<div dir="ltr"><div><div>No. There is no telescope that looks at the sky from the outside; it's CCW<br></div><div>regardless of handedness.<br><br></div><div>The following may not be brief, but I think it is clear and complete.<br><br></div><div> - Arnold<br></div><div><br></div>Polygons are defined by traversing their vertices in counter-clockwise direction.<br></div><div>In other words, the inside of the polygon is always on the left side during such<br></div><div>a traversal, regardless of whether the coordinate system is right- or left-handed.<br></div><div>Note that, consequently, expressions for the area of the polygon and whether a<br>particular location is contained within the polygon may depend on the handedness<br>of the coordinate system; celestial coordinate systems are left-handed.<br></div><div>The sides of a polygon are considered part of the inside.<br></div><div>Also, polygon sides in spherical coordinate systems are great circle segments<br></div><div>and segments have to be shorter than 180 d; if longer sides are called for,<br></div><div>an intermediate vertex needs to be added.<br></div><div><div><div><div class="gmail_extra"><br clear="all"><div><div class="gmail_signature" data-smartmail="gmail_signature"><div dir="ltr">-------------------------------------------------------------------------------------------------------------<br>Arnold H. Rots Chandra X-ray Science Center<br>Smithsonian Astrophysical Observatory tel: +1 617 496 7701<br>60 Garden Street, MS 67 fax: +1 617 495 7356<br>Cambridge, MA 02138 <a href="mailto:arots@cfa.harvard.edu" target="_blank">arots@cfa.harvard.edu</a><br>USA <a href="http://hea-www.harvard.edu/~arots/" target="_blank">http://hea-www.harvard.edu/~arots/</a><br>--------------------------------------------------------------------------------------------------------------<br><br></div></div></div>
<br><div class="gmail_quote">On Mon, Sep 12, 2016 at 12:09 PM, Patrick Dowler <span dir="ltr"><<a href="mailto:pdowler.cadc@gmail.com" target="_blank">pdowler.cadc@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">So just to be clear and minimalistic, I think the only correction to<br>
the text in rev 3530 is that the view if from outside looking toward<br>
the origin. Is that correct? I will also add a reference to STC but I<br>
think this change would make the DALI text minimal and consistent.<br>
<br>
Pat<br>
<div class="HOEnZb"><div class="h5"><br>
On 9 September 2016 at 01:02, Marco Molinaro <<a href="mailto:molinaro@oats.inaf.it">molinaro@oats.inaf.it</a>> wrote:<br>
> Hi Pat, Arnold, all,<br>
><br>
> 2016-09-08 18:56 GMT+02:00 Patrick Dowler <<a href="mailto:pdowler.cadc@gmail.com">pdowler.cadc@gmail.com</a>>:<br>
>> So, should I change the wording to say "when viewed from outside the<br>
>> unit sphere" and add STC reference? I want to say the minimum and<br>
>> leave the definitive specification in the hands of STC, but if we make<br>
>> readers go look it up in STC they will be annoyed.<br>
><br>
> I agree on referencing STC (even if it's not trivial because it would be nice<br>
> to have a pointer to 2.0 or general STC), but summarizing it in DALI.<br>
> I'm ok with the wording, is the group agrees (warning: if no one protest<br>
> means agreement).<br>
><br>
> Maybe, Arnold, can you suggest something<br>
> short and clear to put in DALI?<br>
> The reference text you replied is probably bullet-proof but too long for<br>
> the DALI spec (at least that's my opinion).<br>
><br>
> Cheers,<br>
> Marco<br>
><br>
>><br>
>> Pat<br>
>><br>
>> On 8 September 2016 at 01:00, Felix Stoehr <<a href="mailto:fstoehr@eso.org">fstoehr@eso.org</a>> wrote:<br>
>>> Dear all,<br>
>>><br>
>>> after some side-discussions with Marc I believe the following is true:<br>
>>><br>
>>> - STC-S declares clockwise and counterclockwise with respect to the<br>
>>> celestial coordinate frame (north up, east to the left as seen from earth)<br>
>>><br>
>>> - this is consistent with the footprintfineder.py output and thus all<br>
>>> the MAST/ALMA/ESA spectra<br>
>>><br>
>>> - area calculations will be correct in this definition and coordinate frame<br>
>>><br>
>>> - it is a bit counter-intuitive, because it means that anticklockwise<br>
>>> polygons in that definition are clockwise if you look up on the sky from<br>
>>> earth.<br>
>>><br>
>>> It might be worthwile to declare in WD-DALI-1.1 the coordinate system in<br>
>>> which the clockwise/anticlockwise are defined, i.e. in which system to<br>
>>> "hold the clock".<br>
>>><br>
>>> Best regards,<br>
>>><br>
>>> Felix<br>
>>><br>
>><br>
>><br>
>><br>
>> --<br>
>> Patrick Dowler<br>
>> Canadian Astronomy Data Centre<br>
>> Victoria, BC, Canada<br>
<br>
<br>
<br>
--<br>
Patrick Dowler<br>
Canadian Astronomy Data Centre<br>
Victoria, BC, Canada<br>
</div></div></blockquote></div><br></div></div></div></div></div>