Affiliation
American Association of Variable Star Observers (AAVSO)
Tue, 06/16/2015 - 17:21

Hi,

 

During the summer months, I reside at my mountain home in Grand Lake, CO. I am never here when M67 or NGC 7790  are at optimum positions in order to obtain standard stars for TCs. Is there any way I can use a well-positioned Landolt field with TG to obtain my TCs? It appears that my only choices for standard star fields are currently M67 or NGC7790.

 

Thanks,

 

Keith Graham    

Affiliation
American Association of Variable Star Observers (AAVSO)
TG and Landolt field stars

Right now there is no way to use Landolt fields with TG.

I'm willing to add this capability to TG.  It requires the Landolt stars be identified in VSP as "standard field" stars.  Currently they show up as comp stars in VSP and the program can't distinguish them as standard stars vs. comp stars.

I'm not sure who to address this to, but if VSP could be modified to identify the Landolt stars as standard stars (e.g. they are retrieved from VSP when "std_field=on" is specified in the VSP call), I can make the addition to TG.

Gordon

Affiliation
American Association of Variable Star Observers (AAVSO)
Landolt FOV's as Standard Filed Charts

Keith,

Nothing is automatic.  We are a volunteer organization when it comes to sequences, both the data that Arne gathers (yep folks, Arne is a big time volunteer these days) and the sequences that are generated.  Not much has changed since you were a sequence team member (if my memory serves me correctly).

There is a formal process for sequence requests, updates and corrections. 

If no sequence data exists at all for a fov then the observer uses the following access to request an original sequence:

http://www.aavso.org/request-comparison-stars-variable-star-charts

If a sequence exists but has errors or is inadequate, for whatever reason, for the observer then the observers should file a CHET:

http://www.aavso.org/chet

NOW for the purpose of having the VSP respond with a Standard Field Sequence using existing Landolt stars I will make an exception and accept such requests through the first link for individual FOV’s… please do not ask that it be done for all FOV’s with Landolt values… just those that you might anticipate using that already have existing data (check by using the coordinates, which you would have to supply me anyway as well of what your FOV size is).

Adding the trigger keyword to the file the VSP would use to generate a standards fov is not a trivial matter in terms of time consumption and the sequence team has limited resources and therefore such requests should be for FOV's the observer intends to use for TC computation.

However,  before any such submission please read my below remarks RE using Landolt stars for the purpose of computing transformation coefficients.

Landolt FOVs for TC use.   Without any doubts this is golden data, by the way, HOWEVER..........

Some general remarks… typically you will not find a large number of comps in any specific Landolt fov; the question could arise as to whether or not a sufficient number of comps exist (can very well be dependent upon the individual observers fov) and whether or not there is adequate color representation (B-V).

The previous mentioned SA110-503 is an interesting example.    An F scale chart has only 8 comps (one of which is fairly red at a B-V of 2.3 while the “bluest” star has a B-V of only .568.  An E scale chart shows only 10 stars while a D scale chart shows a pretty good mix with 20 stars, for those that might be lucky enough to have a full degree FOV on their images (this many stars appears to also be the result of over lapping targets within the larger fov).

It crossed my mind that staff at HQ could probably add the key trigger word for Starndard  stars FOV into all existing Landolt stars within the DB… I am just not certain that this is a wise option as it might imply, potentially wrongly, that a specific fov with a standards fov has been approved for TC use.  On the other hand Arne might very well disagree with this and I welcome any such disagreement; always something new to learn if one keeps an open mind and this could then possibly lead to all Landolt stars being so noted.

By the way, it is not up to me to decide how many comps an observer uses or how well they distribute the colors…. I am just cautioning that when choosing  Landolt FOV’s for TC use that some caution be exercised; I endeavor to fill all requests without judgement.

FYI, I do not speak for the sequence team.  My remarks are my own and may or may not be in agreement with other sequence team members or polices of the AAVSO.

Tim Crawford, CTX

Sequence Team

Affiliation
American Association of Variable Star Observers (AAVSO)
Landolt Fields for TG

Hello Tim

I think the original request by Keith was a fair one, to have TG transformation fields that could be used in the summer after M67 is past prime.  

I would suggest to the sequence team that two suitable fields be chosen, one midway between M67 and NGC 7790 for summer use, and another midway between NGC7790 and M67 for winter use.  Note, as you are I am sure aware, I am adopting the Northern Hemisphere definition of summer and winter. 

I would in no way suggest that we use lots of Landoldt fields for transformation, and not all of them for sure.  

