Which catalog for differential photometry

Affiliation
American Association of Variable Star Observers (AAVSO)
Wed, 10/01/2014 - 09:14

I'm beginning to do photometry by using DSRL and software AstroArt5
To achieve differential photometry which is the catalog to be used? AstroArt has pre-installed the catalog GSC1.1 but man can use also different. I tested the catalog APASS but then I saw that it is not usable (at times) for stars brighter than magnitude V = 10. 
Another question: if I use the Green channel of my DSRL without making corrections, the report must indicate filter Tri-G, is that right? 
Is there a way to import the table generated by the photometric variable star plotter in the photometric reduction software? 

Thank you all, 
Gianluca Paone (PGIA)

Affiliation
American Association of Variable Star Observers (AAVSO)
Other question

Thanks Michael, 
just another question. I saw that the same star in the catalog APASS is indicated with a V magnitude different from that of the chart VSP (sequence table 13620ADL). Example: the star 000-BBB-563 in VSP is magnitude V = 8.134 but instead in APASS is magnitude V = 8.729. What is the correct value to use?

Affiliation
American Association of Variable Star Observers (AAVSO)
other question

Hi Gianluca,

APASS has a fixed exposure time that, for most fields, saturates at about V=10.5.  So the star in question is saturated in APASS.  In general, always use the VSP values for comparison stars unless you have experience in selecting comparison stars and good familiarity with the available photometric catalogs.  The sequence team does an excellent job of selecting stars for you!

Arne

Affiliation
American Association of Variable Star Observers (AAVSO)
Thanks of all,
I think I

Thanks of all,

I think I understand !

II have now finished my first photometric reduction of T Cas. The results I have just uploaded to webObs. My code is PGIA. You can see if I reported everything necessary?

Thanks again,

Gianluca Paone

Affiliation
None
Which catalog for differential photometry

The responses given, while useful, did not completely answer the question for me.  In preparation for the upcoming course in DSLR photometry, I decided to download the photometry data for SS Cyg.  Alas, it did not include B-V data for the target itself.  Those data are required to transform the G data into V values, if I understand correctly.

Since the course material intimates that the B-V value for UGSS may be a function of magnitude,  I was anticipating  that the photometry table would contain some sort of table of B-V for SS Cyg itself.  I would find it very useful if a single source of B-V for cataclysmic variables could be provided.

Thank you!

B-V for transformation of DSLR G to V

Hi Stephen,

your post raises a number of issues which will be covered in detail in the DSLR course, but I'll try to answer here briefly. Firstly the transformation procedure using catalogue B-V values was used in analysis spreadsheets developed for the Citizen Sky eplison Aurigae campaign. I'll also discuss the standard transformation procedure which does not use the variable's catalogue B-V index.

The Citizen Sky technique is perfectly adequate for variables that don't change colour significantly. This is not the case for SS Cyg where B-V changes from about 0.62 normally to about 0.34 in outburst. 

In this case it would be better to use the blue and green channels of your DSLR to get an instrumental b-g colour index.  A calibration from b-g to B-V can be done using non-variable stars in the image field of view (if it is wide enough) so you can get a good approximation of the target B-V from your own data. 

However, transformation of DSLR instrumental magnitudes is feasible only for stars that don't have significant emission or absorption lines in their spectra. I don't know too much about dwarf novae but I if they are anything like regular novae then they are not suitable for transformed DSLR photometry. For these you should report the TG (tricolour green) magnitude. In the course I'll show an example using my data for nova Cen 2013. Cheers,

Mark

 

Affiliation
American Association of Variable Star Observers (AAVSO)
Emission Lines in SS Cyg

It has strong emission lines at least during protions of its cycle. I haven't found any spectra on line during quiessence but here is one early in  decline after an

 outburst. http://www.astrosurf.com/vdesnoux/catvar/catvar.html 

and here is one during outburst. The spectral lines appear much more prominent after outburst than during, which makes sense if the disk becomes optically thick during outburst. 

 http://www.britastro.org/vss/SS%20Cyg%20spectrum.htm

Brad Walter, WBY

Affiliation
American Association of Variable Star Observers (AAVSO)
Which catalog for differential photometry

Sometimes when there are no reference stars available on VSP and AAVSO cant supply any for me on request. I find this is often the case with eclipsing variables and LMXBs. I use the UCAC4 Zacharias 2012 catalogue on the Vizier website http://vizier.u-strasbg.fr/viz-bin/VizieR. Has a list of V magnitudes next to many stars. Not sure if thats acceptable for AAVSO or not but there is not much else to go on other than derive the magnitudes yourself with absolute photometry. Perhaps someone could comment on validity and a better option if available.

