Update stripe 82 tutorial

[raddick]

Here is the email thread from the SDSS helpdesk that inspired this issue:

It is perhaps interesting to the helpdesk, that I got the idea to use
the STRIPE field from this tutorial:
http://www.sdss.org/dr12/tutorials/get_stripe82_images/

While the tutorial shows how to obtain images for a certain stripe the
outcome might not be what people expect if queries where STRIPE=82
returns objects outside -50<RA<59 or -1.25<DEC<1.25.

---

Stripe 82 is somewhat special in the history of the SDSS,
since it is an equatorial great circle stripe (along with its
counterpart stripe 10 above the Galactic plane), the SDSS 2.5m
telescope doesn't need to track to observe this stripe, it
can just drift scan with no forcing of the telescope errors.

For this reason, stripe 82 was observed beyond its standard
stripe limits, and those data were added to the database.

As you note, the 'official' limits of stripe 82 only cover about
-50 < RA < 60, much more of the stripe beyond RA > 60 has been
observed.

Likewise when the spectra were being obtained for SDSS
and its followup surveys, the photometry on stripe 82 was
a convenient place to design spectroscopic plates, including
special program plates not part of the regular SDSS survey.
Some of these special program plates are described in the
documentation on the SDSS web pages.

They were included for completeness in the database since
the imaging falls along the equator, with -1.25 < dec < 1.25,
even though the RA is beyond the nominal stripe limit.
The select algorithms for targets on these plates is often
different that that used in the regular SDSS legacy survey.

http://www.sdss.org/dr13/algorithms/legacy_special_target/

http://www.sdss.org/dr13/algorithms/legacy_target_selection/

You may exclude them from your query by using an RA cut,
for instance, where ... and (ra < 60 or ra > 270).

---

I am working on objects in SDSS stripe 82. I use the sql tool to identify these objects by the STRIPE field in the RUN table. A typical sql query might be:

-- BEGIN SQL QUERY
SELECT
sp.objid,
sp.specObjID,
sp.ra,
sp.dec

FROM
SpecPhoto as sp JOIN
RUN as r on (sp.run = r.run and sp.rerun = r.rerun)
WHERE
stripe = 82
-- END SQL QUERY

That indeed returns stripe 82 objects. However, when I verified their positions I realized that a lot of objects are not within stripe 82 boundaries. In particular their right ascension is not restricted to -50°<RA<60° but there are also about 6500 objects returned which have 70°<RA<103°.

Can you give me a hint why that is?

Thank you in advance