BVR transformation and instrumental magnitude

[chrismlt]

Hello Karen,

I'm involved in the somewhat difficult process of transforming my data in the standard BVR system for reporting to the AAVSO with higher precision.
I will use some standard fields to do that, and image them - most probably the large open cluster Melotte 111, which is conveniently located in the evening sky.

https://app.aavso.org/vsd/stdfields

It is recommanded to extract the photometry of a larger number of standard stars.

I dont' think there will be a function to extract all the stars from a serie of images, so I will have to select manually a few dozen of those standard stars as comps, and maybe a fictive variable T.

Do you think this procedure is correct ?

Are the rel_flux_T1, rel_flux_C2, rel_flux_C3, and so on, correct to be used as intrumental magnitude for those standard stars ?

Any suggestion will be greatly appreciated.

Sorry to be possibly off topic,
Christophe

[chrismlt]

Helle again,

I've seen the same question was asked in 2017 :

http://astroimagej.170.s1.nabble.com/Instrumental-magnitude-td937.html

The process is discussed here (AAVSO) :

https://www.aavso.org/instrumental-magnitude

were instrumental is expected/needed to derive the transformation coeff in standard colors.

There is also this webpage :

https://en.wikipedia.org/wiki/Instrumental_magnitude

https://image-analysis.readthedocs.io/en/latest/06_photometry_intro/instrumental_mags_and_the_beauty_of_photometry.html

Most probably those values are already available in the measurement tables.
I initially thought about rel_flux_T1 C2 C3 ... but Source-Sky_T1 C2 C3 seems better candidates.


So I suppose, Source-Sky is the flux expressed in ADU. Then, it fits in the formula :
m=-2.5log 10 (f)

Then I've got the instrumental mag for each standard star of the serie.
It's only a matter of excel table and (big) patience.




I hope this will clarify.
Christophe

[karenacollins]

Hi Christophe,

It looks like you have done some research and now have a better understanding of what you need.

I agree that if you are wanting to measure some arbitrarily scaled instrument magnitude, the Source-Sky value is what you'd want to use.

In the AAVSO link you provided above, some users were suggesting to divide the flux by your exposure time before performing the magnitude conversion.

AIJ cannot compare fluxes across different images, so yes I think you'd need to do that in a spreadsheet program. I think measuring accurately across different images has some potential pitfalls. You'd want to make sure your atmospheric transparency is close, and the airmass of the observations as well.

There is always the option of measuring a real apparent magnitude from the same images using the process in the first AIJ link you provided. But, you'll need to look up some apparent magnitudes of other stars in the field in the same filter that you observed in. I think the AAVSO has a database of star magnitudes to pull from, but I don't know the details.

Karen

[Dennis Conti]

Hi Christophe,

I don't know if this is helpful, but if you are trying to submit a variable star report to the AAVSO, there is an option on AIJ's Multi-plot Main screen (File->Create AAVSO Variable Star Report...) for doing so. This makes use of AAVSO sequenced stars in the FOV of the target star. If there is a target for which there are not yet a series of sequences stars, you can make a request to the AAVSO sequence team to do so. See the following user guide for this AIJ option:https://astrodennis.com/AAVSOReportAIDHelp.pdf.

Dennis Conti
[e-mail]

[chrismlt]

Thanx Karen for the suggestions/advices.

To perform the transformation of the BVRI data obtained in my system (which include the camera, optic and filters) to the standard BVRI system, I will need to image a standard field, when it transits at meridian, so as not to include atmospheric deviances. The airmass will be constant.
The idea was about getting some 20 one-minute exposures, and to get the mean value of thoses 20 individual images, before computing the instrumental magnitudes.

The AAVSO provides some maps and UBVRI mags for a large number of stars in all standard fields :
(E.g.)

https://app.aavso.org/vsd/stdfields  (link provided in a previous mail)

https://app.aavso.org/vsp/chart/?title=Melotte%20111&ra=186.125&dec=25.858333&fov=180&maglimit=14.5&special=std_field&all=on&north=up&east=left
(map)

https://app.aavso.org/vsp/photometry/?title=Melotte+111&ra=186.125&dec=25.858333&fov=180&maglimit=14.5&special=std_field&all=on&north=up&east=left
(photometry table)

Clear Skies,
Christophe

[chrismlt]

Hello Dennis,
Yeah I know your work in AIJ about the macro for creating the report. Very interesting and useful function. ;-)

I didn't know about the possibility to request a star sequence when inexistant in some fields.
Who is the right person to contact for such a request ? Sebastian Otero ?

Thanx in advance,
Christophe