AAVSO Alert Notice 545 announces an observing campaign from Dr. Colin Littlefield and colleagues on the intermediate polar FO Aqr; time-series observations are requested. Please see the notice for details and instructions.
Many thanks and good observing!
Elizabeth Waagen, AAVSO HQ
I think I need some help on the FO Aqr campaign. For my photometric observations I have always used BVRI filters, and when appropriate have used TG to transform the data. I have never used CV observations, so I am unclear as to how I should go about taking unfiltered images with a V zero point per Dr. Littlefield’s request. I do have a clear filter, but I do not know how to acquire and apply a V zero point to those C images. I use Maxim DL for image acquisition and AIP4WIN to reduce the images. Can someone either please enlighten me as to the process or give me a link that will explain the process?
Josch is correct. "Unfiltered with V zeropoint" (or "CV" for short) means that you observe without a filter, and then use V magnitudes for your comp stars when performing photometry. The benefit of this technique is that it maximizes the signal from the target star, enabling an observer to use shorter exposure times to better detect rapid variations.
(Also, just to clarify the Alert Notice: I don't hold a PhD yet.)
I processed my first time series of FO Aqr, using Luminance images and V mags for comp and check stars. MaxIm DL 6.13 will happily generate an AAVSO report, but WebObs chokes on the Lum filter -- not allowed. Hrmpf! How do we report the data, then?
You will have to report your observations as "CV" (meaning clear with V-zeropoint) in order for WebObs to take them. It is my understanding that a "Luminance" filter is essentially the same as "Clear". The difference is that "Luminance" is wavelength-restricted.
I think this means editing my FITS header information, since we have Luminance coded in, and neither MaxIm nor WebObs will be fooled. I don't know of any way to do this retroactively, and I have some hesitance about doing this, since the edits would need to be consistent across MaxIm DL, ACP, ACP Scheduler, VPhot, etc... If I forget one, that could screw up our robotic operation.
I wonder if I can just use a find & replace sort of edit on the AAVSO txt file only?
You've entered specific filter names into MaximDL, which are then used by ACP to create FITS images with that filter name. Even during the basic photometric extraction, the MaximDL name should get carried through - after all, that is what you wanted when you set it up, it is what you need in order to properly dark subtract and flat-field your image, and certainly want it to be that way for non-photometric projects like deep-sky imaging. Where the change occurs is when you create the AAVSO report by "standardizing" the measure - using a comp star with a given magnitude to create a magnitude for submission. At that point, extra information is added: you have the original filter (whatever you named it), along with the bandpass and magnitude of the comp star. That is why WebObs is looking for something specific to indicate how the estimate was made. In this case, it is "CV", for "clear" and "V-band comp star values". You could also have reported it as CR if you had used the comp star Rc-band values, since the luminance filter covers both of those bandpasses and most CCD sensors are most sensitive near the center of the Rc bandpass.
So I agree with Sara - find some way of modifying your Extended File Format text file to replace "LUM" with "CV" before submission. Luminance to the AAVSO means nothing, because you are not saying anything about the comparison star values that you used.
I'll just edit the AAVSO report now that I figured out how to do it.
I had no idea it was so easy to edit the .txt file in Notepad! Once I poked around a bit, POOF! all instances of "Luminance" were magically replaced with "CV" and WebObs was happy. Clearly, that's the way to go, while keeping my FITS headers as thay are.
My data for FO Aqr agrees well with that of other observers, and that sure is an interesting star! I tried 3 different exposures, not sure what my saturation would be, so the next time, I'll just use one of them, and I'll be able to get better time resolution with a higher cadence. Thanks for your explanations, as always.
Luminance works with FO Aqr because it is a cataclysmic variable, which means in general that (a) its color doesn't change during either quiescence or outburst; and (b) it has a color fairly similar to the comp stars. So in this case, your measures will have some offset from those taken with a V filter, but that offset remains reasonably constant and could be removed by a researcher. It often helps to use luminance or some other wide filter when working with faint objects (a larger-aperture telescope is really the better choice!). Where using luminance as a surrogate for V fails is for the rest of the astrophysical zoo. A star that changes color during its cycle, like an eclipsing binary with dissimilar temperature stars, or a pulsating star that gets hotter near maximum, will give offsets that aren't constant, and so your light curve will look different from everyone else's. Other stars, like novae, have huge emission lines that fall outside of the V filter but impact unfiltered measures. It gets even worse for red stars like Miras - then you have a huge color difference between the variable and the comps, you have to worry about second order extinction effects, etc. If you are observing unfiltered, try very hard to stay near the zenith and don't get into high airmass regimes.
