Help Me Calculate my actual F-Ratio from my Jupiter Image

[Xiga]

Arghh, this is driving me nuts! lol. I recently took some videos of Jupiter using my Canon 60D, x2 Barlow and C8 SCT (with crayford focuser) to try and work out what my actual Focal Ratio is, as I'm curious to know just how much the Crayford is increasing the F-Ratio, but i'm coming up with a figure of F30 which surely can't be right, so hopefully someone here can help me out with the numbers. With any luck i've just calculated something wrong!

Ok, so the videos i took were on May 14th around 11pm UK time. According to Stellarium, Jupiter is about 39.2 arcseconds in size at that time. 

As i mentioned above, my equipment is a C8 SCT (2,032 FL), Canon 60D (4.31um pixels), an X2 barlow, and a caryford dual-speed focuser (link below)

http://www.rothervalleyoptics.co.uk/skywatcher-dual-speed-2-crayford-focuser-for-sct-telescopes.html

Ok, so according to my calculations, here's what we have:

Size of Jupiter in Photoshop: 265 pixels (see link at the end of this post)

Apparent size of Jupiter in arc seconds: 39.2

Image Scale: =39.2/265 = 0.148

Focal Length in Image = 206.3 * 4.31 / 0.148 = 6,011

F-Ratio for my C8= 6,011/200 = 30.1

How can this be? I would imagine my standard x2 barlow (which screws directly to my T-ring) and then goes into the Crayford focuser (attached to a 2" visual baclk) would only add a nominal amount to the expected F-Ratio of F20 and maybe bring it up to around, say, F23ish, but not F30 surely? 

Here is a link to a sharpened Best 30% stack straight out of Autostakkert! Using the PS ruler tool i estimate the diameter to be in the region of 265 pixels (give or take a few), whereas i was expecting it to be more like 180 pixels wide (if taken at a true F20).

https://onedrive.live.com/redir?resid=C14ECDF7700B5618!3047&authkey=!APakNBBIiIdFApo&v=3&ithint=photo%2ctif

Hopefully someone can help me out with this, as i've gone over this time and time again and i can't find where i'm going wrong.

Cheers all!

[TyReay]

I love this and use it all the time. Hope it helps.

http://www.12dstring.me.uk/fov.htm

[wxsatuser]

I'm not an expert but is the real magnification of the Barlow mucking up your calculations?

The Barlow will have a fixed focal length but the distance between the lens and chip will alter the magnification.

Formula for Barlow real mag = lens to chip distance/focal length of Barlow.

[Xiga]

I did some more reading and came across this link which does a good job of describing (in sections 6 & 7) the recommended and actual focal length needed. This confirms my calcs that I am actually getting F30, which was definitely too much:

http://www.frankbrandl.com/htm/webcam.htm

I think you’re right wxsatuser it’s definitely the barlow! I just remembered that I ‘tweaked’ my x2 barlow some time ago. I actually own two x2 barlow’s, both are cheapo models but one is Fully Multi-Coated whereas the other has a T-thread. So to get the best of both worlds I simply unscrewed the lens elements and swapped them over, so I could take advantage of the better lens and still screw it on to my Canon’s T-ring for a more secure fit. Both units are about the same length in total, so I assumed that this wouldn’t mess up the effective focal length of each one, but it now looks as though I was wrong to assume so, as it looks like it is essentially now an x3 barlow! Does this sound possible/reasonable, given the unit lengths are pretty similar?

Annoyingly this means I’ve been over-sampling on my last couple of night out. I will have to remember to swap the lenses back from now on when I am aiming for F20, but on the bright side it sounds like I just got a free x3 barlow in the process.