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Chris,
If the full frame (assuming 24 x36 mm) has 24Mp and the cropped frame (APS-C, 15 x 23 mm) the same pixel count, the pixel size in the APS must be smaller.
Using the 1500mm fl, with the full frame the FOV would be larger than the same using the APS. at 1000 fl.
If you used the full frame at 1000mm the FOV would be larger than the FOV in the APS.
You are comparing an image 15mm wide with an image 24mm wide.
You need to compare them at the same focal length.
As Covington says about comparing full and APS frames in his "Digital SLR Astrophotography", p17:
"This has nothing to do with zooming (varying the focal length) in the normal sense of the word."
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