Affiliation
American Association of Variable Star Observers (AAVSO)
Mon, 01/19/2015 - 20:11
Unless I am missing something that should be obvious where do I find the BV for the target variable star? The target doesn't seem to be listed on the photometry table and some stars don't show up on google. In particular I am looking for the value for R Peg.
Hi Ronald,
As of now, VSX does not include color information for the stars, only the Vband min/max values and the spectral type. I usually go to VSP and look at the light curve, where you will find R Peg to have a (B-V) of approximately 2.0.
One of the products of the APASS survey will be mean magnitudes and colors of all variables that are brighter than V=17. Those will be added to VSX sometime in the next year. Until then, look at the AID through the use of VSP to get good approximations.
Arne
An other way to run down the B-V value of stars is to use VizieR:
http://vizier.cfa.harvard.edu/viz-bin/VizieR
In the search by positon box simply enter the name of your target and check the various catalogs that pop up. Usually there will be a couple that have B/V info... just make sure the coordinates match your target. Also Keep the serach radius short do you do not get a lot of data you are not interested in.
Ad Astra,
Tim Crawford, CTX
Sorry for making this late reply, but I think it's an interesting problem to solve.
Often the color of a variable star is just as variable as its luminosity, so looking up the color in a catalog sometimes is not really satisfactory, right?
If you cannot actually measure the color index (e.g. you lack a B filter), the question is : what do you need the B-V color index for? If the answer is that you need it for transforming your results to a standard color system because your filter doesn't exactly match the photometric standard filter you want to report for, then I guess it's fair to say that you will need the B-V color index of the target star only with an accuracy of (roughly) 0.1 , as this index will be multiplied with a transformation coefficient that is usually of order 0.1 and then added as a correction term to the magnitude, so any error in the B-V value of 0.1 will then propagate to an error of just 0.01 in the magnitude, which often is better than the uncertainties in the magnitudes of comparison stars.
Which makes me wonder whether a B-V value could be approximated with DSLR measurements using the tri-color RGB Green and Blue filters. It is well known that the TG filter is reasonably close to Johnson V and that the TB filter is absolutely *not* close to the photometric B standard. But since we need B-V only to an accuracy of ca 0.1 mag, maybe it's still worth looking at (TB-TG) . For example when I correlate (TB-TG) with calatog values of (B-V) for well characterized comparsion stars, I find that for my DSLR (EOS D1100) I get
(B-V) = (TB -TG) * 0.55 (+/- 0.1)
Because you cannot help to make multi-filter exposures with DSLRs, it's quite trivial for DSLR oberservers to check this for themselves even with archived images.
I remember that Roger Pieri has done some extensive work on the B and R filters of DSLRs and I'm quite sure this (B-V) = (TB -TG) * k rule will not work well for certain stars with certain spectral features, but still I'd like to hear from others about the practicality of estimating color to within 0.1 mag with DSLRs.
CS
HB
Hi Heinz-Bernd,
You are correct that some stars are not suitable for DSLR color photometry, i.e. stars with bright emission lines or large absortion bands. But excellent B and R photometry is possible for many variables.
I use a modified version of the Citizen Sky Intermediate tutorial which includes a calibration of the catalogue B-V versus measured b-g curve (simple linear plot) using the Check star and 6 (or more) comparison stars. In the attached plot blue circles are data for Check and Comps, the green trianle is catalogue B-V of the variable star (QZ Car in this case), and the red empty circle is the calculated B-V of the target using the calibration curve.
In this case the measured and catalogue B-V for the target are within a few millimag. Of course this technique relies on the Check and Comp stars being suitable for DSLR, if any of them were significantly off the fitted line I would reject them as not being suitable as Check or Comp stars. Cheers,
Mark
Thanks for the
Hi Mark!
Thanks for the example data, very impressive and convincing, and quite in line with what I've seen in my observations so far (on a limited number of different targets, tho). I think this is a significant thing to look into, as you get those TB measurements for free with DSLRs.
CS
HB
If you want to generate transformation coefficients for your DSLR now is a great time to do it. M67 is well positioned just east of the meridian at about 22:30. that gives you a whole bunch of standard stars to use in ploting your color indexes against the J-C indexes. take data near the meridian on at least 3 nights and average the coefficients you get and you are all set. I don't know if TransformGenerator will work for this task. Perhaps someone who knows can chime in. If it does, that make the process even simpler. Particularly if you use VPhot to do the photometry. It already has positions loaded for all of the M67 standard stars so you don't have to spend hours clicking away.
Brad Walter
Thanks, excellent point, I hope to do this this week (forecast looks good for Wednesday and Thursday).
CS
HB
Hi!
Is it just me or is the website that is supposed to have the reference photometry data for M67 from Arne currently offline (or super-slow)?
http://binaries.boulder.swri.edu/binaries/fields/m67.html
Is this mirrored somewhere else?
