Beginner seeking advice on selecting exoplanets for detection (transit)

[Craig Towell]

Hello,

I have consumed quite a few online guides and tutorials about beginner exoplanet transit detection, but the one thing I’m missing is what mag star and transit depths I should aim for with my equipment.

Can anyone give me a realistic indication of what I should be looking for in the chart here ar.astro.cz/en/Exoplanets/TransitsPredictions with regards to star magnitude and transit depth?

For info I’m using a 102mm refractor and OSC CMOS sensor (for now).

Will HAT-P-20 b be a realisitc target for aperture?? (mag 11.3 and 2% depth)

Or should I be looking deeper transits and/or brighter stars?

Thanks in advance for any help received

Craig Towell

[Dr Paul Leyland]

I have no personal experience with a 102mm aperture but my 400mm can do useful work on a 1% depth at 13th-14th magnitude. Simple scaling says that you have 1/16 the collecting area, which corresponds to three magnitudes in brightness.

Accordingly, I expect that you may have a chance with HAT-P-20 but it’s not certain. Certainly well worth giving it a try.

Regardless, it’s easy enough to test the capabilities of your equipment. Choose a star of a particular magnitude and see how long an exposure is require to reach a SNR of at least 500 and preferably 1000. If it is a couple of minutes or less, and preferably less, you can measure a transit of depth 2% or better. Repeat with fainter stars until you determine your limiting magnitude.

Good luck!

Paul

[Craig Towell]

Thanks for the response Paul, that is very helpful.

I also have a 305mm/1500mm Newtonian I could use, however the FoV would be tiny (0.28 deg x 0.16 deg), not sure if that would be enough to get any comparison stars in shot.

[Dr Paul Leyland]

Craig: that FOV is comparable to my 400mm, which is 16.45 * 13.18 arcmin, or 0.27 x 0.22 degrees. I have not yet had any problem finding a suitable comparison star but, to be fair, stars do become much more common at fainter magnitudes.

Once more: suck it and see is my recommended approach. Remember that you can do all this sort of experimentation at your convenience – you do not need to wait until a transit is imminent. If you do so for all the stars which you are likely to observe over the next few weeks you can choose which telescope to use as appropriate.

[Craig Towell]

Thanks Paul that is very helpful. I had a short clear spell when I got home from work and managed to capture a series of images of varying durations of a star field.

I’m just looking at the images now and identifying the stars in the image and their magnitudes.

What do you use to measure the SNR of a particular star? I have pixinsight and I can see the ADU values of the centroid and the background, is it simply a case of taking the ratio of those two values?

Thanks again
Craig

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