Failing rectangle astrometry

[Jerry Zhang]

 Hello,

I am trying to precisely solve a rectangle optical field (2.8' x 1.4') but always failed, although I have tried almost all the parameters and constrains, for example 

solve-field xxx.fits --ra 277.126 --dec 26.84 --radius 0.5 --overwrite --scale-high 0.1

This is the field:

And this is the correspondent PanSTARRS field, you can see the field is not crowed and positions of the stars are quite clear.

I wonder if it is due to the shape of the field?

Thanks all,

Jerry

[Dustin Lang]

Hi,

That's quite a small field.  By default, solve-field's --scale-low is 0.1.  So you would have to set "--scale-low 0.03" for it to have any chance of working.

cheers,

dustin

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[Jerry Zhang]

Hi Dustin, 

Thanks for the suggestions.

Finally it failed to manage to solve, no matter how I set --scale-low .  Is that just because the field is too small to have enough reference stars in the catalog? Which catalog is astrometry using? 

Jerry

[Bryan]

Jerry

I recall the default indices for a local install are 2_MASS.

Astrometry.net code README — Astrometry.net master documentation

Bryan

[Dustin Lang]

For a field as tiny as that, you'll want the "5200-series (light)" linked here,

http://data.astrometry.net/

specifically the index-5200-31.fits file for your field's RA,Dec.

cheers,

dustin

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[Jerry Zhang]

Hi Dustin,

I used the Gaia catalog index 5200 series but still failed.... I think in the field there are at least 10 Gaia sources according to Aladin.

Could the reason be the rectangular shape of the field?

Best,

Jerry

[Dustin Lang]

I think you need to set the --radius if you set --ra, --dec.  A fairly large radius (1-5 degrees) should work fine.

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[Mark Copper]

I have difficulty with small fields as well. Generally, I had been trying 2.6' x 2.6' ROI to keep sizes down, but even with the entire camera field at 7.3' x 4.8' I've had trouble. And that's using the 5200-series indices. Especially challenging are sparse fields and fields with a bright star. And this even when the coordinates of the field are known within a few minutes.

So it got me to wondering whether it would help to modify the "odds-to-XXX" default parameters, whether XXX was tune-up or solve or reject or stop looking. For example, the odds-to-solve parameter defaults to 10^9 (to 1?). Would it be practical to ask for lower odds? If I *think* I have a target star lined up, and I *know* I'm in the neighborhood, and yet the field doesn't solve, would I get any useful information by setting odds-to-tune to 10^4 and odds-to-solve to 10^6? Practically speaking, that is.

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[Dustin Lang]

Hi,

Generally, I would expect the larger 7.3x4.8 fields to be more successful -- 2.6x2.6 is getting down to where the Gaia catalog starts to run out of enough stars.  And yes, usually you need 15-20 stars to get a solution.  Reducing the --odds-to-* could potentially help with that part, and if you're using --ra, --dec, --radius (and --scale-low, --scale-high) to cut down the search space, the chance of getting a false positive match shouldn't be too bad.  I would be interested to hear if it works if you try -- I haven't messed with that much :)

If you want to post a failing image that you think should work, please go ahead and I may be able to comment further.

cheers,

dustin

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