes drives can “learn” about these gear errors and make periodic error corrections (PEC) To get long exposures, one can attach an electronic camera and have do the guiding for you (autoguiding) To get long exposures, one can attach an electronic camera and have do the guiding for you (autoguiding)

© Sierra College Astronomy Department7 Lab II-E: Guiding Deep-Sky photography through the telescope See section 8.16 BASIC TECHNIQUE 15 See section 8.16 BASIC TECHNIQUE 15 One needs to attach the camera and eyepiece to the back of the telescope (see Figure 8.25) One needs to attach the camera and eyepiece to the back of the telescope (see Figure 8.25) –Off-axis guider –It’s best if you can move the telescope slowly in RA and Dec (see hand controller with Celestron)

© Sierra College Astronomy Department8 Lab II-E: Guiding Deep-Sky photography through the telescope – day procedure Set up the telescope (aligned to north) and point it at the library (or some other building) Set up the telescope (aligned to north) and point it at the library (or some other building) Put a vertical edge of the building in the middle of the field Put a vertical edge of the building in the middle of the field Now turn on the clock drive and note how it takes building to leave (or encompass) the field Now turn on the clock drive and note how it takes building to leave (or encompass) the field Shut the clock drive off and return the field to the building Shut the clock drive off and return the field to the building

© Sierra College Astronomy Department9 Lab II-E: Guiding Deep-Sky photography through the telescope – day procedure Put a vertical edge of the building in the back middle of the field Put a vertical edge of the building in the back middle of the field Now turn on the clock drive and press the left arrow of the controller and note how note how it takes building to leave (or encompass) the field Now turn on the clock drive and press the left arrow of the controller and note how note how it takes building to leave (or encompass) the field Shut the clock drive off and return the field to the building as before Shut the clock drive off and return the field to the building as before Now turn on the clock drive and press the right arrow of the controller and note how note how it takes building to leave (or encompass) the field Now turn on the clock drive and press the right arrow of the controller and note how note how it takes building to leave (or encompass) the field

© Sierra College Astronomy Department10 Lab II-E: Guiding Deep-Sky photography through the telescope – day procedure Practice guiding with the sun Practice guiding with the sun –Put the solar filter on the telescope –Point the telescope towards sun –Make sure eastern or western edge of sun is in the center of the field –Turn on clock drive and sun’s edge in field