Experiments with some really faint stuff

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Experiments with some really faint stuff

Unread post by Pete » Sat Dec 14, 2013 4:28 pm

Log of 12 Dec 2013

Last night I’d opened up the observatory to host the visiting children of a neighbor, and we viewed the moon through cloud. Tonight’s forecast is for near perfect conditions, if you overlook the temperature being only 18°F.

Objective: Experiment with various scope/camera setups to determine the feasibility of detecting Osiris, an exoplanet associated with star 51 Peg (HD 209458). HD 209458b has become the de-facto standard “test case” for detection of exoplanet transits. 150 LY distant, Osiris is a “hot Jupiter” circling 51 Peg every 3.52 days. The necessary photometric resolution of around 0.02 magnitude is far better than anything I’ve attempted to date.

Procedure: Initial imaging attempt using bias/dark/flat reduction failed, as my flats weren’t binned 1X1. Between 18:00 and 19:46 three sets of 50 images are taken using auto-dark.

1. Set 1 consisted of 50 exposures with an auto-dark, binning 1X1 with 2 second exposure, -30C cooling, 1Hz binning at full 14-inch aperture
2. Using a mask with a 4” diameter off-axis hole changed the scope from f/10 to f/35. This permits longer exposures that will theoretically reduce scintillation noise. Set 2 consists of 50 images taken at f/35, 15 seconds exposure, 1X1 binning, -30C cooling 1 Hz guiding.
3. Retaining the 4” aperture mask, set 3 was 50 images with 5 second exposure, 2X2 binning, -30C cooling, 1 Hz guiding. It didn’t occur to me to change over from auto-darks to a full bias/dark/flat image reduction.

Conclusions: Photometric analysis of 150 images is going to take a bit of time as I don’t own Canopus software, and magnitudes are logarithmic rather than linear. However a quick run thru Astrometrica shows the 1X1 binning to produce a very large illuminated area requiring at least a 12 pixel-centroid capture zone rather than the normal 4-pixel centroid. Not even sure what impact this will have. Bottom line is that I’ve some initial data to use in evaluating photometric resolution.

At 20:00 it’s still quite early and the sky’s really really good. I’ve dressed warmly with an electrically heated garment under my 40-below rated one-piece snowsuit and Mickey Mouse boots on over heavy socks. There’s no wind but it’s biting cold at 17°F. Because of the high bright moon the plan is to limit work to just 2 asteroids.

2013 WS43 is a small 173 meter sized Apollo now .12 AU distant and a relatively dim 19.6 magnitude. Discovered only 3 days ago, its uncertainty level is an 8 out of a possible 9. Movement is 2.76’/hr forcing a relatively short exposure so as to stop motion. Imaged from 20:41 – 21:07 hrs at 2 minutes exposure, 2X2 binning, -30°C cooling, 3 Hz guiding. Using Astrometrica’s track & stack capability, frames 1-3, 4-6 & 7-9 show dRA = +0.4’ and dDEC = +0.1’ Stacked image S/N runs from 4.7 to 6.3, and FWHM from 3.9 to 5.0 (a little weak but quite adequate).

2013 VY13 is an Apollo discovered 20 days ago and rated as potentially hazardous. This 379 meter sized asteroid is now .3 AU distant and 19.5 magnitude. On such a dark still night 19.5 shouldn’t be a problem. Uncertainty level is 6. Motion is only 1.25’/hr. Yesterday I’d set up a number of darks, including an unusual one for 4 minutes. With my setup, 4 min is optimal exposure for 1.25’/hr  Imaged starting at 21:30 hrs using 2 min exposure, 2X2 binning, -30°C cooling, 2 Hz guiding. Using track ‘n stack, frames 1-3, 4-6 & 7-9 show dRA = -0.1’ and dDEC = -0.2’

But wait, there’s more!!! The 2013 VY13 image field is lousy with asteroids, some brighter and some dimmer. I’ll work them from brightest to dimmest, as frames will accumulate with time.

49850 (1999 XM94) is a 6 km sized main belt asteroid now 1.1 AU distant and an easy bright 17.3 magnitude. RMS residual on the ephemeris is an unusually high 0.55” Using frames 1, 4 & 7 from the 2013 VY13 field shows dRA & dDEC = 0.0’ The FWHM of 4.0, 3.8 & 4.0 shows the seeing to be quite steady.

