CzeV627 Delta Scuti Variable Star

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oopfan
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CzeV627 Delta Scuti Variable Star

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Post by oopfan »

Last night was an all-nighter. I started with WASP-3 b, an Exoplanet in Lyra and ended with CzeV627. a Delta Scuti variable star in Cygnus. Here are the results of the latter.

CzeV627 was discovered by Czech astronomer Bohuslav Hladik in 2014. (In the attached screenshot I included what I believe to be his discovery data.) It's brightness varies by a mere 0.05 magnitudes every 2 hours. It is of type Delta Scuti (DSCT). I am not an astrophysicist but I believe that pulsations are due to the cyclical build up of opaque plasma in the chromosphere followed by ejection. DSCT stars are short period. Many of them you can capture in one session. Some have relatively large amplitude changes like YZ Boo but a great many, like CzeV627, are virtually indistinguishable from a regular, constant-brightness star. Furthermore, low-amplitude DSCT stars are susceptible to changing amplitude and sometimes will go dormant. Catalogs list CzeV627 with a depth of 0.05 magnitudes but I have seen light curves where it is significantly lower followed by an increase. Last night's data indicates that the depth is currently 0.03 magnitudes. By the way, 0.03 magnitudes is just about the upper limit on the depth of Exoplanet transits.

In the attachment the plot of magnitude versus time is upward sloping since my data was not "de-trended" for changes in Airmass. Through my work with astrophotography using a mono camera and LRGB filters, I know that the atmosphere attenuates light at different wavelengths. Red light more readily passes through the atmosphere but blue light suffers most as the distance increases from the zenith. So what happens is that stars suffer from a color shift. Every star shifts red as they grow closer to the horizon. Since hot blue stars have a lot of energy in blue wavelengths they appear less bright in a monochrome camera. Red stars don't suffer that much. In the attachment there is a screenshot from AstroImageJ showing three stars with "apertures" placed on them. The green aperture is CzeV627. The two red apertures are constant brightness "comparison stars". Each of these three stars have a different color index. Ideally I would want to select comparison stars with the exact same color index as the variable star but this is almost never possible. AstroImageJ can perform "de-trending" for Airmass but I did not employ it. Up until now I have not seen this effect since all previous variable stars I have measured have much larger magnitude changes.

You may notice that the stars are severely out of focus. That is on purpose. The exposure is 150 seconds. At perfect focus the stars would have been severely saturated. To mitigate saturation I de-focused. The brightest pixel in the variable star is now just below saturation. Thanks to Robin, I learned that this is a very effective technique for increasing accuracy. Notice the third column in the spreadsheet labeled "Source_AMag_Err_T1". Values are around 0.0024 magnitudes. That's incredible compared to other photometry I've done at perfect focus where the error was 0.05 magnitudes. That is a 20x increase in accuracy by doing two things: (1) de-focus, (2) increase exposure to bring the star just below saturation. Of course, it comes at the price of long exposures so there is no free lunch!

AFTERTHOUGHT:
After reviewing the data from my first exoplanet transit HAT-P-5 b seen here: https://u235-varstar.now.sh/light-curve/hat-p-5-b I now see that no such upward trend in the data exists. What is different? Most everything is similar except for: de-focusing and exposure. Frankly I don't believe that de-focusing is the culprit. That leaves exposure. For HAT-P-5 I decided upon a non-aggressive exposure that put the maximum pixel value at around 30,000 ADU. From tests that I ran a year earlier I knew that the sensor becomes non-linear at about 55,000 ADU. Last night I threw caution to the wind by flirting with that level. And of course one of the hazards of imaging east of the meridian is that exposure determination is made through the thickest airmass. This meant that as CzeV627 approached the meridian the star became brighter which pushed it farther into the non-linear zone. By the end of the session I could see that a single pixel had reached saturation at 65,535 ADU. No worries, I thought, there are plenty of pixels within the aperture that are below saturation. So that is my theory. Next clear night I will lower the exposure to test my hypothesis.

EPILOGUE:
If as I suspect that the culprit is overexposure then this is a lesson to heed for those in the audience who are Lunar and Solar imagers. We all agree that saturation is to be avoided but one should also consider the deleterious effects of running your highlights too hot. Beginning at around 55,000 ADU you will lose detail. Your highlights will become flat and uninteresting. Don't get me wrong. It's OK if you want to show detail in shadow and the mid-range but know that if you don't crop out highlights then it will distract from your composition.

Brian
CzeV627 (DSCT) 2020-06-27 B Morgan.jpg
CzeV627 (DSCT) 2020-06-27 B Morgan.jpg (263.17 KiB) Viewed 2254 times
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Re: CzeV627 Delta Scuti Variable Star

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Post by admin »

Hi,

another very interesting write-up and I'm glad to hear that the defocusing works well in practice.

Did you do your own measurement of sensor linearity or use the one built into the SharpCap sensor analysis? One of the things I've noticed with the sensor analysis is that some cameras have difficulty with exposure times – for instance there are some where the exposure time is rounded to the nearest tenth of a second when the exposure goes above one second... This sort of thing can play hell with linearity calculations.