So use luminance, or unfiltered, when you understand the issues involved and you really need that additional throughput. Use filtered photometry for all other cases!
Submitted ~325 more data points from last night: 20 sec Lum images with 17" f/6.8 yields an SNR of ~100. What an interesting system!
I worked with a college student on another "polar" (AI Tri) last fall semester, which also had a rapid fluctuation as this one does. Mind bending to try to appreciate all that goes on with these uber-high magnetic field stars.
I am still confused as to how to use the C filter data and V comp star mag to derive the CV mag. I use AIP4WIN to reduce my date. Below is a line from the my date for FO Aqr from Oct. 7, 2016.
FO Aqr;2457669.57412;0.242;0.043;CV;NO;STD;000-BJN-854;13.361;000-BJN-853;11.667;1.6458;na;X16353BIR;(No comp star data for CV filter.)
Note that the mag for FO Aqr is 0.242 and the mag for the comp is 13.361. Since there are no C filter mags given on the photometry chart, I am assuming these mags are instrumental mags. Is this correct? Then would I simply add those two mags to get the CV mag? If so, I could do this using Excel by creating a new column for the results and replace the original results the with the new ones. Or is there a simpler way to do this?
Although I haven't used AIP4WIN, the mag for FO Aqr (0.242) seems to be a differential magnitude in relation to your comp star (V= 13.898 +/- 0.017, per the chart provided by VSP). So 13.898 + 0.242 = 14.140, which is very consistent with data from other observers. I would assume that AIP4WIN lets you specify a magnitude for the comp star. If it does, just specify your comp magnitude as 13.898. You should then have CV magnitudes (a.k.a. unfiltered with V zeropoint). As Arne described earlier in this thread, this enables the use of shorter exposures so that we can better detect and study the rapid variations in this system.
My colleagues and I would like to thank everyone who has observed FO Aqr. Our first paper about the ongoing low state has been accepted for publication at The Astrophysical Journal, and 17 AAVSO observers are co-authors on the paper. A preprint of the accepted version will appear on astro-ph within the next few days.
We are still actively monitoring the ongoing recovery, so please continue observing this system at fast cadences. We prefer cadences of 30 sec per image or faster if you can achieve an acceptable signal-to-noise ratio at such a cadence. The average magnitude of FO Aqr has recovered to V~14.2, so this should be feasible for many observers.
We'd like to monitor FO Aqr until it fully recovers, and if the recovery continues at its current rate, we expect this to occur in about two months.
I can jump back on this as soon as we get past this bright Moon phase.
BTW: I seem to have captured a geostationary satellite in a number of my time series... Anyone else notice a relatively bright object trailing through a few your FO Aqr images? I'll have to go back and look, but it seems to be in the same place in the sky each time. The target is probably west of the satellite by now, but I found that to be quite interesting.
Attached image is from 9-12-2016 at 2:54:23 UT (JD 2457643.6210995372); exposure is 20 sec in duration. North to left, west at top, as shown. The satellite (?) was almost eclipsed by GSC 5804-1408 -- missed by just over 17 arc seconds. This streak appeared on 3 successive images, displaced by about 10 sec of read-out time. I saw this on images from 3 or 4 different nights. Anyone else?
FO Aqr is at -08:21 declination, and you are at +44 latitude. You can do the math, but I would expect the geostationary band to be pretty close to that field center. It was at -5 declination from Flagstaff.
What is fun is to shut off the telescope drive and do imaging of the satellite. They are often about 9th magnitude and very easy to do, and during some seasons, actually get eclipsed by the Earth.
However, in the meantime, it is good that New England is getting clear weather just when needed for the campaign!
AAVSO Special Notice #425 announces the schedule (November 13-14) for XMM-Newton observations of the intermediate polar FO Aqr, the target of the campaign announced in AAVSO Alert Notice 545. Time series photometry is needed for simultaneous coverage during the XMM-Newton observations and after. Please see the Special Notice for details and instructions.