Not sure that a Landoldt field is the best one.  Arne may wish to weight in on that one.  I also think that we should provide some southern Hemishpere clusters for transformation.

Cheers

Gary

Affiliation
American Association of Variable Star Observers (AAVSO)
Suitable FOV's for TG

Gary,

I see that M11 (RA 18, but a bit low with a DEC of -05 for NH observers) has some excellent Henden Photometry.  While pretty crowded in the central regions I think I can build a suitable - Standard Field - data base for transformation coefficient use.

I will try and have something finished with in the next day or two.

However, I would sure welcome some other cluster suggestions that might be suitable for using, if anyone has some.

Tim  Crawford, CTX

Sequence Team.

 

 

Affiliation
American Association of Variable Star Observers (AAVSO)
landolt fields

Many of the equatorial Landolt standards are in VSP; you have to call up the field by coordinates rather than name.  An example of this is the field near SA110-503, at 18:43:11.7 +00:29:43.  All that needs to be done is to add a remark using the VSD admin tool indicating that these are standard stars (STD_FIELD).  Perhaps one of the sequence team could do that.

Arne

 

Affiliation
American Association of Variable Star Observers (AAVSO)
Landolt Fields

This would be a great addition to TG if the Landolt fields could be included. I have called up the fields by coordinates in VSP, but, of course, there is no photomtric data nor IDs for the standard stars in these fields. And having these standard stars included on the charts would make it a breeze to create STAR files for AIP4WIN using Photomcap.

Keith

Affiliation
American Association of Variable Star Observers (AAVSO)
VPhot w/standard stars

I think the same flag for Landolt field standard stars should work for VPhot and facilitate use of TG.

Ken

Affiliation
American Association of Variable Star Observers (AAVSO)
Landolt standards

Great news! Are new(er?) Landolt northern standards around +50 declination added to the VSP, too? While those areas are not so throughly observed (according to paper http://adsabs.harvard.edu/abs/2013AJ....146..131L , http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=J/AJ/146/131), they too contain both bright and faint(er) stars and are very suitable when observing from latitudes 40+ N. At my latitude, celestial equator is 32 degrees above the horizon in the south... at airmass 2, many of those new standards pass close to zenith.

Besides Landolt areas, there are many secondary standard fields observed/created by Stetson http://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/en/community/STETSON/standards/ . In principle, they should be good ones as well.. Downside - many stars in those fields are relatively faint. Stetson has NGC 7790 and M67, too ;-)

I can not comment if it is possible (or even wise) to mix Landolt and Stetson areas. Anyone here knows?

Best wishes,
Tõnis

Affiliation
American Association of Variable Star Observers (AAVSO)
Landolt Standards

While it is possible to mix the standards from Landolt with the secondary standards from Stetson, I believe you will find many opinions as to the wisdom of such a practice.  The Landolt areas were carefully observed as standards for several decades.  The stars were observed during those years on instrumental systems using a minimum number of changes in filters and detectors.  The original sets of stars were selected to give a wide range of colors and magnitudes and were then followed through many different seasons and observing runs and always compared against a basic set of UBVRI standards.  The Stetson fields resulted from numerous science fields and standard fields that were observed over many years with a wide variety of telescopes and instrumental systems.  Great efforts were made to smooth out systematics so that the values represent the best consensus values obtained from the various telescopes under a wide variety of conditions.  It probably comes down to what level accuracy you really need in order to transform your data to a standard system.  If you are working in a narrow color range with ordinary targets, you can usually get away with a few convenient shortcuts.  On the other hand, if you are working with extreme colors and more exotic targets, it is usually best practice to stick as close to a single set of standards in a well defined system as possible.  The Stetson fields are excellent and represent a great resource in many cases.  I like to stick with Landolt standards when possible just because they were observed as standards many years ago and have been checked repeatedly.  In many cases the observations span a period of more than 40 years.

Cheers,

Mike

E Region fields with TG

Hi All,

While adding the equatorial and northern Landolt fields how about adding the E Region standard fields at -45 degrees as well? These are extremely useful for southern hemisphere photometrists. Cheers,

Mark

Affiliation
American Association of Variable Star Observers (AAVSO)
Landolt Fields

Hi Tim,

Thanks for your reply and explanations. I am most aware of and certainly agree that the sequence team is a volunteer organization, and I have to say that it has been doing a fabulous job all along of creating sequences. Admittedly, I was unaware of what it takes to put a Landolt field into VSP. I asked the question only because it appeared to me from earlier posts that more fields could be incorporated into TG with the appropriate data. So I was only wondering what, if any, progress has been made in this direction. Arne’s comments made it appear to me that this should not be much of a problem, but your explanation enlightened me that this may be no light task.

