Thu, 08/28/2014 - 22:32
I will soon be dipping my toes into transformed observations with V and Sloan i' filters.
In the past all of my observations have been in un-transformed V-band. As such, I've simply generated a finder chart then clicked on the associated link to generate a sequence for the same.
What would be a comparable procedure for generating a finder chart that would include comparison stars and and associated sequences with Sloan magnitudes and error?
Initially I'd guessed I could query the APASS database but then is that a valid sequence and, if so how do I tie those to a finder chart?
Thanks in advance,
John Ott
Thank you John
Your post was Very Helpfull. It made possible to me to finalize the report about HD355501. acessing the Sloan r' reference magnitude.
Best Regards
João (John) alveirinho Correia
HI John,
The current VSP is restricted to UBVRI magnitudes. We are in the process of updating VSP, and as part of that, we will give the user the option of choosing which filters to display on the photometry table. That will include all of the Sloan filters, with the majority of that data coming from either APASS or SDSS. However, the new VSP is a few months away. That means for the time being, you have to retrieve your own calibrations. Use the VSP/VSD comparison stars, but then data-mine to find the Sloan magnitudes for those stars. Certainly APASS can be searched, either through Seqplot or from our web search form or VizieR. The magnitudes you find there are good enough to be used for comparison star values, but since they don't show up in VSP yet, we request that you place what magnitudes you used in the "notes" section of your report.
Arne
Thanks Arne,
So once I've located comp stars that match the VSP chart....
If using Vizier (SDSS catalogs) I might do something like the following:
For each comp star: report the i'mag (Model magnitude in i' filter) and e_i'mag (Mean error on i'mag)?
In the notes section, include the filter designation, the photometric catalog used;i.e. V/139/sdss9, the objID or..... instead/in addition to the objID, report _RAJ2000 and _DEJ2000 and corresponding AUID for each comp star?
If using APASS:
For eachcomp star: report the Sloani' and i'err.
In the notes section, include the filter designation, catalog;i.e. APASS, Data Rel 7, and RA and DEC shown there and corresponding AUID for each comp star?
I can see that one needs to carefully match - as close as possible - the RAs and DECs for each star.
--John
It's been recommended that since many modern surveys are using Sloan g',r',i' that new filter purchases should involve one or more of these filters. In that spirit I purchased a Sloan i' figuring I could use that in combination with my existing Johnson V filter to do transforms.
I started out taking images of NGC 7790 with that filter combination and find it exceedingly difficult to match-up AAVSO finder chart labels and field photometry AUIDs to what is output from APASS. I have put a lot of effort into trying to cross-reference (via RA and DEC) VSP Field Photometry to APASS so that I'd have valid AUIDs & Labels. However my transforms have a lot of scatter in them so I suspect that I may not be getting a good map-over between APASS stars and VSP charts. Especially in crowded fields, it seems like trying to match-up RA and DEC between these two systems might be a very error prone way of doing things.
My question is, what kind of experiences have other people had involving tranforms and Sloan filters?
Thanks in advance,
John Ott
OJJ
Hello John,
maybe you could get some help from program called Topcat? It is just great for manipulating catalogs. See for example: https://www.youtube.com/watch?v=GnYTiWALrm8 (author uses even AAVSO data to explain things :-), tutorial has three parts! ) and https://www.youtube.com/watch?v=XMFxVpP_F1o
The only trick would be: how to get CSV-data out of the VSP? I do not know how to do that without extra processing of the output files. Maybe data wizards in/of AAVSO can comment about that?
After getting VSP table in CSV-format, you can crossmatch two catalogs based on coordinates. In the end you have join of two tables, containing also AUID-s you want.
I'm actually curious, what e.g. Arne thinks about using direct Sloan magnitudes (where they exist). Huge amount of catalogs from Virtual Observatory are accessible with Topcat, also SDSS catalogs.
Best wishes,
Tõnis
Tõnis,
Thanks for the information on Topcat. I'm looking into it. It looks very interesting!
There is a way to get .CSV data out of VSP I've done it before.
