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VPHOT tells me there are no variables in my field - but there are

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dcrowson
VPHOT tells me there are no variables in my field - but there are

In wanting to catch Wolf 359 (CN Leo) for a NASA citizen science project, I've been pointing at this variable. I see that there are observations but nothing since 2015. I'm confident I have the star in my field but VPHOT is telling me there are none. There are sequence stars. See this thread about it - https://www.aavso.org/wolf-359-lghtcurve.

I'm looking for direction.

Dan

MZK
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Share Images

Dan:

Share a few images with me at MZK.

Ken

MZK
MZK's picture
Position?

Dan:

IF the target has moved significantly (beyond the VPhot search radius, which you can set) from its J2000 position, VPhot can of course, not find it!

Mark its current position and make it a target in your sequence, as long as you are sure that you select the correct star!

Ken

dcrowson
Position

I'm sure the star is there as it is in the center of the image. That is what has me confused. I'll share a couple images.

Dan

MZK
MZK's picture
Cursor Position

Dan:

On your image in VPhot, load the AAVSO Comps. Do you see all the comps that are shown on the CN Leo VSP Chart identified correctly on your image? If so, VPhot is finding stars at their expected positions.

On your image in VPhot, move your cursor to the known J2000 coordinates (RA/Dec) of CN Leo. Is the cursor now over the visible star that you know is CN Leo, or some place else? What does this mean? Has CN Leo moved away from its J2000 coordinates?

Should VPhot identify CN Leo at its old reported J2000 coordinates? Yes, BUT is there a visible star there? I think you may acknowledge that there is no faint star at this location. In fact, if you look at many of your images, you may note that there is a faint star near that position in some of your images. There was such a faint star in one of the images you sent me.

When, I selected that image, CN Leo was shown with a little red circle (unidentified) around this faint star. Why can I see it and perhaps not you. Go to Tools/Settings. Note the value in the sequence and catalog import. Is your value 20? Change this setting to a small number like 3. Try again.

Report what you find.

What do you think you can do to select the correct star AND make it a target in your own sequence?

Ken

dcrowson
The comps do show up as do

The comps do show up as do the sequence stars.

When I set the sequence and catalog import to 3, it does show a circle around a spot -

When I pull and image from a sky survey using simbad coordinates, it centers on the same spot. The issue is that CN Leo is one of the three stars a little up and to the left of the red circle.

I'm pretty sure my image is centered on the correct star and I think the simbad coordinates are incorrect thus leading vphot to not detect it.

Dan

MZK
MZK's picture
Let's talk off-line?

Dan:

VPhot is working properly. Let's talk off-line. If you wish, send me an email at kenmenstar@gmail.com and we can trade phone numbers? Then we can talk?

Ken

TYS
TYS's picture
CN Leo

Dan, this might help. I compared your image to an AAVSO chart of CN Leo and I think I identified it. This star is quickly moving.

dcrowson
That looks correct to me but

That looks correct to me but the question is still how can I get VPHOT to recognize it so I can submit observations?
 

Dan

spp
spp's picture
Richard has found it.

Dan,

Please share you image with me, SPP.  I think I know how to fix this.

I think your target is the fairly bright star that Richard has circled.  In the star chart there are only 2 stars inside the circle.  If you look at your image, I think you will see three stars.  The bright star at the up left should be CN Leo.

To get VPhot to recognize this star do the following:

Click on that star.  This opens an information box.  At the top enter the name, CN Leo.  Just below the name, click on Fixed Target.  Then, at the bottom, click "update".

If you can find the B-V color somewhere, you could enter that later.

Phil

 

MZK
MZK's picture
Click on it!

Dan:

Play around. Click on the star on the image that you know is CN Leo.

See what happens. You CAN figure this out without Phil telling you.

Ken

dcrowson
Ken,

Ken,

I figured that. I just through VPHOT would deal with it as it always has. It somewhat throws a loop in the process if one can't trust it.

I submitted.

Dan

TYS
TYS's picture
You can trust VPHOT.

You can trust VPHOT but if the star (CN Leo) is moving VPHOT cannot locate it. CN Leo is one of the fastest moving stars visible from Earth.

MZK
MZK's picture
J2000 Coordinates

Dan:

Current catalogs typically use the J2000 Coordinates to locate stars. This is true of the catalogs that VPhot and most other software use. The last major coordinate epoch was J1950. So we will probably wait another 30 years to get the next catalog epoch?

So if a star moves much faster than the vast majority of distant stars, everyone must make an adjustment. Fortunately, there are few of these.

Ken

MZK
MZK's picture
More Common Problem

Dan:

A more common problem with location of a target is the presence of a nearby brighter star. IF the search radius is set too high, the centroid of a faint target star may be shifted to a very close brighter star. This often happens for all of the stars in the "double trouble" campaign.

It is ALWAYS important to confirm that the star identified as your target is in fact the correct one. Routinely take such care by looking at your images or accept the embarrassment of misidentification and an erroneous magnitude. 

Ken 

Eric Dose
Eric Dose's picture
"...if a star moves much

"...if a star moves much faster than the vast majority of distant stars, everyone must make an adjustment."

Really, it would be much better if the chart system made the adjustment.

MZK
MZK's picture
Ok Eric

Hi Eric:

Some planetarium software will adjust to JNow coordinates on the basis of proper motion data to position the star in the center of the image but I'm unaware of photometry software that will do that with existing catalogs.

Do you know of some? If you can write an algorithm easily to do that to an existing catalog, I'd listen!

Ken

Edit: See comment/post below

 

MZK
MZK's picture
Jnow? Equinox vs epoch

Eric:

Actually thinking about this last night. Ugh!  ;-(

I think Jnow vs J2000 is normally just a correction of the equinox due mainly to precession of the earth? So, relatively easy for a planetarium program to adjust?

But, correction for actual proper motion of every star would require data on the proper motion of each star and adjustment from the current epoch (J2000). Not sure this is easily available?

So how would you adjust the catalog in real time for each star? Can you?

Ken

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