Hello everybody!
I was just checking gam Cas’ light curve, and found some unusual feature that happen at the beginning of this year and lasted for quite a few weeks:
Can somebody explain what happened here? Was this an outburst of some kind?
Cheers!
Enrique
(BETB)
One observer (VOL) observed once a night over the period shown, looks really good huh.
Most of the others that look noisy are mine and I took many images each night to test to see how noisy DSLR with a wide-angle lens would be. Nothing strange going on really, just shows the difficulty of observing such a bright object with comparison stars far away and of different color and a DSLR with wide-angle-lens.
VOL used Eta Cas as primary comparison and Tycho 2 magnitudes apparently. Eta Cas is an F9V class star btw not well matched to Gamma Cas. VOL used a DSLR as did I. No PEP over this interval but which is probably the best way to observe Gamma Cas.
I used Delta Cas from the VSP chart sequence. Delta Cas (000-BBC-894) specifically. Its spectral class is A5V which is a better color match to the variable Gamma Cas which is classified as B0.5IVe. My check 1 was Beta Cas and check 2 was Eta Cas. I did the the following comparisons of Gamma against Delta, Beta against Delta and Eta against Delta.
Most, if not all, of the variation during each night in my observations are simply photometry noise and not real variation. Any bias comes from possibly the different catalog sequences–but there is not enough information to be sure what value from Tycho 2 was used by VOL. VOL data also says it has been transformed but only V magnitudes are in the database. The photometry errors for almost all of my measures were 0.002 magnitude. Way underestimates the real photometry noise using a DSLR and wide-angle lens!
I’ll stand by my measures. Anyone who wants to use my data for Gamma Cas should probably make nightly averages, however, or use them to get a better estimate of the real expected scatter.
There are some TG observations not shown on the plot above that show similar scatter and bias, btw.
Some of this was already discussed in this thread… VSP catalog stars from Yale Bright Star Catalog - B closure problems
More on my observations: I used 20-s exposures which should minimize normal atmospheric scintillation, “flickering”. I also stacked two or three images together depending on the night. Total integration of 40 to 60s.
I spot checked a few of the images and none of the stars of interest are anywhere near saturation. Delta for example was right in the middle of the dynamic range around 30K DN. Gamma in the same image is around 45K DN.
The stars are a little non-round but the apertures selected allowed for all the stars light to be inside the aperture but there might still be some small aperture shape dependency. These observations were made in the middle of the winter too so it might have been a little windy!
I have no explaination for the scatter at this point.
I used 20-s exposures which should minimize normal atmospheric scintillation, “flickering”. I also stacked two or three images together depending on the night. Total integration of 40 to 60s.
Jim,
Have you tried stacking more than two or three images? Maybe ten, or even more? With a 14mm wide angle lens the stars are not likely to drift out of the field. My guess is the night to night precision may improve.
My DSLR photometry method with the camera in a fixed position on a tripod was to take 10 or 15 images and average the calculated mag rather than stack images, allowing standard deviation to be determjned. Exposures had to be shorter than yours, 10 -15 seconds, because of the longer focal length lens (200mm). The ISO was set to the value at which the gain was 1 e- per ADU (400 for my camera).
Hello all,
I observe gamma Cas with a DSLR Canon 600D and 1:2.8 f=28mm lens which is heavily defocused, star image has a radius of 16 pixels. 10 trailed images with 10 second exposure are measured and averaged. Eta Cas is used as a comparison star, I think delta Cas is a variable.
The accuracy is around 0.02..0.03mag when compared to PEP observations Smith+Henry 2021 (Automated Photometry of γ Cassiopeiae: The Last Roundup - IOPscience)
Best regards
Wolfgang Vollmann (VOL)
Well I guess my photometry is no good then… no surprises there… Have fun.
I also deleted all the values from the db.
I reprocessed my Gamma Cas measures using now Eta Cas as the primary comp and I get basically the same magnitude for Gamma Cas. Brighter than what VOL gets. B shifts some but that still points back to the question I had on the other thread about the catalog magnitudes and their accuracy. The catalog errors are very large for the stars 0.1 to 0.141 magn. in the AAVSO sequence.
Wolfgang: You report only V band measures but not B. Your data records say you transformed the reported measures. Did you but just not report the B magnitudes?
Any chance you can give me the Tycho 2 catalog magnitudes you used, V and B and the R if you’ve got it so I can see if there is any bias in the catalog magnitudes used?
Also are you stacking or doing photom. on single images?
Thanks.
Hey Jim.
The magnitudes of such bright stars are known to a large degree of accuracy, GCPD values are usually very good and Hipparcos can provide good confirmation. Tycho-2 only comes after those.
That doesn’t mean that the AAVSO sequence has those values because some sequences were created many years ago and may even come from sources like the BSM, where the magnitudes are okay but the errors are largely overestimated, just because they were not errors listed.
So if you find some discrepance, just let us know what the star in question is so we can check.