Kind regards,
John Ott
OJJ
Hi John,
The paper by Dunckel in the following Proceedings discusses this topic in some detail including the errors. That author also finds the scatter to be considerable.
http://www.socastrosci.org/images/SAS_2014_Proceedings.pdf
For the transformation coefficients themselves, primary photometric standard stars will give more accurate results than secondary standards. The primary Sloan standards are listed at the address below. I think that the stars marked 'SA' are also Landolt's Johnson-Cousins standards.
http://www-star.fnal.gov/ugriz/tab08.dat
Just to be clear, my response was to your question "what kind of experiences have other people had involving tranforms and Sloan filters'. I have performed Sloan transformations with good results using primary standards. However, I am not very familiar with AAVSO procedures, so there may be other issues that I don't understand.
Tony Mallama
My main headache right now is trying to get from APASS RA and DEC to the AUID. Afterall in AAVSO-land that is what we report Comp and Check stars as.
So again. I have a V-band filter and a Sloan I-band. To transform these two, I'm assuming use of the latest APASS release. Problem is, that data hasn't any AUIDs and the database that does doesn't accomodate Sloan filters.
I've tried mapping RA and DEC in VSP to those in APASS and it can be done but, it is very tedious and error prone. Arne suggested that by now VSP might include Sloan filters but thus far I don't see it.
Any suggestions how to easly get comp/check AUIDs with Sloan magnitudes?
Thanks,
John
Hello John
Congrats on being an early adopter of sloan filters. You are probably a couple of years ahead of us at this point, and nothing is really ready.
Now if you do BVRI, then we have two very nice tools--TG and TA (Transform Generator & Transform Applier) that do all this for you in BVRI for clusters NGC7790 and M67, using instrumental mags from VPhot, along with a sequence file that makes finding and matching RA and DEC unneccessary.
My suggestion is to do BVRI and take advantage of the tools, or you will have to roll you own as you are doing, but very few of us are doing that, and very few are submitting Sloan data at this point. Personally, I have >65,000 observations over the past 21 years, and not one of them is in Sloan. So far, none of our campaigns have asked for Sloan. If you get through all that, the Light Curve Generator does not offer that as an option, so you won't even be able to plot your results--not sure about VStar whether it has Sloan or not. Have to check,.
The transition is coming, but its not here yet. Sloan was devised for Galaxy Study, not VS study, and the legacy of BVRI is still with us. This is certainly slowing things down, as we heard 5 years ago that things were "Going Sloan". Its kind of like the "Metric Conversion" of the 80's.
Gary
Thanks for the information Gary.
I suppose I should have read between the lines with Arne's suggestion that new puchasers of filters consider buying Sloan filters since it is the wave of the future. He may have been thinking Sloan would come along faster than it has too.
I'll have to shelve the Sloan SI and buy a Cousins I-band filter if I want to do transforms with my existing V-band filter.
Thanks again,
John Ott
Hi Gary,
No doubt this question shows my ignorance on the purpose of APASS, but for me it begs the question, why put the APASS data out there for AAVSO access if Sloan was mainly devised for Galaxy study and there are no AUIDs associated with APASS yet?
Thanks,
--John
Hi John
We do expect a transition to Sloan, just not sure when. Since most all surveys are now sloan, future work will be that. So the galaxy guys don't care about variable stars for the most part, so they abandonded BVRI. This is where the brilliance of APAS and Arne come in. APAS bridges the two systems so that we can connect in the future. No one else can do this. This is why the Professionals are coming to APAS and why we are seeing it cited on papers, and posters in professional literature and conferences.
A decade from now, I expect that we will have a ton of AUID's on Sloan only PT sequences. BTW, at the Arlo Landolt Conference in May, Dr. Johnathan McDowel made a good scientific argument and an impassioned request to keep observing in V, so that the 500,000 plates that Harvard has going back 100 years is connected. There are lots of Gaxaxies in the Harvard stacks whose images will be invaluable for getting pre SNe projenitors.
John and all, its a unique time that we will see this transition.
Gary
Thanks Arne, Gary and others for the education on topics related to Sloan fllters, transforms, APASS etc. It is encouraging that as Arne said, I "can easily transform Sloan r,i to Cousins Rc,Ic, so you get the best of both worlds" or not transform and still use the i' filter.
