Another wonderfully clear night

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Pete
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Another wonderfully clear night

Unread post by Pete »

Tuesday, 16 Oct 2012

The sky’s been clear all day and the forecast is for relatively clear sky for 3 days, with tonight being primo. The day’s been spent scraping and repainting the dome interior and not wanting to wait to enjoy it I start a set of -25°C dark frames around 16:00. It’s in the low 50s as dusk falls. But between all the turn on procedures it’s fully dark by the time imaging starts.

137032 is a 3.2 km sized Apollo now .28 AU distant and 16.1 magnitude. Discovered in 1992, it still has an uncertainty level of 2 due to occasional gravitational perturbation. Apparent motion is 5.5’/hr. Imaged from 18:58 – 19:12 hrs at 30 sec, 2X2, -20C, 2 Hz. The camera can now reach -25C but I’ve already got a 30 second -20C reduction programed and I want to get going. Frames 1, 6 & 12 show dRA = =0.4’, dDEC = +0.1’, and magnitude = 16.2 R.

141851 is a 1.9 km sized Apollo now .44 AU distant and 18.2 mag. Although this asteroid has been observed at 8 oppositions the uncertainty level is still 3 and the RMS residual 0.51”. Motion is 4.4’/hr. Imaged from 19:19 – 19:34 at 1 min, 2X2, -20C, 3 Hz. Stacking, frames 1-3, 4-6 & 7-9 show dRA & dDEC both = +0.3’

219071 is a 2.5 km sized Apollo now .5 AU distant and 17.8 mag. Motion is 2’/hr, uncertainty is 1, and RMS is .52 Imaged from 19:40 – 19:57 hrs at 2 min, 2X2, -20C, 3 Hz. Frames 1, 3 & 5 show dRA = +0.2’ and dDEC = + 0.1’

2012 RV16 is an 830 meter sized Apollo that I’ve been chasing lately. It was discovered 31 days ago. Distance is 0.13 AU, motion is 10.6’/hr, uncertainty is 5. Imaged from 20:04 – 20:11 hrs at 30 sec, 2X2, -20C, 2 Hz. Frames 1, 3 & 5 show dRA = + 0.9’ and dDEC = + 0.8’

4947 Ninkasi is an Amor discovered in 1967. Tonight it’s supposed to be .36 AU distant and 18.1 mag. Uncertainty is 1, RMS is .54, and motion is 2.1’/hr. Imaged starting at 20:17 hrs at 2 min, 2X2, -20C. But I can’t find it anywhere in the camera’s FOV, even when using 4 different stacks of 3.

2012 QY14 is tonight’s challenge object. This 659 m sized Amor is .34 AU distant and only 19.6 magnitude. Motion is 1.655’/hr, uncertainty is 4, and orbital arc used for ephemeris calcs is 54 days. Imaged from 21:00 – 21:49 hrs using 3 minute exposures. The focus has gone to Hell, which is particularly un-good when trying for something this dim. Using stacking, frames 1-3, 4-6 & 7-10 show dRA = +0.2 and dDEC = 0. S/N and FWHM are 4.0/4.5, 5.2/3.4 and 3.2/2.7 respectively. The asteroid shows up as a very faint somewhat disperse object, and even with adding a 4th frame to the final group it’s so weak that I’m hesitant to submit it. But after all this work, I do.

Refocused, doubling S/N. Boy was I off. And at 44F the temperature hasn’t changed all that much. Didn’t realize that a 10°F drop in temp would throw the focus off that severely.

2012 QB17 is a 659 m sized Amor now .21 AU distant and 18.3 magnitude. Discovered 51 days ago, uncertainty is 4 and motion is 1.7’/hr. Imaged from 20:05 – 20:34 hrs at 3 min, 2X2, -20C, 1 Hz. The asteroid is visible but weak in single frames. Frames 1-2, 3-4 & 5-6 show dRA = +0.3’ and dDEC = + 0.2’

244783 is a main belt asteroid that happens to be in the same FOV as QB16. It’s 5.7 km in size, 18.7 magnitude, 1.4 AU distant. Uncertainty is 1 and movement is 1.0’/hr. Frames 1-3, 6-6 & 7-9 show dRA and dDEC = 0.

2012 SA59 is a 1.1 km sized Amor now .37 AU distant and 18.6 mag. Discovered 16 days ago, uncertainty is 7. Motion is1.04’/hr. Imaged from 22:47 – 23:10 hrs at 3 min, 2X2, -25C, 2 Hz. Stacking helps here. Frames 1-2, 3-4 & 5-6 show dRA = + 0.5’ and dDEC = +0.2’

That’s it for the night. After today’s long bike ride, I’m wiped. I’m in at 23:15, after observing a beautiful red meteor zip across the zenith. 24 observations, including the weak set, are forwarded to the MPC.

Conclusions & lessons learned….. Not sure. Didn’t realize the scope was so temperature sensitive. And wondering about the relatively poor capture of faint objects in such a clear sky. Possibly the sky still isn’t “winter clear” and possibly in the past I’d use very weak images. Hmmm. As for not finding Ninkasi, it was a crowded field. Either the asteroid was hidden by multiple stars or the MPC database was off ( which seems to happen once in a while).

hgp logged 17:30 hrs, 17 Oct 2012
Pete P.
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AndyG
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Re: Another wonderfully clear night

Unread post by AndyG »

Pete, when I was looking into automating my focusing I came across a couple of write-ups describing focus vs temperature change. I plugged the formulas into a spreadsheet, give it a try if you want. You would enter your scope's OTA length, aperture, and f-ratio, and it will calculate the temperature change after which you would need to refocus according to the Jerry Lodriguss formula and the goldfocus.com formula attributed to Don Goldman and Barry Megdal.

For my refractor, the formulas seem to under-estimate the temperature sensitivity, giving 6-10 deg F, whereas in practice I find I need to refocus for smaller temperature changes than that. The formulas indicate a much greater temperature sensitivity for the 11" f/10 SCT example scope in the write-up, suggesting re-focusing with temperature differences of 0.7 deg F! The difference between the refractor and SCT results was mostly due to the magnification factor from the secondary. Not having an SCT I have no idea if this is accurate or not in practice.
Andy
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Pete
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Re: Another wonderfully clear night

Unread post by Pete »

Andy, you continue to amaze me with how much you've learned in the short time you've been stargazing with us. I'll certainly be looking at delta-t from now on.

Pete
Pete P.
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