Can't get blue camera exposure name for plates with no red camera exposure

[dmargala]

Plates 7334 & 7336 do not have r2 exposures (see [boss-pipeline 4152] for more details). The b2 exposures do exist so I think it should be possible to access them.

import bossdata
finder = bossdata.path.Finder()
mirror = bossdata.remote.Manager()
spec_name = finder.get_spec_path(7334, 56656, fiber=501, lite=True)
spec_file = bossdata.spec.SpecFile(mirror.get(spec_name))
frame_name = finder.get_plate_path(7334, spec_file.get_exposure_name(0, 'blue', 'spFrame'))

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-6-e5d206bae0a3> in <module>()
----> 1 frame_name = finder.get_plate_path(7334, spec_file.get_exposure_name(0, 'blue', 'spFrame'))

/Users/Daniel/source/bossdata/bossdata/spec.pyc in get_exposure_name(self, sequence_number, band, ftype)
    377             raise ValueError('Invalid band "{}". Expected blue or red.'.format(band))
    378         camera = band[0] + self.spec_id
--> 379         return self.exposures.get_exposure_name(sequence_number, camera, ftype)
    380 
    381     def get_raw_image(self, sequence_number, band, flavor='science',

/Users/Daniel/source/bossdata/bossdata/spec.pyc in get_exposure_name(self, exposure_index, camera, ftype)
    222         if exposure_index < 0 or exposure_index >= self.num_by_camera[camera]:
    223             raise ValueError('Invalid exposure_index {}, expected 0-{}.'.format(
--> 224                 exposure_index, self.num_by_camera[camera] - 1))
    225         if ftype not in ('spCFrame', 'spFrame', 'spFluxcalib', 'spFluxcorr'):
    226             raise ValueError('Invalid file type ({}) must be one of: '.format(ftype) +

ValueError: Invalid exposure_index 0, expected 0--1.

And using bossplot:

$ bossplot --plate 7334 --fiber 501 --verbose --band blue --exposure 0
Plotting exposure[0] spectrum for 7334-56656-0501.
Exposure summary:
offset camera science flat arc
------ ------ ------- ---- ---
The value --exposure 0 is outside the allowed range 0--1 for this target.