Having only submitted non-transformed V for many years then delving into transforms and the many catalogs is an eye opener and a more than a bit humbling. I can see I need to learn quite a bit more about the purposes and applications for SDSS, Panstars, APASS, Tyco etc. Any suggested reading for bringing myself up to speed would be welcomed!
--John Ott
The sequence team page on AAVSO has information regarding the magnitude ranges over which various survey data are useful. If you use Vizier on the Strasbourg astronomical Data Center website
http://cdsweb.u-strasbg.fr/
you can pick an object, and select what kind of information you are looking for by wavelength, type of astromic information you are looking for and get a list of data from the various surveys that fit that bill. You can also request information from a specific survey. Vizier also has descriptions of the various surveys and links to information about them so that you can read the survey descriptions that tell you the limitations of the survey. There is a chapter in Galactic Astronomy that gives overview information on various filter systems. This is a very "weighty" tome that you may want to request from the library rather than purchase. It's one of the Princeton series on Astrophysics and isn't easy reading, but it is crammed with useful reference information. I read through it once. It gave me a headache, but I still use it regularly as a reference. It is also getting a little dated. I think it was published in the mid to late 1990's.
Egad! Writing that something from the 1990s might be a bit dated, gave me a jolt.
Brad Walter, WBY
Hi John,
This sounds fine. For the VizieR reference, whatever is the minimum amount of information that uniquely identifies the star.
In the future, when the APASS Sloan data is included in the comparison star database, this step will go away.
Arne
A simple question to the colleagues familiar with imaging and photometry with Sloan type filters, and to everybody concerned:
What is the most reliable database to use for creating u’g’r’i’z’ sequences – APASS (DR8), UCAC4 (APASS DR7) or SDSS (DR7, DR8 and DR9) ?
Why I am asking this is explained below.
A sample from different sources only for filters SG, SR and SI:
ID of the star: SDSS J130024.36+302808.0 (UCAC4-603-050773, GSC 02532-00946, 2MASS J13002436+3028079)
Coordinates:
Decimal: 195.10145083,30.46896854
Hexagesimal: 13:00:24.34,+30:28:08.28
The SG (g'), SR (r'), SI (i') data:
APASS (DR8): SG=12.387; SR=11.849; SI=12.208
SDSS (DR8 and DR9): SG=15.06; SR=11.97; SI=14.02
Yep, what a surprise, only 2m difference between sources - let's play dice what to choose
OK let’s continue and to add a data from UCAC4 (APASS DR7):
UCAC4 (APASS DR7): SG=12.42; SR=11.865; SI=12.798
Oooooops, the star have got different color index (SG-SI)
SG-SI= +0.179 (APASS DR8)
SG-SI= - 0.378 (UCAC4 or APASS DR7)
Acc. 2MASS the star is about 5 700K in temperature but acc. the APASS releases it should be simultaneously about T=5 700K and T=26 000K
Let’s extend the first simple question: What should to be done?
Regards,
Velimir
Hi Velimir,
Somewhere on the SDSS page (or perhaps in the VizieR readme file) there should be listed the saturation limits for the SDSS catalogs. This is usually around 14th magnitude, so the example you gave is of a saturated star and so difficult to compare with the other catalogs. If the SDSS star is NOT saturated, then I think the main SDSS catalog is your best choice for SDSS photometry. PANSTARRS will be another choice in the near future, but again, watch the saturation limits.
For stars brighter than 14th, APASS is your best choice at this moment. DR8 is available from the AAVSO website and DR9 (an enhancement of the southern zones) will be available shortly. DR10, which will be available later this year, will include z' and Y as well as stars as bright as V=7.
Arne
Thank you Arne,
Obviously for the stars brighter than 14th should be used APASS DR8.
APASS DR8 is not listed in VisieR? Do you think we will not get a trouble with the referees of our papers?
Regards,
Velimir
Hi Velimir,
You should not get in trouble referencing APASS DR8. If you do, send the referee to me.
Seriously, there are some AAS meeting abstracts that can be referenced for APASS, along with a Munari paper, and any of those should be satisfactory.
