Any number of references I have seen seem to equally state that there is / is not field curvature. Can anyone you provide some definitive data / reality re the Intes MN-66?
I am now turning to the long-planned investigation of spectroscopy. The device of initial interest is the LHIRES III. One concern is the amount of back-focus that might be required - is anyone able to advise where-is or how to find the focal point in my system - MN-66 (152mm F/6 with a JMI NGF DX1 fitted).
The JMI info. is in the attached .doc file.
If you tell me the f-ratio of MN-66 I can reverse-engineer the design and show you Zemax plots.
Yes, Maksutov Newtonians suffer from both residual field curvature and coma, unless made in a bit special way (Wright variant with oblate primary), which I think is not the case with MN-66. But at f/6 (which I'm guessing is the second 6 in 66) aberrations will be moderate, depending how far you go off axis.
How big is the chip you intend to use ?
Manufacturer states F/6. Also states focal length of 900 mm and clear aperture of 152 mm which makes about F/5.92?
Initially I will be using a DSI Pro II monochrome which has a Sony ICX429ALL (Interline) Image Sensor. Sony data sheet shows:
Chip Size: 7.40 mm (H) x 5.95 mm (V)
Number of pixels: 795 (H) x 596 (V) approx. 470K pixels
Number of effective pixels: 752 (H) 582 (V) approx. 440K pixels
By definition, I believe that being interline, each second column is light shielded and inactive reducing the overall extrinsic QE by a factor of two (Howell, S. 2006. Handbook of CCD Astronomy 2nd ed.)
(The LHIRES III spectrograph is optimised for an F/10 (F/8 - F/12) scope so I will probably need to add in a [short] Barlow. Hence the need to have an understanding of how much travel is available / needed).
with such a small chip (less than 5mm off axis) an f/6 Maksutov Newtonian is certainly more than ample good enough to place almost 100% of energy within one pixel (~9 micron).
The Barlow might make that slightly worse (they will all introduce some chromatic residuals) but it will still be adequate for the purpose.
For the record, at 5mm off axis MN66 C-F spot is something like 7.6 micron RMS radius. At 3mm off axis (with a mild Barlow, that will be the field used) it is 6.4 microns RMS.
Any number of references I have seen seem to equally state that there is / is not field curvature. Can anyone you provide some definitive data / reality re the Intes MN-66? 
very much for the details, and your time and effort.
