I am trying to determine the “true” magnifying factor of a Celestron X-cel LX 2X Barlow for use with a LogitechPro 4000 webcam (the Logitech CCD's pixel size = 5.6 micron) in planetary imaging. I am using the Barlow without an eyepiece inserted at the prime focus of my Celestron C8 f10 CAT, then directly inserting the nose piece of the webcam into the Barlow - with or without an appropriate extension between it and the webcam. The amount of distance between the Barlow optics and the webcam sensor will determine the resolution of the image. That distance will depend on, among other factors, the focal length of the Barlow.
For planetary imaging a resolution of around 0.3 arcsec/pixel may be close to optimum for my setup. The resolution ("sampling") of a webcam sensor is (206 x Pixel size)/FL, or 1154/FL for the Logitech, where FL is the amplified (or increased) focal length of the C8 produced by the Barlow. For my setup, FL = 2000 x A, where A is the amplification of the Barlow. (Note: A is not the same thing as the magnifying power of the Barlow, "2X" as indicated on the side of the Barlow.) More precisely A is a function of both the Barlow focal length and the distance between it and the sensor. The equation (from Thierry Legault, Astrophotography) is : A= (d/f) + 1, where d is the distance to the sensor and f is the Barlow focal length. I can somewhat control the distance so that if I know the Barlow focal length I can determine the resolution for any given distance. There will be an optimum distance (or, amplification) for my setup to achieve a reasonable resolution at the sensor.
So, what is the Barlow focal length? I have read that most Barlow's have a focal length between 50 and 100 mm, but that is a large range resulting in a large range for the distance needed to achieve an optimum resolution. For example, if the Barlow focal length is 50 mm then to get 0.3 arcsec/px I would need to separate the Barlow and sensor by about 46 mm. But for a Barlow focal length of 100 mm I would need a separation of twice that (i.e., about 92 mm). That is approximately the difference between using the Barlow alone versus the Barlow plus extension.
Even though a Barlow is a negative lens it nevertheless does have a focal length. So, my question is: how can I determine its focal length? This specification is nowhere to be found, either in the manufacturer's literature or markings on the body of the Barlow. Does anyone know what is the focal length of this Celestron Barlow, or how it can be directly measured?
Celestron Orange Tube C8 and f6.3 focal reducer
Williams Optics Z61 with WO field flattener
Pentax K7 DSLR
LogitechPro 4000 webcam
Edited by seckroaad, 27 October 2020 - 03:48 PM.