Hi!
Following my recent post on challenges with target star identification in time series, I have similar problems with single images, on and off. Here is an example with N Vul 2021. I have a saved sequence, where VPhot easily identifies the comparision stars but not the target - it shows a red circle encompassing the target... Search radius set to 15 pixels.
Further: on loading variable star catalogues rather than my sequence, there is no N Vul 2021 at all, not in GCVs and not in VSI. And no star designated TCP XXX (the previous designation of the nova) at this position at all.
See attached screenshots for details.
Lesvephotometry has no problems identifying and measuring the target with VSX data.
Any ideas on what is going on here? My images here were plate solved with ASTAP.
Magnus
Magnus,
Ken can probably tell you why this is happening. Adjusting the target centroid position with the Pixel Tool usually works when this happens to me:.
Zoom in close on the target. From the menu above the image display click on Pixel/ADU mode, then click on the target star in the red circle. From the box at the upper left click Go. Adjust the centroid position, then click apply and OK. From the menu bar click Back to Regular Image. Again from the menu bar click Regular mode. If the correct target name doesn't appear, click on the star and name it. It's in the VSX by the name N Vul 2021 . You must then Save the sequence.
Phil
Hi!
Thanks for your suggestion!
However, I can't move the aperture, since it is not there. If I go to Pixel/ADU mode and click ON the target star (so the cursor changes from a + to a poining finger - then I am sent to Sinbad database (a new window opens), and is informed that there is no such identifyer as NOVA Vul 2021.
If I click just beside, when cursor is a +, then I am sent to the close-up where I normally can adjust my apertures. However, there is no aperture to adjust - as there is if there is a green one placed on the image. So doesn't work.
Interestingly, this happens in my V-band images from yesterday - worked nicely a few days ago. And the B-band from yesterday works nicely, the target is directly identified from my sequence. Same plate solving process, only difference is another filter.
I've shared a V-band image with Ken.
Magnus
"I've shared a V-band image with Ken."
Good. I would certainly defer to Ken on this. That said, I would also like a crack at this just in case it also happens to me some day. Could you please also share it with SPP.
Phil
Hi!
I've shared a V and a B image with you. So, in this one, the target (N Vul 2021) is NOT identified when I load Variable Star INdex, nor when I load GCVS. However, a lot of other targets are identified by VSI, and there is a lot of comparison stars identified when loading comparison stars. In my B-band images, the target is identified, and according to the coordinates given by hovering over it, it is at the exact same spot as the (non-identified) target in the V-band image.
Do you get the same when trying?
Magnus
Yes. This is a puzzle. I'll work more on it later this morning
Phil
I found a couple of odd things:
In your B image the VPhot reported position of the target is very close to the position given in VSX. Rounding to the closest second, it matches the VSX position in RA and differs by 2 seconds in Dec.
In your V image the VPhot reported position for the target differs from the VSX position by 1 second in RA and 15 seconds in Dec. Perhaps there is something wrong with the plate solution in the V image or there is some distortion in the V filter in this part of the image.
We should expect that enlarging the search radius would be able to correct for this. When I go to Tools>Settings>Search Radius, reset the radius to 15 pixels (more than 40 arcsec), then Save, nothing changes. The little red circle, which I assume represents the search radius, does not enlarge. Do you see any effect in the search aperture after resetting the search radius?
I suggest an experiment:
Upload unsolved B and V images, and let VPhot solve them. See if the problem goes away.
Phil
Hi again!
Thanks for your suggestions!
How do you determine the coordinates in VPhot? I find that I get confusing results. When I hover over the star in zoomed out mode, I get almost exactly the same as in VSX. When I zoom in, this changes and I get a different Dec value. I also found that the value given by hovering over the star in zoomed out mode was different from the one when I clicked on it and the small window with details presented itself. Further, even though I had the crosshairs over the star when I clicked, the marked spot is then off, not containing the star.
I tried uploading not solved and then updating WCS in VPhot. I used a B-band image to update from (VPhot does not do a proper plate solve).
ANd I tried plate solving in Pinpoint and uploading. Same result.
I get a feeling that there is something odd with the geometry of the image - not least since the marked spot was not where I clicked. However, my later images work well....
Magnus
I agree.
Load either of the images you shared with me, (B and V). Zoom out to show the whole image, then load the VSX catalogue. There are dozens of VSX variables in your field. On one side of the image the VSX variables are identified (shown green). On the other side the variables are not identified (shown red). This is happens in both images. There may be something crooked in the optical system.
Phil