Skip to main content

Transformation coefficients. Am I doing it right?

wthb
wthb's picture
Offline
Joined: 2011-01-28

Hello all!

I have, quite recently, been affected with the "photometry bug". Up till today, I have been using iTelescopes and Sierra stars for that purpose. 
Last week, I received my new ccd-cam (QSI6893) and BVR-filters. (Oh, happy day!) My telescope is a Skywatcher Explorer 190MN.

To get things right from the beginning, I'm trying to determine the transformation coefficients for my system. Due to the fact that I'm a newbie, can someone take a look att my procedure and results? It's a lot to learn, mostly in a non native laguage, and my mathematic knowledge is somewhat out of date. Better to discover my mistakes before reporting any observations.

I have taken photos of ngc7790 and Landolt SA114. The images have been calibrated (bias-dark-flat), platesolved and uploaded to vphot. In vphot I have loaded the compstars and created a sequence with only compstars to read the instrumental magnitudes. The images for ngc7790 was taken in an airmass of ~1, FWHM ~3. The images for Landolt SA114 in an airmass of ~2, FWHM ~5-6. The Landolts is a little bit low in my sky, but I'm living on North 60. 

I'm attaching a spreadsheet with my measurements and calculations. The values for Tbv and Tvr seems to look quite similar for the two different fields. The values for Tb, Tv and Tr seems to be a little bit more scattered. Maybe that will be averaged out after analysing more standard fields? Also the value for Tr seems a little bit high, have I missed something?

So again, I would be very happy to have this checked.

 

Clear skies,

Tomas Wikander
Borlange Sweden
WTHB

AttachmentSize
Trans coeff Landolt SA114 NGC7790 2012-11-18.xls67.5 KB
Transformation Coefficients
HTY
HTY's picture
Offline
Joined: 2010-07-23

Hi Tomas,

It looks like you're off to a good start with your transformation coefficients.   I haven't had time to look too closely at them but quickly I have these suggestions:

1.) Plot the relationships graphically.  I saw a few outliers that were very far from the regression in the NCG 7790 data in the V-R vs v-r plots that you should probably check the identification and/or the photometry for.  They may be causing some of the inconsistensies that you are seeing.  It may be the 145 star which looks like it is actually a close double if I'm looking at the correct star.  You may not want to use that one but don't take my word for it.  Check your images.

2.) Try to pick a wider range of colors in the NGC 7790 images.  I think that because you only took the preassigned comp stars, you got a limited range of colors.  Have you seen this link?  http://binaries.boulder.swri.edu/fields/ngc7790.html  .  If you can get a few of the redder stars, you'll have a greater color range to work with.  Just click on the other stars in the image on VPHOT and make them comp stars.  Then you'll get the instrumental magnitude for those also.  That will give you more of a range to see if there is a trend for V-v vs. B-V, etc.

That's all I can see so far from a quick look.  I highly commend you for starting out computing your transformations right at the start.  You will want to test your camera to find the linear range also.  Many cameras do become non-linear before they saturate and you will want to know what that range is.

But walk before you run.  You are doing a nice job so far! 

....Tim (HTY)

Transformation Coefficients
CTX
CTX's picture
Offline
Joined: 2010-07-08

Tomas, You have obviously put a lot of thought and effort in your pursuit and are building a good foundation for the subject. I am not sure that I can truly answer your concerns, “Am I doing it right,” as I am not a guru when it comes to transformation coefficients while other members are more so.

However, I have had a lot of experience in computing them and am happy to share with you some observations, as it were. Please do not take offense to any of my remarks as none is intended; I too am still learning.

I am assuming that your BV filters are Johnson prescriptions and your R filter is a Cousins prescription?

1) To Quote Arne (26 Jul 2007): "A Transformation coefficient generally tells how your instrumental value differs from the true value as a function of star color. This is usually a small correction; important if you need high accuracy but unimportant if your target and comparison stars are similar in color." Arne

Uncertain as to whether or not Arne would alter any of this today (or the next included quote… it is always possible with the passage of time that re-editing is in order) but I keep in the front of my TC notebook as a reminder.