I totally agree with you that there is definitely no need for all Landolt fields to be placed within TG. I found your comment about using M11 or some other cluster very interesting. Somehow I thought M67, Landolt Fields and NGC 7790 were the only standards we should use for TCs. This is why I suggested Landolt Fields. But if other clusters can be used, then I am all for it. In looking at various Landolt fields, I see that there are very few stars in each one. Would this be an issue? When using M67, we are obtaining instrumental mags from 30-60 stars. So just how many stars do we need to obtain reliable data that will result in accurate TCs? If more stars will give better ultimate results, then I can see where a cluster would be more advantageous. On the other hand, I thought Landolt stars were the golden standards to be used when deriving TCs So I must admit that I am a bit confused . If clusters are used, would this mean more work to be certain the photometry is very reliable for selected stars? If so. how can we be certain as to their photometric accuracy. Also, I was under the impression that Landolt fields are used so everyone using them would be on the same page with the standards.

I am probably missing something here, so I am entirely open to someone setting me straight.

Cheers,

Keith

 

Affiliation
American Association of Variable Star Observers (AAVSO)
Additional clusters vs M67 and NGC 7790

Hello All

Based on the past year of experience, I think I would prefer using lots of stars in the determination, with lots of spread in color.  That means staying up until the wee hours at certain seasons, but for me, that would allow me to drop out outliers, and still have a very good population.  I like to start with 100 stars or so and drop out those faint and close ones that cause the aperture jump to a brighter/wrong star.  In the case of M67, this still leaves me with about 60 really good ones that do not show up as outliers.

 

Gary

 

Affiliation
American Association of Variable Star Observers (AAVSO)
Clusters

Hi Gary,

Your preference for many stars does seem very reasonable. When I created my TCs using M67, Gordon (in another thread) suggested I could use the first 30 stars of the 64 to ease the job of star selection. He mentioned that he got very good results with 30 stars from M67. I follwed his suggestion and my results appeared to be good as well. This is why I asked the question regarding Landolt fields or clusters. Each Landolt field shows less than 10 stars and many times 5 or less. So Iam wondering just what would be the best approach for the already overworked sequence team? Would clusters or Landolf fields better additions to the TG? Perhaps that should be determined first so the sequence team is working on the better solution.

Cheeers,

Keith

Affiliation
American Association of Variable Star Observers (AAVSO)
Reply to Keith RE Landolt and other questions

Keith,

I apologize for taking so long to get back to you.. I was out of town most of yesterday.

Arne’s comments made it appear to me that this should not be much of a problem, but your explanation enlightened me that this may be no light task.

It is not that it is such a major task for the one field… but it could turn into one if ALL landolt fov’s were to be so marked (which in any event is something that HQ could probably accomplish quicker, if that is desired)

I found your comment about using M11 or some other cluster very interesting. Somehow I thought M67, Landolt Fields and NGC 7790 were the only standards we should use for TCs.

M67 and NGC 7790 both have extensive Hendon USNO 1M calibrations in relatively small fov’s that have a large quantity of stars.  They are a “natural,” therefore for Transformation Coefficient calculations and were the FOV’s so designated to have the trigger word inserted so that they would show up as a Standards Field when the appropriate coordinates are entered and the option selected.

M11 is simply another relatively small fov that has extensive Hendon USNO 1M calibrations and has been previously suggested as another option for TC computation.

On the other hand, I thought Landolt stars were the golden standards to be used when deriving TCs So I must admit that I am a bit confused . I

Landolt data is  golden BUT the majority of LANDOLT FOV’s have a limited number of calibrated stars so might not be the best option because of that with many of the FOV’s.   As I mentioned, SA110-503 has a decent quantity of stars if you have a one degree FOV and there might be others.  I am curious how you came by the information that Landolt stars were the best option for TC computation?  The data is certainly golden but as I keep bringing up the quanity in any given fov is limited. 

It is the Landolt data that enables the all sky photometry that creates the calibrated data from all sources, afterall!

if clusters are used, would this mean more work to be certain the photometry is very reliable for selected stars? If so. how can we be certain as to their photometric accuracy.

It is not so much that clusters be the source, it is simply that they can present a large number of calibration opportunities with relatively small FOV’s.   You have my personal guarantee that along with Landolt standards no calibrations are more golden than Hendon USNO 1 meter.  smiley

Realistically, the observer only needs a few such Fields spread out in RA for TC computation.

