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AltAz Imaging

bengriffin

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jgs001

jgs001

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That all depends... DSS will derotate the frames and align them all up, as long as you don't have rotation within the frames themselves. It worked well for me, up to 2 minute subs on M31... You just need a seriously large amount of shorter subs to make it work... I was regularly using over 100, 30 to 60 second subs.

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bengriffin

bengriffin

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It worked well for me, up to 2 minute subs on M31... You just need a seriously large amount of shorter subs to make it work... I was regularly using over 100, 30 to 60 second subs.

I will have a go at that!

Did you take regular darks also?

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bengriffin

bengriffin

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I did, but I've found that with some of my images, dark's have actually reduced the fainter data (I don't know why). I always shot flats though.

Okay - DSS is pretty impressive! Here is my first stab - I didn't have any flats or darks - which is noticeable, but I am really impressed! There's a lot more nebula visible, even though I've had to bring it out a bit due to the very high dynamic range between the faint outer nebula and the stars and surrounding nebula of the Trapezium. I guess I lost quite some detail around the Trapezium in this second picture.

bengriffin-albums-orion-nebula-first-light-dark-field-picture3660-celestron-9-25-sc-unmodified-canon-450d-4-exposures-400-800-1600iso-15s-30s-stacked-using-dss-post-processed-using-aperture.jpg

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Tim

Tim

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That's an excellent shot of the trapezium, really good, loads of detail there.

I'd have thought you wuold get away without darks with 30 second subs on a 450D, espcecially with the weather as it is at the moment....BRRRRRRRR. Taking flats will help though, and it is worth reading the blurb with DSS, it tells you how to make the best out of your data with proper calibration.

With a lot more subs, as John says, most of that noise in the image would turn into signal, and be a much smoother version, but that trapezium area really is good, even the small faint stars are showing their colour, and the SII rich ribbon is shining through very nicely indeed. You should see what some of the astrophotography software could do with data like that! Well done!

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bengriffin

bengriffin

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TJ, JGS,

Thanks for your encouragement!

I am back in Central London now, where the sky tends to be so yellow that I just see stars poking through the murk - but when I get back to France, I shall have some more goes.

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bengriffin

bengriffin

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Being nosy, what scope was that with? You have a very small filed there considering the sensor size - was it an SCT maybe?

Arthur

You guess wisely! It's a Celestron CPC 925. I'm not convinced by the goto stuff - but the optics seem to be pretty good (and it seemed like a great buy with a dramatically reduced price at FLO).

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bengriffin

bengriffin

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Here's another shot having tried some different settings with DSS, and using local adaptation (with a custom curve) on Photoshop. I can see that even with 4 subs, it's not bad - but a good deal more and I would be swimming in signal, and that's with an unmodified DSLR...

bengriffin-albums-orion-nebula-first-light-dark-field-picture3668-celestron-9-25-sc-w-unmod-canon-450d-4-exposures-400-800-1600iso-15s-30s-stacked-using-dss-post-processed-using-photoshop.jpg

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bengriffin

bengriffin

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Ah. Then you may want to consider one of the many models of focal reducer to the equation - you get a wider field and more light too (so maximising the shorter exposures). You have about 20% of the nebula there... it is rather big!

Arthur

Oh - that's excellent advice, Arthur! Thanks for that. I actually cannot remember if I was using a Barlow for this - I wanted to concentrate on the trapezium area - but I hadn't thought of adding a focal reducer to gather more light.

How about tiling as an alternative? Is it too much work? I guess that the point of a focal reducer is to increase the amount of light - rather than just to fit the nebula into the frame...

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Milamber

Milamber

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By tiling I think you mean a mosaic, that's an option but it does mean a lot of work just to get the frames to align and balance together. With the camera you are using, shooting in RAW, you can add a couple of stops or take a couple off for the trapezium area - that's the point of RAW really, the fact that you can pre-process the image yourself rather than let the firmware do it. I would go so far as to say *always* shoot RAW if you can, for the extra size files it is well worth it.

To be honest, get a small fast refractor and a flattener and you would be surprised, with the SCT you are always going to be fighting against that f/10 when shooting big targets like this. Maybe going for smaller nebulae or even globulars would be better objects to point the Celestron at initially - M42 is a favourite though and you can always come back to it for a bit more data.

