I hope someone can help me with this because I seem to be going round and round.
I've got a WO FLT98, RFIV and a Canon 1000d, and I'm trying to work out the correct setting for the RFIV (it was three marks on the barrel - 66, 76 and 86).
I get very few nights out, so I want to make sure that I'm in the right ball park with this setting, so that I can spend as liitle time as possible refining it in the garden.
The WO website states that the RFIV spacing for the FLT 98 is 76.8mm - so on my first (and only(so far)) night out with the scope I set the scale on the RFIV to about 10% of the way between the 76 and 86 setting.....and didn't get a particularly flat field according to CCD Inspector. So I've been doing some research and here's where I've got;
Flange Spacing EOS Camera = 44mm
Thickness of T mount = 10mm
Therefore the spacing of the face of the T mount to the EOS chip is 54mm.
Spacing for RFIV and FLT98 = 76.8mm
The difference between the camera spacing and the RFIV spacing is therefore 76.8 - 54 = 22.8mm.
So if the spacing of the RFIV is measured from its face which bears on the T mount, I need to add 22.8mm of additional spacer between the RFIV and the camera - which I can't think is correct.
So maybe the published spacing is measured from the rear element of the RFIV? Maybe that's why WO have posted this on their website: http://www.williamoptics.com/accessories/images/FLAT4_Layout.jpg
This shows that they've used the end of the T tread as their base line for measurement and the face of the RFIV is 4.5mm "in" from there, with the centre of the rear lens element being 24.1mm and the edge of the rear lens element being 16.6mm from the same point when set at 76mm.
Since the T mount butts up against this face of the RFIV, it seems logical to use this face as the baseline for measurement so, at a setting of 76mm, the rear lens element is..
24.1 - 4.5= 19.6mm at the centre and
16.6 - 4.5 = 12.1mm at the edge
..from the face of the RFIV.
Ok, so do we use the centre measurement or the edge measurement?
If it's the centre - then to acheive the 76.8mm we need an extra 22.8 - 19.6 = 3.2mm, so need to set the RFIV to 76+3.2 = 79.2 on the RFIV scale.
If it's the edge measurement, then we need 22.8 - 12.1 = 10.7mm so the RFIV needs to be set to 76+10.7 = 86.7, but the RFIV won't go past 86.
So, (sorry this has taken a while!) to achieve a distance of 76.8mm am I correct in saying that the RFIV should be set to 79.2 (i.e. about a third of the way between the 76 and 86 setting)?? If this is right, what's the point of the numbers on the RFIV scale?
That's made my head hurt - can anyone help please?