Affiliation
British Astronomical Association, Variable Star Section (BAA-VSS)
Sat, 02/03/2024 - 19:32
I just used TG to calculate my transform coefficients using standard fields SA20 and GD2.
Neither are listed at the top button selection so I used "Landolt Field" option and was able to add files to TG for processing.
Are they Landolt Field?. Is this OK? VPhot only found 3 stars for my GD2 images.
It seemed to work fine and matches the results using the more populated field of NGC7790.
Kevin
Can somebody explain the notation used
E.G I have outputs from TG
Tbv,
Tb_bv
Tv_bv
I went to update my telescope info in VPhot and the fields are a bit unclear. Are the filter letter drop downs the lower case "subscripts" attached to the letter T?
Maybe there is an explanation somewhere.
Kevin
PS It just got more confusing. On the VPhotTwo Colour Transform page the values to be entered are labelled
TBV =
TV =
Kevin;
Yes there is a difference in labeling for the 2-color fields. Geir took an unfortunate short cut in labeling there. However, don't think so much!
Use the pull down boxes in the transform fields in the Telescope Setup page and select the filter colors that make most sense compared to the TG output.
You will get it right. Trust me!
Ken
PS: IF you fail after 'trying', I'll give you the answer or skype?
Kevin:
SA designates Selected ('Areas') fields (edit per Brian). GDs are in other standard fields.
I'm not sure Gordon Myers included GD fields under Landolt Fields in TG. I would think not?
There may be normal aavso comps displayed in the GD2 field on the image.
Ken
PS: IF the coeffs are the same it is a bit of luck with fewer comps but the SA comps do have 'very accurate' magnitudes. How many comps were in the SA field? Fewer than NGC7790, I assume? The key is how well distributed the colors were!
Minor nomenclature clarification: 'SA' stands for 'Selected Areas', which were defined by Kapteyn like 100 years ago, and designate regions that were intended to be intensively observed and sampled in various ways to help delineate galactic structure. In the early going, Arlo surely chose stars in those regions to standardize in UBVRI because there was a fair amount of existing data (spectra, proper motions etc) for them. In later work, in order to find 'very blue' stars to standardize (rare in the Selected Areas), he often chose known white dwarfs or hot subdwarfs and observed those along with ordinary stars around them to create sequences with a fair range in color. Some of these are even in Landolt's first 1973 standards paper (well worth reading and re-reading). The 'GD' stars are blue stars from the Lowell proper-motion survey, mostly north of the Equator, and similar stars in the south with different acronyms. Several these are now HST spectrophotometric standards that Arlo helped calibrate.
The Landolt GD 2 sequence stars, although perhaps adequate, are fairly faint (13< V< 15), and the color-range of the stsrs besides the white dwarf is limited (no A/F dwarfs, no red giants). By calibrating other brighter stars in this field against the Landolt stars in the same images and in other fields on several nights, one could greatly extend and fill-in the color range.
\Brian
Thanks for the history lesson, Brian. Interesting to learn. ;-)
Ken