AUTOSTAR INFORMATION Last updated: 31 July 2001

Box Scan Focal Lengths

Sent:	Monday, July 30, 2001 12:16:27
From:	dpersyk@worldnet.att.net (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).

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