DR9 will be included in VizieR.
Arne
Hi John,
I'm starting work on APASS DR10 in September. I'm expecting it to take about 4 months to reprocess the 400,000 images, so that DR10 should be released in early 2016. Once it is released, I'm hoping to add Sloan magnitudes to the majority of the VSP comparison stars (this takes staff time, so will have to be approved by Stella). If this happens as planned, you would have easy access to Sloan photometry in about 6 months.
Right now, you can do things manually, by using the APASS web form to query for individual sequence stars. You can query Landolt/Sloan equatorial fields and get sufficient data to derive the transformation coefficients for your Sloan system. The process is tedious, but possible. I think the bigger problem is that tools like the Light Curve Generator have not been updated to include the Sloan filters directly.
You can get astrophysical information with any filter set. If you want to work with Panstarrs or LSST data, you will be using Sloan filters, so I still feel the Sloan filters will be important for amateurs in the not-distant future.
I think the way I'd handle your Ic vs i' problem right now is to rename your i' filter "I" in your image files, and use the standard TG/TA transforming tools. i' is close enough to Ic that normal transformation will handle it just fine, and untransformed data using Ic comparison stars should also work. Perhaps put a note in the comments field that you are using the Sloan i' filter, but otherwise proceed like you were doing normal Johnson/Cousins photometry. I don't think you need to purchase a new Ic filter. Then, next year you can submit new data either as Sloan transformed into Cousins, or Sloan native, as you prefer.
Arne
Hi John,
A few thoughts to add:
I am imaging exclusively in SLOAN filters (g’, r’and i’ - ASTRODON), and I am using VPhot. To create my own sequences I am extracting the necessary data from the different sources using RA and Dec data. For the stars fainter than 14m I use SDSS data (the saturation limit of SDSS is 14m), and for stars brighter than 14m I am using APASS data from AAVSO access form. Sometimes is difficult to decide the right data from APASS so in this case I am using UCAC4 (APASS DR7) data instead.
SDSS and UCAC4 data are available through VizieR and it very easy to use RA and Dec to find the right object. When APASS will become available through VizieR it will be great step ahead.
Usually I do not transform my data, because I am using ensemble photometry and to transform the ensemble data is still impossible through TG and TA instruments.
Velimir
Hi John,
I guess I don't understand fully your questions. Stars in the APASS catalog don't have AUIDs because these identifiers are reserved for AAVSO targets for which data is submitted to the AID, and for comparison stars used for those targets. Not all of the 60 million objects in the APASS catalog fall into those categories, just as they don't for the main SDSS catalog, Panstarrs catalog, GAIA or Tycho. These are reference catalogs from which AUID stars are selected.
APASS contains both Johnson B and V, as well as Sloan g,r,i photometry. So the B&V magnitudes are obviously available for direct use for the millions of archival variable-star measurements. The Sloan filters are there to provide calibration for the professional community, as well as for those bright measurements by the small-telescope community that support telescopes like LSST. As mentioned before, you can easily transform Sloan r,i to Cousins Rc,Ic, so you get the best of both worlds.
Arne
This is kind of an old thread , but I have sloan g,r, and i and I was wondering if there has been any updates on standard fields for sloan filters. I would like to be able to transform the measurements.
Thanks,
Mike
I would be curious about this, too. So far as I know there are no consistent Sloan 'standards' fields. The Sloan survey telescope data are on a system slightly different than the "primed" standards among the Landolt stars published by J. Allyn Smith et al:
https://ui.adsabs.harvard.edu/abs/2002AJ....123.2121S/abstract
...and those are different again from Pan-STARRS 'refcat2' and from SkyMapper DR4, both supposedly tied to the GAIA spectrophotometry. All have slop of at least +/- 0.05 mag in various filters amongst each other. I think as a stop-gap, one should adjust to the 'refcat2' zero-point since it covers the whole sky. Fainter than mag 11 or so it seems to be OK in g,r,i. If you submit/publish any data, be sure to specify reference stars and adopted values plus whatever transformations you derive so that external comparisons can be made to some extent.
\Brian