Pinpoint with UCAC4 can't minimize RA when Center on Target


#1

Usually Pinpoint with UCAC4 gets me within a few pixels of my Reference Data FIT image within one or two tries. Lately it’s converging to with 20 pixels on Dec but over 100 pixels on RA with 5+ tries. The only change I think I made since I captured my Reference Data was to rotate the QSI 683 10 or 20 degrees. Could that be related? Not sure what else would cause this as it’s been working perfectly for many objects.


#2

You should never use UCAC4 for solving. More info on this can be found on the PP forums.


#3

See following. Resolves all the time at 1400 mm F/L. Just doesn’t always converge to 50 px. What’s better for this F/L? Maybe a mount resolution issue? Using G11.


#5

USNO UCAC4 (PinPoint 6 or later only)

This is a large catalog just over 8Gb), but it is “the one to use” if you are doing precision astrometry with PinPoint 6 or later. You can download it in bits and pieces at night if needed. Get it from CDS at ftp://cdsarc.u-strasbg.fr/pub/cats/I/322A/UCAC4. When you go to this page, you will see subfolders u4b and u4i. Create a folder on your drive called UCAC4, then create subfolders u4b and u4i under it. Now copy the contents of these folders from the CDS site into the corresponding folders on your hard drive. Run the Catalog Checker to validate your installation.

Additionally, the US Naval Observatory will send anyone a free 9GB (double sided) DVD of the UCAC4 database. To obtain a copy, send an email request to brenda.hicks@navy.mil with UCAC4 in the subject line. Be sure to include your US Postal mailing address. When you get the DVD, create a folder somewhere, say My Documents\Catalogs\UCAC4. Then create two sub-folders “u4b” and “u4i” under your new folder. Copy all of the stuff from the u4i folder on side 1 of the dvd to your u4i folder. Then copy the stuff from the u4b folders on both side 1 and side 2 into your u4b folder. Finally run the Catalog Checker to validate your installation.


#6

A good general rule of thumb is:

  • USNO when your images are smaller than 15 arc-minutes
  • GSC when they are larger

#7

Thanks Ken. At 1400 mm I’m in the USNO group. Never had a failed solve with UCAC4. Looks like the inability to get within 50 pixels is because I rotated the camera 10-20 degrees since I took the reference.


#8

I’m not sure why angle would have anything to do moving between 2 points in space. It would certainly affect rotation, but not the centering aspect.


#9

There is also a post by Bob Denny (PinPoint author) that suggests NOT to use UCAC4 for pointing.

"We recommend against USNO UCAC4 for plate solving for telescope pointing because it lacks stars fainter than mag-16, for only about one hundred million stars, where the smaller USNO A2.0 has five times as many stars and includes many more faint (down to mag-18) stars. This can make plate solving with A2.0 more reliable. The small errors in a few individual stars (proper motions) in A2.0 are irrelevant for telescope pointing. "

He goes on to say
"USNO A2.0
This is a large catalog (6Gb), and while old, this is still the “one to use” for common plate-solved telescope pointing applications because it goes deeper than the others, and includes bright stars. The fact that some nearby stars’ proper motions are out of date has an insignificant effect on the centering accuracy of the solution. You can download it in bits and pieces at night if needed. Get it from ftp://ftp.nofs.navy.mil/usnoa/. Copy the zonexxxx.acc and zonexxxx.cat files in this directory to a directory on your system. Set your catalog type to USNO A2.0. Set your catalog path to the directory into which you copied the filies from USNO.

Alternatively (and more painfully), you can get this catalog from http://usnoa2.dc3.com/. You’ll have to right-click and save link as… or save target as… for each file, then after that you have to unzip all of the .CAT files. Time consuming. I don’t have FTP capability at the high-speed service on which this resides, and I don’t want to clog up the local office net service as we use it for our phones etc. It’s a one-time project…"


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