CS
HBE
CS; I had the same experiance, link leads to nothing and I could not find it with a search either. A bit more typing but I am using the AUID # as a label.
Bikeman;
The current standard data for M67, recently re-generated by Arne, is available on the VSP. Use ev cnc as the variable name (in the cluster). M67 doesn't work since it is obviously not a known variable star. Remember to check want 'standard field' in advanced options section. Can get either chart or photometry table by proper check box. The photometry table will report comps as 'standard'.
BTW, won't the cluster be quite small in the field of your dslr? Hard to resolve many comps? And they range from 10-17?
Ken
You can also enter the coordinates, 8:51:22.83 11:48:02.01 and specify a standard chart. I did not consider that you might be using a DSLR with a relatively short (<, say, 500 mm ) focal length lens or telescope. If you are imaging with short focal length, then the stars of the central part of cluster may be too densely packed. You might try making two VSP plots, one at about 30 min FOV which will allow you to see stars that are less tightly packed around the perimeter of the dense, central area of the cluster, and a second chart at about 75 min FOV which will allow you to see about 16 additional outlying standard stars. I would also suggest that if you are imaging with short FL then you probably don't need to set the magnitude limit as high as 16, perhaps 13 or 14 and that will cut down the clutter substantially. You might also try the option to include lines with magnitude labels. I find that helpful identifying the correct star in the denser parts of the cluster. If you plate solve, VPhot will pick the standard stars by location, but you will have to choose which ones to use in TG because, if I recall correctly, it will pick all all the stars in your magnitude range even ones that suffer from crowding due to your short FL.
Brad Walter
Thanks for all the good hints. I'm doing this with a 750 mm Newtonian illuminating a 12M pixel APS-C sensor, so the resolution of the cluster stars should be kind of ok, and photometry down to ca 14 mag is quite feasible. CS HB
For illustration, this is a 1:1 crop of M67 thru my telescope/sensor combination. That should allow to use quite a few of the standard field references, too many actually to do this manually for my taste, I guess it's time to use SourceExtractor again.
CS
HB
If you plate solve the image you can upload VPhot and it has a sequence with all of the comp stars pre loaded
There is (or was) an excellent on-line plate solving website that works very well, better than Maxim using Pinpoint. I tried to access it a few minutes ago without success to see if it accepts images in any form other than fits. It may just be undergoing maintenance but the "URL cannot be found on this server" message was ominous.
If you can plate solve your image than VPhot has the sequence for the standard stars of M67 and NGC 7790 pre loaded so you don't have to click on anything manually. You should check your plots for outliers. If I think but am not positive that Transform Generator will work for a DSLR as long as you have more than and band you can work with either b or r if you are doing transformations for a DSLR. You need to confirm this with DSLR knowledgable guys like Mark Blackford. TG has a great intuitive and easy visual method that displays the 3 sigma band on the plot. But only exclude outliers once. Don't do it iteratively.That is a statistical no-no.
Brad Walter
Yeah I tried nova.astrometry.net which seems to be back now after some maintenace earlier this weekend. FITS is ok as a format, I use IRIS to split my RAW images (after vonversion to DNG) into the RGGB channels and then AIP4WIN to stack them, and finally save as FITS. I will try VPHOT later on.
CS
HB
I tried again, as well, and logged on to let you know it was back up and saw your last post. I am glad that it was only down for maintenance or whatever and not GONE.
Brad Walter
So finally I put all the pieces together. Astrometrý.net gave me the plate solution, VPHOT did the photometry for the standard field comp stars, and after excluding faint and blended stars I got what looks like a credible transformation for the DSLRs Green filter to V :
It would be really interesting to take this DSLR transformation, one step further. That is, using VPhot for your r, g;b subframes, measure the selected comp instrumental magnitudes for each bayer filter (assume g=v), download them and import them into TG. Then you could use TG to calculate all the transform coeffs. Are you willing to try? You would need to set up TG on your computer and run the data. Not really difficult and we can provide help.
Ken
Hi!
> It would be really interesting to take this DSLR transformation, one step further
> [...]Are you willing to try?
Definitely, and this was my motivation to get involved in this thread in the first place. I have this suspicion that especially the RBG Blue filter in DSLRs can be really useful if used with care, and ignoring it altogether and just use the Green channel is a big waste. I learned about TG during the 2014 fall meeting of the AAVSO and wanted to try it ever since, I just lacked the time and good calibration data..until now.
I'd also like to make some more measurements of M67 but weather here in Germany is not favorable lately.
CS
HB
Hi HB:
GREAT! If you haven't already installed TG, go for it. It normally installs without problems. Don't give up if your hardware/software gives you a problem. Ask for any help.
