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Field curvature measurements have been made. The results are sorted here with more recent images near the top and highly curved fields at the bottom. CCDWare's CCDInspector has been used to measure different optical systems and CCD camera combinations. CCDInspector measures the FWHM values of several thousand stars in the field and fits a 3-D polynomial surface to them showing field curvature and or collimation accuracy as a function of stellar profiles.
09/03/06 Takahashi BRC-250 f/5 1268 mm Focal Length & SBIG STL-11000M:
The left image shows a 3-D view of the image of the Eastern Veil Nebula complex on the right. The image on the left shows a very flat field as a result of the superb optics of the BRC-250 that was designed for 35 mm and larger formats.
06/29/06 Takahashi BRC-250 f/5 1268 mm Focal Length & SBIG ST-10XME:
The left image shows a 3-D view of the image of M 17 on the right. Compare this image with the M17 shown below.
Nikon ED IF f/2.8 300 mm Focal Length, 107 mm Aperture Lens & ST-10XME:
The left image shows a 3-D view of the image of the M8 & M20 complex on the right. The image on the left shows a very flat field as a result of the superb optics of the Nikon lens that was designed for the 35 mm format. The ST-10XME chip occupies a little more than the center quarter of the 35 mm format.
Nikon ED IF AF-I f/4 600 mm Focal Length, 150 mm Aperture Lens & ST-10XME:
The left image shows a 3-D view of the image of the Crescent Nebula NGC 6888 on the right. This shows a very flat field over the surface area, 2184 x 1472 pixels, of the ST-10XME chip. The Nikon lens, again, shows off it's superb optics and is well matched with the ST-10XME camera.
Meade 10-inch f/6.3 Schmidt-Cassegrain Telescope (SCT) & ST-10XME with a Meade 0.63x focal reducer/field flattener:
The left image shows a 3-D view of the image of M65 & M66 on the right. The curved focal plane inherent to SCT's becomes evident. Even with the reducer, the field is not as flat as seen above. Critical focus was achieved closer to one edge that another. The collimation was not as precisely centered as it could have been. The resulting field is both curved and tilted.
Meade 10-inch f/6.3 SCT 1600 mm Focal Length & SBIG ST-10XME:
The left image shows a 3-D view of the image of M 17 on the right. This shows a severely curved field over the surface area of the ST-10XME chip. No focal reducer or field flattener was used. A slight tilt is also present in this field as a result of less that ideal collimation.
Meade 10-inch f/6.3 SCT without a reducer & CSU Fresno's large format SBIG STL-11000XM:
The image on the left is a 3-D view of the plane of focus of my 10-inch SCT. This shows the severe field curvature over the large surface area, 4008 x 2672 pixels, of the STL-11000XM chip. The image on the right is the same as shown below but with the corners and center enlarged. Note that the stars are not symmetrical. The large CCD chip of the STL-11000 is flat but the plane of focus is not in a SCT. Other more expensive optical designs, such as the Ritchey-Chrétien, produce a flatter focal plane and more desirable stellar profiles. CCDInspector can also measure collimation errors and showed that the optical system was well collimated for this image.
