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Old 04-01-2012, 10:48 AM
pjphilli (Peter)
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Canon dslr can't hack it?

HI

I image in suburban Sydney where light pollution is a problem. For the past year or so good clear skies are a rarity when clarity appears to be affected
by high atmospheric moisture. I find that to image object such as the Horsehead and Pleiades that I have to use 500sec images. I am using a modified Canon 400D, ISO 1600, light pollution filter, on a 200mm RC at f8. The 500sec images come out with quite milky and no amount of stacking or stretching etc can produce a decent composite. Shorter image durations just do not show much feint nebulosity. My thoughts are now wandering to getting a QHY8-L ccd camera. Would this give me significant improvement over the Canon 400D in current sky conditions?

Cheers Peter
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Old 04-01-2012, 12:05 PM
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Part of the problem is the dynamic range of the image as well as the sky gloow you are getting.
Increasing the ISO increases the amplification in the camera but doesn't change the amount of light. there is an ideal ISO for these cameras that will give you the best dynamic range. I remember seeing a review (But cant find it at the moment) for a 40D that had ISO 800 as the sweet spot.

These cameras are 12 bit imagers so have a range of +/- 4096 for each pixel in RAW settings. I don't know what the well depth for the sensor is. This number is then multipled by a factor and this is the "gain" of the camera. This is what changes with the ISO rating. If the well depth is 30000 (as an example) and the gain is 0.25 then the total depth is 7500. this is higher than the dynamic range of 4096 for the image so data is lost.
try using a lower ISO and see if you can get a better dynamic range in your images.
Obviously moving to a dedicated astro imager would improve things as well.

Last edited by Terry B; 04-01-2012 at 03:12 PM. Reason: typos
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Old 04-01-2012, 01:32 PM
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I'm in a similar situation as Peter, but just to clarify, would the same targets be viable from dark skies with the dslr or is it to do with the 12bit resolution vs 16bit of astro ccd cameras and amount of data lost?
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Old 04-01-2012, 03:18 PM
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Quote:
Originally Posted by alistairsam View Post
I'm in a similar situation as Peter, but just to clarify, would the same targets be viable from dark skies with the dslr or is it to do with the 12bit resolution vs 16bit of astro ccd cameras and amount of data lost?
I am no expert on this. Dark skies always helps but the 12 bit conversion does limit your ability to image dim objects as the total levels of the thing you are imaging may only be a few points higher than the background. This makes it hard to stretch the image. This is one reason why the 40D was a big improvment over its predecessors.
The imager is still useful but not as much for dim object pretty pics compared to a dedicated 16bit camera.
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Old 04-01-2012, 03:26 PM
pjphilli (Peter)
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Terry - Thanks for explaining the trade off between sensitivity and well depth and your further comment. I will give a lower ISO a go and see if this makes a difference.
Alistair - I am sure a dark sky would help but I am confined to backyard imaging! Terry's comments regarding resolution and ISOs seems relevant.
Cheers Peter
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Old 04-01-2012, 03:34 PM
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mill (Martin)
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Quote:
Originally Posted by alistairsam View Post
I'm in a similar situation as Peter, but just to clarify, would the same targets be viable from dark skies with the dslr or is it to do with the 12bit resolution vs 16bit of astro ccd cameras and amount of data lost?
From a dark site the dslr can perform almost/aswell as a lot od ccd camera's (depending on type of ccd camera afcourse).
From a light polluted site it is more advantagious to use a ccd camera.
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Old 04-01-2012, 03:55 PM
Poita (Peter)
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Quote:
Originally Posted by mill View Post
From a dark site the dslr can perform almost/aswell as a lot od ccd camera's (depending on type of ccd camera afcourse).
From a light polluted site it is more advantagious to use a ccd camera.
Well, it can perform well if modified, and had cooling added, if it hasn't been then the DSLR will have considerably more noise and much less sensitivity to H-Alpha light.
Without the internal filters removed

http://www.hapg.org/_frontlook/Premod.jpg

With Internal filters removed.

