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I once got a recommendation to use the live view (on-screen preview) feature to aim the camera (EOS 70D; astromodified) when shooting astrophotos. But even with the largest aperture (f/4) and ISO (12,800) I'd only see one fairly bright star (in my case: Aldebaran; 0.87 mag) on the LCD and nothing else. Magnifying the preview does not help either. Neither does increasing exposure time (I tried it out with 20" and 30"). The lens was zoomed at 24 mm all the time.

I'm trying to aim on the celestial object (crab nebula) I'd like to shoot.

So: Am I doing anything wrong, or is this normal behavior of the live view? Does this mean that I can't use live view to adjust my camera before doing a shot of the night sky?

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  • Do you have "Expo. simulation" enabled? Dec 29, 2021 at 7:51
  • What, exactly, are you trying to adjust? Focus position? Exposure brightness? Framing (what is included and what is not included in the picture)? Something else?
    – Michael C
    Dec 29, 2021 at 16:42
  • @RomeoNinov No, at least not deliberately.
    – Neppomuk
    Dec 29, 2021 at 22:09
  • @MichaelC I'm trying to aim on the celestial object (crab nebula) I'd like to shoot. This is thaw I need live view for.
    – Neppomuk
    Dec 29, 2021 at 22:11
  • @Neppomuk What kind of telescope is your camera attached to? How is it attached?
    – Michael C
    Dec 30, 2021 at 3:38

3 Answers 3

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The crab nebula is magnitude 8.5. That's very dim. To image very dim deep sky objects like the crab nebula, you're going to have to do things a little differently than your current approach.

Rather than a lens, you almost certainly need a small telescope with a much wider objective than a 24mm f/4 lens, which has an entrance pupil of only 6mm in diameter. A small Newtonian reflector typically has a 10cm primary mirror. That's almost 17 times in diameter and 278 times more area to collect 278 times as much light as your 24mm f/4 lens. It will also typically collect that light from a narrower slice of the sky, which is also what you need for tiny targets such as M1, which has an angular size of barely one-tenth degree (6 x 4 arcminutes). Your 24mm lens shows about 60 degrees of sky diagonally, or about 600X the size of M1. So at best, at 24mm you can hope for the crab nebula to cover only about 10 x 8 pixels on your 70D's sensor.

Even then, you're going to need pretty dark skies to pull magnitude 8.5 objects out of the background sky glow. If your sky is dark enough, longer exposures with a tracking mount can capture such dim objects, but getting them onto the screen in Live View in real time is probably a pipe dream.

What you can do is use the brighter objects nearby (like, for instance, Aldebaran) to aim in the vicinity of your target and then take a test shot and examine the results to see if you're aimed where you want. If you're using a tracking mount set up properly, wherever the scope is pointed will be maintained while you're looking at your test image(s).

You can also use the brightest stars in the sky to manually set focus. I will "barrel sight" my lens at the brightest star in the sky and move the focus ring from past infinity back until the star is visible and as sharp as I can get it. Even with the brightest stars, if they are out of focus by very much the light from them will be spread so thin that you won't see anything at all. I'll then use 5X magnification to refine focus in very small adjustments. I'll then use a slightly dimmer star at 10X to get the focus as sharp as I can.

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  • Yes, I even think that Aldebaran was visible in the live view.
    – Neppomuk
    Dec 30, 2021 at 21:40
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M1

Before delving too deep ... consider the size of M1. It is 6 arc-minutes edge-to-edge. Here is a simulated view of it that I generated using Starry Night Pro Plus:

Field of view for M1 through Celestron C14 with APS-C sensor

The above field of view simulates what you could capture using a camera that has an APS-C size sensor (e.g. my Canon 60Da) if attached to a Celestron C14 Schmidt-Cassegrain Telescope. This is a 14" aperture f/11 telescope with a 3910mm focal length.

Here is another field-of-view generated for a 540mm refractor:

Simulated view of M1 at 540mm using APS-C sensor

While the above image used a telescope, you could achieve similar focal length with a camera lens. But ... you can see at those focal lengths M1 is just a smudge in the center of the frame. You wouldn't expect to see detail.

When using shorter focal lengths I tend to look for wider-field subjects (or sometimes several nearby subjects).

Focus

Canon uses "exposure simulation" when using live-view. I use live-view only to focus the camera, then use software to control a series of long-exposures for imaging.

Everything in space is focused at infinity. You can achieve focus by picking anything in space (it doesn't need to be near the object you plan to image), then move to your subject (without changing focus) and begin shooting.

To focus a Canon camera, put the camera in Manual exposure mode, set the ISO to max ISO, set the aperture to wide-open (if using a lens ... can't adjust this when using a telescope), and set the exposure duration to 30 seconds. This maximizes the brightness of the live-view display to make it easier to see stars (you'll only see the brighter stars ... so pick a bright one.)

