jimh76 wrote:1. For the centre ORTFs, is it ESSENTIAL that they are exactly 17cm and 110 degrees
Those are the specifications for the ORTF array. If you rig the mics with a different mutual angle and/or spacing it won't be an ORTF array... But it will still be a near-spaced array and it will capture stereo, albeit with slightly different characteristics to the ORTF format... And those different characteristics may be beneficial.... Or not! The most likely obvious resulting from altered measurements will e a different stereo recording angle.
Like Mike, I use a template. I simply drew a pair of lines crossing at 110 degrees, and then placed some crosses on the lines where they are 170mm apart. I then printed it into transparent film, but ordinary paper works too. If the template is placed on a table you can assemble the ORTF array precisely, upside down with the mics placed appropriately on the template sheet. The benefit of the transparent sheet it that it can be held over an already rigged ORTf array to check nothing has moved!
2. Should the ORTF go quite near the orchestra or should they be further away, or is the only difference how much of the room they pick up?
The stereo recording angle for an ORTF array is about 90 degrees. So, if a sound source moves left from the centre line in front of the mic, it will sound like it is moving left from the phantom centre of your speakers. When that sound source is at an angle of 45 degrees to the ORTF array, the sound will have reached the left-hand speaker and be fully left.
What that means in practice is that if you want the orchestra to fill the stereo image, the ORTF array has to be positioned such that the orchestra fits within an arc spanning +/-45 degrees from the centre line. If the array is closer than that point, the orchestra will have an exaggerated spread for instruments near the centre, as well as their being over-emphasised (louder and closer), with bunching of instruments at the left and right speakers.
Conversely, if the array is too far back, the orchestra won't stretch fully between the speakers, and the outer regions to left and right of the orchestra will carry the room reverb and audience noise.
Of course, moving the array closer to or farther from the orchestra will also change the overall perspective -- the balance of direct sound to reverberation. As a result, it is usually necessary to decide first where the mics need to go for a good perspective, and then what kind of array to use to provide the appropriate stereo recording angle... Bearing in mind that different arrays will also change the perceived perspective to some extent, so there's a bit of iteration required to fully optimise things!
3. Does the lack of bass of using cardiods at distance affect the sound to a large degree.
It all depends what you mean by a lack of bass! If you used a pair of SM58s as an ORTF array it would sound extremely thin and unsatisfying. But normal, single-diaphragm studio cardioid mics should be flat down to 40Hz or lower when used at a distance. Closer placement results in the bass boost due to the proximity effect.
...and you can always use some EQ if necessary...