Eta cas itself (V= 3.47) has a K7V companion (V= 7.51) which is 13" away. The combined brightness is 3.44 V. So the value to use will depend on you solving them or not.
Cheers,
Sebastian
Hello Jim,
I use 3.47mag V as combined magnitude of the eta Cas pair. This was cited as “standard” some years ago in GCPD, there are newer/other values.
I take 10 images in rapid succession (defocused, no tracking). No stacking is done, every single green image is measured with Muniwin (refer to https://c-munipack.sourceforge.net/). This is easy and quick. Then the measures are averaged.
For transformation I use an approximation: my Tc transformation coefficient is 0.11mag per 1 mag B-V difference. For eta Cas B-V=+0.59mag, for gamma -0.05mag. The color index difference is 0.64mag, so 0.64*0.11mag = 0.07mag is subtracted from gamma Cas TG magnitude to get approximate V. I tried with transformation of tricolor magnitudes but this did not give better results.
I also do differential extinction correction which corrects up to a few hundreths of a magnitude when gamma Cas is low. I try to observe year round as much as possible.
Best regards
Wolfgang
3.47 is not the combined magnitude, as I mentioned above. It is the magnitude of the brighter star. Results in the GCPD are labelled as AB and give V= 3.44.
It is not a big difference, but it is good to know where some differences may come from.
Cheers,
Sebastian
In re gamma Cas comp stars and gamma Cas UBV photometry over the long-term, folks may well have a look at the recent masterful summary paper by Hrvoje Božić and collaborators, which appeared on astro-ph as:
The related data files have appeared just this week at VizieR:
Note that the photometry of the comparison stars is based on a very large number of nights with rigorous and homogeneous reductions against primary UBV standards over some decades. For gamma Cas, their comp stars are specified as:
Name RA (J2000) Dec V B-V U-B rms errors; number of obs
HD 2626 0 30 19.9 +59 58 39 5.940 0.013 -0.350 0.010,0.015,0.017; n=161
HD 2011 0 24 47.5 +61 49 52 5.385 0.012 -0.151 0.009,0.013,0.016; n=117
If I were doing this, I would not use any stars besides these (and basically ignore whatever a AAVSO chart says). The reason for doing so is to enable new data to link as directly as possible to the long runs by these (and others) doing properly calibrated photometry of this and other Be-type stars.
\Brian
HD 2626 is in VSX as CzeV4197 so it is only good for visual use (not certainly to observe the 3.7 mag. brighter gam Cas!!). It has a maximum amplitude around 0.014 V. Not good if you look for short period small amplitude activity in gam Cas.
HD 2011 is okay.
Very good…! I’m just trying to figure out who is zooming who! 
The other problem I see may come from such a large difference in magnitude between the two HD stars mentioned if used as comps for Gamma Cas.
So anyone have any comments on the original Delta Cas I used as primary comp. The color is a better match to Gamma Cas than Eta is the only reason I picked it.
Obtw, I don’t defocus nearly as much as Wolfgang. I do defocus a little tho. I actually turn the drive off on the old iOptron Skytracker mount and let the field drift a few seconds, usually 3 to 7 seconds, between each exposure. This should and seems to sample the stellar psf by the Bayer matrix sufficiently.
I’ll give them a try next.
Thanks for the comments.
del Cas is a DSCT variable with an amplitude of almost 0.01 mag (V= 2.67-2.68) So again, not the best comp star for high precision photometry.
I guess only one other comment:
As it seems almost all stars in the universe are variable at some level is there a solution “we”, the AAVSO needs to think about to solve the stable comparison problem? So Delta Cas is a DSCT. Would it be possible to account for the variability with a LC model. Complicated and wouldn’t account for sudden period changes, etc. Color changes in each filter color would also be complicated.
Maybe I’ll try a comp star variation model to remove the basic DSCT variation in Delta Cas just to see what happens to the Gamma Cas LC.
Something to think about for the future.
The discussion started because folks are clearly having issues with instrumentation, the reduction process, and calibration — pretty basic stuff. It will thus be a challenge simply to show that they can detect any variation in the well-vetted stars I suggest.
In re the delta-mag, given that the professional folks are using old-fashioned single-channel photoelectric photometers, surely modern detectors can do at least as well here in the technical vastness of the future? With a CCD I regularly use comps a couple magnitudes fainter than the target. I don’t see any issue there.
\Brian
Brian, I totally agree either DSLR or CCD with some basic effort gives excellent results. However, I’ve never seen bias like what I’m seeing with Gamma Cas, esp. in the B closures, CH1 - CHK2, Var-CH1, Var-CHK2. In time-keeping we called that a three-cornered hat analysis and my data does not close very well.
I’m gonna try the two HD stars but it will require more processing and I also need to go back and extract photometry for the two HD stars which I didn’t do the first time thru… I should have enough SNR for it not to be a problem.
Lightning and thunder nearby so I’m off for now! 
