Astronomy shots - DPChallenge Forums

Canon RF 100-500mm f/4.5-7.1 L IS USM

06/23/2007 07:47:09 AM · #4

That is a galaxie, but it is our galaxie, the Milky Way. I don't think that lens would have enough magnification to show any other galaxie better than a blurry spot.

06/23/2007 08:22:10 AM · #5

Originally posted by cloudsme:

That is a galaxie, but it is our galaxie, the Milky Way. I don't think that lens would have enough magnification to show any other galaxie better than a blurry spot.

M31 in the constellation Andromeda would, at 50mm, (80mm equivalent in 35mm terms) show some detail. The FoV of the lens will be about 25Â° and M31 spans over 2Â°, so yes, you'd see some detail. Those in the Southern Hemisphere also have the Magellanic Clouds, which are irregular galaxies gravitationally bound to our own.
On an APS-C camera, the 50/1.8, stopped down to f/2.8 or f/4, may provide a good image across the frame. On a 35mm-format camera, even the 50/1.4 gets pretty bad in the corners. Focus is critical, and not as easy as you'd think.
In order to get exposures of more than a couple seconds without star trailing, you'll need to track the sky, either with a mount intended for a telescope, or by using a "barndoor" mount (google it). You need pretty precise polar alignment and tracking to achieve crisp images of several minutes or more. The longer focal length you use, the more precise the tracking needs to be.

06/23/2007 08:30:59 AM · #6

A few examples here. M31 shot with a 70-200 lens @ 200mm. Most of the wide angle shots are with either the Canon 15mm fisheye or the Canon 24mm f/1.4 L

rami

Canon EOS-350D Rebel XT

Canon EF 50mm f/1.8 II

06/24/2007 10:47:05 AM · #7

can you actually get a decent shot without modifying the camera ( removing the IR filter ) ...
cool responses ... thanks

06/24/2007 10:52:29 AM · #8

Originally posted by rami:

can you actually get a decent shot without modifying the camera ( removing the IR filter ) ...
cool responses ... thanks

Absolutely!
Where the IR modifications help is for imaging nebulae. The nebulae often have relatively strong emission in the IR.
For galaxies, there is some benefit from IR mod, but not as much as for the nebulae. For star clusters, planetary imaging, and general wide angle night sky shooting, IR modification is not at all necessary.
I work with unmodded cameras (previously the Canon 10D and now the 5D) and I'm quite happy with my results, though my location is problematic (light pollution).

rami

Canon EOS-350D Rebel XT

Canon EF 50mm f/1.8 II

06/24/2007 10:52:41 AM · #9

kirbic ... NEAAAAAAAT pictures.... why is the galaxy shot taken in four exposures ?? to avoid trailing ? ... if so how can you combine multiple exposures.. is it something done in the camera or can you do it in photoshop

Canon 12x IS USM (S3 IS built-in Lens) -- 36-432mm (35mm EQ)

06/24/2007 11:18:34 AM · #10

Originally posted by rami:

The four exposures help to reduce noise and can provide a little extra detail. it's especially important with astro images, because the contrast can be very low. Processing involves "stretching" the data to increase contrast. Having multiple frames, and working in 16-bit per channel mode, helps avoid artifacts.
The image "stacking" can be done in Photoshop, or it can be done by special purpose programs designed specifically for stacking astro images. In Photoshop, once they are aligned, combining them is easy. The first (bottom) image in the layer stack stays at 100% opacity, the second gets set to 50%, the third at 33%, the fourth at 25%, etcetera. Random noise is reduced by the square root of the number of exposures, so four exposures reduces random noise 2x (by half).

ETA: If you didn't already, be sure to look at the "original" resolution version of those shots. They really lose something in the small versions.

Message edited by author 2007-06-24 11:22:00.

GeneralE

Canon PowerShot S3 IS

06/24/2007 11:37:12 AM · #11

Originally posted by kirbic:

The image "stacking" can be done in Photoshop, or it can be done by special purpose programs designed specifically for stacking astro images.

For example, Registax.

darnok

Nikon D850

Nikon AF-S Nikkor 16-35 f/4 ED VR

06/24/2007 11:42:36 AM · #12

Originally posted by kirbic:

Focus is critical, and not as easy as you'd think.

Wouldn't you just set the focus to infinity?

06/24/2007 12:20:51 PM · #13

this might sound stuped butt... how can a 50mm lens see that far?

Tamron SP AF 28-75mm f/2.8 XR Di for Canon

06/24/2007 12:25:30 PM · #14

Originally posted by Raziel:

Originally posted by kirbic:
Focus is critical, and not as easy as you'd think.

Wouldn't you just set the focus to infinity?

