I'm not sure whether this is the correct forum for this question, but Eps Cep is a SPP (a Delta Scuti), so here goes... please note that I am new to photometry and am just trying to practice on some well-known variables, the eventual goal being to study exoplanet transits.
Last night I collected 64 images of Eps Cep over about 90 minutes, in hopes of gathering enough data in one session to construct its light curve, since its period is given as just under an hour. The camera was a Canon EOS Rebel T6i, lens was a 250mm, f/5.6, ISO 200 and 30 second exposure. Star tracking was done with an iOptron SkyTracker and was adequate. Plate solving using astrometry.net was very quick. Defocusing seemed just about right to me, but some of the pixels in Eps, Del, and Zeta Cep are saturated.
The major problem, though, and what seems like a possible showstopper, is the presence of a faint star a couple of arcminutes from Eps Cep, roughly in the direction of Zeta Cep. It is so close that sampling the dark sky background would likely be difficult, and defocusing more to eliminate saturation would only make that problem worse. I'm concerned that reducing ISO or shortening exposure time would result in the plates being unsolvable.
So I have two questions:
1. Can anyone suggest a solution to the problem? How much shorter dare I make the exposure before not enough stars are visible for the solver to work? If I increase focus to make the stars smaller, that would require an even shorter exposure time to avoid saturation, which goes back to my concern about solvability.
2. What IS this faint nearby star? It is too far from Eps Cep to be the "faint companion star" described by Wikipedia. Stellarium shows an unnamed and undesignated mag 6.5 star about the right distance from Eps Cep, but in the wrong direction: it's shown as roughly along the line from Del to Eps Cep, but on the "far" side of Eps Cep. Is Stellarium's database possibly inaccurate? My plates show no star where Stellarium says that one should be.
Thanks in advance.
I don't have a good answer to your first question. Sometimes with close neighbors one can do aperture photometry by placing a big aperture around both stars. However, the amplitude of eps Cep appears to be small, only about 0.06 mag, making it a tough target, and I am not optimistic that such an approach will work. How good are your photometric results for similar but isolated stars? Do you get useful results for stars with such small amplitudes? A different planetarium program, such as Starry Night, might ID your unknown star. Unfortunately, I don't have access to my planetarium program at the moment.
Hi, and thanks for your reply.
I haven't done any other stars yet, because my aim was to do photometry on an extreme SPP, where the period is so short that I can easily gather data over at least one complete cycle in a single session. Eps Cep is the shortest period pulsator I could find. It's true that its amplitude is small, but my ultimate goal is to study exoplanet transits, where I believe the amplitude would be even smaller in most cases. A Jupiter-sized planet transiting a Sun-like star would cause a dimming of only 0.01 mag. If I can't detect 0.06, how can I hope to detect a transit? In any case, as far as I know, all Delta Scuti variables like eps Cep have small amplitudes, and I don't know of any other kind of variable that has such short periods.
Originally I started out working with Del Cep, but its period is over 5 days, and with our weather it would take several weeks to gather enough data to construct a light curve, if it's even possible to do that with images taken at random points in a cycle.
Yeah, I have Starry Night too... but I can no longer run it on my Mac as it is a 32-bit app, and I just updated the OS to a recent version that no longer supports them.
There are some very short period variables with larger amplitudes than eps Cep that might serve as test stars for you. I could perhaps suggest a few, but would have to know how faint you think your targets can be. My guess is that you want relatively bright targets, but you probably have a better idea than I do as to how bright they should be.
Since I need to find any target naked eye (not using a go-to scope for this stage), 4th or 5th mag is about my limit. My site is pretty dark, but not the best, and then of course there is the Moon for another couple of weeks now. I just realised that del Sct itself might be a better target, after the Moon passes 3rd quarter or so - the Moon was another reason I was trying to stick to far northern targets. If you know of others that would be farther from the Moon now and visible soon after sunset, I'd still be interested.
Your brightness constraint is a serious one and most of the targets I had in mind appear to be too faint. However, if I understand correctly, you would like a bright target of short period, preferably in the north, and you want to test whether you can detect low amplitude variability. Would beta Cep, after which the class of beta Cep is named, be a suitable test case? It has a period of about 0.19 day but an amplitude of only several hundredths of a magnitude. Otherwise, I might suggest trying a star with longer period and combining observations over several nights to get a light curve. Delta Cep, which you already mentioned, could be a test star in that case, with, of course, a much larger amplitude.
I am not sure I understand how this is done. From what I understand, AIJ (one of the few photometry packages I can run as I do not have easy access to a Windows system) basically just plots light vs. some form of JD. So observations over several nights would have long gaps during daylight even if one could stay up all night snapping images. The other way might be to reconstruct a light curve from observations taken at random times over many nights using an algorithm that tries to compute a period (e.g. Phase Dispersion Minimization, PDM) and then plots light vs. phase. I've taught a lab that simulates that kind of photometry, but I have no idea if AIJ can do that and find the documentation very weak.
There is also the issue of weather; here in Vermont it can be a week or more between good observing nights, even in the summer. That's the other reason I decided to stick with very short period variables, maybe 3 hours max. Beta Cep might be an acceptable substitute - Stellarium, at least, doesn't show any stars within a few arc minutes. Thanks for the suggestion!
Hi Elisabeth:
One thing you can try is shooting a constant star of similar magnitude to your desired targets. Do the photometry on the star and see what the scatter looks like. If it is more than 0.01 you're going to have difficulty detecting the variation in a del Scuti. For exoplanets you'll probably want to get that scatter down to a few thousandths of a magnitude or better.
Using VSX (https://www.aavso.org/vsx/index.php?view=search.top) you can search for target stars by brightness and period. There is a class of del Scuti called High Amplitude Del Scuti (HADS) which have amplitudes of up to a few tenths of a magnitude instead of hundredths but there don't seem to be any bright ones (none brighter than mag 5, only 3 between mag 6-7.) However if you search for variability type DSCT you will find several bright ones with magnitude ranges above 0.1 mag, including del Scuti itself.
To look at the light curve of a star that varies over a longer period you can 'phase' the observations: subtract integer multiples of the period so that all the obs are spaced over a single cycle. The spreadsheet command is MOD(JD-epoch,period)/period (you can get the period and epoch for any star you are likely to observe from VSX) which collapses all the observations to the range 0..1. You can then plot this and get a nice light curve.
Richard
Yes, I…
Hi Richard,
Yes, I understand subtracting multiples of the period, that always seemed like "cheating" to me though, and obviously wouldn't be legit for a candidate exoplanet where the period is only provisionally guessed or even unknown (thinking way ahead here). The other issue would be even getting the data over several nights. As I think I wrote somewhere, often times a whole week will go by here in Vermont with no nights clear enough for observing. Since the beginning of summer we've only had 7 good nights so far, two of them this past week. So I think I need to focus on very short period variables.
Del Sct has been on my radar, yes. But its location makes it a tough target for the next couple of weeks because of the Moon. Thanks for the suggestion of using VSX to find other possible targets - I wasn't aware of the HADS category.
All that said, since my ultimate goal is exoplanet transits, I need to know if my camera can detect variations of a few hundredths of a mag. Trying a constant star (with a constant comp star in the same FOV) would be a good way to suss out the noise, thanks for the suggestion.
Liz