Wed, 08/03/2016 - 14:10
Hi all.
I'm observing this star since June for the second year and now it plays show in the tricolor blue band.
Blue measurements are often higher (0.15 mag) than green ones (nearly constant) and sometimes there is a jump below the corresponding green measurement.
It's very excinting to follow this variable star.
I'm doing DSLR out of focus transformed measurements with a SONY Alpha 2000 and 50mm or 200mm lenses on a Skywatcher StarAdventurer.
CTIA, Tiziano Colombo
Italy
Very good Tiziano.
I am very interested with your results. I have a question. What is the -13.44 in the reported observation. Sorry, I should already know this.
55 (-13.75)
57 (-13.44)
Also, can I tell from the report, which 57 is being used ? Is it 000-BCB-499 or is it 000-BCB-480 ?
Also, do the 40's in the FOV saturate your DSLR?
Ray
Hi Ray,
when you convert ADU into magnitude, you compute -log(ADU) that produces a negative number representing the brightness level of the stars in magnitudes.
57 corresponds to 000-BCB-499
both 40 stars are not saturated; it's likely a consequence of a slight defocusing, low ISO (100) and short exposure time of 20 sec.
Tiziano
Still confused here. AAVSO shows a comp star as 57 (-13.44).
To me, that means the comp star had magnitude of 5.7. and the (-13.44) doesn't compute.
Why does one report ADU's ? We know the magnitude is 5.7.
ln(65536) is only 11.09. Log(65536) is only 4.82.
So I am guessing that (-13.44) is -1 times the log of all the ADU's in all the pixels of the star. But it doesn't seem to tell me if one of the pixels is saturated or above the linear regime.
Also, why not report which comp is being used? (000-BCB-499)
Are you counting ADU's in the star image pixels and using the ADU count for one of the I's in the magnitude EQ?
Sorry, I am so mixed up
Thanks
Ray
Hi,
when you measure the luminosity of a star you count ADUs and convert to mags using the expression -2.5 times decimal log of ADUs as you can read in the Photometry or DSLR AAVSO manual. The expression you cited in your last post in fact is useful to determine the difference between the instrumental mags of the star with respect to instrumental mags of the comparison star, that must be algebrically added to the tabulated mag of comparison to obtain the var star mag.
In this way you have a differential mag; to determine a transformed mag you have to take into account also the colour difference b-v proportional to the B-V tabulated trough the experimental regression slope coefficient.
When I participated to the Epsilon Aurigae campaign carried on by John Hopkins, he taught me a nice graphical system to know if the image of a star is saturated: doing at least two graphical sections of the star image, if the central and higher part of the graph is flat then the star is saturated.
The value 65536 is the saturation value for a single pixel and it is the threshold of the graphical section I said before and not the aperture photometry value of the whole star image. My aperture photometry measurements exceed 1000000 ADU in the blue band and 400000 ADU in the green band. In fact I have to sum over all pixels that compose the spot of the star and furthermore I use the binning 2x2 for V2048 Oph, so four previous pixels are summed and the new threshold becomes 65536*4.
Definitely you are right when you say I'd have to specify which 57 I'm using; I thaught that if I maintain the same cfr star it's important to verify that mags change of the var star don't correspond to the analogous variation of the cfr star.
Best regards
Tiziano Colombo
OK, Thankyou Tiziano
I will read the Photometry or DSLR AAVSO manual to see what they are doing.
I have not done DSLR work, but assumed that DSLR files could be converted to FITS and just uploaded to VPHOT. It seems like you are only using raw DSLR images and counting ADUs. That should work OK too.
Ray
Molto buono Tiziano .
Sono molto interessato con i risultati . Ho una domanda. Qual è il -13,44 nell'osservazione riportato . Spiacente , dovrei già sapere questo .
55 ( -13,75 )
57 ( -13,44 )
Inoltre , posso dire dal rapporto, che 57 viene utilizzato ? E ' 000 - BCB - 499 o è 000 - BCB - 480 ?
Inoltre , fanno di The 40 del FOV saturare la vostra reflex digitale ?
Ray