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 Post subject: Comet Holmes
PostPosted: Thu Nov 15, 2007 1:30 pm 
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Hi folks,

After a bit of encouragement from Gordon I went after Comet Holmes last night. Sky & Telescope have a nice web page up here at the moment describing how to find it but if you live much to the south of the equator I'm afraid you will be out of luck. NASA's Jet Propulsion Laboratory also has an interesting web page here for the more numerically inclined, and it includes a pretty neat Java applet showing where in the solar system the comet is (it's well outside the orbit of Mars and about 1.6 times further from us than the Sun is).

The following image is a 33% crop (clickable if you want to download the 100% crop for a bit of pixel peeping) of a 180 second exposure with my Canon 40D set to ISO 1250 and Daylight white balance and with my EF 70-200mm f/4L IS USM lens set to f/4 and 200mm. Apart from the crop the only post-processing was a very small tweak to the levels with Photoshop to darken the sky slightly but essentially what you see is what came out of the camera. No noise reduction or fancy image stacking and dark frames for this one!

Image

To give you an idea of the scale, the comet's apparent size is a bit smaller than the Moon's ½°. Of course it helps hugely that the camera was piggybacked on top of my telescope to take advantage of the mounting's ability to compensate for the Earth's rotation. One note about the lens: it was pretty much vertical during the exposure and I'm delighted to note that there is absolutely no sign of "zoom creep".

As a bit of fun, I took the image above and simulated what I think you would see if you fixed a camera to a tripod. I assumed you could set ISO 3200 and with an exposure of 35 seconds at 200mm and f/4 you should, after a bit of processing, see something like this 25% crop:

Image
I've probably got the trail (motion blur) direction wrong but you get the drift. :roll:

Why not have a go and see if you can do better? For Nikon D80 owners you might want to review this post which describes how to defeat the long exposure noise reduction issue.

In any event, if it's a clear night and you live in the Northern Hemisphere why not grab a pair of binoculars and try some Mark 1 eyeballing. When you've ticked off the comet there's lots else to see at this time of the year, particularly if you wait until the Moon has set.

Bob.

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Olympus OM-D E-M1 + M.Zuiko Digital ED 12-40mm f/2.8, Lumix 7-14mm f/4, Leica DG Summilux 15mm f/1.7 ASPH, M.Zuiko Digital 45mm 1:1.8, M.Zuiko Digital ED 75mm 1:1.8.
Leica D Vario-Elmar 14mm-150mm f/3.5 - f/5.6 ASPH.
OM-D E-M5, H-PS14042E, Gitzo GT1541T, Arca-Swiss Z1 DP ball-head.
Astrophotography: TEC 140 'scope, FLI ML16803 camera, ASA DDM60 Pro mount.


Last edited by Bob Andersson on Thu Nov 15, 2007 5:20 pm, edited 2 times in total.

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 Post subject:
PostPosted: Thu Nov 15, 2007 1:52 pm 
Another great shot Bob. You mentioned that you need to set the white balance to daylight, Gordon also said this on another thread. Why is this important and what happens if you don't?


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PostPosted: Thu Nov 15, 2007 2:01 pm 
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Hi Graham,

I set Daylight white balance in the hope that I will get fairly natural colours. It's very hard to know exactly what colour most astronomical objects should be because for most they are so dim that the colour receptors in the eye don't kick in. Except for a few objects like the Moon and planets the way colours are represented in a lot of astronomical imagery is pretty artificial anyway as colour is often exaggerated to highlight particular details.

Bob.

_________________
Olympus OM-D E-M1 + M.Zuiko Digital ED 12-40mm f/2.8, Lumix 7-14mm f/4, Leica DG Summilux 15mm f/1.7 ASPH, M.Zuiko Digital 45mm 1:1.8, M.Zuiko Digital ED 75mm 1:1.8.
Leica D Vario-Elmar 14mm-150mm f/3.5 - f/5.6 ASPH.
OM-D E-M5, H-PS14042E, Gitzo GT1541T, Arca-Swiss Z1 DP ball-head.
Astrophotography: TEC 140 'scope, FLI ML16803 camera, ASA DDM60 Pro mount.


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 Post subject:
PostPosted: Thu Nov 15, 2007 2:12 pm 
Thanks for the answer Bob.


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 Post subject:
PostPosted: Thu Nov 15, 2007 2:20 pm 
Hi Bob
I noticed you captured the comet with a light blue colour, while the local version was kind of brownish, Here

Any idea why was it so ?

DavidL


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 Post subject:
PostPosted: Thu Nov 15, 2007 5:10 pm 
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DavidL wrote:
Hi Bob
I noticed you captured the comet with a light blue colour, while the local version was kind of brownish, Here Any idea why was it so ?

