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Image Analysis Question

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WWAA
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Joined: 2012-08-13

Please forgive my ignorance but I'm very new to photometry as well as using VPHOT.

I've gone through the video tutorials (twice!) and read the on-line help within VPHOT but still have loads of questions.  I'll try to limit them!

My images will all be from the same scope, SRO50, which is one of the AAVSO's robotic scopes.  The observing program is a single star (AZ Cas) in each of UVB.

Should I create a separate sequence to use for each of the filters?  Or is 1 sequence to use on all 3 ok?  I noticed the FWHM for the target, check and comp stars seems to get saved with the sequence.  But I've discovered if I pull up a different image than what I used to create the sequence, and load the sequence, on that image, the FWHM values are a bit different than if I re-load the GCVS and Comp Stars catalogs on that new image.  Because I have to redo all the work relabeling the comp stars as well as calculating and adjusting the aperature radius, I can see great benefit in using sequences.  However I'm concerned that I will lose precision with this seeming difference in FWHM between images.  The stars are relatively bright (9, 10 & 11 mag) and it was suggested I actually use 2.5*FWHM, so maybe the minor differences in FWHM between the images is insignificant?

Or are there clues like a significant difference in airmass that would suggest creating a new sequence?  Or should I create a new sequence every few weeks?

VPHOT is a great tool but it's so easy to click-click-click and have a report.  My biggest concern is poor data accuracy from my ignorance.

Any help or suggestions would be greatly appreciated!

Wayne Westlake

WWAA wrote: "Should I
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roe
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Joined: 2010-07-25

WWAA wrote:

"Should I create a separate sequence to use for each of the filters? "

There is no need.  When you load the AAVSO comp stars, all their photometry comes with them.  VPHOT is smart enough to apply the correct photometry according to the filter designation in the FITS header of the image.  You might want to make sure that each of the comp stars you choose (and you don't have to choose all that are available) have the photometry you want to use (ie, U, B and V).

You will want to adjust the aperture, gap and sky annulus for each star to avoid interfering stars and VPHOT will remember them for you.  Remember, VPHOT stores all these according to their RA and Dec, hence the need for the plate solution, so it will place them accurately for different images.

I believe the default aperture used in the anlysis is 1.5*FWHM so the aperture used might well vary with different images according to how sharp the stars are.

aperture sizes
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HQA
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Joined: 2010-05-10

roe wrote:

You will want to adjust the aperture, gap and sky annulus for each star to avoid interfering stars and VPHOT will remember them for you.  Remember, VPHOT stores all these according to their RA and Dec, hence the need for the plate solution, so it will place them accurately for different images.

I believe the default aperture used in the anlysis is 1.5*FWHM so the aperture used might well vary with different images according to how sharp the stars are.

Jim's reply may be misinterpreted, so let me re-state it slightly differently.  For a given image, you MUST use the same measuring aperture size for each star.  You cannot use one aperture size for the target, and a different aperture size for the comparison.  You might want to look at all of the stars that you are going to use on an image, and choose one aperture and sky annulus that does the best general job for all of those stars.  You can have a star or two in the sky annulus if necessary, as the algorithm for determining sky will usually do a good job of removing that contamination.

If the measuring aperture is already defined in terms of FWHM, it is already taking the sharpness of the image into account.  Why you might want to use 1.0*FWHM or 3.0*FWHM is because of the brightness of the star and its contrast with respect to the sky background.  There is a "curve of growth" plot in VPHOT that will tell you the aperture size to use to optimize the signal/noise for a particular image with specified target and comparison stars.  However, using the default gets you pretty good results, especially when you are new to the game.

Arne

HQA wrote: "Jim's reply may
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roe
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Joined: 2010-07-25

HQA wrote:

"Jim's reply may be misinterpreted, so let me re-state it slightly differently."

Yes, sorry about that.  Actually, with VPHOT you cannot vary the aperture size across an image, just the exclusion gap and sky annulus on a star-by-star basis (which, IMHO, is an advantage over Maxim's photometry tool).

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