Retry read errors spherex_psf.md

Author: troyraen

tutorials/spherex/spherex_psf.md

@@ -55,8 +55,10 @@ The following packages must be installed to run this notebook.
 ```
 
 ```{code-cell} ipython3
+import http.client
 import re
 import time
+import urllib.error
 
 import astropy.units as u
 import matplotlib.pyplot as plt
@@ -139,12 +141,21 @@ As we do below, you can use `hdul.info()` to print the list of FITS layers of th
 ```
 
 ```{code-cell} ipython3
-with fits.open(spectral_image_url) as hdul:
-    hdul.info()
-    cutout_header = hdul['IMAGE'].header
-    psf_header = hdul['PSF'].header
-    cutout = hdul['IMAGE'].data
-    psfcube = hdul['PSF'].data
+Max number of times to retry transient read errors.
+max_retries = 3
+for attempt in range(max_retries):
+    try:
+        with fits.open(spectral_image_url) as hdul:
+            hdul.info()
+            cutout_header = hdul['IMAGE'].header
+            psf_header = hdul['PSF'].header
+            cutout = hdul['IMAGE'].data
+            psfcube = hdul['PSF'].data
+        break
+    except (urllib.error.HTTPError, http.client.IncompleteRead):
+        if attempt == max_retries - 1:
+            raise
+        time.sleep(10 * (attempt + 1))
 ```
 
 The downloaded SPHEREx image cutout contains 5 FITS layers, which are described in the [SPHEREx Explanatory Supplement](https://irsa.ipac.caltech.edu/data/SPHEREx/docs/SPHEREx_Expsupp_QR.pdf).