Affiliation
American Association of Variable Star Observers (AAVSO)
UCAC4 V magnitudes

Well, actually most of the UCAC4 V mags come from the AAVSO, since UCAC4 takes most of them from APASS and from Tycho (not Tycho-2) for the bright stars.
But beware of two things:

*There are 8 and 9 mag. stars from APASS which are saturated and shouldn't be used (stars brighter than 10th mag.).

*And the bright stars taken from Tycho have Vt magnitudes listed as if they were V, which is plain wrong.
So don't use anything brighter than 10 or so from UCAC4.

Contacting the Sequence Team would always be the best choice in these cases.

Cheers,
Sebastian

Affiliation
American Association of Variable Star Observers (AAVSO)
UCAC4 V magnitudes

Yes  I see the ones I have been using say APASS when I put my cursor over V mag in the UCAC4 2012 catalog. An example is V821 ARA an LXMB. There are no reference stars in the AAVSO chart. I normally request reference stars from AAVSO and they always come back saying they are not available. Cant recall if this is the case for this object or not but perhaps in this case I did not ask. But in other cases when I have asked I do use this list and use magnitudes around 12 to 14.

Affiliation
American Association of Variable Star Observers (AAVSO)
Benefits of Using the Sequence Team

There are other benefits of using the sequence team

1. Publication: Once done,  the chart and sequence are available to everyone

2. Consistency: The sequence team is very experienced at picking stars from the best catalogs available for that part of the sky and that particular magnitude range and transforming catalog magnitudes correctly, if necessary, to the standard system. When making a sequence, they take into consideration whether this is a variable that will be observed by visual and PEP observers as well as CCD observers and pick stars that will be useful for all observing methods that make sense for that star. 

Brad Walter, WBY 

Affiliation
American Association of Variable Star Observers (AAVSO)
V821 Ara

Hi Peter,

I see when plotting the field of V821 Ara that, as you mention,  no comparison stars are found.  Remember that the Sequence Team has created sequences for thousands of variables, but there are always more that need sequences.  When you run into one of those cases for a star that you want to study, go to

Observing->Variable Star Charts->Request Comparison Stars

from the AAVSO home page tabs.  That gives you instructions as to how to request a sequence for your new target.

Arne

Affiliation
American Association of Variable Star Observers (AAVSO)
B-V DSLR for transformation of G to V (color change)

Good morning Gianluca Paone.

I read your question a few days ago. I have been developing software for photometry transformations DSLR cameras, webcam, ccd color. this is already finished but their windows are not fully translated into 25 languages ​​supported. due to the way it performs the transformation, the fact that the object change color, is not a problem, but still not traslate her manual to English and several vieotutoriales perform only me now are translating. besides that I'm recording three more video tutorials.

to summarize, the photometry can be performed in AstroArt 5 if it is a single star, if lots of stacked images is done in iris. The software has a escripts generator to generate hundreds of orders automatically supported by iris, with the intention of additions per group (which can be interleaved manner (12345, 23456, 34567, etc ....).

due to the difference between the DSLR and the system channels this has been designed to work with these elements. you can use a relationship with slope change for the transformation color-color on all channels, and quadratic relationships to V-v, B-b, R-r.

forum in Spanish I've been writing about this, but I have thought this forum carry it when videos and tutorials are finished. although this wastes time to those interested in trying it.

however, if you want to try here is the link for download.

http://olichris.jimdo.com/rgb-fotocalc-software/

http://olichris.jimdo.com/app/download/5753897818/RGB+FotoCalc.zip?t=14…

 

 

Affiliation
American Association of Variable Star Observers (AAVSO)
B-V DSLR for transformation of G to V (color change

However, Oliver, won't there still be a problem if emission lines exist that are in the pass band of the color CCD sensors and not in the normal J-C pass band? Transformations apply corrections that are linear approximations. There is an assumption that the signals have continuous functions in the frequency domain. I don't think they can correct for something that resembles an impulse in the frequency domain that is detected by one set of filters and sensors and is missed or almost completely missed in the other. I think the same issue would apply to broad  absorption bands well. I don't think narrow absorption lines would be a problem since they don't have much effect on the total energy from a black body radiation curve over the relatively much wider pass bands of photometric filters. 

Brad Walter. 

Affiliation
American Association of Variable Star Observers (AAVSO)
emission

correct Gary, my comment was a general. focused rather the fact that Gianluca Paone looking color indices of objects for use in the transformation of its magnitude. check the links with ss espectos cyg in Pussycat state spectrum is fairly flat, but in the burst H-beta emission of 486.1 nm, is very prominent, and it matches (would have to check exactly), but coincides with the flight of the G band DSLR with respect to V, which would have influence (though not if largely), possible best not transform as Mark said, because rather than corrected, one could instead introduce some error in the result.