Affiliation
American Association of Variable Star Observers (AAVSO)
Tue, 04/30/2024 - 16:23
Would you please explain how to add airmass in the FITS header, if VPhot does not calculate one?
Position of telescope is known, well as altitude of star. Maybe there is simple formula or algorithm? Every time VPhot reproach me for the airmass absense, but does not want to help!
Your image acquisition software is what normally adds AIRMASS to the fits header. No idea I’d there’s a way to get VPHOT to add it - I doubt it very much.
Gary
The Transform Applier application, which is part of VPhot, will compute airmass for the target and check star.
I'm looking into porting the formula into VPhot: if the information is all there (jd, lat/long, ra/dec) then its pretty straight forward.
George
Thank you, George, I hope very much to add airmass to TG images. As I understood, TA requires 3 bands, but it's problematic in my case.
TA works with 2 bands just as well. So you can transform TB/TG to B/V and get your airmass too!
George
There are two reasons for my one-band TG observations:
1. FILTERS. TG points are comparely close to B, but TB and TR are too far from R and B for reliable transformation.
2. LAZINESS. I use stacked images got with planetary mono camera and old simple HEQ-5, so the process of science images preparations is not shorter than the observations.
So I hope, airmass correction will inrcease TG accuracy but I do not dream more about transformation of CCD filters to Johnson.
Mikhail,
You note that you have a mono camera and report TG observations. Do you use a geen astrophotography filter?
Roy
Of course, I use green filter, just SVbony CCD G but not Johnson V.
Without green, with UV-IR cut, estimations lays between V and R.
If you decide not to determine transformation coefficients, you don't know how your green filter performs.
Sure, you can compare your variable star magnitudes with those of other observers, but you don't really know how accurate they are, and there may be a range of values among different observers.
Without TCs the following test is useful. Select a pair of comp stars, with B-V values that differ by 0.5 or more, designate one as the target and measure it's magnitude. For even more information, select several pairs of stars, with a range of B-V differences. Unless your Tv_bv is near zero (probably unlikely for a green astrophotography filter) you should see a range of errors, with the smallest error for the pair of stars with the smallest B-V difference.
My system has a Tv_bv of -0.145 with the green astro filter. This means that for a target-comp B-V difference of 1.0, the error will be 0.145. The relationship is linear, so a B-V difference of 0.5 will give an error of about 0.07. In order to have an error as small as 0.01 with my filter, I would need to find a B-V var/comp difference of only 0.07.
I write about the above issue often, and therefore probably bore readers of this Forum, but no-one else addresses it. It is important for those observers who submit TG observations.
Roy
Thank you, Roy, for the important note!
Yes, indeed, systematic TG error is clear visible in comparison with V. In the most cases my TG is very close to untransformed V and a bit of lower of reliable transformed V evaluations. I do not know, will airmass correction improve TG, but I hope to use it.
However in the case of deep red stars, included to SNEWS program, I got huge difference betweeen TG and V (look please V386 Cep and AG Cam light curves and compare with untransformed WX Cas).
Generally my exotic method gives realistic results no worse of visual. TG is broadly used by DSRL observers for timing of minimums, but for precise standard photometry it suits less. What to do, "it's not a bug but feature"...
Mikhail
I'm adding the AIRMASS…
Hi,
I'm adding the AIRMASS parameter during the calibration in AstroImageJ. Then, I upload the calibrated files in VPHOT and do photometry.
Yes, Nikola, AIG is powerful multifunctional tool!
Regretfully my IQ is not so high for intuitive operation with AIJ, but I will be happy to learn it in the Choice course "AIG for beginners" :)
I completely understand you! It is not intuitive. I had to learn it along the observations of exoplanet transits, where it is a mandatory tool.
By the way, some acquisition software also automatically adds AIRMASS directly into the original raw FITS file. I use an open source platform INDIGO under Linux and it does that.
The question has connection with airmass: how low shoud be star for adding L comment code? 10 degrees? 15? 20? Is there a standard or recommendation?
I don't know of a strict rule for when to put an L in a comment. I think it's the decision of the observer. As a rule of thumb, it's not a good idea to do a photometry when the object is below 25 degrees above the horizon, but it depends on the site and atmospheric conditions. You can check FWHM also.
The AAVSO guidance is not to observe variables below 30 degrees in altitude, air mass ~2. The assumption is that above 30 degrees all comps and target stars can be considered to have effectively the same air mass, thus air mass can be ignored in the analysis. Of course, if your observing project requires observations at less the 30 degrees you can do this, but then you have to deal with air mass in the analysis.
Arne Henden posted an explanation of why this works for most of us (considering typical altitude of observing sites, FOV, etc.) in one of the AAVSO forums. It is interesting reading if you can find it.
Phil