whats the secret to astrophotography?

[EDL]

My guess is a lot less noise with stacked short shots vs one long one, plus sharper focus.

 

[TCampbell]

Canon has a 60Da camera. The a is for "Astrophotography," something to do with the coatings on the sensor.

Canon U.S.A. : Professional Imaging Products : EOS 60Da

I have this camera.

Prior to digital, "film" photography didn't have a problem with UV or IR light because the film itself was designed to be sensitive to the wavelengths you were interested in using. Digital sensors, on the other hand, actually ARE sensitive to both UV and IR light (the spectrum that the sensor can detect goes beyond what humans can see). The "problem" with a sensor that can see more than the human eye is that a lens refracts light like a prism. A low quality lens can cause color "fringing" on the edges of subjects within an image - where the edge has a blue fringe on one side and red on the other. This is because the periphery of the lens works like a prism and tries to split light into it's constituent wavelengths. Since UV and IR are even farther out of the spectrum, the fringing from UV and IR would be even stronger than it is for blue & red. This results in images that look "soft" even when they're correctly focused.

To combat the problem, every camera manufacturer adds both UV and IR filters inside the camera and these are located directly in front of the imaging sensor (not on the lens.)

That's for "normal" photography. But for astrophotography, the camera won't really image UV light (because UV light is the easiest to block -- very little UV light arrives). The IR spectrum, on the other hand, penetrates clouds & dust as well as the earth's atmosphere more easily. You can get stronger astrophotography images if you have a camera which is more sensitive to IR light.

Many astrophotographers will use a standard DSLR, but then modify the camera by removing the IR cut filter and either replacing it with a clear filter or replacing it with a much weaker IR filter. This does, of course, void the camera's warranty.

Canon used to make an astrophotography edition of the 20D -- called the 20Da. They stopped producing that quite a while back, but there was enough demand, that they decided to market a pre-modified version of the 60D called the 60Da. The camera is about 3 times more sensitive to IR light specifically in the hydrogen alpha band.

I've noticed that if I take a photo with a standard DSLR vs. a photo with the 60Da that the exposure times on the 60Da are considerably shorter and look better as compared to an unmodified camera. Very serious imagers use special cooled imaging systems by companies such as SBIG, Finger Lakes, or Apogee, etc. and these cameras are unfiltered monochrome cameras. The astrophotographer uses a filter wheel to create color images by taking multiple images by selecting the bandwidths they want to allow through the filters.

 

[Garbz]

Wait so for shooting on a tripod, do you take one shot in low ISO and then more in higher ISOs for image stacking?

Nope multiple high ISO photos. The idea is the stacking process statistically eliminates noise introduced by the sensor. Go to the deep sky stacker webpage and read through the manual as it gets more complicated than that. The stacking process involves light frames (your actual photos) dark frames which eliminate hot pixels (same ISO and exposure except with a lenscap on) flat frames to compensate for sensor dust and vignetting (same iso photo of a featureless sky) and bias frames to compensate for noise introduced by readout (same ISO and physical temperature but at maximum shutter speed).

The software then cleverly combines all the frames resulting in one noise free HDR picture.

yeah the guide scope is an orion ST80 refractor, with the starshoot autoguider...it works pretty well, however I need to get an extension to mount between the SSAG and scope, without a diagonal on it I don't have the length to reach focus on the SSAG, I've got it to work before rigging it up, but its not ideal. thats one reason I've had some difficulties shooting DSOs....but the SSAG itself and the scope itself are really good for what they cost. I use PHD as the guiding software.

What's the focus issue you're having? I've heard of flexture in the focus tube because it doesn't use a helical focuser like some other guidescoeps, but why would you not be able to focus properly? The SSAG and the ST80 are a matched pair!?

I'm having trouble trying to see what the benefit of image stacking is vs keeping the shutter open longer. If they move in the long exposure wouldn't they move by the time you took several shots?

Oh they do, but the software can always turn your frames for you

The ideal case would be to use an equatorial mount and autoguider to track the movement of stars. That would let you capture the image with long exposures at low ISO with no trails. However in absence of this tech you can capture repeated shorter frames and then stack them. As part of the stacking process you align the stars in each frame.

I forgot to mention something though. This works horribly if you have a lens with a lot of barrel distortion.

 

[Edsport]

Here's a couple shots i posted earlier of the night sky...

Taken with my Canon 350D and 75mm-300mm kit lens @ 220mm piggybacked on a Meade LX 200 classic for tracking. Unguided...
Exposure 4 mins x 13 stacked
ISO 800.
F 5.0.
220.mm.
M31 - Andromeda galaxy.

Taken with my 350D and 75-300mm lens @ 280mm...
Exposure 5 mins x 180 stacked.
(This is an edit, it should state 180 secs x 5 exposures).
M42 - Orion nebula.

 

[Aloicious]

yeah the guide scope is an orion ST80 refractor, with the starshoot autoguider...it works pretty well, however I need to get an extension to mount between the SSAG and scope, without a diagonal on it I don't have the length to reach focus on the SSAG, I've got it to work before rigging it up, but its not ideal. thats one reason I've had some difficulties shooting DSOs....but the SSAG itself and the scope itself are really good for what they cost. I use PHD as the guiding software.

What's the focus issue you're having? I've heard of flexture in the focus tube because it doesn't use a helical focuser like some other guidescoeps, but why would you not be able to focus properly? The SSAG and the ST80 are a matched pair!?

they are great, but the ST80 is made to have some kind of diagonal on it. and without one (or an extension) the drawtube doesn't extend far enough to reach focus at the SSAG's sensor. some of the guys in my astronomy club use it (w/ an extension or diagonal), and they work great together, I just havent got around to ordering one yet.

 

[Aloicious]

great images Ed!

unless I'm mistaken, that should be Andromeda (M31) and the Orion nebula (M42). correct?

for those who aren't into astronomy, M31 is our closest neighboring galaxy.

 

[Aloicious]

I'm having trouble trying to see what the benefit of image stacking is vs keeping the shutter open longer. If they move in the long exposure wouldn't they move by the time you took several shots?

the stars do move, but with stacking exposures you'll have several images without star trailing that you'll align together before stacking. if you just leave the shutter open you'll get star trails like this, which is one long exposure:

which isn't a problem if that's what you're going for (like I was), but if you're wanting to get stars as single points of light, and really pull the details of the milky way, or galaxies, etc, then you either have to stack images, and if you need/want to do long exposures, then you have to have some way of tracking the movement, like an equatorial mount, autoguiding (if needed, which depends on the focal length you're using, and how long exposures you're doing)...

 

[Edsport]

great images Ed!

unless I'm mistaken, that should be Andromeda (M31) and the Orion nebula (M42). correct?

for those who aren't into astronomy, M31 is our closest neighboring galaxy.

You're correct. I edited my post and named the photos. Thanks for the Like...