100734 (1998 DB1) is a 5 km sized main belt asteroid now 1.6 AU distant and 18.9 magnitude. RMS residual on the ephemeris is 0.48” Using frames 3, 9 & 15 from the 2013 VY13 field shows dRA & dDEC = 0.0’

93119 (2000 SJ59) is a 4.4 km sized main belt asteroid now 1.8 AU distant and 19.6 magnitude. Motion is only 37”/hr. Using frames 1-4, 5-8 & 9-12 from the 2013 VY13 frame set shows dRA & dDEC = 0.0’

132844 (2002 RW39) is a 4.4 km sized main belt asteroid now 2.2 AU distant and 20.3 magnitude. This may be the deepest attempt I’ve ever made. Motion is 32’/hr. Using the 2013 VY13 frame collection, frames 1-6, 16-21 & 22-25 show dRA & dDEC = 0.0’ The S/N is a bit weak running from 4.2 down to 3.8, and because this object was starting to occult a star track the last set was only 4 frames. But the numbers are good, and I’ve established a new benchmark at 20.3 magnitude.

2007 TS204 is a 4.0 km sized main belt asteroid 1.8 AU distant and a dim 19.8 magnitude. Apparent motion is only .32”/hr, but I’ve been imaging 2013 VY13 long enough to have established a good track. I’ve been working these extra objects in the frame from brightest to dimmest, and TS204 is out of sequence as the early frames found it positioned over a star. Frames 9-14, 15-19 & 20-24 show dRA & dDEC = 0.0’, with relatively high 9 S/N and tight FWHM of averaging 3.8 The astrometry on this one is strong.

330305 (2006 TA76) is a 1.7 km sized main belt asteroid 1.3 AU distant and an extremely dim 20.4 magnitude. (If successful this will be the deepest I’ve ever gone.) Motion is 42”/hr. Using the accumulating images from 2013 VY13 and stacking, frames 1-10 & 11-20 show dRA in dDEC = 0.0’ My third set of confirmation images ( 21-30) shows nothing, but I think the asteroid’s hidden in a star trail. S/N was 6.9 & 6.6 on the first 2 stacks, with FWHM of 3.2 and 4.2 These are good enough numbers that I’ve got confidence in them.

2007 VZ40 is believed to be 3.8 km in size, 2.1 AU distant, and 20.5 magnitude. However it was only observed 15 times over an orbital arc of only 18 days, and it was last observed on Nov 5, 2007. i.e., this one’s been lost. Using frames 1-14, 15-28 & 29-41 I can’t find it. This one stays lost.

It’s fortunate that 2013 VY13 was relatively slow so that I selected a longish 4 minute exposure. And it’s also fortunate that it’s to the east of the bright moon, so that instead of setting on me it transits the median. Because it’s going on about 3 hours now and I’ve some 40+ frames to work with, permitting deep stacking. The next is the deepest I’ve attempted on anything – 20.7 magnitude.

2011 GA30 is a 1.3 km main belt asteroid now 1.15 AU distant and a faint faint 20.7 magnitude. Observed a total of 43 times during 3 oppositions, uncertainty level is 2. Motion is 41.7”/hr.
Stacked frames dRA dDEC S/N FWHM
1-12 0.0’ 0.0’ 6.3 4.1
13-24 0.0’ 0.0’ 3.0 6.3
25-39 0.0’ 0.0’ 5.0 3.6
Although I’ve been burned before on S/N of 3 or lower blinking shows the second set of measurements to be right on track, and with acceptable values on either side of it, I’ve confidence in the numbers. Yet another new magnitude benchmark 

The observatory’s shut down at 00:45 because I’m tired and because there are no more asteroids in the field of view I’ve been working for hours. Coincidently, clouds are rolling in from the west – nature’s way of telling you to knock off. The temperature’s held steady at 17°F all evening, but 90% of this evening’s work has been done from inside the house via Windows Remote 

Conclusions & lessons learned:

Won’t know about exoplanet capability until I’ve had a chance to analyze the data, and running astrometry on 150 individual measurements is going to take a chunk of time.

Conditions were perfect this evening.

The re-addition of the old dew shield with the new on-axis counterweight was critical to tonight’s success. No stray light from Luna or the dome interior reached the camera.

After several years I’ve not made one discovery. The big automated professional scopes are doing well however. Perhaps it’s a matter of not detecting apparent motion due to the short time frame used to image a piece of sky. Maybe I need to repeat observations over a number of nights using sets of 10 minute exposures stacked 3 deep…… Now that idea smacks of desperation.

Looking at the bright side, I’ve hit 20.7 magnitude!!!!

hgp logged 16:00 hrs, 14 Dec 2013
Pete P.