[pvelez (Pete)]

Dumb question....

If I have a telescope with focal length 1200mm and add a barlow that gives me a 1.6x magnification boost, does my focal length increase by the same factor ie I go to 1920mm?

I am trying to plate solve guider images with my new LISA spectrograph. I understand that the guider optics magnify the image on the guide camera sensor by a factor of 1.6.

I have an RC8 with a focal length of 1620mm reduced to 1200mm or thereabouts with a focal reducer. That should give me a pixel size of 1.27 arcsec/pixel. But because of the guider optics, that should drop to 0.79 arcsec/pixel as the effective focal length is now pushed out to 1920mm.

How's my logic?

If I can platesolve my guidecamera images, I can move my scope to centre my target on the spectrograph slit easily using Maxim - without it, there is a hell of a lot of trial and error involved with the gamepad controller.

Pete