Ad Astra

Tim

Affiliation
American Association of Variable Star Observers (AAVSO)
Landolt

Hi Tim,

Your answers to my questons tell me we are on the same page. And I am glad Arne stepped with his comment about the 6 clusters. Like you, I agree that many stars are far better than just the few that Landolt fields provide, and Gary has indicated likewise. Somewhere along the line (can't remember where or when) I got the impression that Landolt fields along with NGC7790 and M67 were the ones to use in order to keep TCs standardized. Thanks to your comments and to those of others on this thread, It inow clear to me that there are other options of which I was previously unaware. It makes perfect sense to me that more stars are better when deriving TCs and that Landolt fields do not provide these numbers where as Clusters will provide them in a reasonable fov. I will assume that the 6 clusters Arne intends to submit to VSD will be spaced so one will have access to clusters year around. Then adding these clusters to TG will give 8 options from which to choose instead of  a zillion Landolt fields.  It sounds like this is all coming together very well.

Cheers,

Keith

Affiliation
American Association of Variable Star Observers (AAVSO)
Landolt standards in VSP

What is needed to create a new standard field for VSP?

Best wishes,
Tõnis

Affiliation
American Association of Variable Star Observers (AAVSO)
Update From Arne RE Standard Fields.

Just received the following email  information  from Arne Hendon which should resolve any further need for Standards Fields:

"M11 is one of the 6 standard clusters that we will be providing users, and I'm handling each one separately on my own, checking for internal consistency before submitting to VSD.  The 6 clusters (like NGC7790 and M67) are being submitted by me after vetting."  Arne

Tim

Affiliation
American Association of Variable Star Observers (AAVSO)
STAR Files

Well now, once I get questions answered, others start to pop up.

Gordon made a STAR file for M67 that is used for developing TCs when using AIP4WIN (as described in Appendix C of the TG Users Manual).. It appears to me that creating this file was no small task. I do not see such a file for NGC7790, but such a file would be necessary when using NGC7790. Since I do not use either Maxim or VPHOT, I do not know if such a file is necessary to derive TCs using them. If so, this would mean even more work. When the six clusters are added to the standard star list and incorporated into VSD, then this would mean six more star files must be created for AIP4WIN users (and possibly for Maxim and Vphot users).

I now see the work load that would be involved on behalf of AIP users. So my questions are:

  1. Has such a STAR file been created for NGC7790?
  2. Who will be “assigned” the task of creating STAR files for the additional six clusters (and NGC7790 if not already created).

As Tim pointed out, I was an original member of the sequence team, but I have not been active for a number of years (I have since been putting my time and efforts into high and low res spectroscopy),  Since I was the originator of this thread and since I am a user of AIP, I would be willing to help with creating these files, but I may need some help to jump start my brain to bring sequencing back to the surface. The one danger I see in making these files is the probability for typos, so I wonder if these files could be generated somehow by deriving the information from VSD and inputting the data directly into the STAR files. Perhaps Gordon could let us know how he generated the M67 STAR file.

Cheers,

Keith      

Affiliation
American Association of Variable Star Observers (AAVSO)
NGC7790 STAR file for AIP4WIN

Keith,

The NGC7790 STAR file for use with AIP4WIN is attached. (Remove the .txt - I had to add it to be able to post the file.)

Creation of additional STAR files for new clusters probably would not be too difficult.  Originally  I did a lot of Excel manipulations working off stardard text files listing the reference stars, locations and magnitudes.  But now the magnitude values in the STAR file are no longer used.  TG cross-maps the old "Boulder" star id numbers to the correct AUID's, and gets the current standard reference values directly from VSP. 

I assume new clusters will use AUID's, so creating an AIP4WIN STAR mapping file may be trivial.

My first task will be to add the new clusters to TG once they are entered into VSP.

And consider looking at VPHOT...

 

Affiliation
American Association of Variable Star Observers (AAVSO)
Star Files

Hi Gordon,

Are you saying that TG uses the STAR file info to cross reference with VSP to obtain the most recent values (mags, AUIDs etc) ? If so, that means that any updates to cluster data in VSP info will automatically be applied to future TCs. Very slick. 

Cheers,

 

Keith

Affiliation
American Association of Variable Star Observers (AAVSO)
Yes, VSP latest values used..

Keith,

Yes.  Everytime you run TG, it goes to VSP and uses the most current reference magnitudes.  So any updates to reference magnitudes in VSP are always used.

Gordon