Try out your existing M67 images rather than wait for more? You may want to make any fits header changes from g to v early in the process. VPhot and TG understand b, v; r filters. In VPhot, I think you may want to select and save a subset (not too faint or merged) of all standard comps in your own sequence. Do you still have a copy of the handout from the talk at the Fall Meeitng? It discusses downloading instrumental magnitudes and importing to TG.
Good luck with this endeavor that will potentially help other DSLR observers too.
Ken
[quote=MZK]
Hi HB:
GREAT! If you haven't already installed TG, go for it. It normally installs without problems. Don't give up if your hardware/software gives you a problem. Ask for any help.
[/quote]
Yeah, I'm used to that..I work as a scientific software developer ;-)
[quote=MZK]. Do you still have a copy of the handout from the talk at the Fall Meeitng? It discusses downloading instrumental magnitudes and importing to TG.[/quote]
Yes, I still have that handout. I'll report back when I have some results, but tonight might be a chance for some clear skies ... TG and VPHOT are for the cloudy nights ;-)
CS
HB
I will chime in to agree that it would be very interesting to see what the full set of transforms that TG would give. I bet getting the full set using TG is MUCH easier than calculating one transform manually.
By the way that looks like a pretty reasonable slope. More than one would expect for a monochrome CCD with J-C filters but not a very large amount greater - and that makes sense.
Brad Walter
So finally my B-channel image made it thru the VPHOT queue , and the fit of (instrumental - catalog B mag) vs (B-V) looks surprisingly good (same subset of stars as in previous fit for V , Green magnitudes).
Currently downloading TR and its dependencies, more later.
Hi HB,
your blue curve looks very good. Is this from a single image or the average of several?
Try the red channel, I think you'll find it can be used as well for many stars. It would be interesting to see at what B-V values the linearity starts to breakdown. This is an experiment I've wanted to try for a couple of years but haven't found the time yet. Cheers,
Mark
If youTry Transform Generator (TG) you will NEVER go back to doing it with spreadsheets. I urge you to download it and try it. Take it from a guys who has done it manually a number of times and was very skeptical about using a canned package. It is simply fabulous with a great tool for excluding outliers. Point and click.
Brad Walter
Attached are a couple of the plot screen shots from TG 5.3 showning 3 sigma lines original and after excluding outliers (which you only do once on the origninal. the latest version handles the 3-sigma guide lines a little differently. I don't think it moves them after points are excluded.
Then it allows you to select the night you want to use and averages the transforms for you as in the last screenshot
Based on requests from several users, 3 sigma error lines were added to the TG plots. Here's how they work:
The initial plot of a transform shows the 3 sigma error of the linear data fit in purple. As a user removes outliers, the original purple lines remain, but new dotted blue lines are added indicating the 3 sigma error using the remaing points in the transform calculation. See the attached screen capture.
FYI, on a separate item, George found a labeling error on the plots. When points are removed, the message shows the wrong star label. This does NOT affect the transforms - it is just a label problem on the plot. We're testing a fix that will be released shortly.
So finally, after much distraction from the daytime job ;-) I managed to put my M67 RGB DSLR images thru the TG software.
As expected from previous Excel-based attempts, the red channel might not be useful.
The results are below. Tv_bv seems a bit high, I think I'll do soem more checks on this.
What this does seem to confirm tho is that the RGB-Blue filter is quite well-behaved in terms of transformation.
I also enclosed the raw photometry output from VPHOT. I used a SNR threshold of 120 which gave me enough stars to work with, I'm not confident in the actual magnitude of teh SNR values from VPHOT (gain just estimated).
Airmass is never taken into account, neither by VPHOT or TG, right?? The timestamp in the input image was probably wrong and so must be the airmass calculation.
CS
HB
HB:
I was anxiously waiting!
Looks like err and r^2 implies you had tight plots. How did they look? Did you have to delete many outliers? Can you share? Did you really use SNR=120? Lose a lot of other comps although the brightest stars are the ones that show the widest color difference. What was the range of color difference for your comps? As you know, it's easy to change SNR in TG and see the impact.
Why do you think the red channel may not be useful? I think you have said this before but I'm not sure you provided a reason?
We really need another brave sole to try this test as well to see if the transform coeffs are typical for a DSLR? Anyone else have a DSLR attached to their scope?
"Never" is a long time, but airmass is not used by VPhot or TG for these calculations!
Ken
I was thinking about sharing the plots but instead attached the raw input files, so the curious can, for example, chose outliers and SNR threshold on their own judgement :-) .. Just select VPHOT as input format and load all three files at the same time into TG.
The fit and the number of outliers look worse for those transforms involving the red filter, so I'm not too optimistic about the R channel's usefulness. But B looks promising.
SNR: The actual SNR values will be wrong, but still the higher, the better, and I picked the threshold so that I get at least ca 10 comp stars. Maybe that's not the best choice, but the raw VPHOT files contain a lot more comps so everyone can try for him/herself.
