The Starry Night, 60

:: home ::




              <<   41   42   43   44   45   46   47   48   49   50   51   52   53   54   55   56   57   58   59   60c   60   61   62
              63   64   65   66   67   68   69   70   71   72   73   74   75   76   77   78   79   80   81   82   83   84   85   >>   SRCH


8/15/2011. The sky is clear, moony, and unsteady (stars are 2.5 - 3 arc seconds). Guiding is all over the place. Achieved images are not going to tell the tale, but I think the visual inspection of the "frame and focus" and "fine focus" images did. Tonight it's the seeing rather than imprecise collimation keeping me from accurately measuring the offset from L to Ha. I'm going to leave the telescope to collect 900s subs of M57 through the Baader 7nm H-a filter until dawn. Haze and moonlight notwithstanding. Autopsy to follow.



M57, outer shell.
15x900s Baader 7nm H-a
Tightly cropped.


2.8-3.0 FWHM stars. And there's no particular difference from side to side, so the focuser may not be too far out of square after all. A litany of troubles means this is not the final word: too much haze, guidestar image was large and elongated (too far off-axis, out of focus, residual collimation trouble, something), subs could've been longer, chip could've been cooled below -15°C. The end result is a very dim image of the outer shell of M57. I have some doubts about the linear feature marked at lower left. Lots of things to improve here.


8/16/2011: Clear skies were predicted for tonight -- and the sky was mostly clear until shortly after astromical twilight. By then, I'd lined up on M51 and taken four short exposures (300s each) to check up on the summer's supernova. Seeing sucked. I went to 2x2 binning since 0.76 arcsec/pixel was way, way oversampled for the conditions. When clouds moved in (as we knew they would), I grabbed a dark and flats and called it a night. The flats were made with the two stage diffuser hung on the dewshield, illuminated by slewing toward the galleria windows and cranking up the lights (note to self: excellent flat!). The final image is nothing special, but it's something (I'd've been thrilled with this last year). I'd worry more about the softness if not for yesterday's reassuring M13. Here's a sure sign of autumn's advent: I could chill the chip to -20°C for the first time in weeks.



M51 w/Supernova
4x300s L
Guided with DSI in OAG, 2s at a time.
Stars are 3.4 - 3.6 arcsec FWHM.
Whattaya gonna do with seeing like that?


8/17/2011: Better, much better. In the twilight, I focused on Arcturus and found a very tight image, in the 1.3 arc second range. While twilight deepened, I checked the offset from Baader L to Baader Ha and found it to be at most 10 RFU's (outward from L, as if it really matters). I shot flats and darks until the sky was dark enough to commence to imagin', and then I decided to see if I could steal one last shot of M51 and its supernova before they are lost to the NW treeline. The RG & B frames were messy, elongated in declination (usually a good sign that polar alignment is off). I adjusted polar alignment with the bore 'scope as best I could, then after noticing the elongation of the guide images, refocused the guide camera. That's two variables at once, sorry, but time is short. The axis ended up within 10-15 arc minutes of where it was to begin with; the guider's images are much sharper.

Listen. Tonight the sky has given me a new commandment: thou shalt focus the guide camera. The guider's images were stretched out, maybe 2.5 or 3 times longer than wide. Any scintillation produced hot spots within that bar of light. And, naturally, the guider tried to follow the centroid which moved along the bar with the sparkles. After focusing, I got much less "Sierran" plots from PHD's corrections graph. And much rounder stars:


m51 color

M51 w/supernova
6x300s L, 300s RGB
1.9 - 2.3 arcsecond FWHM


That would need a lot more exposure to rival my best image of the galaxy and its supernova (the SN has faded considerably during a weeks-long cloudy spell), but it's not bad for a short exposure plucked off the NW treetops under summer haze, through twilight's last gleam-ing.

I wanted to try something higher in the sky and new to me. How about M56, a lovely globular in Lyra, all but buried in a bright starfield? It's not often imaged and reasonably resistant to Moonlight. It's also near the zenith with plenty of guidestars. Focus seems durable: I did not refocus for M51 nor for M56. In the globular's field, when I checked focus, I saw FWHM values of about 1.8 pixels, or about 1.3 arc seconds. Good enough, said I, and left the focus buttons alone.



