The Starry Night, 48

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05/07/2011. Wasn't supposed to be clear, but it was.

I checked for a detectable exo-planet transit in a part of the sky I could see and found one marginal candidate: WASP-13b, in Lynx, a hot Jupiter set to cross its sun's face starting at 10:30PM my time. As hot Jupiters go, it's small in proportion to its star, so the dimunition of starlight is not particularly great -- only a little over 8 milli-magnitudes -- but you have to start somewhere to get the kinks out of the method and the tools.

wasp13bAccording to Guide, WASP 13b shared the field with NGC 2832, a bright-enough galaxy. I recalibrated the mount on Regulus and commanded a slew to NGC 2832, which landed the field on the chip without drama. The scattering of background galaxies turns out to be picturesque.

I made several exposures to find one that put the host star's brightness just above midrange in the 16-bit digitization from the ST2000. Thirty-second exposures with 1x1 binning gave me readings of around 45,000, which I believe is safely below the camera's anti-blooming rolloff. There were plenty of comparison- and check-star candidates in the field. I slewed to the zenith, captured a flat, and returned. (Note to self: should I bin 2x2 for this application? Does that give me a deeper effective electron well, with 4x more available steps with which to resolve very shallow minima? Or am I missing something, as seems likely?) The photo at right is a stack of 7, 120-second exposures binned 2x2.

There were some high clouds blowing through, but the sky between them seemed very transparent. This sky was not ideal for hunting the shadows of hot Jupiters, but it was at least good enough for a live rehearsal. I began taking 30s exposures at about 10:10 and continued for about an hour and a half. That interval included the ingress of the planet and a good bit on either side, but did not extend to mid-transit, let alone egress which would, in any case, be behind the western treeline.

I have got finally to learn to use Excel. Data collection in Maxim seemed to go perfectly well, but I need a better way to display my catches, and I need to understand better how, exactly, to make best use of the check and reference star(s). This run was good practice at least, and maybe the start of an exo-planet transit is buried in the data. I need an unambiguous target with which to practice so I can see what works and why. So far, so good.




The thin line is the measured brightness of the host star for WASP 13b. Other lines represent the reference star and check star and relations between their measured brightnesses. If I understood it better, I'd explain it better, I promise.

The huge hiccup at center right is presumably due to a passing cloud, or a bit of undigested beef. There is more of wavy than of wavelength about that one.

[5/22: I have finally fumbled my way through a really basic tutorial on Excel, read more about differential photometry, and found a VBS routine that averages FITS files in arbitrary-sized gruops for photometric measurement. Taking all that data in blocks of five images, selecting three reference and one check star: indeed, the target star fades! But I think it's luck and noise combining to taunt me. For one thing, ingress seems very protracted; for another, the dip is closer to 20 mmag than to 8; and finally, the check star varies more than the subject star -- the data taken together behave well enough, though, so maybe, I dunno, let's play with it some more, catch some more of these on clearer, steadier nights and attend to the workflow for better S/N.]



After the exo-planet adventure, I decided it was at last time to photograph M51, one of the showpieces of the northern sky and one that every astrophotographer must eventually shoot. There are so many superb M51 photos out there that I have hesitated to cut my teeth on it. It's one thing to make a shabby photo of something obscure and find a way to admire it, but a shabby picture of a famous target will always call to mind a library of excellent photos. That would be much more discouraging.

I did thirteen 300s exposures to start -- one hour's worth plus one to grow . The extra was in case I needed to discard a frame owing to satellite trails, unaccountable drift, or whatever. The results looked pretty good, but the sky was clouding over, so I shut the telescope-controlling computer off and planned to call it a night. I played with the initial dataset some, and then went out to bring the bird feeder in. There was not a cloud in the sky. It only took about ten minutes to get the kit going again. I collected luminance data from about 1:15 until dawn. Pushing the data hard (thank you, flat box!), lets me show faint streamers that result from the two galaxies' mutual gravitational influence. Tides of stars have been tossed from each and form an extensive and complex coccoon around the pair:


m51 deeper

Messier 51, an interacting pair of galaxies in Canes Venatici
58x300s L (4h 40m total)
A-P 5-inch F6, ST2000XM
A-P Mach1 GTO


05/27/2011. Wasn't supposed to rain, but it did. Well, yes, it was, but the wind was wilder than I expected it to be and it somehow lifted the cover off the telescope in the middle of the night. You know, the cover I have to walk around a couple of times and tug and lift just so in order to remove? A gust of wind picked it up and set it down neatly beside the telescope where I found it in the predawn dim about ten days ago. After exposing the telescope, the rain put an inch of water in the power box, soaked the electronics, and sopped into the Protostar black flocking paper inside the tube. Can't complain, not with the kind of wind damage friends and friends of friends are putting up with this spring, but aye god, dang. The A-P electronics are fine. The cameras don't seem any wetter than after a night of heavy dew, but the tube refuses to dry out. So this is a good time to try out the new Protostar flocking boards by replacing the lining. I bought 30 inches of the heavier variety since I figured I'd make a shield for the 200mm F2 while I am at it. In retrospect, the thinner material might have stuck to the tight curves of the 6-inch tubing better. This works well enough; I cut it on the mat cutter to 18-something inches in 5 and 6 inch widths (for the tube ahead of and behind the single baffle in this early A-P tube). It conforms reasonably to the walls under the force of its own spring-outie-ness. I glued a 5.5-inch strip inside the dew shield and did that only because it's out there in public view and I want it to look neat. Vanity, vanity -- all is vanity.


5/28/2011. I'm thinking of rebuilding the SN10 yet again. I saw the Blacklite tubes and connectors Protostar sells while looking for the flocking boards. They look very inviting and the aluminum truss lobster-trap arrangement is just too bulky and heavy for what it houses. I'm only thinking of it at the moment, because I really don't need a new build-it-yourself hobby at the moment. But it would be a lot cheaper than the AT10RC I can't keep my eyes off'n of and it would give me 10-inches of aperture at just over 1000mm FL at F4. After allowing for reflection losses, I'm not convinced it would be worlds deeper than the 5-inch refractor, but it would keep me occupied and put a currently unused 'scope back into service by making it lighter, less bulky, and by shielding the Schmidt corrector from dew. I may buy a tube connector to use as a dew shield and give it another trial under the stars to see if this seems a worthwhile project before getting more involved.

I found Bruce Gary's Excel spreadsheets and an artificial star plugin for Maxim DL to aid in exoplanet transits as well as a lucid discussion from Peter Kalajian who used an ST2000XM with good success for that project. More when I know more.


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                   © 2011, David Cortner