Fixed Target Confirmation Chart FAQ

The following are some standard questions that observers have about their fixed target confirmation charts. If you do not find the answer to your question here, please contact your Program Coordinator.

• What should I be checking when I review my confirmation charts?
• Why doesn't my target appear to be centered on the cross hairs?
• Why are the coordinates listed at the top of the chart not the ones I supplied?
• How well centered does my target have to be?
• How close can a brighter neighbor be?
• How is proper motion handled?

• What should I be checking when I review my confirmation charts?


• Correct field

Just because there is a target centered on the cross hairs doesn't mean that it is the target you intended. If you correctly entered the coordinates of another target the only way you will know is if you check the rest of the field. If your target has no visible component you can still check that the other stars in the field are correct.

• Target well centered

If your target is not perfectly centered in the cross hairs, see the questions below for why it may not be centered and how well centered it must be.

• No brighter close neighbors

If there are any brighter close neighbors, see the question below for more information.

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• Why doesn't my target appear to be centered on the cross hairs?

If you have checked your coordinates and you believe them to be accurate, here is one reason that good coordinates might not look good on the confirmation chart.

• Is your target very bright?

Very bright targets can look miscentered on the guide star survey plates. (This is why we suggest that you use Hipparcos Input Catalog coordinates for bright targets.) Here is an example chart for a very bright (V=4.83) target. Note that the diffraction spikes are not even centered on the burned out blob. It will be clearer what happened when you look at this other example chart (target has V=8.3). Note that there is an off centered halo around the star. This is due to a reflection which is always somewhat off center except in the very center of the original plate. So, in the first example, the image is so burned out that the halo is burned out also. The diffraction spikes are the only clue as to where the original target lies.

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• Why are the coordinates listed at the top of the chart not the ones I supplied?

All confirmation charts are made with coordinates that are Equinox of 2000 and Epoch of 2000. If you supplied coordinates with an Equinox that was not 2000, the coordinates will have been precessed to the reference system of 2000. If you supplied coordinates with an Epoch that was not 2000 AND you supplied a proper motion for the target, the coordinates will have been moved to the position that is predicted by that proper motion for the year 2000.

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• How well centered does my target have to be?

This depends on the instrument you are using. Note that on the smaller of the two confirmation charts, the cross hairs are 15" apart and each pixel is 1.7". So just falling within the crosshairs is generally not good enough.

• STIS

In a STIS ACQuisition exposure (either point or diffuse), a 5"x5" subarray will be searched for the brightest target. This means that you have an error budget of 2.5" to be assured that your target will fall in the subarray, but even targets measured on the guide star survey plates can be off by an arcsecond. So, now you are down to about the size of a pixel. So, targets to be acquired by STIS should appear well centered in the cross hairs on the confirmation chart.

If your target is not well centered due to visual magnitude, you will need to be extra careful double checking your measurement or catalog entry.

• FGS

Please consult your contact scientist.

• ACS & WFC3

This is not an issue for ACS or for WFC3 as there is not an on-board target acquisition.

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• How close can a brighter neighbor be?

This depends on the instrument you are using.

• STIS

In a STIS ACQuisition exposure (either point or diffuse), a 5"x5" subarray will be searched for the brightest target. Adding the diagonal of the subarray to the possible error from the coordinate measurement indicates that you should not see an object of equal or brighter magnitude within about 5" (3 pixels on the confirmation chart). If there is such an object, you should discuss with your contact scientist alternate target acquisition scenarios.

• FGS

Please consult your contact scientist.

• ACS & WFC3

This is not an issue for ACS or for WFC3 as there is not an on-board target acquisition.

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• How is proper motion handled?

For a target with proper motion (that is a target with at least one of RA PM or Dec PM specified), the user-specified RA and Dec is still listed just below the title, but is keyed with an orange circle. That position is followed immediately by the user-specified Epoch. To the right, the computed position of the target at the image's epoch is shown, keyed with a magenta plus sign, followed immediately by the Image Epoch. Below both positions, and centered, are the user-specified proper motion parameters: RA PM and Dec PM. In the two pictures, the magenta cross hairs are now centered on the computed position and an orange circle is drawn around the user-specified position. An orange line connects the two positions.

See this example of a confirmation chart with proper motion.

Please ensure that the magenta cross hairs point to your object of interest - if not double check your coordinates, epoch, and proper motion. Incorrect target specification can cause a target acquisition to fail.