Classic Knife Edge
Variant of Classic Knife Edge
Robert Reeves Film Review
Kodak Wratten Filters
Figure Periodic Error of Telescope
2) I use a Celestron 6mm Guiding Eyepiece whose reticle has 16 graduations across the field of the reticle. Divide the number of reticle graduations into the Field of View found above to calculate the individual reticle graduation's field of view.
3) While the telescope is driven watch the movement of the star in RA closely. You can quite accurately estimate the amount of Periodic Error of your telescope.
Stars travel westward through the sky at the rate of one revolution every 23 hours 56 minutes (86,164 seconds). (drift time) x cos(star’s declination) x 360° Field of view in degrees = -------------------------------------------- 86,164 seconds per day
... On my 6" A-P f:7.3, FL=1,125mm the star corsses the 6mm Celestron Guiding Eyepiece in 51sec.
... There are 16 graduations within the reticle.
... This gives 0.2043deg, or 00h 12m 25.8s for the 6mm FOV
... Divided by 16 graduations gives 0.0128deg, or 00h 00m 46.08s or 46 arc.sec per graduation.
... Although 46arc.sec is too large to use to determine the Periodic Error of a quality mount inserting a 2x Barlow before the guideing eyepiece will bring the 46arc.sec down to a more usable 23arc.sec. If you do insert a Barlow be sure to retime the star's travel across the guiding eyepiece.