Sunset in Kruger

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I tried photographing moon and Saturn with a 8'' Dobsonian (without tracking). And it was a breeze. But I had to follow the '600 rule', i.e I only exposed it for less than (600/f) sec ~ 1/2 sec. But now I want to photograph deep sky objects, and since there is no tracking the most I can do is give exposures of less than one second. Would these small exposures be good for stacking? What should be the ISO ? What else can I do to take better deep sky shots.?

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4 Answers 4

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There is no substitute for per-image SNR (Signal to Noise Ratio). A half-second exposure following the 600 rule won't be enough. You have to factor in read noise as well as photon shot noise. Photon shot noise can be delt with via stacking, but read noise...read noise diminishes detail and in the deeper shadows, can eliminate it entirely. With half-second exposures, your overall SNR is going to be so low as to not be worth it. You could stack 1000 frames with half-second exposures, and it wouldn't come anywhere close to stacking 10 frames with much longer exposures.

Also, keep in mind, increasing ISO does not actually increase sensitivity. The interchanged use of "sensitivity" and "ISO" has lead to a grave misunderstanding of what ISO really is. Increasing ISO does not actually improve your SNR, it simply amplifies a lower signal by a certain factor. (Increasing ISO does marginally improve IQ, by amplifying the signal before read noise is added, but it is still quite marginal overall.)

The only way to improve the end results is to gather an overall image signal that is sufficiently strong enough to make read noise a relatively inconsequential factor. You still want to stack, as it is difficult to maximize SNR even with longer exposures, and photon shot noise will always be a problem that can be minimized with stacking. To that end...either using a camera with bigger pixels that have a naturally higher SNR, or using a tracking mount, are really the only ways to improve your deep sky shots.

IT'S ALL ABOUT SIGNAL TO NOISE RATIO!

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Great Answer!! So without tracking what else can I do with the telescope apart from taking Planetary Photos? –  Alok May 1 '13 at 2:10
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Enjoy the deep night sky with your eyeballs! :) You could probably get some pretty sweet shots of the sun as well, if you use proper filtration. –  jrista May 1 '13 at 2:13

You could try something like DeepSkyStacker. I have not personally used it, but I had it recommended to me by someone after some night shots I took. My understanding is that you take short photos from a fixed position and it can apply the appropriate position adjustments and stack them for you to produce a final image.

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If it's any help, I use a Canon EOS 7D DSLR and a Skywatcher Explorer 200P (8" Reflector) telescope on a non-tracking Equatorial Mount and have had some moderate success by limiting the exposure time to no more than 1/10 second (halve this if using a x2 Barlow), use an ISO (approximates to sensor gain) of 1000 to 1600 and take upwards of 100 to 250 separate images in Camera RAW, more if you can manage 500 to 2500 would be optimal but time and CF Card space comes into play.

I have found live view with the rule of thirds grid on helpful as I aim to keep the main area of interest within the centre box of the grid of nine boxes - this gives the stacking software an easier job registering and stacking the images plus gives a big enough margin to allow for later post processing image cropping.

Some stacking software can't handle Camera RAW so convert the images to TIFF (no compression) first with something like Photoshop Elements. Then with something like Deep Sky Stacker stack your multitude of images using Intersection Mode and Align RGB Channels in Final Image.

Warning an appreciable amount of PC Memory is needed and even with a fairly quick processor it takes a while. Finally use Photoshop or the GIMP to maximise the dynamic range and adjust the brightness, contrast and colour balance to tease out the level of detail required.

Hope this is of use...

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Do you have a sample image you could include? Would be nice to see what sort of results you can get with that method. –  MikeW Mar 31 at 21:07

One rather obvious way to improve the result is to use a very low magnification or to just take the picture without a telescope. Many deep sky objects can be seen much better at low magnifications. E.g. the whirlpool galaxy has an angular size of 11 by 7 arc minutes, so it's a rather large extended object (given the enormous distance to us). At high magnifications the surface brightness becomes very low making it very difficult to get a good signal to noise ratio.

Of course, the reason why you end up with a better signal to noise ratio at lower magnification is due to the pixels gather the light from a larger part of the galaxy, therefore you will sacrifice resolution. However, you can then still get to a higher resolution using super-resolution techniques which involves sorting out the images that are shifted by some fraction of a pixel in the horizontal and vertical directions, stacking them separately and then combining everything to a higher resolution image.

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