M56, a globular cluster in Lyra
6x300s L, 300s RGB
Smallest FWHM stars yet.

All I had to do was ask for sub-arcsecond imagery? Some of the stars in the best L-channel subs of this image measure 0.85". 1.2" - 1.3" is more typical. I took another half hour of L data while waiting for Comet Garradd to step onto the stage. Increasing moonlight and haze may make that a relatively unrewarding target, but it's worth a try.


comet Garrard

Comet Garradd
45x60s L, manually aligned on the nucleus in Maxim DL
The field is about 20 minutes of arc tall.

I made a few 150s exposures at first, but they allowed obvious trailing. I dropped the exposure to 60s, during which time the comet moved a little over 2.5 arc seconds. The resulting images of the comet's central condensation are still measurably trailed in comparison to the field stars which are typically well under 2 arc seconds. I shot until clouds cut me off after about 50 frames. I collected a total of 62 one-minute exposures just in case the clouds relented. They didn't. I closed the shop and produced the quick look above. Below is a tighter, full-res look at the nuclear region, unsharp masked and stretched without mercy to try to show some detail.


Is the bright, nearly circular shell within the enhanced area real or is it an instrumental artifact? I'm thinking it's legit. Similarly bright stars in star-aligned images don't behave anything like this when treated the same way. Let's say it's 10-12 arcseconds across the longest chord. At the comet's distance (1.39 AU) that comes to something like 7 - 9,000 miles.


8/21/2011. I badly misjudged the sky tonight. At about 11:30, I went out to retrieve the bird feeder and was amazed to be under one of the most transparent skies in months: the Milky Way shines through Cygnus down through Aquila and into the pines. I had no clue during this evening's walk that the sky would be this good; in fact, I had every reason to think mist would rise from the creek and the river. The ground is saturated, and the sky is lit every thirty seconds or so by lightning too distant to make thunder, and the lively stars suggest that the seeing is abominable. Still, I feel as if I am wasting a good night by not pulling the cover off the telescope and having at it. One real concern is that while everything else could be plugged in, synchronized, initialized, and working in ten minutes, the fans will take an hour to move most of the warm air out of the tube and two to let the optics do their best. Beyond confession, tonight may be about inspiration: I need to rig up something to speed that process along, a front-mounted fan to help exhaust warm air. [Rain overnight, but the insight about needing to speed the cooling process endures.]


8/22/2011. Clear, dry, and cool. I tried a couple of frames after carefully collimating with the hall of mirrors. Almost good enough. Two small tweaks with two knobs finished the job. I aimed at the other globular in Hercules, Messier 92, for an hour with decent results. The seeing is not all it might be, but transparency is excellent:



M92, the lesser globular cluster in Hercules
18x150s L, 300s RGB
Same scale as M56 above and M13
on previous pages.


And then off to NGC 6888, the Crescent Nebula. Here's the "morning after" look. I left the telescope to collect H-a overnight. I'll need more tonight. More and better RGB would be useful, too. And try to get the chip colder and use longer subs. Or maybe shoot broadband G and B and use the accumulated Ha for R. O-III would be good, but I'd rather use gear I actually have, the alternative being unproductive. Last night's result is shown small because of noise:



NGC6888, the Crescent Nebula in Cygnus
19x900s H-a, 300s RGB
Overstretched to see what's in there;
more signal coming soon.


ngc 6888 bw

NGC 6888
29x300s L + 19x900s H-a
(August 21, 23. 7h10m of exposure.)

This is about all I'm going to get here without using an O-III
filter. Wonder how the old Lumicon visual O-III would do?


And finally for this evening, a casual all-sky photo of the open-air observatory with the Ritchey aimed into the central starclouds of Cygnus. It's only slightly more vivid than the naked-eye view on this first autumnal night of the year. Note the glow of Hickory / Charlotte in the upper left and the substantial portion of the sky hidden by the pines. S'OK, a lot of imagers contend with far worse in the way of nighttime skies.


all sky

An all-sky view
Canon 5D, 1600 ISO
8mm Peleng Fisheye, 15s F4


:: top ::


                   © 2011, David Cortner