Sunday, December 27, 2015

A new DSLR. Now what?

I received a DSLR for Christmas. This is a place to express my ignorance on how to use it for astrophotography! If you have answers, please comment!

The Camera
I have a Canon EOS Rebel T5i (also known as the 700D). This is a crop-frame 18MP camera that came with an 18-55 mm zoom lens. It currently has an 8GB SD card.

There's targets which I photographed adequately and others that I attempted to photograph poorly with the iPhone that should be within reach of this camera: 1) Moon, 2) Messier objects, 3) Planets, 4) Milky Way, 5) Constellations, 6) Star Trails, 7) ISS/Satellites/Flares

1) The Moon
With the current lens fully zoomed, you can make out general surface features. This wide field of view should allow capture of halos and the glow of a moon peeking through clouds.
  • Do the higher-quality capture options actually mean more MP saved and more detail? or is it more a factor of compression ratio?  Just how much more hand-held detail will I see with a "raw" photo versus "L1"
  • How do I "flatten" the exposure so that I don't have to choose between clouds and surface features? Can the T5i do HDR?
For a full lunar disk, I'll need a T-Ring and T-Adapter to attach the T5i to my NexStar 8" SCT for prime-focus photography. I've located the Celestron OEM pieces to purchase.
  • How do I calculate the field of view I'll get? I suspect that the full lunar disc will exceed the crop-frame size of the camera.
    • Update: A link from David Blanchflower (@DavidBflower) and some basic trig helped me calculate field of view for my sensor attached to my Celestron 8" SCT with 2036 mm focal length at 0.63°x0.42°, enough for quarter moon but not full. 
  • Will my current focal reducer do the trick? Is the T-adapter even threaded to accommodate a focal reducer (or other filters)?
For lunar close-ups, I could crop photos, or figure how to magnify. This requires another adapter, the "Tele-extender". I know little about this, other than it should allow an eyepiece between the T-adapter and the telescope.
  • Will any of my eyepieces work in this configuration?
  • Is there a better way to magnify?
With the brightness of the full moon, very short exposure times should be possible, so tracking should not be needed.

2) Messier Objects/Deep Sky
To capture star clusters and galaxies, I need more magnification than my current zoom lens, but I think many popular targets require larger field of view than the full moon. This list tells me (?) that the Andromeda Galaxy, M31, is about 1 x 3 degrees, which is 2 x 6 full moons. The Great Orion Nebula shows at about 1 x 1.5 degrees, or 2 x 3 full moons.
  • How do I expand the field of view of my telescope/prime focus combination further?
For dim objects such as these, gathering lots of light will be necessary. The rough "rule of 500" implies that with the 2032 mm focal length of my 8" SCT, I should only be able to capture about 1/4 second exposure before drifting stars become evident. Clearly this won't provide the rich colors and clear images I'm looking for. So I guess I'll have to finally figure out if the old telescope can be properly aligned for tracking!
  • Will stacking a large number of 1/4 second exposures still give me a reasonable result as compared to stacking less, longer exposures?
I also need to learn about "dark frames", how to take them and how to use them. And, will Registax be sufficient software for me, or is there other free/paid software I need to look into.

3) Planets
My results with the iPhone were okay for an iPhone. I could recognize that Jupiter had bands and the red spot was vaguely discernible, but no shadows from Jupiter's moons were visible. Saturn definitely had rings, but no ring divisions were to be seen.

With prime focus, without a focal reducer, I'll definitely have more pixels to play with. A 2x barlow should (?) increase the image size even more. At this point, I suspect that taking video of these bright objects should yield lots of clear, stackable frames.
  • How do I increase magnification even more with prime-focus photography?
  • Is video the best bet? Or should I be taking individual frames?
    • Update: Video from the camera will be taken at lower-than-full resolution, so still images are better. Using an intervalometer to capture many short exposures will provide raw material for stacking.
4) Milky Way
Using a tripod and the current lens zoomed out to 18mm, I should be able to expose for up to 30 seconds before significant star trails form. For long exposures on the iPhone, I use the NightCap Pro app, which combined many shorter exposures into one long exposure. Now I've got to figure out the proper ISO and aperture to set in order to not over or under-expose in these long exposure pictures.
  • How do I figure out the proper settings to use for a Milky Way exposure?
    • Update: This tutorial gives some great detail on tools and techniques.
  • How much do I reduce the exposure to compensate for light pollution in my cul-de-sac?
  • Is a light pollution filter necessary, or can I wipe most of it out in post-processing?
    • Update: Light pollution can be handled with a special filter that selectively removes sodium and mercury spectra. Also can be minimized in post-processing with some artistic adjustment.
  • How do I take a dark frame for noise reduction?
    • Update: Lens cap and same exposure settings!
  • When stacking multiple frames, how do you align both a fixed horizon and a moving milky way in multiple exposures?
  • What should I look for in a wider-field lens?
    • Update: I calculated the field of view of the 18-55mm lens. At 18mm, I see 63.5°x45.0° while zoomed to 55mm gives 22.9°x15.4°.
5) Constellations
Tripod-mounted shorter exposures should expose larger constellations. I think this will be a more straight-forward trial-and-error process, and one in which I can achieve quick success.

6) Star Trails
As near as I can figure, the camera can be set up to take up to 10 consecutive exposures at a maximum 30-second exposure each. That's only 5 minutes.
  • How do I keep the shutter open longer in "bulb" mode? What accessory do I need to hold the button down?
    • Update: An intervalometer is the tool I need now have!
  • Will a very long single exposure flood the frame with noise?
  • When stacking multiple exposures, will there be a "pause" in between exposures with a break in each star trail?
    • Update: Yes, there will be a pause. Not sure how to get rid of it.
7) ISS/Satellite Flares
I've been quite successful taking long exposures of the ISS and satellite flares with the iPhone. I'm debating any advantages of DSLR images instead. I'll be facing some of the same challenges with the Star Trails images: how to set exposure, keep the shutter open long enough, and stack multiple frames for one continuous trail.

Also, the field of view of the iPhone is wider than I'll get with this lens.
  • Any recommendations on wide-angle or fish-eye lens specs?
I have a lot to learn, and am looking forward to successes, and dreading the stupid mistakes I'm sure I'll make. But I love to learn, and this will present some interesting challenges.

Please comment below, or tweet at @BeckePhysics with any suggestions or recommendations! I appreciate your help.