While transformed data is to be prized…. with a good system the corrections tend to be quite small, especially if stars are of similar color… we are typically talking ~ corrections of maybe a few thousands to a few hundredths in my experience. Which raises the question as to whether or not they are really that important with average situations in that the value with all our observations, IMO, lies with the changes from observation to observation to observation, etc., but then this is a potential argument for another occasion.

2) IMO, M67 is one of the better options for a FOV for TC computing. It contains a large number of stars, with a variety of colors, for which Arne has generated very solid photometry: Besides the Landolt fov's there are others suggested, such as NGC 7790, M11 & SA110-503 but, again, IMO M67 is more ideal: http://binaries.boulder.swri.edu/binaries/fields/m67.html It is strongly recommended that transformation coefficient’s be recomputed on an annual basis and for a minimum of three different nights averaged together.

3) To Quote Arne, Again, 12/28/08….. Arne to aavso-photometry group “RE Transformation Coefficients-Color Index’s The normal guideline is that the color index you choose needs to include the wavelength range of your filter. This is to remove as many systematics as possible - always interpolate, never extrapolate. Your chosen color indices will work; the ones for B and V are pretty much standard. I prefer to use (V-Ic) as the color index for Rc and Ic. (V-Ic) has more change with star color and so results in a better transformation. For example, an A0 star has (V-Ic) = (Rc-Ic) = 0.00. However, for a G2 star, (B-V) is about 0.63, while (Rc-Ic) is about 0.33, or half as large. The transformation coefficients determined using (V-Ic) are more accurate since fitting something with wide range + error is easier than fitting something with smaller range + same error. (V-Ic) is also more sensitive to interstellar extinction. Using (Rc-Ic) means you use Rc, a bandpass that has Halpha in it, a very prominent line that can be in emission or absorption, causing an error in transformation. This is very obvious with novae, where Halpha emission can dominate the continuum in the Rc bandpass.” That being said, my use of (V-Ic) is a preference, not a rule. (Rc-Ic) will work.” Arne

I include this second quote from Arne because it does point out one of the weakness’s with Rc filters. IMO, if using just three filters then the best combination would be BVI and not BVR. You might consider changing out the Rc for an Ic, if funds permit.

4) I have often stated that working up transformation coefficients is as much an art form (qualitative judgment) as science (quantitative judgment) because the computed values are going to depend upon:

a) The specific FOV chosen

b) The number of stars included

c) The color mix of the stars included

d) The number of nights averaged

Change any of the a) - d) “values” and you are going to generate different transformation coefficients every time!

5) IMO, your latitude is to high for using any of the Landolt FOV’s as the air mass is going to be to large for “good” photometry data; and I will not comment further on that FOV.

6) I have never fully trusted the NGC 7790 FOV for TC computation. Whenever I have tried to use it I have had poor results with the data. Whether this is the fault of the actual photometry, the limited number of brighter comps or the colors of the comps I was using or even possibly the air mass from my location I do not know and it never really seemed worth the effort, for me, to resolve the reasons as M67 is available. I simply avoid, therefore, NGC 7790 anymore and wait for M67 to cycle back each year and use Arne’s solid photometry from that FOV (probably a mix of about 25 comps).

My copy of the NGC 7790 photometry data shows 31 available comps. M67 data shows 64 available comps (no, you do not need to use them all and I am not suggesting that) and they are all brighter comps, except for 13, than what the NGC 7790 photometry represents. I do note that your mix of stars appears to be tilted towards the blue end (B-V values) and this will have an effect upon the final data.

It can be quite a challenge to balance colors with some red and blue stars included and probably more of the in- between colors. If it were me I would include some redder comps within the mix. Which ones and how many… I can’t frankly tell you, you have to experiment but then on the other hand If it were me I would be using only M67 for the effort.

