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PostPosted: Sun Oct 26, 2008 10:27 pm 
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First I played with star trails... nothing special, just whatever's visible in my garden. Picked my 50mm prime, then for no apparently reason I set it to f/8 and left it on continuous at 30s exposures. The following is the result after about an hour.

Image

Note to self: use a wider angle lens next time, and maybe point it somewhere more interesting. Also remove the 1st test shot I did hence the gap.

With a wider angle lens and longer exposure, I think I can get a "rainbow" like effect as this is close to south facing. It's interesting the different colours that come out in PP.

While that was going, I noticed the big dipper above the houses across the road so had a go at photographing that too. Here's a 50% crop from one.

Image

Question for the more experienced star gazers out there. Above the square bit there's two specks close together, and the orange blob caught my attention. I've tried to look them up using the software I got with my scope. The fainter whiteish one fits an item listed as SAO 15580. But there's nothing obvious where the brighter orangey speck is.

As luck would have it, in the time it took me to look at the pics and notice it, the clouds had moved in before I could get my scope out.

I believe I have ruled out a sensor hotspot or lens flare by moving the positioning. The orange speck stays there. Note I did not do any dark frame subtraction on this, and the camera seemed to generate a lot of them.

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PostPosted: Mon Oct 27, 2008 2:05 pm 
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Glad I have inspired others :D :D :D

The first shot looks pretty cool, where are you based? You certainly managed to capture a lot of stars even at f/8 imagine what you would get wide open. This is something I want to try with my fifty on the next clear night. I assume the orange is from the glow of street lighting? How many shots did you combine? Its an interesting thought as to the pattern you would get facing south, wouldn't it be an upside down rainbow?

The second shot is also very nice. This is something I want to do next once I have had enough with trails. That is to build up a collection of constellations.

As for the ID's I wouldn't know where to begin this is something I wish to improve on in the future, together with a new telescope.

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PostPosted: Mon Oct 27, 2008 8:27 pm 
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Good shots!

My first gues son the blob would have been some galaxy or nebula, but checking a star map doesn't reveal any likely candidates - see:

http://en.wikipedia.org/wiki/Image:Ursa ... on_map.png

http://www.astro.wisc.edu/~dolan/conste ... ts/uma.gif


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PostPosted: Mon Oct 27, 2008 9:16 pm 
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Digz, I'm normally not that from from you. Just along the road in Swindon. Tonight I'm in Sunderland on work so I've left all my kit at home. I found one advantage of DSLRs over an ultra compact. It's a lot harder to misplace! I normally carry my compact with me on travel, but I couldn't find it after a good search. I'll probably find it in a travel bag I haven't used in a while.

Anyway, I forgot to mention that was at ISO100 too! The final combined image was pretty dark looking, so I had to turn it up a bit using 'chop later. The glow is almost certainly light pollution. I was pointing fairly high up where it appeared clearer. It was about an hours worth in 30 second exposures, so around 120 shots. I only took one dark frame which didn't remove all the bright spots. Is it best to do it before or after shooting? Even both? The resize down took them out though.

As for the rainbow effect I'm hoping for, think of it this way. Looking at the poles you'll get circles. Consider trying to look at the south celestial pole from the Northern hemisphere. A bit tricky I know. But if we can see low enough, then we should get some circles that way. As they go over the horizon, that should be the rainbow shape.

Back to the orange blob. If it's a regular item then I'll have other chances to look for it. I even took screenshots of maps and resize/rotate/overlay it to try and figure out where it was, where I found nothing of significance to explain it.

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PostPosted: Tue Oct 28, 2008 7:22 am 
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I took some night shots a few nights ago. This one turned out the best.

Image

Full size can be viewed at my picasa page.

It didn't turn out too well as there was a lot of noise, even at ISO 100 and I removed it by increasing the amount of blacks in the scene :) I'll take more shots sometime after I get my remote trigger as I figure I need a longer exposure to get brighter stars.

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PostPosted: Tue Oct 28, 2008 12:55 pm 
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Not too far at all Popo.