The comps are in the color range B-V = ca 0 ... 1.6
I hope this is useful, I'd really like to compare this to other DSLRs.
CS
HB
Of course, duh! Done. Thanks.
Ken
I've worked up a set of transforms for my Canon T5 at 300mm (zoom)
Tbv 2.866 +/- 0.080
Tb_bv 0.446 +/- 0.025
Tvr 2.151 +/- 0.085
Tr_vr -0.556 +/- 0.085
Similar to yours.
This was just a first try.
George
Hi George, Ken and Heinz-Bernd,
I intend determining tranformation coefficients of my DSLR+ED80 system using VPhot and TG. All I need is a clear evening without the moon too close by so I can image M46 while it's still reasonably well placed. Weather here in Sydney has been very poor recently and doesn't look too promising for a few days at least. Cheers,
Mark
I am looking to update the TA (TransformApplier) program to handle DSLR data. So by all means develop your transform coefficients! TA would recognize the DSLR filter set (TB,TG,TR) and know that you are transforming to B,V,R.
One issue I'm still working on is that we were warned that the TB and TR response are significantly different from the Cousins B and R and that you should only try to apply (and develop) coefficients on stars that are not abnormal (ie, do not have strong absorbtion/emmision lines). So, is there list of normal spectral classes that would work and is there a way to query for a star's spectral class? That's what I need to investigate.
Any suggestions on how to do this and make TA a tool for DSLR photometry? I'm looking for input.
George
SGEO
Hi George,
I am not using the classical transformation with DSLR but another technique I dub "VSF" (for Vmag Synthetic Filter). This is in fact just a color temperature correction like done in photography. It is a combination of the RAW B,G,R DSLR outputs like Gc=G+a.R-b.B that at photon/electron count level. Then the Gcm mag is conputed from Gc as usual. I did an assessment of this technique through a computer simulation based on the spectra of the Pickles library and the reponse curves of the filter system of the DSLR. It shows a significantly better correlation with the Pickles data than the classical transformation. You could find the description of it and the results for Vmag in my paper:
http://www.aavso.org/sites/default/files/jaavso/v40n2/834.pdf
I use that technique for number of years with excellent results and it's much easier to apply than the classical transformation and its difficult calibration.
Since then the technique has been applied to B-V and V-R. During the Nova Delphinii 2013 campaign we had long discussions about the filters issue and I provided a simulaion result of what it means for the DSLR, the simulation includes both the classical transformation and the VSF.
The correlation between Pickles (B3-V), Bessell like filters (noted Bz-Vz) and the DSLR B-V (Bm-Gm in my table) is good for most stars.
The case of the nova (OTxxx) shows more deviation but not a disaster. What is clear is the huge correlation change between Bm-Gm and Gm-Rm that clearly shows the effect of the huge Ha line variation of the nova.
Not in that table is the case of the M stars. Part of them have a large deviation of their B-V with regard to their temperature, this is due to a significant large band blue excess, not lines. The DSLR Bm-Gm is less affected by such excess than the Johnson filters, resulting in a large deviation.
The DSLR B-V and V-R are very reliable as the measurments are made at the same instant, using the same electronic chain integrated in a single Si chip. That fact implies there are no deviations between the chains due to any factors. It should be much more considered instead of being rejected by number of photometrists. I think we should have a way to directly report such DSLR color index, they are very meaningful.
If you are interested I can re-run the simulation including the M stars of the Pickles library.
In the attached table:
- All stars are normalized at Vmag=1 at AM=0 , by the way the Vmag results are the deviations.
- kt is the classical transformation coefficient for V, KR and KB are the coefficients of the VSF at AM=1
- V-Rc and B3-V are the color index provided by Pickles, the reference.
- Bz-Vz is the classical index from Bessell like filters at AM=1
- Bm-Gm and Gm-Rm are the color indexes from the DSLR using a specific transformation
- Vjm is the Vmag from a classical transformation from a Bessell filter
- Gm is the raw mag from the DSLR
- Gtm is the DSLR V mag using the classical transformation
- Gcm is the DSLR V mag using the VSF technique
Clear Skies !
Roger
Thank you Roger. Fascinating stuff.
As Arne has pointed out, transformation is not exact; the goal is to get closer to the standard system and there are many ways to do it and many ways to add refinements.
Let me study this a bit.(that's a 15 page paper that you sent!).
I'm in the process of adding some DSLR capabilities to TA, but this would be using the classic transforms, moving TG to V, TB to B and TR to R and trying to build in checks to identify when the assumptions about airmass same for comp/star break down. Part of the problem is the limitiations of the AAVSO extended format that we need to work with to feed the data in WebObs/AID.
Cheers,
George