Issues with viewing through DSLR with telescope

[OzEclipse]

I think the issue is the sensitivity/noise of the 40D sensor. Live view is a video feed with a very short exposure time. A blank screen is all I would expect from live view of a 40D through that scope.

1. Use a bright star to focus and to align the finder with the centre of the photographic field.
2. Then focus on the bright star.
3. Next move the scope to Orion centring it in the finder scope.
4. Turn the ISO up very high and do a short exposure. This will let you confirm you are pointing at Orion.
5. Adjust the framing and reshoot the test shot.
6. Turn the ISO down and take the real extended time exposure of your subject.

[TCampbell]

I'll offer up a few single straight-out-of-the-camera unprocessed images...

M42 is the middle star in the 'sword' below Orion's belt. Even in binoculars you can quickly tell it doesn't resolve as a pinpoint of light but more like an unfocused patch. Under higher magnification you can visually see the cloud (it's much easier in darker skies ... but it is bright enough to be visible even in urban skies.)

All images are shot with a Canon 60Da at ISO 800 using a TeleVue NP101is which is an f/5.4 refractor. The scope was on an equatorial mount and tracking. These "as shot" without any additional processing (other than converting the CR2 RAW files into JPEG and reducing in size to make them web-friendly).

10 seconds: The core of the nebula is visible here, but not the outer extents.

1 minute: More of the nebula is visible, although the core is starting to over-expose.

4 minutes: Much more of the nebula is visible, the core is over-exposed to the point of being completely blown out. Even longer exposures would reveal more although you can start to see the light pollution taking its toll.

From this you can see that a 1 second exposure would be pretty tough to see much (especially in an f/9 scope compared to what you see in a 10-second image taken by this f/5.4 scope).

With your Canon 40D, ISO 800 is the ideal ISO to use for deep-sky astrophotography (see: http://dslr-astrophotography.com/iso-va ... n-cameras/ ). This has to do with how "gain" works on cameras. ISO is a term originally used to describe the sensitivity of photographic film. But it was carried over to digital cameras because photographers were familiar with the concept. Technically this isn't how digital cameras work. The sensitivity of the sensor never actually changes ... it is whatever it is for the given camera model. ISO is really a "gain" (amplification of the signal) applied to the data AFTER the exposure is over but before the image is saved to the storage device.

Cameras can do this two ways... one method is called "upstream" gain ... the other is "downstream" gain. The difference is whether the gain is applied before vs. after the analog signal is converted into digital information.

It turns out many Canon cameras (including yours) do a combination of "upstream" and "downstream" gain. Below ISO 800 they mostly do "upstream" gain (amplification of the analog data). Above ISO 800 they mostly do 'downstream" gain (multiplication of the digital value of the pixel). At ISO 800 they do a little of both. Boosting "downstream" gain results in a loss of dynamic range (think of this as the total possible number of gradients from black to white). ISO 800 is the ISO that lets you get away with the most gain possible ... while sacrificing the least amount of overall dynamic range on YOUR particular camera (it will be different for other camera models.)

Generally speaking, when you shoot M42, you usually take a variety of exposure durations and then blend them (using a technique called high-dynamic range or "HDR"). Software handles the magic ... but it lets you take the data regarding the brightest parts of the photo from your shortest (dimmest) exposures ... and also takes the data regarding the dimmest parts of the photo from your longest (brightest) exposures ... and blends them to get something with much more tonal range.

Tips:

If the telescope is not focused then you'll struggle to see anything at all even if it's pointed bang-on accurate for your object. HOWEVER... everything in space focuses at "infinity" (the same distance). SO... once you attach the camera, point to anything you can find in space that is nice & bright ... the moon... or the brightest star you can find. Then adjust focus to get it as sharp as possible.

If pointing to a star, switch on "live view" mode on the camera, then crank the ISO to the max and set the exposure time to 30 seconds. Canon cameras have something called "exposure simulation" in live-view. It will brighten what you see in live-view so that stars that might otherwise be too dim to see will be clearly visible. Adjust focus to try to make the stars as pin-point as possible. You can even magnify the live-view ... go ahead and magnify to the max (10x) and tweak the focus.

Be patient as you focus. When I started, I adjusted focus that it "looked good to me" on the really tiny screen on the back of the camera and spent all night imaging. THEN I got the camera inside and unloaded the images onto my computer and ... every image was mushy. I was close ... but not sharp. I've since learned that it pays to be really patient to get the best possible focus (even if it takes you 10-15 minutes to do just this one thing ... your patience will be rewarded.)

Having achieved focus ... point the telescope to the object of interest and shoot a 30 second exposure (still at max ISO). The image will look really "noisy" because the ISO is too high... but the point of this is to make sure the telescope is properly centered on the object you are trying to capture. Tweak the position of the telescope until it is framed nicely.

NOW return the exposure to sane settings (ISO 800 for your camera) and whatever exposure duration you intend to use. This can be challenging because at ISO 800 in an f/9 scope... a 5-minute exposure is not out of the question. And the problem with 5-minute exposures is that your telescope wont track accurately enough without using an auto-guider AND since you have an alt/az mount ... you'll experience field rotation (the object will appear to "twist" slightly over the duration of the photo). You may be forced to deal with much shorter exposures (e.g. 30 second or less) and just shoot a LOT of them. Equatorial mounts (especially "solid" equatorial mounts) are preferred for astrophotography. Giant professional observatories actually do use alt/az mounts ... but they have camera mounts that incorporate a field-rotator to de-rotate the "twist" that alt-az mounts experience. The amount of "twist" depends on where the object is located in the sky. The twist is faster near the zenith ... slower near the horizon.

[Baurice]

M42 is now difficult from the UK, as are M45 and the Hyades. I would suggest the Beehive Cluster (M44) is a good target. I have a 75-300mm zoom lens with my DSLR and only rarely use it with a telescope.