Arthur

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bengriffin

bengriffin

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Dave - I don't mind at all - nice work. I'm not really familiar enough with this stuff, so I was playing down the saturation on the image.

Arthur, you were right - I meant mosaic. And likewise, I always shoot in RAW regardless of context. I consider it sage advice. I chose M42 because I am more familiar with it than anything else, and I guess I knew I would get something! Being away from the computer, I only had the tiny LCD back to go by.

In France I am in the 'net wilderness. Next time I am over there, I will plan my shoots a it more, take more notes, choose a subject carefully, ensure that I have several charged camera batteries kept nice and warm, and try out a focal reducer.

On a more general note, is ISO1600 a better choice than ISO800? I tried a variety of exposures and ISOs - and DSS seemed to be able to accomodate them well, but I'm interested in views on that. Also, trying to get back on topic, I didn't notice any inner image rotation at 30s (I didn't have any bulb capability during the shoot) any idea about what sort of exposure times I can get to without getting rotation artefacts?

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Milamber

Milamber

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The ISO depends on your camera... some handle higher better than others (eg. my D1x only goes up to 800, I can push past that but it's not pretty) and really it's a case of suck it and see. I would say go with the lowest you can get away with though... but sometimes you just have to dial in a big number and pray!

Rotation on M42... well, the closer you get to the pole the more you will get, M42 is pretty safe in that respect - M81/82 on the other hand would be pushing things. By that I am talking about recovery as well as rotation, close to the pole there is less relative movement of the object but more actual rotation. Rotation of the image is harder to process away than linear issues.

Arthur

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jgs001

jgs001

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The field rotation effects are dependant on where in the sky the object is. Low in the east and west, and you should be able to get up to about 2 minutes. As the target gets higher or more North or South, the sub lengths come down. I've read that the sub length before field rotation becomes a problem is not linked to focal length. I was using 45 second subs on M42, as it never really was low enough in the east for my purposes. I was able to hit 2 minute subs on M31 around October, but the drive train on a NexStar SLT mount isn't accurate enough to sustain that, and was losing 50% of subs to drive errors.

As for the ISO to use, this has been debated back and forth with various recommendations and suggestions. Using the standard photographic theory, dropping the ISO from 1600 to 800 would require you to double the exposure time to get the equivalent exposure. I tend to use 800, but there are thoughts you should use 200 as that provides the greatest dynamic range, I'm just not sure. Probably best to run some tests and see what you can achieve with various settings. When I was using AltAz I used 1600 most of the time however.

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bengriffin

bengriffin

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Ah - yes, I think I understand - you are implying the rotation is an effect due to the ratio of the Alt vs the Azi motors, so that the closer to zenith, the more noticeable (ie closer to 1:1) the ratio becomes? Or is it that the closer to the pole, the smaller the arc of the rotation is? Or both?!

My pictures were pretty close to the zenith - I had to wait until the moon had set - and there was ice everywhere.

Re. ISO - I found with regular tests that the ISO 1600 is very noisy - but it really does pick up a lot of light, and the problem is that the dynamic range falls out with close dark values. Erm, this is Canon Raw - so that eg taking a low light picture at ISO 100, and then upping the exposure in post-processing shows a lot of very undesirable compression artefacts; whereas with a high ISO rating, there is a thick but even blanket of colour noise.

As for exposure times, as I said, I was most restricted by the fact that I had no bulb/timer-remote, so I could only dial in 30 secs - which I did at ISO800.

Knowing that there is a debate is useful - I will continue to experiment for my own equipment.

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bengriffin

bengriffin

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I hope you don't mind, but I have stretched your image a little and upped the saturation.

I have the advantage of having the 32bit tif.. I added some more saturation, and used a crop of the trapezium at full resolution.

bengriffin-albums-orion-nebula-first-light-dark-field-picture3669-trapezium-crop.jpg

I believe that I could probably get much more from the images - even the four paltry exposures can reveal a lot of detail. Having seen how much information the pros get from their images is quite inspiring - especially as eg the titan lander images cost quite a lot...

I'm reasonably happy for a first stab - not quite the HST lightshow, but it begins to show some of the structure that the HST reveals around the trap.

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