The Coal Sack

[2020BC (Bill Christie)]

Subject: The Coal Sack
Exposure: 16 x 4 mins (64mins) 800 ISO 10 Darks 10 Flats
Camera: Canon 350D unmod
Lens: Canon 50mm f1.8 @f6.3
Mount: Meade LXD-75 unguided
Location: Canberra, PM 15Jul07
Process: Alignment, flats and darks in MaxDSLR. Photoshop curves, etc.
Crop to around 9 degrees from 25 degrees widefield.

Field of view is roughly 9 degrees.

To view larger image click here (1.1MB).

[little col]

wow , really like that widefield image , just shows what can be done with an unmodded camera and a standard lense

[2020BC (Bill Christie)]

Thanks. Yes, if one buys a Canon DSLR you buy the 50mm f1.8 EF II lens. You just do. It only costs $AU 135, and for almost nothing you get a fast, very sharp lens which can produce some really good wide shots.

[Dietmar]

hi Bill,

very nice image!
btw: how come there are 10 spikes?
after reducing it - it is 5 spikes...why?

[sheeny (Al)]

Good stuff Bill!

Al.

[2020BC (Bill Christie)]

Originally Posted by Dietmar

hi Bill,

very nice image!
btw: how come there are 10 spikes?
after reducing it - it is 5 spikes...why?

Thanks, folks. Dietmar, I don't know for sure. I seem to remember that the number of spikes will be in sync with the number of edges presented by the blades of the camera's iris, and the smaller the aperture the greater the effect of diffraction - I think...

[Garyh]

Thats very nice Bill, very smooth and noise free!!
You have put the time in and rewarded with a fine image!!
I wouldn`t mind seeing the image uncropped!
cheers Gary

[Dr Nick (Nick)]

There is something about widefields that I like...

And of course this is no exeption!

Well done!

[ballaratdragons (Ken)]

Originally Posted by 2020BC

Lens: Canon 50mm f1.8 @f6.3

Top stuff, Bill.

I know nothing about DSLR's but I need to learn if I am to get one. So, my question is:

How can it be f1.8 @ f6.8.
Wouldn't it be one or the other?

[2020BC (Bill Christie)]

Thanks!

As you know, lenses have an iris to control the aperture. This lets you change the amount of light that is getting into the camera. Wide open, at maximum aperture (f1.8), this lens lets in a LOT of light and is therefore very "fast". However at f1.8 around the edges of the image you get "coma" where the stars are not round but instead look like little comets.

To get nice round stars right across the image field you need to "stop down" the lens by closing the iris a bit.

With this lens you get dramatic improvements with each stop down to around f4, and then after that you get slight incremental improvements. At f6.3, with this lens, I get stars that are pretty sharp right across the full width of the image. I've not tried it any slower.

Originally Posted by ballaratdragons

Top stuff, Bill.

I know nothing about DSLR's but I need to learn if I am to get one. So, my question is:

How can it be f1.8 @ f6.8.
Wouldn't it be one or the other?

[ballaratdragons (Ken)]

Originally Posted by 2020BC

Thanks!

As you know, lenses have an iris to control the aperture. This lets you change the amount of light that is getting into the camera. Wide open, at maximum aperture (f1.8), this lens lets in a LOT of light and is therefore very "fast". However at f1.8 around the edges of the image you get "coma" where the stars are not round but instead look like little comets.

To get nice round stars right across the image field you need to "stop down" the lens by closing the iris a bit.

With this lens you get dramatic improvements with each stop down to around f4, and then after that you get slight incremental improvements. At f6.3, with this lens, I get stars that are pretty sharp right across the full width of the image. I've not tried it any slower.

Thanks Bill.

No, I didn't know lenses had an iris. I always thought the iris was in the camera (like non-DSLR's have). Shows I know absolutely nothing about cameras

[avandonk]

It is very expensive to make irises with curved edges to minimise diffraction effects let alone with many blades. The Canon 50mm F1.8 has five blades giving ten diffraction spikes. You only get these many blades on the very expensive lenses. You could experiment by leaving the 50mm lens wide open and place a circular aperture in front of the lens. Even carefully cut black cardboard would do the job.
I personally think that diffraction artefacts have no place in real images if it can be avoided. If you like them that is fine.

Bert

[2020BC (Bill Christie)]

Originally Posted by Garyh

Thats very nice Bill, very smooth and noise free!!
You have put the time in and rewarded with a fine image!!
I wouldn`t mind seeing the image uncropped!
cheers Gary

Thanks, Gary.

You may already have seen the uncropped image, (a widefield of the Southern Cross and Pointers click here (1MB) )

This Coal Sack image is a crop from a newly reprocessed version of that widefield.

[CoombellKid]

Very nice image Bill

regards,CS

[ispom]

Good shot, Bill,
I've storage it for my collection, not last because of the 10 spikes

[Ric]

That's a lovely view of the Coalsack Bill very enjoyable.

I had to wait until I came to work today, only dialup at home but it was worth the wait.

Cheers