http://www.hapg.org/_frontlook/Postmod.jpg
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Old 04-01-2012, 06:02 PM
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Peter i am talking about a modded dslr and as Peter has stated, his camera is already modded.
A dslr in the middle of summer is not that good but you can take darks and it will still produce good pictures (don't let me get my 40D pics out of the closet to proof it ).
Also a LP filter will help a lot in light polluted skies and i don't know if Peter uses one.
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Old 04-01-2012, 07:12 PM
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BTW the 40d is 14 bits / channel not 12 as its predecessors.
All subsequent released Canon SLRs have 14bits as well.
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Old 04-01-2012, 11:06 PM
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Peter, see my thread here for an example of imaging in light polluted versus dark skies.

In my back yard, I can't even see the horse head in my subs with several minute exposures through a telescope... it's just red from light pollution. I need to shoot lots of frames and stack before I can see it. From a dark site, a 1 minute sub clearly shows the horse head.

According to the maths figured out by some smart people (such as here and here), 1 minute of shooting at the dark site I went to would be equivalent to about 30 minutes in my back yard. My personal experience so far supports this.

By the way, are you dithering between each of your frames, i.e. moving the camera by small amounts so that the target object is always at slightly different locations on the sensor? If not, the effectiveness of stacking will be greatly reduced.
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Old 05-01-2012, 01:36 AM
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Hi

Thats very informative Dave, any links on dithering techniques?
I was losing hope with my canon, but looks like dark skies may compensate if the camera is uncooled or unmodded.
Will Ha filters work with a dslr? Will NB imaging be affected by light pollution as well? Sorry, don't mean to detract.
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Old 05-01-2012, 11:41 AM
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Dithering - lots of links online, a quick Google search should bring them up. Very easy to do manually - in between each sub, stop auto guiding, move the mount slightly (e.g. a random amount less than 30 pixels in both x and y axes), enable guiding, and start the next sub.

As for noise, check out this bottom-right corner crop of a dark frame taken on my 5DmkII at ISO 800 x 301 sec after continuous back-to-back shooting. Subtracting it from a light frame doesn't leave a lot of image left! Side note - without dithering none of that noise would get canceled out.

You can get H-a clip in filters for modded crop cameras. There's lots of into about narrowband online and on here (search for it), but short answer is that very narrowband filters (<= 5 nm) can be good at cutting out light pollution. Some of the nicer RGB filters for CCDs will also selectively block some types of light pollution.

Peter - do you mind posting some of your raw "milky" subs? I just noticed your shooting parameters and unless your light pollution filter cuts out a HUGE amount of light, it sounds like the subs may be overexposed? Ideally, you want the main peak of the histogram to be around 1/3 to 1/2 on the left side.

Edit: I found these slides about narrowband imaging from one of Don Goldman's talks (i.e. the Don in Astrodon) to be very informative. I'm technically minded, though, so YMMV as they say.

Last edited by naskies; 06-01-2012 at 03:56 PM. Reason: added link about NB info
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Old 06-01-2012, 02:57 PM
pjphilli (Peter)
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Thanks everyone for your comments - I will follow your suggestions and give it another try. Cheers Peter
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Old 07-01-2012, 10:59 AM
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Referring to the original post; ISO 1600 is too high for suburban skies. Try using ISO400 and slightly shorter subs. You could also use ISO800 but this will produce some noise, which can be cancelled out by lots of subs and good darks.

A dedicated CCD will give you better results but at this stage I think it is important for you to get images you can process and learn about processing. Good data collection makes for easy processing. So get the data collection going first and then work on the processing.
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Old 08-01-2012, 12:07 PM
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I had a 350EOS modified uncooled worked well from suburban site set on ISO800 I only went to CCD as wanted cooled so could image all year round.

Personally IMO better going OSC CCD than modify a perfectly good DSLR for cooling etc
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