diff --git a/auto_stretch/stretch.py b/auto_stretch/stretch.py
index 0eded17..ad8b918 100644
--- a/auto_stretch/stretch.py
+++ b/auto_stretch/stretch.py
@@ -1,4 +1,3 @@
-from astropy.io import fits
 import numpy as np
 
 """
@@ -8,11 +7,11 @@
 
 class Stretch:
 
-    def __init__(self, target_bkg=0.25, shadows_clip=-1.25):
+    def __init__(self, target_bkg:float=0.25, shadows_clip:float=-1.25) -> None:
         self.shadows_clip = shadows_clip
         self.target_bkg = target_bkg
 
-    def _get_avg_dev(self, data):
+    def _get_avg_dev(self, data: np.ndarray) -> float:
         """Return the average deviation from the median.
 
         Args:
@@ -25,7 +24,7 @@ def _get_avg_dev(self, data):
         return avg_dev
 
 
-    def _mtf(self, m, x):
+    def _mtf(self, m: float, x: np.ndarray) -> np.ndarray:
         """Midtones Transfer Function
 
         MTF(m, x) = {
@@ -61,7 +60,7 @@ def _mtf(self, m, x):
         return x.reshape(shape)
 
 
-    def _get_stretch_parameters(self, data):
+    def _get_stretch_parameters(self, data: np.ndarray) -> dict[str, float]:
         """ Get the stretch parameters automatically.
         m (float) is the midtones balance
         c0 (float) is the shadows clipping point
@@ -80,7 +79,7 @@ def _get_stretch_parameters(self, data):
         }
 
 
-    def stretch(self, data):
+    def stretch(self, data: np.ndarray) -> np.ndarray:
         """ Stretch the image.
 
         Args:
@@ -111,5 +110,5 @@ def stretch(self, data):
         return d
 
 # Wrapper function for simpler interface
-def apply_stretch(data, target_bkg=0.25, shadows_clip=-1.25):
+def apply_stretch(data: np.ndarray, target_bkg: float=0.25, shadows_clip: float=-1.25) -> np.ndarray:
     return Stretch(target_bkg, shadows_clip).stretch(data)
diff --git a/tests/TestFitsGen.py b/tests/TestFitsGen.py
new file mode 100644
index 0000000..d1cad3b
--- /dev/null
+++ b/tests/TestFitsGen.py
@@ -0,0 +1,39 @@
+# Generate a test fits file to ensure autostretch works in production
+
+import numpy as np
+from astropy.io import fits
+
+# Parameters
+ny, nx = 1024, 1024   # image size
+n_stars = 50        # number of stars
+seed = 42
+output_file = "test_image.fits"
+
+# Random seed for reproducibility
+rng = np.random.default_rng(seed)
+
+# Background
+image = np.full((ny, nx), 1000.0, dtype=np.float32)
+
+# Add some stars as Gaussian spots
+y, x = np.mgrid[0:ny, 0:nx]
+for _ in range(n_stars):
+    x0 = rng.uniform(0, nx)
+    y0 = rng.uniform(0, ny)
+    amp = rng.uniform(500, 5000)
+    sigma = rng.uniform(1.0, 2.5)
+    image += amp * np.exp(-((x - x0) ** 2 + (y - y0) ** 2) / (2 * sigma ** 2))
+
+# Add noise
+image += rng.normal(0, 5, size=image.shape)
+
+# Save to FITS
+hdu = fits.PrimaryHDU(image)
+
+hdu.header["NUMSTARS"] = n_stars
+hdu.header["SEED"] = seed
+hdu.header["HISTORY"] = "Created synthetic image with stars for auto_stretch test"
+
+hdu.writeto(output_file, overwrite=True)
+
+print(f"Created {output_file}")
diff --git a/tests/test_image.fits b/tests/test_image.fits
new file mode 100644
index 0000000..1df488f
Binary files /dev/null and b/tests/test_image.fits differ
diff --git a/tests/test_stretch.py b/tests/test_stretch.py
index 067ebba..6e713b4 100644
--- a/tests/test_stretch.py
+++ b/tests/test_stretch.py
@@ -1,5 +1,6 @@
 import pytest
 import numpy as np
+from astropy.io import fits
 from auto_stretch import stretch
 from auto_stretch import apply_stretch
 
@@ -16,4 +17,17 @@ def test_stretch_from_standalone():
     stretched_image = apply_stretch(image) 
     print(f"Image: {image}")
     print(f"Stretched image: {stretched_image}")
-    assert np.shape(stretched_image) == np.shape(image)
\ No newline at end of file
+    assert np.shape(stretched_image) == np.shape(image)
+
+def test_stretch_with_fits_file():
+    file = fits.open("test_image.fits")
+    image = file[0].data
+    s = stretch.Stretch()
+    stretched_image = s.stretch(image)
+    print(f"Image: {image}")
+    print(f"Stretched image: {stretched_image}")
+    assert np.shape(stretched_image) == np.shape(image)
+    fits.writeto("stretched_test_image.fits", stretched_image, header=file[0].header, overwrite=True)
+
+if __name__ == "__main__":
+    pytest.main([__file__])
\ No newline at end of file