Almost, but not quite. Most lenses focus "past infinity" for two reasons:
- Infrared photography requires this, since infrared focuses at a slightly different point than visible light.
- Tolerances involved in lens and camera manufacturing; the extra travel ensures that infinity focus can be achieved with all lens/camera combinations without insanely tight tolerances being required.
Additionally, many lenses are not truly "flat field", meaning that focus at the corners may be slightly different from at frame center. Tweaking is required to get the best combination of center and corner sharpness. This effect is much more prevalent with wide angle lenses.

strangeghost

Canon EOS-7D

06/24/2007 12:32:52 PM · #15

Not all lenses have a mechanical stop at infinity. Many lenses allow you focus a bit "past" infinity. Therefore, you have to visually make sure your focus is spot on. As Kirbic says, this can be the most tedious process in acquiring good astrophotos.

exclamationjay, you're not necessarily seeing "far," Much more important is that you're seeing "faint." Many celestial objects are fairly large. The Andromeda Galaxy, M31 in Kirbic's shot, is about 5 times the width of the full moon! It's absolutely huge, but it's so faint that most people don't even know it's there. Similarly, the Milky Way spans the entire sky and is beautifully detailed, but most who have seen it only recollect a diffuse, low contrast belt in the sky. Any lens you own can potentially do decent astrophotography, either star trails on a fixed tripod or long exposures tracking as the Earth rotates.

Nikon AF-S DX Zoom-Nikkor 18-55mm f/3.5-5.6 ED

06/24/2007 12:45:03 PM · #16

Originally posted by exclamationjay:

this might sound stuped butt... how can a 50mm lens see that far?

Not really a silly question at all. In order for something as far away as M31 (2.9 million light years) to be visible in a lens that takes in between 26Â° and 40Â° of sky, it has to be (1) huge and (2) emit huge amounts of light.
M31's disc diameter is about 240,000 light years. It is simply enormous. It contains 100 billion or more stars, and thus emits tremendous amounts of light. Even so, it is just visible to the naked eye as a "gray smudge" under dark skies. It's very difficult or impossible to see with the naked eye under light-polluted skies.
FWIW, M31 will show up even in images taken with the 15mm fisheye! Compare (warning, large images):

This image of mine was taken at 200mm. This image was taken with the 15mm fisheye. M31 appears at upper right.

Apple iPhone 5

06/24/2007 01:17:30 PM · #17

Originally posted by kirbic:

This image was taken with the 15mm fisheye. M31 appears at upper right.

Very nice! - Is that a satellite trail on the bottom left of the wide-angle shot?

Message edited by author 2007-06-24 13:17:46.

06/24/2007 01:32:29 PM · #18

Originally posted by jhonan:

Originally posted by kirbic:
This image was taken with the 15mm fisheye. M31 appears at upper right.

Very nice! - Is that a satellite trail on the bottom left of the wide-angle shot?

It's actually an aircraft. The interrupted trail (dots instead of a continuous line) is the giveaway.
It's not unusual at all to catch a satellite in one of these shots. I've even caught more than one in a single exposure.

Tamron SP AF 28-75mm f/2.8 XR Di for Canon

06/24/2007 01:43:04 PM · #19

so tonight if i go in my garden, put my camera on a tripod and shutter for 5mins, point the camera at the sky i will get pictures like this?

strangeghost

Canon EOS-7D

06/24/2007 01:45:53 PM · #20

Originally posted by exclamationjay:

so tonight if i go in my garden, put my camera on a tripod and shutter for 5mins, point the camera at the sky i will get pictures like this?

Depending on how dark your area is, and how good your skies are, you'll get something like this:

Bug's shot was in the range of 30 minutes. Note the glowing sky. Any light pollution at all (nearby lights) will really overwhelm the sky at long exposures. You need a relatively dark location. Also, with the moon just past first quarter, moonliight will be as big an issue as artificial light.

06/24/2007 01:52:37 PM · #21

haha cant wait to try! dont fancy walking through the middle of the park at night though..

Tamron SP AF 28-75mm f/2.8 XR Di for Canon

06/24/2007 02:05:31 PM · #22

haha well me and my friend are going the park at mid night tonight (GMT) so ill have the pictures on later.

what should and should'nt i do?

*Not only is this going to be my first time at Astronomy Photograhy it is at night photography haha so i need all the advice i can get =D

Message edited by author 2007-06-24 14:08:15.

strangeghost

Canon EOS-7D

06/24/2007 02:08:50 PM · #23

Originally posted by exclamationjay:

haha well me and my friend are going the park at mid night tonight (GMT) so ill have the pictures on later.

what should and should'nt i do?

Wait until the moon is out of the way (a little past midnight, your time). See if you can make an interesting composition with some landscape elements and the bright stars of the southern sky. Have fun. Take an extra battery!