If using a lens, switch to manual focus mode and adjust the focus so it is near the "infinity" focus for that lens (if you don't do this then blurred stars wont up at all and you'll be looking at a blank live-view screen).

Adjust focus to get try to get the star down to pin-point size. Use the live-view "zoom" feature to magnify to 10x and adjust focus again.

Be patient doing this step... I am guilty of getting it the point where it looked pretty good to me ... and started shooting. Only to get home later and realize that everything was just a little soft. I recommend setting the ISO and exposure settings back to sane values and take a few test shots ... then inspect and MAGNIFY them to verify you really are happy with the focus before you continue.

To get really precise, pick up (or make) a Bahtinov focusing mask (https://en.wikipedia.org/wiki/Bahtinov_mask). The mask causes bright points of light to form diffraction spikes along three different axes. When you achieve perfect focus, the spikes will all converge at a common center point.

The downside of a typical Bhatinov mask is that it is solid with slits cut into it ... and this means it is blocking a little over half of the light. So what few bright stars you can find at all... get even dimmer. LonelySpeck (I am not affiliated in any way ... although I do own one of their filters) makes a clear slide-in filter that is etched much in the same way that a Bhatinov mask is made ... and it does the same thing (produces diffraction spikes that help focus). But since it is a clear filter with etched lines it allows all the light to pass ... so stars don't get dimmer when you try to use it. The downside is that it is a square "slide-in" type filter and this requires a filter bracket and adapter ring to attach to your lens -- these aren't common equipment. Most photographers don't own them and many local camera stores don't even carry them (the major camera stores do).

Tracking

Faint-fuzzies need long exposures. But since the Earth is spinning, it will drift in the field of view unless it is on a tracking mount.

Earth rotates at just over 15 arc-seconds per second. Since M1 is roughly 6 arc-minutes across, each 4 seconds would shift it by a full arc-minute. In 24 seconds it would have moved it's own width.

Exposures for deep-space faint-fuzzies are usually many minutes long ... 5, 10, even 15 minutes.

Tracking during long exposures is complicated by both the precision of the tracking mount and the precision of it's polar alignment. Generally an auto-guider system is necessary (I typically do not use an auto-guider at low-ish focal lengths -- e.g. around 500mm, but do use them at high focal lengths -- e.g. around 2000mm) The guider uses a separate camera and you select a star near the object you want to image. It takes a new exposure ever few seconds (e.g. maybe every 3-5 seconds) and compares the location of the tracked star to a master frame to determine if it is has moved in the frame. If it has, a correcting movement is sent to the mount.

If you are just beginning to learn astrophotography, then starting with larger wide-field subjects is easier because they can be captured with camera lenses or short focal-length telescopes and usually don't need the complication of adding auto-guiding.

Without needing to invest in a solid equatorial telescope mount, you can use a tracking head that attaches to a regular photo tripod. Sky Watcher and iOptron both make tracking mounts that are popular. For telescope mounts, things get expensive. The telescope mount quality is easily the single most important piece of equipment (more than the telescope and more than the camera).

You can search AstroBin.com as a resource for learning. Most astrophotographers who post their work to AstroBin include, telescope, camera, exposure info, and if and what filters may have been used. This can give you a pretty good idea of what you might need to achieve similar results.

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  • I've already got a tracker box (iOptron Pro). It's much exacter than my old Vixen Polarie (especially if you also use the Polar Aligner app), but requires a rail with counterweights when using a heavy camera and lens. So far, I've been photographing rather large targets: the Andromeda galaxy, the veil nebula, the North America nebula, open clusters etc.
    – Neppomuk
    Feb 12 at 23:44
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    @Neppomuk This part of the year you can grab the area of the sky near M42. e.g. I used a 135mm lens and got the Orion Nebula, Running Man, Flame, Horsehead and M72 ... and just a touch of Barnard's loop (I'd have needed a shorter focal length to get all of Barnard's loop) ... all in one frame. Feb 13 at 2:15
  • If only wheather was good enough for that, then yes! Unfortunately, we haven't had clear night skies since Christmas.
    – Neppomuk
    Feb 16 at 21:05
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After giving live view a second try, I found that (at least on the 70D) exposure time is limited to 1/30". This would mean that live view is absolutely unsuitable for astrophotography.

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  • Not 1/30th ... 30 full seconds. But that's just using the camera to control exposure. In bulb mode you can do any duration you want. (note that bulb mode isn't accurate to within fractions of a second ... it's meant for long exposures only.) Mar 6 at 4:36
  • The full 30" you are mentioning are the max. exposure time when taking pictures, but for the LiveView previews, the max. integration time is really 1/30". Even in the OSD, I get the number 30 (instead of 30") after having set exposure time to 15".
    – Neppomuk
    Mar 7 at 20:15

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