I will try and find some time today to look more closely at your SS.

Keep in mind that there is no such thing as a stupid question so keep asking!

Tim Crawford, CTX

Transformation Coefficients
wthb
wthb's picture
Offline
Joined: 2011-01-28

Tim and Tim,

thank you for your opinions and help regarding this, it is highly appreciated!

I have noticed Your problems with ngc7790, and I'll try to nail M67 when the lousy Swedish weather permits. Hopefully before the end of this year, if not decade... M67 reaches 40 degrees above my horizon, hopefully enough to get good data.

I have used the compstars, as linked above, for ngc7790. You were right, the 145 star is a double and I have deleted that one. One thing that strikes me is that the AAVSO compstars loaded from vphot does not seem to correspond with the same stars when listed in the ngc7790 standard field. When values are available from both AAVSO compstars and ngc7790 standard field, I have used the latter. 

I see the problems regarding the use of R-filters, I'll put an Ic-filter on my shopping list. Honestly, I have not checked the specs for my filters. It's Baader Planetarium BVR-filters, but I can't get detailed specifications from their website. I have found a transmission curve for their filters, and it looks like their R-filter is peaking slightly below 600nm. Isn't it so, that Rc-filters peak around 640nm? If that's the case, I have a non-Rc-filter. That could also explain the rather high value for Tr. Does that make my filter unsuitable for reports to AAVSO?

Anyway, I have made a new version of my spreadsheet covering my measures of ngc7790 and sa114. After cleaning up and adding compstars, it looks a little bit less scattered. Is this good enough or does it look strange? As You stated, this is not only science, it's an art form and it surely takes some practice to know it. Of course, I will repeat this for a couple of nigths, to average out any errors.

All the best,

Tomas Wikander

Transformation Coefficients
WBY
WBY's picture
Offline
Joined: 2010-07-24

Does anyone know what the photometric values are for IDs 32 and 33?

Arne, did they miss the page break or were they omitted for some other reason?

 

Brad Walter, WBY

Transformation Coefficients
WBY
WBY's picture
Offline
Joined: 2010-07-24

Odewahn, et al; 1992PASP 104 has good photometry in B,V&R for part of the field covered by Arne's photometry of NGC 7790. they give values for the 32 star as

V=14.248 (0.005) B-V=0.541(0.004) and V-R=0.335(0.004) with either 20 or 21 observations for each. You have to be careful reading the tables of this paper. the "4" often did not strike well and it can be difficult to distinguish it from a "1".

 

If you look up this paper, 29, 30, 82 and 97 correspond to 13, 14, 18 and 17, respectively, in Arne's numbering. Most values for the common stars agree fairly well but there are a couple of exceptions that differ by about 3 quadrature combined standard deviations of the reported uncertainties for a particular color.

Transformation Coefficients
roe
roe's picture
Offline
Joined: 2010-07-25

Following the lead of Tom Krajci I've developed a spreadsheet (ODS but can be saved as XLS) to calculate BVI coeffients from M67 images (I've also done a Landholt field and could easily extend it to others).  I developed a VPHOT sequence for selected stars in M67 that are labeled according the the key image that someone(?) published.  I understand this sequence could be shared with others.  

In use, one uploads B, V and I images to VPHOT (V and I required, B optional), use the aforementioned sequence and downloads the VPHOT reports.  The VPHOT data are manually transcribed into the spreadsheet according the star id tags and the transformation coefficients are calculated essentially instantly.  If there is no B info, the associated transforms are meaningless but the V and I values are good.

I've hesitated to publish these spreadsheets because I heard Arne was going to do something similar (and, to quote Jim Croce, you don't tug on Superman's cape!).  I could, as a fun project, add more Landholt fields around the sky or NGC 7290 or whatever if there is interest.