Many thanks and Good observing,
Elizabeth Waagen, AAVSO HQ
Looking clear as a bell here in Vermont. Dome is open, instruments cooling, new calibration images already taken, now just waiting for twilight to come to an end...
Great -- I hope your imaging is going well. I've got 400 x 20 sec in the can, trying to get another 200 images before its too far to the west. At 20 sec in Lum (Planewave CDK-17 -- 0.43 m), I'm getting SNR values on either side of 100, even with the Moonlight -- started around 113, now falling to ~98 due to higher airmass.
I hope a few people west of here are also imaging so we can get nearly continuous coverage. I wonder how many observers are away from their equipment while attending the AAVSO conference?
I was thinking the same thing about the conference. I got about 3.5 hours of data (uploaded) - see attached. It was 60 seconds in V with a C14. I look forward to seeing how/if your data overlaps mine.
Hopefully the XMM-Newton run took place as planned.
--- Dave LDJ
Our data agrees very well! I was going to post this anyway, so I'll try getting an image from VStar up here.
Your Johnson V data (C-14?) and my CV measures (CDK-17) overlap very nicely, along with the expected time shift due to longitude. Looks like we had very similar conditions, too. I ran 400 images with pretty decent seeing (for here), then refocused, and collected another 225 or so. The second batch were not as good as the first, with bigger error bars, but I posted them since I caught a significant dip in brightness (not shown here). I have not figured out how to transform a time series yet, so I have not done that.
To the west, Geoffrey Stone in California got some very clean data with much smaller errors a little later this AM -- NICE!
We may get more clear skies tonight, but I won't be on target -- my wife has to get a little of my time .
Newbury, VT (observatory is in Peacham, VT, about 20 miles north of me)
Yes they do agree quite well considering yours was unfiltered. The observation period for XMM-Newton would be over by now, so probably not much value in observing it again tonight. I did check the observing schedule for XMM and it still showed that the observation was to take place so presumably our data will have been useful.
Remember that discussion of geostationary satellites? Well, I had one ruin an observation, and I failed to catch it until after submission, when I saw the discrepant point on VStar. I should have checked the data more carefully before submission, but working at 1:30 AM has its drawbacks, as well as long time series of over 600 images.
The attached image does make a good demonstration of the importance of reviewing every image, not just sampling every 10th frame!
I've reported it as discrepant -- my apologies for the extra work, Sara!
Thank you for letting me know about the discrepant observation. I have removed it from the database.
Just as a reminder to everyone, you DO have the power to delete (or edit) your own bad observations yourself. All you have to do is use the WebObs Search tool (https://www.aavso.org/apps/webobs/search/) to locate the problem observation, then click "Delete" next to it. You can also delete multiple observations by clicking on the little boxes in the left column or clicking on the unlabeled box up in the header row of the table of observations to select all. Remember that you must be logged in to the website first and that you can only edit or delete your own observations - not those of any other observer.
I don't mind helping observers to edit or delete their problem observations, especially when it involves a lot of observations on only those that meet a complicated set of criteria, but if it is a simple search and isn't too much trouble for you to do it yourself, you will generally get things cleaned up sooner than if you have to wait for me to get to it.
Many thanks, Sara
BTW, that is a neat image showing the satellite going directly through your target!
My colleagues and I are very pleased with the coverage by the AAVSO. Thanks for the great work, everyone! It will facilitate the analysis of the X-ray data.
Thanks for the feedback. It was fun to observe it knowing that XMM-Newton was on it simultaneously. I plan to continue time series observations for the next while. I presume that is still of value.
--- Dave LDJ
I have attached an updated long-term light curve of FO Aqr. Different data sources are shown using different marker styles. The horizontal dashed line shows the median magnitude of the bright state based on long-term CRTS data. You can see that the recovery has stalled about 0.2 mag fainter than the bright state (as measured by CRTS). The thick, red line is an outlier-resistant linear fit to the recovery, and the bottom panel shows residuals from the fit.
Each data point (except for a few of the blue squares) is the median magnitude of FO Aqr during a time series. This helps to prevent short-period variations from contaminating the long-term light curve.
I know that FO Aqr is getting close to the Sun, but please keep up the good work.