Sent: Monday, July 30, 2001 12:16:27 From: firstname.lastname@example.org (Dennis Persyk) Dick Seymour suggested I forward this to you for possible use on your web site. The attached text file describes a method for redefining the scope focal length so that a box (spiral) scan works with other than the supplied 26 mm eyepiece. I got the idea from Dick on the LX-90 group. Use the file if it is applicable to your site. Box Scan Focal Length Problem: What focal length does one insert into Autostar (Telescope/Focal Length) so that a box scan has the proper pitch when using any given eyepiece? Solution: Let AFOV be the apparent FOV of the eyepiece in degrees Let EFL be the eyepiece focal length in mm Let TFL be the scope focal length in mm Let FOV be the system FOV in degrees Let FOV Min be the system FOV in minutes; FOV Min = 60 FOV Equa 1) FOV = (AFOV x EFL)/TFL Equa 2) TFL = (AFOV x EFL)/FOV Example: What do we input for scope focal length so that Autostar executes a proper box scan for an 8.8 mm Meade SWA eyepiece? First, for the supplied 26 mm Super Ploessl, EFL = 26 and AFOV = 52, so FOV = (52 x 26)/2000 = 0.68 deg = 40.6 minutes. Second, for the 8.8 SWA, EFL = 8.8 and AFOV = 82, so we have FOV = (8.8 x 82)/2000 = 0.36 deg = 21.7 minutes. We need to trick Autostar into thinking that our scope has a different focal length. The box scan assumes the 26 mm eyepiece, so we need to calculate the scope focal length that yields a 0.36 degree FOV with the 26 mm eyepiece. From Equa 2), TFL = (52 x 26)/0.36 = 3756 mm. We key in 3756 mm into Autostar under Scope Focal Length to get the desired box scan pitch. Summarizing, Calculate the FOV for the eyepiece in question using Equa 1. Calculate the TFL using TFL = (52 x 26)/FOV = (1872)/FOV where FOV was determined from Equa 1. Enter it into Autostar. My thanks to Dick Seymour who devised the focal length change method and first described it on the LX 90 group list. Dennis (former ETX-90 owner, now LX-90 owner).
Return to the top of this page.
Go back to the Autostar Information page.
Go back to the ETX Home Page.