Astrometry vs SAO Image DS 9

For those who really understand astronomy and astrometry…

Wow this is DEEP stuff.

The question is: what “position format” to use (1 or 2)? I put photos into and get RA and Dec, I have taken the output FITS files and put them into SAO Image DS 9 to get RA and Dec. Could they just be reasonable and agree? NO! I am using the “FK5” numbers.

In - if you use the World Wide Telescope you apparently get numbers in HH MM SSs but the values for Dec do not show units.

In SAO Image DS 9 there are no units shown for RA or Dec.

Now we should get a lot of hints from the displayed digits.

On two of my recent images, I see:

Begin (observation 1):
Astrometry: RA 06 19 53 Dec +60 32 29
SAO Image: RA 06 19 20.9036 Dec +60 28 20.283

End (observation 2):
Astrometry: RA 05 48 50 Dec +56 35 09
SAO Image: RA 05 50 05.7637 Dec +56 45 14.211

I do try to keep very careful records since I have mixed up the begin and end, I have mixed up numbers (there are a bunch of them), etc. I really need to automate this to eliminate errors!

So, for anyone using a digital camera or who understands how those two sources work, what values of RA and Dec should I use, and what position format should they go into?

I think that I should use position format 1 and get the RA and Dec from Astrometry, the World Wide Telescope option.

I have read up on the Fifth Fundamental Catalog and what FK4 and FK5 are - have looked for help and FAQs on the two sources. I have not found the answers I need.

Can anyone help?


Can you provide the link to the image that solved? It should be accessible there.

In the end it will depend on the epoch of the star catalog used to calibrate the images, but also if the software that you use corrects for precession and nutation.

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Does this link work?

image 22_feb_4 (there are several images there). Submission 3308923 if that helps.

CharlesPhillipsHouston, 17607 (right now?).


Yes, the link works.

I am 99.9% certain that the software uses the J2000 epoch (which is essentially FK5) for its astrometric catalog. This means that when reporting IOD observations you should use epoch code 5 (

The software adds a world coordinate system (WCS) into the new-image.fits file. Most software will correctly use the WCS to transform pixel coordinates to celestial coordinates. However, also adds polynomials to model distortion in the image frame. It may well be that some software does not properly deal with this, which could lead to position offsets when comparing measurements from different programs.

The DS9 software does properly deal with the distortions, so I would advise that you use that.

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As it is with so many things, when you dig into something it is more complex than you had first thought it was.

I will send in some observations based on the numbers from DS9 and will use the epoch code 5 - and see how the residuals look.


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