First attempt at photometry of an exoplanet transit- HAT-P-53 b

[catburglar]

Last night (Friday 1st December) I decided to try and repurpose my EAA set up for a spot of exoplanet transit photometry.

In many ways my set up is not really optimised for this sort of work- it's an alt-az mounted SCT with F3.3 reducer (reduced to approx 1000mm FL) with a whopping 0.32x0.18 degree FoV provided by an uncooled ASI290mm camera. Searching here (https://astro.swarthmore.edu/transits/transits.cgi) for potential targets gave lots of candidates for me to go at...I probably didn't make the best choice in retrospect. The target star was fairly dim (mag 13.7V) and it's track took it to approx 80 deg elevation at meridian crossing exacerbating the problems of field rotation with my alt-az mount...But the skies were forecast to be clear all evening and the target should at least be visible and not blocked by local obstructions for the entirety of the circa 4 hour observation session. So what's to lose?

I did my usual set up- one star align from a parked position and then platesolve/sync the mount position, check focus etc and found the field easily- cross referencing with the AAVSO chart plotted for the same region hoping to find some suitable comparison stars. Given the limited FoV and the problems of field rotation, making sure the comp stars would remain in the images throughout the session was a bit too difficult for me to work out, so in the end I just took a punt on final framing. In the end it didn't matter so much because the tracking was a bit flakey and the images  moved all over the place- I had to do a few platesolve / recenter cycles throughout the session.

Before getting going with the "science images" I did a couple of trial exposures- I'd need to keep them short because of the rotation, but still needed to get a decent SNR- I was hoping to discern variation of approx 1.5%. Since there were no stars with similar magnitude labelled on the AAVSO chart I decided to hedge my bets and use low gain to maximise my dynamic range and avoid saturating any pixels if I did need to use some stars much brighter than HAT-P-53. I ended up using 5 seconds at 110 (unity) gain.

I set up sharpcap to capture the images- made sure things were working ok at the scope end, and then came in from the cold...hoping it would do it's stuff for about 4 hours...I kept an eye on things remotely- recentreing periodically as required. I lost a lot of data in the second half of the transit due to the arrival of clouds, but luckily they cleared before the start of egress, so I was still able to get something useful.

I processed the resulting images (bias, darks and flats) using astroimagej based on the guide from the AAVSO website and here's the result... The data is noisy, I binned to 3min "frames" in AIJ as a compromise- trying to maintain good temporal resolution. The fit to my data gives a transit duration of 2h46m36s- virtually identical to the values I find in the catalogues and the radius of the transiting planet is 1.27 Rjup, versus 1.318 from the pros.

So...in short...I'm chuffed 🙂 ... I'll definitely try this again- I might put my scope on a wedge and apply some PEC training so that I have fewer problems keeping the target and comps on the CMOS chip... And I might find something a bit brighter to go at too...maybe mag13.7 was a bit too much of a stretch first time out... The good news is that there are loads of targets many which aren't too faint and because the periods are short transits happen every couple of days so it fits in with the vagaries of British weather...This could become addictive.

[lunator]

Very interesting read. Congrats on getting a decent result. The BAA variable star section might prove a useful resource as well. 

Look forward to reading more reports.

Cheers

Ian

[catburglar]

@lunator I did look at the BAA/VSS web pages - but the AAVSO pages seemed a bit more coherent with links / guides that went from start to finish. I’d hope to go back to the BAA (I like to keep it local 🙂 ) and contribute something meaningful when I’ve got a bit better. At this point I’m just seeing if I can get to a useful level of precision without needing major upgrades to my equipment.

I started with exoplanets because they sound exotic…However, it seems like similar techniques apply to study of CV’s of which there there are also lots of fairly bright targets that can show clear results from a single session…whenever the weather cooperates.

To me this seems like an almost ‘instantly rewarding’ observational activity, much more so than monitoring many other variable star types- I might get hooked.

[andrew s]

30 minutes ago, catburglar said:

@lunator I did look at the BAA/VSS web pages - but the AAVSO pages seemed a bit more coherent with links / guides that went from start to finish. I’d hope to go back to the BAA (I like to keep it local 🙂 ) and contribute something meaningful when I’ve got a bit better. At this point I’m just seeing if I can get to a useful level of precision without needing major upgrades to my equipment.

I started with exoplanets because they sound exotic…However, it seems like similar techniques apply to study of CV’s of which there there are also lots of fairly bright targets that can show clear results from a single session…whenever the weather cooperates.

To me this seems like an almost ‘instantly rewarding’ observational activity, much more so than monitoring many other variable star types- I might get hooked.

I posted my results on the AAVSO site although there is/was a bug in the search which had not been fixed when I retired from observing. A bonus is NASA may pick up your data! The BAA data base does not take relative flux which is the normal. Nice start, well done.

Regards Andrew 

[melsmore]

1 hour ago, catburglar said:

@lunator I did look at the BAA/VSS web pages - but the AAVSO pages seemed a bit more coherent with links / guides that went from start to finish. I’d hope to go back to the BAA (I like to keep it local 🙂 ) and contribute something meaningful when I’ve got a bit better. At this point I’m just seeing if I can get to a useful level of precision without needing major upgrades to my equipment.

I started with exoplanets because they sound exotic…However, it seems like similar techniques apply to study of CV’s of which there there are also lots of fairly bright targets that can show clear results from a single session…whenever the weather cooperates.

To me this seems like an almost ‘instantly rewarding’ observational activity, much more so than monitoring many other variable star types- I might get hooked.

Try the BAA exoplanet section pages for loads of info and tutorials. Exoclock project may well want your results posted to them too. 🙂

Edited December 3, 2023 by melsmore

[catburglar]

4 minutes ago, melsmore said:

Try the BAA exoplanet section pages for loads of info and tutorials. Exoclock project may well want your results posted to them too. 🙂

Thanks for that link- I missed it completely. I didn't think to look for a specific exoplanet section, I went straight to variable stars. First rule of observation- keep your eyes open!

[catburglar]

I downloaded HOPS from the Exoclock project and processed my images again. There are some helpful features in this application compared to AIJ- identifying a FoV that's in all of the images, alignment of frames that accounts for both rotation and translation, showing trends for PSF and background sky brightness prior so as to remove poor quality frames from the stack and also suggestion of suitable comparison stars based on flux compared to target. There are also some bits that I don't quite understand, so I'll need to do some more research. At this point- I'm not sure how to assess the quality of my data. The HOPS manual state lower values for stdev and autocorrelation are better, but I don't know if there's a threshold that's simply not good enough to be useful, but at least it feels like I'm in the ballpark...