Slit Image Test
Overview Computations Setup Contact Home

Setup

CCD/Film Plane needs to be perpendicular to the optical axis, and the testers longitudinal axis needs to be aligned to the optical axis. This time consuming initial alignment's need be done only once. Subsequently, the attached laser pointer is used to quickly align the tester to the optical axis.

By taking a bit of care initially, Mask mounting can be done subsequently both quickly and accurately


Initial Alignment
   Longitudinal Axis to the Mirrors Optical Axis:
  • Center dot the mirror.
  • Mount the mask with the center hole over the center dot, also making sure the holes are level.
  • Observe the center slit with a 10 mm eyepiece. Rotate the tester (camera tripod works well) horizontally and vertically so as to keep the center slit image fixed with respect to the upper mat cutter blades while moving the tester on the longitudinal axis - Figure 1.
    Figure 1 Mat Cutter Slit Assembly   Mat cutter assembly  

   CCD/Film Plane Perpendicular to the Optical Axis &
   Attached Laser to Optical Axis:
  • Remove the mask, making sure not to move the mirror or the longitudinal axis of the tester.
  • Attach a plastic mirror to the CCD/Film Plane - Figure 2. Direct a laser pointer through the hole of the plastic mirror to the center of the test mirror. Rotate the CCD/Film Plane until the 3rd bounce strikes near the test mirror's vertical centerline.
  • Continue fine alignment, aiming the attached laser at the center dot while rotating the CCD/Film Plane horizontally and vertically (see Figure 6 Tester) until the 3rd bounce strikes the test mirrors center (special thanks to Lonnie Robinson for this improvement).
  • When finished tighten the three CCD/Film Plane Locking Bolts.
  • The attached laser is now aligned to the test mirrors optical axis.
  • Replace the plastic mirror with a card as shown in Figure 3. Mark the card where the laser strikes it.
    Figure 2. Perpendicular Alignment to Optical Axis   perpendicular alignment

3 laser bounce

 
    Figure 3 Optical Axis Alignment   quick alignment  
Subsequent, Quick Alignment of Tester to Optical Axis:
   After initial alighnment, the attached laser pointer is used to quickly align the tester to the optical axis as follows:
  • Center dot the mirror.
  • Rotate the tester aiming the attached laser near the center dot.
  • Rotate the mirror till the reflected laser beam hits the mark on the card as shown by Figure 3.
  • Rotate the tester aiming the attached laser at the center dot.
  • The tester is now aligned to the optical axis.
Mask Mounting Setup
    Mounting the Mask correctly over the mirror is key to Slit Testing. By taking a bit of care initially, Mask mounting can be done subsequently both quickly and accurately - Figure 4.
  • Center dot the mirror.
  • Adjust the strap so that the top of the mirror just touches (using a strip of paper as a feeler gauge) the top retaining bolt which is covered with a hard plastic sleeve.
  • Swing the mirror left so as to rest against the left retaining bolt which is covered with a hard plastic sleeve.
  • Aim the attached laser to the center of the mirror.
  • Adjust the mask so that the Laser Center Target (mounted over the center hole) cross hairs intersects the laser beam, making sure the Mask is level.
  • Tighten the left and right Mask clamps.
    Figure 4 Mask Mounting Setup   Mask Mounting Setup  
Subsequent Mask Mounting
   
  • Adjust the strap so that the top of the mirror just touches (using a strip of paper as a feeler gauge) the top retaining bolt which is covered with a hard plastic sleeve.
  • Swing the mirror left so as to rest against the left retaining bolt which is covered with a hard plastic sleeve.
  • Mount the Mask.
  • Mount the Cover over the mirror and mask - Figure 5.
    Figure 5 Mirror Cover   Mirror Cover  
    Figure 6 Tester   Tester  
Accessories
    With the Foucault Knife Edge find the ROC, and then move longitudinally aft a distance determined from experience to fill the CCD edge to edge with edge Slit image Figure 7
    Figure 7 Tester Attachments   Tester Attachments  
10 mm Eyepiece
    Using the eyepiece adjust the lateral position to center the center slit - Figure 8
    Figure 8 Mounted Slit & Eyepiece   Mounted Slit & Eyepiece  
Camera Figure 9
    Set shutter delay to as long as possible (e.g., 12 seconds) to eliminate camera shake.
Adjust the camera's ASA (e.g., 200) to require longer exposures (e.g., 25 seconds) offers the following advantages:
  • Reduces noise in the image.
  • Suppresses effects of air turbulence i.e., integrates.
    Figure 9 Mounted Camera   Mounted Camera  


References
  [1]   Karine and Jean-MarcLecleire, A Manual for Amateur Telescope Making. http://www.willbell.com/tm/atmmanual/index.htm
  [2]   Kodak Spec KAF-8300CE
  [3]   http://www.filmlabs.org/deadlinks/mounts.htmLens mount to CCD spacing
   Recommend saving this webpage to your computer, since it has vanished in the past.
  [4]   Jim Burrows Sixtests.exe
  [5]   Foucault Test by Leon Foucault 1858 http://bobmay.astronomy.net/foucault/leontop.htm
  [6]  Conics
Optical Shop Testing Edited by Daniel Malacara, Wiley and Sons, 1978, Appendix 1 page 479, 482
Conic approximation
Conic equations
Conic constant - Schwarzschild constant

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