limiting magnitudes for the 5200 series

[Mark Copper]

Hello Dustin,

Do you have a sense what the limiting magnitudes of stars in the 5200 series used by solve-field are?

I'm attaching a fits file that did not solve -- no surprise for multiple reasons -- and a Gimp view of it showing possible stars.

The bright star in the lower left is Arcturus. The star to the right (px 1087, 213) is a magnitude 13.2 star, and the partial image directly above Arcturus is magnitude 14.8. The 13.2 mag star is about 7 arcminutes from Arcturus. (I'm getting this from manual calculations with Gaia data so I could easily be way wrong.)

In the Gimp image there are 4 more stars that pop out, but they must be quite dim. However, there are many more places encircled in the arcturus_v_Light_005-objs.png plot (simplexy finds 145). But even this command line with odds-to-solve lowered to 100 does not solve:

  $ solve-field -O -d 7 -3 14:15:39 -4 19:10:56 -5 1 -z 2 --odds-to-solve 100 arcturus_v_Light_005.fits

So I'm thinking probably the 4 really dim stars aren't in the 5200-series database.

Do you have any thoughts?

Thanks for reading.

Mark Copper

[Dustin Lang]

Hi Mark,

Typically an image needs 10-15 detected stars for Astrometry.net to be able to recognize it.  Looking at this image, I'm pretty hard-pressed to identify that many by eye, so I wouldn't necessarily expect it to solve.

The 5200-series uses stars from Tycho-2 and Gaia-DR2.  It doesn't include *all* stars from those catalogs -- it selects a bright subset to uniformly cover the sky at the scale of the image in question.  So the 5200 scale will contain fainter stars than 5201, which has fainter stars than 5202, etc.  The lowest scale, in some places, uses all the stars available in Gaia-DR2, which goes down to about G=20.

Hope that helps,

dustin