We had good success on the Nova Forum in critiquing observers and improving the photometry. Unfortunately, Nova Del is in its nebular phase, which means almost all of the flux from the star is through emission lines. These make comparison between observers almost impossible, as you can't transform cleanly and your particular filter/camera/telescope combination may have a different throughput for the various emission lines than is the case for the next observer. Continue observing it (please!), but perhaps it would be good to choose another star for critiquing observers.
So I'm going to have a few test stars over the next year, and I offer my expertise in critiquing your observations. These test stars will be non-pathological: no emission lines, no strong molecular absorption lines, no flickering. This limits us to primarily regular pulsators and eclipsing binary stars. However, the goal is for everyone to submit observations that fall within 0.01-0.02mag of everyone else. It can be done!
Rule number 1: you have to make your observations as "sets", with a minimum of 3 images per filter. Results need to be averaged and the report needs to be the average value and the standard deviation from that value. I'll post Richard Sabo's Windows program on the website as a helpful tool.
Rule number 2: you can do single star or ensemble, as the comparison stars for these field have well-determined magnitudes and colors.
Rule number 3: you need to observe in at least two standard filters. I'd choose B and V as the best; V and Ic as second best; any number of filters is ok. I'll probably do BVRI myself, but may include Johnson U, for example.
Rule number 4: you need to transform your data. This is the hardest part of the experiment. You can post untransformed data at the beginning, but learn how to transform, get your coefficients, and replace the untransformed data with properly transformed data before long.
The first object: XZ Cet. This is classified as an anomalous cepheid, with a period of 0.823 days. At 2hrs RA, it is an early evening object. You do not need to have a cadence of more than 10-15 minutes, and once/hour or evena couple of times per night is fine. So you can go off doing other projects inbetween your measures. I've posted a comparison star sequence. None of the stars is comparable in brightness to the target, but you can adjust your exposures so that the target is not saturated, yet you have good signal/noise in at least one comp. Note that most of the comps are to the east/northeast of the variable, so you might want to offset the variable in the field to pick up comparison stars. The variable is 9th magnitude, so watch out for scintillation; it is at -16 Dec, so don't observe past 2-3 airmasses (that in itself may limit how long you follow it!).
Good luck! As observations are submitted, I'll post on this thread any comments that I might have. I expect others to contribute their views as well! If you don't like this star, wait for a month or two and I'll have another one for you.
Arne
Brad,
Yes, I was implying that your near seal level, often humid location does not help when it comes to R band observations. Most amateur R band data taken at or near seal level is simply not very robust, and as Doug indicated, if you have BVIc it doesn't really add anything to the mix.
Mike
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Yes, I was implying that your near seal level, often humid location does not help when it comes to R band observations. Most amateur R band data taken at or near seal level is simply not very robust, and as Doug indicated, if you have BVIc it doesn't really add anything to the mix.
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I am probably missing something here, but Rc is not affected any more by humid conditions than any other passband. Mike, perhaps you are confusing this with Johnson U, which is difficult to do from sea level?
Arne
I agree and disagree with Doug. It is true that V and Ic give you the general spectral distribution of a star, but Rc has some unique characteristics that make it useful. It is a wide filter that matches the quantum efficiency of most CCDs, and therefore gives you the deepest images for an equivalent exposure time. It is not as susceptible to atmospheric extinction variations as Ic. Perhaps more importantly, it is the wide-band filter that includes Halpha, and gives you a rough handle as to whether that line is in emission or absorption and how strong or variable it is. For example, the phased light curve of W Vir (2007AJ....134.1999T) shows relatively smooth light curves at BVIc, but much larger scatter at Rc, indicating Halpha variability. Often on pulsators, you can identify specific phases at which a star shows Halpha emission (presumably shock-induced), which then can be used to plan spectroscopic observations. Rc can also be used for creating tricolor images that closely match the usual RGB system.
If I have a restriction of only having three filters, then BVIc makes the most scientific sense. If I have more space in my filter wheel, and have the funds, then including Rc is perfectly reasonable.
Arne
I agree and agree with Arne! :) I should have said "normally" or "frequently" - or at least "sometimes".
If your interests are stars with H-alpha variability - then you would *always* want Rc.
Cheers,
Doug
If you are interested in Ha, another approach would be to use spectra or narrowband filters to isolate the Ha or other narrow lines. We used the narrowband Ha filters at MMO to separate the Emission and Continuum.
Gary
Thanks everyone for the clarifications re R passband. I think I will keep it in the mix but I am going to modify my routing for the T Taur star campaing to include Ic and not stop at R. What's a few more flats and darks and an hour or two more analysis between friends?
Gary,
Thanks for direct comments re apertures and fixex for the Mewlon for use with the big format camera. I will contact TNR today about the internal corrector.
I am convinced that a few of the odd results I am getting in my growth analysis result from differences in FWHM among the stars on the same image. That was why I was doing all of those FWHM spreadsheets but the thing that really showed up the profile skews was interactive 3D plots in Mira-Pro. My R set of V1264 Tau images from 12/23 had a large spread in FWHM among stars on the same image Using averages of 6 images V1264 FWHM = 4.87 pix, C127 FWHM = 4.3 and K122 FWHM = 5.3. I can improve this with better postioning of stars on the image. This was my first night's imaging for this star and I plunked the variable in in the middle of the image since, from a superficial inspection of the 45' VSP plot, I thought I was going to use the 126 and 127 stars. Hoever, 126 has interference from a star just to the north. The centers are only about 9 arsec apart (14 pixels for me), ~2.7 FWHM. I might be able to use Steve Howell's small aperture growth curve method to separate these two stars. It would probably work at 3 FWHM. It fails at around 2 FWHM. You could use the star just to the west of 126 to obtain the growth curve. It is similar magnitude and close but not close enough for interference. I am going to try that for the heck of it just to see how it works out. If that works out and gives beter results, I may recalculate the mags and re-submit using the 126 and 127 stars. However In the future I will set the center of the image equidistant from the three stars.
Brad
Arne,
I am collecting data on NGC 7790 with a new camera and filters to calculate transformation coefficients. In the process of doing that something occurred to me while looking at the data for the various field standard stars (see attached). From examination of the error terms, it is clear that the errors for color indices are calculated empirically as the standard deviation of the color index values calculated from individual measurement sets rather than, for example, SQRT(Verr^2 + Berr^2). So what error do I use for, say, B when I have the V error and the B-V error? My automatic response would be SQRT (B-Verr^2+Verr^2) but that only applies to independent error terms and the V and B-V error terms in the table are not independent since they involve the same measurements. Also since they aren't independent, in this case it seems to me adding the two together in quadrature will overstate the error. However, it is equally clear that I can't use SQRT(B_Verr^2 - Verr^2) since the B-Verr isn't SQRT(Verr^2 + Berr^2) and is nonsense in a substantial number of cases in which the B-Verr is smaller than the Verr.
So for my Berr example, do I just pick the larger of the two errors, Verr or B-Verr as the uncertainty in the standard star B magnitude?
This is a matter of curiosity and it is something that it seems to me one has to deal with regularly using standard star field magnitude data, which typically give V errors and the errors of the various color indices rather than errors for the individual filter magnitudes.
Brad Walter