I also have a spreadsheet to apply the transforms to data.  The coeffients are entered once and the instrumental values are entered as needed and the transformed values are calculated, again essentially instantly.  As best as I can figure out, it requires the instrumental values to be measured with only one comp star.  Arne tells me if you want to use an ensemble, you have to do the different comp stars individually and then average the transformed values.  Looks like a LOT of work and I haven't figured a way to automate it.  Maybe a programming guru could apply his/her talents to the problem?

Jim Roe [ROE]

Transformation Coefficients
CTX
CTX's picture
Offline
Joined: 2010-07-08

Thomas,

 Your Baader R filter should be fine… don’t worry about it.

Now that you have added more stars and more color variety your data looks better, over all to me.  Some of your corrections seem a bit “high” but after looking at both your Landolt and NGC7790 data I would say they are at least in the same ball park, all things considered, and in ~ agreement with one another, which is a good verification of your process.

 If it were me I would use the NGC7790 computed TC’s simply because they are based upon a much larger population of stars than the Landolt FOV as well as a much higher airmass.

 Even though some of the TC’s might be considered a bit high, they are what they are and yours is a good example of why we do them.  Your data is reflecting how well your Scope and CCD combination is showing how your instrumental values differ from the true value as function of star color.

 It is also possible that M67 may not get high enough for you in your location and that NGC7790 with a larger data base may be the best choice; however, I would still urge you to try M67 if at all possible.

 It will be interesting for you to now apply your TC’s to a specific FOV.

Good Observing,

Tim Crawford, CTX

Mentoring Team


PS- feel free to email me directly if futher questions: tcarchcape@yahoo.com

Transformation coefficitients
eavesm
eavesm's picture
Offline
Joined: 2010-07-26

Hi all, sorry if I am missing the obvious but surely Tv, Tb and Tr change with airmass do they not (because of extinction) - i now cannot understand how to input a value for these into Vphot telescope set up as the value would change depending on the airmass of any particular image? - any help would be much appreciated.

Cheers

Martyn

Late answer
rmu
rmu's picture
Offline
Joined: 2010-07-23

Hello Tomas,

Now I am doing photometry on Landolt standard fields too, to compute transformation coefficients as you. Thanks to your message I am discovering the NGC7790 standard field, how difficult is to find such a positive declination field! All Landolt fields are on the celestial equator, wich is not accesible from my observing site, and I could only use M67.

I find your calculations on the xls very good. As I am running several imaging sessions for the M67 field, I find that band coefficients Tb, Tv, Tr are quite sensible to conditions, as color coefficients varies less over observing sessions. That's because I think your values fluctuate in a normal way. The only point is that it seems you have used few stars to calculate trasformations.

I think you should credit more the NGC7790 results, because Landolt SA114 it's too low for your location. Have you ever try with M67?

Best regards

wthb wrote:

Hello all!

I have, quite recently, been affected with the "photometry bug". Up till today, I have been using iTelescopes and Sierra stars for that purpose. 
Last week, I received my new ccd-cam (QSI6893) and BVR-filters. (Oh, happy day!) My telescope is a Skywatcher Explorer 190MN.

To get things right from the beginning, I'm trying to determine the transformation coefficients for my system. Due to the fact that I'm a newbie, can someone take a look att my procedure and results? It's a lot to learn, mostly in a non native laguage, and my mathematic knowledge is somewhat out of date. Better to discover my mistakes before reporting any observations.

I have taken photos of ngc7790 and Landolt SA114. The images have been calibrated (bias-dark-flat), platesolved and uploaded to vphot. In vphot I have loaded the compstars and created a sequence with only compstars to read the instrumental magnitudes. The images for ngc7790 was taken in an airmass of ~1, FWHM ~3. The images for Landolt SA114 in an airmass of ~2, FWHM ~5-6. The Landolts is a little bit low in my sky, but I'm living on North 60. 

