diff --git a/jwst_mast_query/jwst_download.py b/jwst_mast_query/jwst_download.py
index 01bd552..c29759b 100755
--- a/jwst_mast_query/jwst_download.py
+++ b/jwst_mast_query/jwst_download.py
@@ -16,6 +16,7 @@
 
 from jwst_mast_query.jwst_query import query_mast
 from jwst_mast_query.pdastro import makepath4file,AnotB
+from jwst_mast_query.utils.constants import PARAM_DEFAULTS
 #from pdastro import makepath4file,AnotB
 
 
@@ -62,10 +63,12 @@ def define_options(self,**args):
         #    outrootdir = '.'
         #print('Default output rootdir: %s' % outrootdir)
 
-        parser.add_argument('-n','--skipdownload', action='store_true', default=False, help='skip downloading the data.  (default=%(default)s)')
+        parser.add_argument('-n','--skipdownload', action='store_true', default=PARAM_DEFAULTS['skipdownload'],
+                            help='skip downloading the data.  (default=%(default)s)')
 #        parser.add_argument('-o','--outrootdir', default=outrootdir, help=('output root directory. (default=%(default)s)'))
 #        parser.add_argument('--outsubdir', default=None, help=('subdir added to the output root directory. If None, then propID is used (default=%(default)s)'))
-        parser.add_argument('--clobber', action='store_true', default=None, help='existing files are overwritten, otherwise they are skipped (default=%(default)s)')
+        parser.add_argument('--clobber', action='store_true', default=PARAM_DEFAULTS['clobber'],
+                            help='existing files are overwritten, otherwise they are skipped (default=%(default)s)')
 
         return(parser)
 
diff --git a/jwst_mast_query/jwst_query.cfg b/jwst_mast_query/jwst_query.cfg
index 2b44b74..8d23ff0 100644
--- a/jwst_mast_query/jwst_query.cfg
+++ b/jwst_mast_query/jwst_query.cfg
@@ -87,6 +87,13 @@ date_select:
 # If left empty, the tables will not be saved.
 savetables:
 
+# If clobber is True, all files will be downloaded. If false, only files that do not already exist in the
+# local directory will be downloaded.
+clobber: False
+
+# If True, the data will not be downloaded. Only the summary tables will be produced
+skipdownload: False
+
 # columns returned from MAST to the obsTable
 # The default set in the code is mastcolumns_obsTable=['proposal_id','dataURL','obsid','obs_id','t_min','t_exptime']
 mastcolumns_obsTable: ['proposal_id','dataURL','obsid','obs_id','t_min','t_exptime','instrument_name']
diff --git a/jwst_mast_query/jwst_query.py b/jwst_mast_query/jwst_query.py
index a9202ef..da4d516 100755
--- a/jwst_mast_query/jwst_query.py
+++ b/jwst_mast_query/jwst_query.py
@@ -336,8 +336,6 @@ def define_options(self, parser=None, usage=None, conflict_handler='resolve'):
         parser.add_argument('--webpage_thumbnails_height', default=PARAM_DEFAULTS['webpage_thumbnails_height'], help=('Height in pixels of the resized jpg images '
                                                                                                                       'to be inserted into the index.html summary '
                                                                                                                       'file. (default=%(default)s)'))
-        parser.add_argument('--skipdownload', action='store_true', default=PARAM_DEFAULTS['skipdownload'], help='If set, no files will be downloaded. (default=%(default)s)')
-
         return(parser)
 
     def get_arguments(self, args, configfile=None):
@@ -431,7 +429,7 @@ def subenvvarplaceholder(paramsdict):
             # skip config file
             if arg=='configfile': continue
 
-            if argsdict[arg] is not None and argsdict[arg] != []:
+            if argsdict[arg] != PARAM_DEFAULTS[arg]:
                 if args.verbose>2:
                     print('optional args: setting %s to %s' % (arg,argsdict[arg]))
                 self.params[arg]=argsdict[arg]
diff --git a/jwst_mast_query/scripts/jwst_download.py b/jwst_mast_query/scripts/jwst_download.py
index ef87668..0288c28 100644
--- a/jwst_mast_query/scripts/jwst_download.py
+++ b/jwst_mast_query/scripts/jwst_download.py
@@ -30,7 +30,7 @@ def main():
     if download.mk_all_tables(showtables=True):
         sys.exit(0)
 
-    if not args.skipdownload and len(download.ix_selected_products)>0:
+    if not download.params['skipdownload'] and len(download.ix_selected_products)>0:
         download.download_products()
         print('\n######################\n### Downloaded Selected Products:\n######################')
         download.productTable.write(indices=download.ix_selected_products,columns=download.params['outcolumns_productTable'])
@@ -40,7 +40,7 @@ def main():
             sys.exit(0)
 
     # Make the webpages
-    if args.makewebpages:
+    if download.params['makewebpages']:
         download.mk_webpages4propIDs()
 
 
diff --git a/jwst_mast_query/utils/constants.py b/jwst_mast_query/utils/constants.py
index c994978..503e9d5 100644
--- a/jwst_mast_query/utils/constants.py
+++ b/jwst_mast_query/utils/constants.py
@@ -30,7 +30,7 @@
                   'sortcols_summaryTable': ['date_start','proposal_id','obsnum'],
                   'configfile': cfgfilename,
                   'token': defaulttoken,
-                  'instrument': 'nircam',
+                  'instrument': None,
                   'obsmode': None,
                   'propID': None,
                   'obsnums': None,
@@ -38,9 +38,9 @@
                   'obsid_list': [],
                   'sca': None,
                   'verbose': 0,
-                  'filetypes': ['uncal'],
+                  'filetypes': ['fits'],
                   'guidestars': False,
-                  'lookbacktime': 1,
+                  'lookbacktime': 1.,
                   'calib_levels': None,
                   'Nobs_per_batch': 2,
                   'obsnum2outsubdir': None,
@@ -50,6 +50,7 @@
                   'makewebpages': False,
                   'outrootdir': None,
                   'outsubdir': None,
+                  'clobber': False,
                   'skip_propID2outsubdir': False,
                   'skip_check_if_outfile_exists': False,
                   'skip_check_filesize': False,