calculating image scale

[neil phillips]

Hi all hoping some one can help me figure out what focal length i have been filming at recently

Ok heres the info, I have a 12" f6 scope native focal length of 1800 mm, into this i stacked two barlows a Televue 3x and a celestron 2x, However the calculations are more complicated than that. i have a 2" extension on my focusser and find focus with the stacked barlows about two thirds out.

I have been told that a 2" extension on my focusser will make the Televue 3x about a 4x. so 4x with Celestron 2x barlow, we get the sum 4x2 = 8 or in this case 8x barlow power.

but still thats not the whole picture, because of the long barrel of the Televue barlow from the Celestron lens to the Televue lens i measure just over 3" so to find what the celestron is multiplying by we need to know what a 3" gap does to the celestron correct ?

If i ignore that for a bit and do a sum 4x televue x2 = 8x

8x 1800 mm focal length of the scope = 14400 mm focal length, but of course i havent yet taken into account the 3" gap between the barlows. So am i right when im told the 2" extension on the focusser is upping the Televue 3x to a 4x ?

if that is correct i just need to figure the effect of a 3" gap between the TV lens and Celestron lens. Can anyone confirm or help with any of this cheers if you can

Neil