I would try bumping the ISO up to around 800 (maybe try 400 to start.) this will obviously show more stars but it will also mean that you wont have to tweak the exposure so much. The shots I have posted are all at the original exposure (i.e. 25sec f/3.5 or 5.6) with no adjustment during RAW conversion. When you use a higher ISO and then tweak exposure you will obviously get noise. Also DSLR are more prone to digital noise especially when the sensor gets hot (hence the reason for using several smaller exposures and combining.) This is why we use dark frame subtraction. I typically take one shot straight after i have finished my series and that's it. However I have heard of people taking several shots but I don't know what the advantages of this is. It is, however, important to note that the dark frame should be acquired after the main shoot and not later on. It is important to get a record of the hot pixels that will correspond to what you were shooting.

I think I understand what you are saying about the rainbow effect and it sounds theoretically possible. As for the orange blob it is interesting.

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PostPosted: Tue Oct 28, 2008 1:24 pm 
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I don't think the difference for sensor heat noise is significant between one long exposure and continuous shorter ones. But I like that with shorter ones you have more chance of rescuing something should something go wrong during the long exposure.

I will certainly try turning up the ISO.

On the dark frame, I have to admit I forgot when it was over so there was maybe a few mins before I took the dark frame. If the hot spots were due to heat that may have cooled off by then. Will have to remember to do it quicker next time.

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PostPosted: Tue Oct 28, 2008 1:49 pm 
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As I understood it you will get more noise (become apparent) on a 5min exposure than a 30sec one because the sensor will heat up more because it is active for 5mins.

It would be interesting to see if anyone can put us straight on this matter.

As for the dark frame just keep the lens cap handy and once the last shot is taken, put it on and shoot one (or several) more :)

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PostPosted: Tue Oct 28, 2008 2:39 pm 
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Ok, we're thinking of different things there. I'm thinking that, 10x30sec shots taken continuously woudn't be different from 1x5m as far as heating is concerned.

For the trails I took it as given that the short exposures had to be continuous else you'd get gaps. Therefore as far as heating noise is concerned they would be practically the same.

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PostPosted: Tue Oct 28, 2008 4:25 pm 
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popo wrote:
Ok, we're thinking of different things there. I'm thinking that, 10x30sec shots taken continuously woudn't be different from 1x5m as far as heating is concerned.


That's a fair comment actually and I never though of it that way. Some more research is the order methinks.

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PostPosted: Wed Oct 29, 2008 7:28 am 
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But given how cold it is on a clear night, would that mitigate some of the sensor heat? :)

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PostPosted: Sat Nov 01, 2008 12:07 am 
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Trails attempt 2. I used 16mm lens this time so much wider angle. But my focus was quite out when I looked afterwards. With the resize down you can't notice it so much :D

Image
218 images combined with 1 dark frame, ISO400 f/4.5 30s each. No processing other than combine and resize.

This is taken from the same spot as the previous attempt. I aimed a bit lower to catch the top of the trees. The opposite neighbours have a gigawatt garden lamp which lights up half the known universe every time a cat walks past, hence the rather bright trees.

I'm not sure what the non-trail blobs are. They are NOT hot pixels as they are defocused in the full size version. They likely appear in a single exposure only from the short trails they make.

To my eye, the blurry blue line starting about 2/3 up on the left also looked blurry to my naked eye. I looked it up and they turned out to be the Pleiades. Got my scope out, and found it had too much magnification so I couldn't see them all at once. Put scope away and just used a 300mm lens which filled the frame much better.

Image
ISO1600 f/5.6 1s resize only

Orion was rising as I was packing up, but it was too low in the light pollution for me to bother with this time.

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Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
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Last edited by popo on Sun Nov 02, 2008 12:26 am, edited 1 time in total.

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PostPosted: Sat Nov 01, 2008 10:54 am 
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Can those non-trail specks be sensor dust? With the cumulative effect of 219 images I expect the effect on the image to be quite different than with a single exposure.

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PostPosted: Sat Nov 01, 2008 11:32 am 
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Dust should appear dark, not light. And at f/4.5 it'll need to be grain sized to be visible.

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PostPosted: Sat Nov 01, 2008 12:00 pm 
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Nice shots. I was again last night trying my nifty fifty but the intervalometer was playing up :-(

Some of those spots are plane trails (the ones forming a straight line) as for the others I dont know. I dont think they are dust as I had a dust spot on my sensor and stacking over 100 images the dust spot is recognisable as a dust spot.

Its quite cool as we were both looking at the same part of the sky last night as I saw the Pliades and toock some shots which I will post later on.

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