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Old 10-11-2014, 05:26 AM
Placidus (Mike and Trish)
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47 Tucanae - inner 4 min arc - a blue star

Trying to solve a focus-shift-with-altitude issue, we took this ultra-short live sports action photo of the inner 4.5 minutes of arc of 47 Tucanae. Has anyone ever noticed the extremely blue star toward 5 o'clock? It was there two weekends in a row, so probably not our first supernova.

L 4x60sec, RGB 2x180sec each. Aspen CG16M on 20" PlaneWave. North up. Full moon.

(You've seen 47 Tucanae before, but the whole shot is here.
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Old 10-11-2014, 08:29 AM
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RickS (Rick)
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It does look like the odd one out, Mike!
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Old 10-11-2014, 09:19 AM
Placidus (Mike and Trish)
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It is a corker, but seems it's been there rather a while. This article from ESO shows the blue star in exactly the right place. I was able to register the two images star by star. Mind you, their image is a bit on the over-saturated side.
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Old 10-11-2014, 09:29 AM
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Have you checked with Simbad???
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Old 10-11-2014, 10:02 AM
Placidus (Mike and Trish)
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Quote:
Originally Posted by Merlin66 View Post
Have you checked with Simbad???
Thanks for the good suggestion. Never tried Simbad (and would probably need the harbour-master's help) but no need - as mentioned the ESO article shows a 5 minute total exposure on just the core, using the 8 metre VLT in 2001. The conspicuous blue star is shown very clearly. My guess is it is just a particularly impressive "blue straggler" that isn't normally seen because people (quite reasonably) burn out and/or desaturate the core trying to get a good overall shot.

Cheers,
Mike
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Old 10-11-2014, 10:25 AM
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mithrandir (Andrew)
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Quote:
Originally Posted by Merlin66 View Post
Have you checked with Simbad???
Are your two images oriented the same way?

A quick Aladin+Simbad search only shows up a few blue star candidates - one B8 and six F[568]
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Old 10-11-2014, 10:27 AM
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Aladin - GALEX UV images.....
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Old 10-11-2014, 10:32 AM
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Originally Posted by Placidus View Post
Thanks for the good suggestion. Never tried Simbad (and would probably need the harbour-master's help)...
Mike, you can download Aladin (it needs Java) from http://aladin.u-strasbg.fr/aladin.gml (or run it online) and it will do all the hard work getting the data from Simbad.

You open your plate-solved FITS in Aladin, and use "File->Load Catalog->Simbad database" to select the sort of objects you want. Note that it defaults to a 14' radius around the image centre. I asked for stars and it gave me ~4600 from which I picked out any OBAF candidates.
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Old 10-11-2014, 12:50 PM
Placidus (Mike and Trish)
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Quote:
Originally Posted by mithrandir View Post
Are your two images oriented the same way?

A quick Aladin+Simbad search only shows up a few blue star candidates - one B8 and six F[568]
Hi, Andrew,
The two images are both North up. They look quite different because the "whole thing" image has been stretched a lot more to bring out the halo.

You're way out-gunning me here, but by do-it-myself plate-solving my photo against the PPM catalogue, I measured the blue star at 00:23:58 -72:05:29 J2K, which plausibly makes it the 13 mag B8 star in your list. That was fun. Many thanks!
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Old 10-11-2014, 03:46 PM
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Originally Posted by Placidus View Post
Trying to solve a focus-shift-with-altitude issue,
A focus shift with the hedrick focuser or the rotor focuser? These things have lead screws (as you know), what are you observing?
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Old 10-11-2014, 07:01 PM
Placidus (Mike and Trish)
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A focus shift with the hedrick focuser or the rotor focuser? These things have lead screws (as you know), what are you observing?
I suspect its the Aspen 10 position filter wheel. The front and back plates on that are ridiculously floppy. I was worried when I first took it out of the box. I might be able to stiffen the arrangement with some external struts.

I'm seeing about a 250 micron change in position beween zenith and 60 deg south. The sweet spot on the CDK20 is very generous and it barely affects the central stars, but the corners on the full frame chip start looking pretty dodgy unless I refocus. Luckily it can be done automatically under scripted control.
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Old 10-11-2014, 07:06 PM
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Good work Mike! I've never noticed that little bloke before. I might have a look at my individual subs of old tuc later and see if you can see it. There is no sign of it in my finished image for the reasons you mentioned above. Amazing how something like this can be missed in an object so often imaged. One of the many reasons I love this hobby.
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Old 10-11-2014, 08:05 PM
Placidus (Mike and Trish)
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Quote:
Originally Posted by Merlin66 View Post
Aladin - GALEX UV images.....
Thanks much, Ken. Still trying to assimilate this hence my delayed reply. The crosshairs that you've placed seem to be on a very bright UV source, but the coordinates seem different to what I measured, which seems to agree very closely to Andrew's late class B star. Perhaps a source can be bright in UV but dim in blue, and vice versa. We're out of our depth here. But thanks muchly for having a look.

Cheers,
Mike and Trish
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Old 10-11-2014, 08:07 PM
Placidus (Mike and Trish)
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Good work Mike! I've never noticed that little bloke before. I might have a look at my individual subs of old tuc later and see if you can see it. There is no sign of it in my finished image for the reasons you mentioned above. Amazing how something like this can be missed in an object so often imaged. One of the many reasons I love this hobby.
Thanks, Rex! Well put.
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Old 10-11-2014, 09:18 PM
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Thanks for sharing this Mike, it's actually pretty cool, I just had a look at some of my subs and it is clearly visible. I don't have the resolution that you obviously do but it's still pretty cool, so thanks again,
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Old 10-11-2014, 09:50 PM
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strongmanmike (Michael)
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Great looking Tuc there Mike Yes, the blue star is indeed intriguing but I also find the obvious blob of yellow stars around the core just as fascinating. I highlighted them in a shot I took of 47 Tuc a few years ago and closer in. At the time I (and others) thought it was more in the processing but your image shows them very clearly as well...?

Mike
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Old 11-11-2014, 05:39 AM
Placidus (Mike and Trish)
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Originally Posted by strongmanmike View Post
I also find the obvious blob of yellow stars around the core just as fascinating ... closer in. At the time I (and others) thought it was more in the processing but your image shows them very clearly as well...

I hope that my quick "live sports action" shot starts to look a bit more like yours when it grows up and gets more hours. The yellow inner core has always been very salient to me visually (16" Dob) - looking like a rising moon, or perhaps a chinese lantern, in the centre of the largely colourless halo. I'd wondered whether the visual effect was just because only the core was bright enough to stimulate the cones, but it seems real photographically.
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Old 11-11-2014, 05:42 AM
Placidus (Mike and Trish)
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I just had a look at some of my subs and it is clearly visible ...
Superb! Thanks for checking.
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Old 12-11-2014, 10:25 PM
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Nice capture Mike.

An interesting view of a popular object.

Ross.
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