Cheers, Robin
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Re: CzeV627 Delta Scuti Variable Star

#3

Post by oopfan »

Hi Robin,

I ran the linearity test according to the steps described by the AAVSO.

I just finished sampling the max pixel values of my CzeV627 lights. The first frame was 62,480 ADU and the last was 65,230 ADU. That is not to say that all pixels in the star's disc are that value, probably only one. I have to be much more careful with this going forward. A lesson learned the hard way! At least now I know how the non-linear zone can manifest itself.

I'd like to salvage this data. I could do a least squares fit and then rotate the points level. Is that cheating? The peak-to-peak amplitude will be under its actual value. I'll fully disclose what I did to the data. It's just going up to my site, nowhere else.

Brian
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Re: CzeV627 Delta Scuti Variable Star

#4

Post by admin »

Hi,

Just to check I've got my head around this correctly – your plot is of magnitude so the lower values are actually brighter, meaning the trend is for the star to get dimmer over time which is potentially caused by pixels approaching saturation and therefore responding less, correct?

One thing to be aware of is that the same saturation could be happening your reference stars (depending on their relative brightness to the target star). I suppose that in theory you could take the pixel values from the images and then correct them based on your sensor linearity calibration results (at least as long as they're not completely saturated) and then feed the corrected values into ImageJ. Unless someone has already written software that does this though it would be an awful lot of work.

I don't see any particular harm detrending the data - you are essentially assuming that the offset varies linearly with time, which is probably at least moderately true (all physicists will tell you that for a small enough effect you can ignore the higher order terms of just deal with the linear ones!)

Cheers, Robin
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Re: CzeV627 Delta Scuti Variable Star

#5

Post by oopfan »

Hi Robin,

The comparison stars were all fainter, so no saturation there. You are absolutely correct, the comparison stars are running linear but not the target so it appears as if it is growing fainter as time marches forward to the zenith. And you are correct, I forgot to flip the vertical axis scale.

I've attached the "de-trended" data. Meh. The amplitude is too low. I think I'll just re-image it at the next opportunity.

Thanks,
Brian
CzeV627 (DSCT) 2020-06-27 B Morgan reworked.jpg
CzeV627 (DSCT) 2020-06-27 B Morgan reworked.jpg (85.69 KiB) Viewed 2207 times
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Re: CzeV627 Delta Scuti Variable Star

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Post by umasscrew39 »

Brian

This is so cool! I had no idea this kind of stuff can be done. I did some background reading on the techniques used, what kind of data are generated, and how to interpret the data. That way, I can at least follow your work on this!!

Bruce
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Re: CzeV627 Delta Scuti Variable Star

#7

Post by oopfan »

Hi Bruce,

I am thrilled that you find this interesting. I certainly do. Ever since I was a little kid I imagined traveling to these distance star systems to see the physical processes that astrophysicists and cosmologists imagined. Theories come and go but that is the nature of science.

You can start learning more using Wikipedia. Simply do a search for "Variable Stars". It will generate a deluge of information. I suggest reading the first couple sections and then refine your search to "Delta Scuti Variable Stars", "Eclipsing Binary Stars", "Exoplanets", and "Cepheid Variable Stars". That last topic is fascinating in that you can measure interstellar distances by simply measuring the star's brightness fluctuations. The first three topics every one on this forum can measure in a single session. My telescope and camera aren't special. You guys and gals have far better equipment. The good thing about Robin's forum is that we can learn from each other and work through problems together.

The software you need is free: AstroImageJ. With it you will be able to create calibration masters, and then process your "science" images (a fancy name for your lights,) and perform photometry. There is no stacking required. Each calibrated frame yields one data point. If you are ready I can do a search for candidates. Once you run through the process a couple times it becomes easy, and then you will want to tackle more challenging targets.

As you read more about the topics don't be put off thinking that you need a set of "photometric filters". Everything I do today uses a standard luminance filter used for AP. Yes, there are advanced techniques that require the use of photometric filters but we aren't going there. We are in it to have fun!

Brian
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Re: CzeV627 Delta Scuti Variable Star

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Post by umasscrew39 »

Thanks for the info, Brian. Now that we are in hurricane season in Florida and lots of cloudy/hot/humid nights :-(, I have plenty of time to read up on the topic and the software :-).

Bruce
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Re: CzeV627 Delta Scuti Variable Star

#9

Post by oopfan »

Let me know if I can help, Bruce.

I finally got my wish for a drought following years of incessant rain. Now the days are beautiful but the nights are afflicted with high altitude thin clouds, enough to damage any attempt at imaging.

Brian
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Re: CzeV627 Delta Scuti Variable Star

#10

Post by oopfan »

Finally! Some clear skies last night but wouldn't you know it? A 90% Moon!

I decided to give CzeV627 another shot. This time I got the exposure right. You can see the results here:

https://u235-varstar.now.sh/phase-plot/cze-v627
https://u235-varstar.now.sh/gallery/cze-v627

Thanks,
Brian
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