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VPHOT Chart differs from VSP Chart

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msheald
VPHOT Chart differs from VSP Chart

Hello! I tried to use VPHOT this morning to analyze images of RU HER. When I clicked on the chart, the one that came up differed from what I downloaded from VSP.

    The VSP coordinates for RU HER are 16:10:14:52 an +25:04:14.4 while the chart that came up from VPHOT were 16:10:27.98 and 25:07:55.4.

    Additionally, a couple of comps were not loaded when I loaded the AAVSO sequence - 144 and 126.

    Thank you and best regards.

Mike

MZK
MZK's picture
VPhot Chart

Mike:

I suspect the difference is due to the fact that on VSP you used the target name to get the chart but VPhot looked for the RA/Dec of your image to get its chart/comps. I suspect the center of your image is just a little different in RA/Dec.  That is my guess without seeing one of your images (share to MZK)? 

Were the two missing comps at the edge of FOV? Or were they fainter (144 anyway) and excluded by your SNR setting?

Those are my initial guesses?

Ken

msheald
Coordinates of RU HER Differ between VPHOT and VSP

Hello! I'm sorry, I mispoke.

    The coordinates for RU HER differ between that charts generated by VSP and VPHOT.

    The chart for VPHOT lists RU HER as 16:10:27.98 and 25:07:55. VSP lists the coordinates of RU HER as 16:10:14:52 an +25:04:14.4. There is a star in each position for each chart; however, VPHOT and VSP list diferent coordinates for RU HER, so are identifying different stars.

    VSX lists the coordinates consistent with VSP.

    I suspect that 144 and 126 are not listed by VPHOT because they would be off image if the VPHOT RU HER coordinates are used. However, they are on the image if the VSP/VSX coordinates of RU HER are used.

    I can manually supply the coordinates for RU HER and the comps based on VSX/VSP, but I thought I would ask about the different chart and coordinates generated by VPHOT for RU HER. Best regards.

Mike

msheald
VPHOT Chart

Hello! I forogt to specify - the VPHOT chart I access comes up when I click on an image that has been plate solved and then click on Catalogs:Show AAVSO Chart. Best regards.

Mike

MZK
MZK's picture
VPhot Chart

Hi Mike:

Did you not agree with my response in Post #2 above?  We can talk off-line if desired?

Ken (MZK)

msheald
Hello! Thank you for your

Hello! Thank you for your note. I'll share the uploaded image of RU HER, and I would appreciate your guidance.

    I was concerned that the listed coordinates for RU HER seemed to be different between the chart that VPHOT brought up and the chart that VSP brought up. Perhaps this is the image center rather than the coordinates of RU HER? Best regards.

Mike

 

MZK
MZK's picture
VPhot Chart Center

Mike:

As you have hypothesized, the RA/Dec reported on the VPhot Chart corresponds to the center of your image. I assume you have moved the VPhot cursor on your image and determined that. When VPhot queries the target and comp databases, it uses the center of your image as the reference point. Since there is no guarantee that a VSX target or AAVSO comps may occur at the center of any given image, this is appropriate. The database query looks for targets/comps near this central point and within the FOV.

However. the confusion that results (in this case) is that on the VPhot Chart, the center of the image is shown with a cross. Additionally, the name of a VSX target (in this case – RU Her) that also exists on this image (but not at the center) is reported in the title of the chart along with the central RA/Dec. Thus, when you compare the VPhot Chart with the VSP Chart, the crosses do not exist at the same point! This does lead to confusion. I’ll put this on the “to fix” list since it is a bit ambiguous.

However, note that VPhot does place VSX targets (RU Her and another), AAVSO comps, and apertures in the correct place on your image, based on their RA/Dec. The remaining issue is for the observer to look at the VPhot Chart and understand this discrepancy and handle it appropriately. The easiest alternative is to accept that VPhot is putting the apertures in the correct place and continue with photometry. Your questioning of the chart ambiguity is good but fortunately the results are correct.

The 144 comp was not identified since it is too faint. I do get 126 on the image you shared with me. If you improve your scope pointing, this will be less of an issue.

HTH, Ken

msheald
Thanks! I appreciate you

Thanks! I appreciate you clearing up my misunderstanding. Best regards.

Mike

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