DavidL

My guess is that their colours could be off because the comet is fairly low to the horizon in Malaysia and so is artificially reddened. I have seen another photo of Comet Holmes taken with an EOS 5D which also showed a cyan colouration.

Here's a cropped thumbnail I made from a stack of just two RAW images with dark frame subtraction and other post processing which shows a green/cyan tinge.

Image

I didn't include this image in the original post because in one of the two original photos there are worse than usual tracking issues. I was also a little surprised by the greenish colour but since producing it I see that there have, indeed, been reports of a greenish tinge, though possibly not as strong as shown here. I also specifically wanted to demonstrate what is possible without getting involved in heavy duty software manipulation!

So why (ignoring your brown example if I may) are we seeing such different colours? There is an emission line component to the total light received from the comet rather than just a smooth spectrum. This is the comet's spectrum (an image I wish I could take credit for!)

Image

You can see a number of emission lines there but the most significant ones are at the high frequency end of the green part of the spectrum with some less strong ones in the cyan and greenish-yellow part of the spectrum. They could easily trick a colour DSLR sensor into producing inaccurate colours even without the added complications of "white balance" when using JPEGs out of the camera.

In summary, it ain't easy! :lol:

Bob.

_________________
Olympus OM-D E-M1 + M.Zuiko Digital ED 12-40mm f/2.8, Lumix 7-14mm f/4, Leica DG Summilux 15mm f/1.7 ASPH, M.Zuiko Digital 45mm 1:1.8, M.Zuiko Digital ED 75mm 1:1.8.
Leica D Vario-Elmar 14mm-150mm f/3.5 - f/5.6 ASPH.
OM-D E-M5, H-PS14042E, Gitzo GT1541T, Arca-Swiss Z1 DP ball-head.
Astrophotography: TEC 140 'scope, FLI ML16803 camera, ASA DDM60 Pro mount.


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 Post subject:
PostPosted: Thu Nov 15, 2007 9:34 pm 
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Hi Bob, that's a really nice shot - I'm glad you got a chance to photograph it!

I'm also really impressed by the 70-200mm f4 lens - it just goes from strength to strength that one. Normally zooms are out of the question for long exposures like these due to creep, but there's no evidence of it - and the star images also look good...

I was going to suggest you consider the 200mm f2.8 prime, but apart from the extra stop in aperture, it looks like you're getting superb results with the 70-200mm.

As you know, comets also change shape and brightenss throughout their journey, so do please keep us updated! You may also see some evidence of a detached tail if you do a much longer exposure.


Last edited by Gordon Laing on Fri Nov 16, 2007 12:27 am, edited 1 time in total.

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 Post subject:
PostPosted: Thu Nov 15, 2007 11:34 pm 
Thanks Bob and Gordon for the clarification and information. !
BTW, how did you managed to tracked it Bob ?

And Yes, please do post some more photos of the comet.


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 Post subject:
PostPosted: Thu Nov 15, 2007 11:41 pm 
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DavidL wrote:
...BTW, how did you managed to tracked it Bob ?...

The comet is a long way away so the only tracking needed is the usual sort where the camera follows the motion of the stars. Hence piggybacking the camera plus lens on the telescope's motor driven mount which nicely decouples the optics from the Earth's rotation.

Bob.

_________________
Olympus OM-D E-M1 + M.Zuiko Digital ED 12-40mm f/2.8, Lumix 7-14mm f/4, Leica DG Summilux 15mm f/1.7 ASPH, M.Zuiko Digital 45mm 1:1.8, M.Zuiko Digital ED 75mm 1:1.8.
Leica D Vario-Elmar 14mm-150mm f/3.5 - f/5.6 ASPH.
OM-D E-M5, H-PS14042E, Gitzo GT1541T, Arca-Swiss Z1 DP ball-head.
Astrophotography: TEC 140 'scope, FLI ML16803 camera, ASA DDM60 Pro mount.


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 Post subject:
PostPosted: Fri Nov 16, 2007 4:06 pm 
Great Shot Bob!


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 Post subject:
PostPosted: Fri Nov 16, 2007 4:35 pm 
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jay1 wrote:
Great Shot Bob!

Thanks. Enjoy the new camera.

Bob.

_________________
Olympus OM-D E-M1 + M.Zuiko Digital ED 12-40mm f/2.8, Lumix 7-14mm f/4, Leica DG Summilux 15mm f/1.7 ASPH, M.Zuiko Digital 45mm 1:1.8, M.Zuiko Digital ED 75mm 1:1.8.
Leica D Vario-Elmar 14mm-150mm f/3.5 - f/5.6 ASPH.
OM-D E-M5, H-PS14042E, Gitzo GT1541T, Arca-Swiss Z1 DP ball-head.
Astrophotography: TEC 140 'scope, FLI ML16803 camera, ASA DDM60 Pro mount.


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