[New content] Companion tutorial on Chapter 1 of Carroll and Ostlie

[kakirastern]

To work on companion tutorial for BOB Chapter 1 - Celestial Sphere

Contents include:

• WCS
• plot dist in different projections. alt/az, ra/dec, glat/glon
• proper motion, ephemeris
• observation planning tool
• some problem demonstrating cos dec issue

Whether there will be a narrative is to be determined. The initial workflow will be on Google Colab, and a link will be posted in this conversation later.

[kakirastern]

As promised the working draft of the tutorial on Google Colab can be acessed at https://colab.research.google.com/drive/18FcSOaAq5wu5PfAHAPtIgPUvscJ4EpkJ

[kakirastern]

Working checklist can be viewed at this Google doc.

[eblur]

This is a good start! I added my touch to the introduction and the learning goals. Let's keep on going.

[kakirastern]

@kelle mentioned in late 2019 that there are existing notebooks dealing with the observation planning tool item such as with astroplan, so can leave that out for now in the proposed tutorial.

[eblur]

Okay! I have polished some of the text and section headings. Major comments that I could not address myself are typed up in bold.

@stargaser I think it would be helpful to provide a 1-2 sentence description of what a World Coordinate System is in general, with some links to documentation or resources. I have marked that location in bold for you.

@kelle please check this out at your nearest convenience

[kelle]

This is looking super good!

• I made some changes directly, adding more text and explanation for folks that are brand new to astro, and followed Lia's lead and made some comments in bold.

• I think this is actually shaping up to be multiple tutorials. It could be a series. Eg., WCS 1: Reading and plotting, WCS 2: Finding separations, WCS 3: Calculating proper motions etc. What do y'all think? If we agree, Kris can make new colab notebooks for Sections 3 and 4 but we can keep discussing in this issue.

[kakirastern]

Agree with @kelle that this notebook could be shaped up into multiple tutorials.

[stargaser]

I've rearranged the introduction of WCS and added some references, and I'm working through the examples. I've added some notes in bold to the tutorial. Here are a few points:

• The first example of HPLT-TAN is new to me -- I'd prefer a simpler example, perhaps repeating the 2-D image example.
• The image of the Helix Nebula has a complicated header, apparently being an image made from a photographic plate, and includes some header keyword combinations that violate the FITS WCS standard. Many of those trigger warnings when the WCS object is created. I would prefer that we either simplify the header or choose a different image. At a minimum, the warnings need explanation. I too have a soft spot for the Helix Nebula -- perhaps a nice Spitzer image that was used for a press release? (though that would go against AltAx in next point). Or an image already available with get_pkg_data_filename (see below).
• I found the checklist for this tutorial which specifies showing ra/dec, glon/glat, and alt/az. I recommend that Section 3 show first celestial and galactic coordinates. AltAz can be shown but I am certain that atmospheric refraction is included and that leads to a 0.8% discrepancy in the sky separation, which I found alarming and that I expect astronomers will find unsettling without explanation. I urge to show the separation in celestial and Galactic coordinates and demonstrate that they are the same, and then explain why AltAz is different.

I agree with splitting Section 3 and Section 4 into separate tutorials. Then we could use an image without header problems, for example one available using astropy's get_pkg_data_filename, for this part, and the UK Schmidt image for the Section 3 tutorial with AltAz.

[kakirastern]

Let me see what I can do with the Helix nebula image regarding its current FITS header... One thing I could do is to simplify it manually to conform to the current FITS WCS standard.

[eblur]

Hi folks, should we add Issue #443 content to the first notebook in this series?

[kakirastern]

@eblur Yeah, I agree it would be a good idea!

[eblur]

Hi @kakirastern and @stargaser, this looks great. Just a few big comments that I can't do from my end:

(1) Learning goals should be a bulleted list. Just make them a one-line summary of each section.

(2) Section 3: How did you know to use 'fk5' as input to the get_transform method? Please describe for the readers.

(3) Section 3: Most people don't draw scale bars as arrows. Would the following code snippet work to eliminate the arrow heads?

ax.arrow(337, -21.2, 0, 0.1, head_width=0, head_length=0, fc='white', ec='white', width=0.003, transform=ax.get_transform('fk5'))

Also please make a vertical scale bar (shown in code snippet), so we don't have to deal with the confusion between arcmin in degrees and minutes of hour angles. We can reserve the hour angle conversion to the Exercise (which I have already done in the text).

Also please describe the main inputs to the arrow method, so that people understand why there is a 0.

[kakirastern]

@eblur Will follow up on your comments soon once the Learn Astropy co-work hour resumes for the summer