I'm attaching a spreadsheet with my measurements and calculations. The values for Tbv and Tvr seems to look quite similar for the two different fields. The values for Tb, Tv and Tr seems to be a little bit more scattered. Maybe that will be averaged out after analysing more standard fields? Also the value for Tr seems a little bit high, have I missed something?

So again, I would be very happy to have this checked.

 

Clear skies,

Tomas Wikander
Borlange Sweden
WTHB

Transformation coefficients
CBLA
CBLA's picture
Offline
Joined: 2011-07-16

I have been working over the past few days to determine the transfomation coefficients for my system (I have a B and V filter).  I realize that this is a topic that pops up periodically, so I tried to learn as much as I could from other people's experiences in the forum.  I wound up imaging M67 using the process described in Priscilla Benson's paper.  Attached is my excel worksheet with my results. I feel pretty confident that I did the process correctly which means I probably made a mistake.  I would appreciate feedback on any glaring errors.

I am unsure of the best practice for using the coefficient information.  I have already entered my values into VPhot (Tv, Tb, and Tbv).  For a single target should I take 3 images in each filter, stack each group of 3, and then perform the transformation?  Typically, I have used time series to get a better idea of the uncertainty, but that doesn't seem to be an option here.  I just want to make sure that after having gone through the process of figuring out the coefficients I am using them correctly.

Thanks

Landolt +50 standards
Tonisee
Tonisee's picture
Offline
Joined: 2011-06-23

Tomas and others, there is a recent paper by Arlo Landolt, certainly worth to view.

http://adsabs.harvard.edu/abs/2013AJ....146..131L

It's extremely useful for those living close to 60N latitude! What's really nice, it contains quite a lot of areas, most of them have also couple of brighter stars (V~8). Authors have done great job, publishing also a table for different telescope FOV-s: area center coordinates, size, brightest star, maximum colour etc.

By the way, a great addition to thise "newer" areas is SA98.

Tõnis

VPHOT sequence for M67
nmi
nmi's picture
Offline
Joined: 2010-08-01

Jim,

I'm interested in the VPHOT sequence for M67.  How can I obtain it?  I'd also be interested in a copy of your spreadsheets if you would care to share them.

Thanks for your comments and help

Mike

standard cluster sequences
HQA
HQA's picture
Offline
Joined: 2010-05-10

Hi Mike/Jim,

I've actually been creating standard sequences for a number of clusters.  Matt Templeton did a search of clusters to find some that looked good for transformation, trying to pick some in the north and some in the south.  We're obtaining photometry using AAVSOnet for the new clusters. I also have existing photometry for M67, NGC7790, IC4665, etc.  So as time permits, I've been adding that photometry to VSD/VSP so that programs like VPHOT have the stars available.

In the case of M67, I uploaded today a set of 213 calibrated stars.  If you search VSP with a center of 8:51:24 +11:48:00, and select "would you like a standard field chart?" (and I strongly suggest selecting "none" for "what other variable stars should be marked?" because there a bunch near M67), you will get a chart with the calibrated standards marked.  These will be good for any transformation analysis.  I confess that I have not compared this calibration (from NOFS/BSM/AAVSOnet) with published photometry; that might make a good datamining post by someone.

Coma Berenices has also been entered; the other clusters will be entered as time and calibration permit.

Arne

VPHOT sequence for M67
roe
roe's picture
Offline
Joined: 2010-07-25

NMI wrote:

"I'm interested in the VPHOT sequence for M67.  How can I obtain it?  "

I shared it with you in VPHOT.  Let me know if you got it.  As to the spreadsheet , send me your email to Jroe@jamesroe.com.

 

Found different NGC 7790 reference numbers...
mgw
mgw's picture
Offline
Joined: 2010-08-01

I made an error on my previous note - I just found the NGC 7790 star numbers in VSP are different than those on the original field data.  I'll run tests later to compare with yours and get back to you.

AAVSO 49 Bay State Rd. Cambridge, MA 02138 aavso@aavso.org 617-354-0484