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Help would be much appreciated...


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Last night I did (for me) a fair imaging session.

2 targets.

1) The Fireworks Galaxy 54 frames of 30 seconds each.

With this I can stretch it enough that I can see the galaxy, I just can't get a decent image.  I assume it is just too faint for this type of imaging.

2) The Iris Nebula

The reason I switched to this is I assumed it would be an easy target,    This one I did 35 subs, again at 30 seconds.  My issue with this is that I can see data there, but the core is just overblown and I can't figure out how to seee the outside without killing it.

Both these were stacked in DSS with 30 Bias, 25 Darks, and 27 flats. 

I haven't included my processed images, as let's be honest they are junk, I was just wondering if anyone would be kind enough to have a look at the DSS outputs (here reduced from 58 meg to 15 meg each) and let me know if there is data and I am just not processing right, or whether I pushed for targets I just cannot get with my setup.

I was going to go for 150 subs of the fireworks to be honest, but I honestly did not know if it was in frame or not!  It is only in processing I can see my target was actually quite good, and if I had known I would have done another 100.

Equipment is as in my sig.Fireworksmall.tif

Irissmall.tif

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30 sec subs just won't hack it on something like this, and it doesn't matter how many of them you stack. It needs a (much) longer integration time to pull this one out of the background murk. 10 min at least but of course any improvement will be incremental with increased exposure, so it depends on what your tolerance for a 'good' result is. The flats don't seem to be working too well either.

Just to prove there is actually something there I looked using StarTools..

Fireworksmall_zps2d151142.jpg

I'll look at the Iris data later...

ChrisH

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Again, same applies as with the Fireworks galaxy. Not enough signal and the background isn't flat, so any attempts to pull an image out is made that much more difficult. This is just a rough job to see what is in there, it could be made a bit better probably but you're on a hiding to nothing trying.

Irissmall_zps4b2f107d.jpg

ChrisH

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just on the equipment used and the settings, already mentioned you will need a lot longer subs, upping the ISO will increase the data but also the noise you will need to run some tests to get a balance, a Focal Reducer will half the sub time or double the data captured, properly the best improvement would come from guiding to get 10 minute or more subs this would allow lower ISO with lower noise......The finder scope used as a guide scope will be the cheapest option, you will need a adapter supplied by Modern Astronomy and a guide camera i used a lodestar there are cheaper type just need the c-thread on the end to screw the camera to the adapter the adapter will replace the viewing end of the guide scope which unscrews....and a PC/Laptop......

004GUIDESCOPEFINDERSCOPE.jpg

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Cheers chris. Thought that might be the case.

Oh well, I will try something much simpler tonight.

Well your focus looks good with little trailing, and (I assume) you are trying to image with an 8" SCT @ f/10 unguided - which is 'challenging' to say the least. So as suggested above, add a simple guider to your rig so you can extend exposure times and you should get very good results. Perhaps use an f/6.3 reducer, that will vignette with a DSLR I think but you can still crop it down to a very usable image or you might even be able to process it out.

ChrisH

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Sorry but increasing the ISO won't get you any more data. See here: http://www.blackwaterskies.co.uk/2014/01/do-high-isos-make-dslrs-more-sensitive.html

You really want more subs and longer exposures, so guiding is the way to go.  In theory you could take shedloads of 30 second subs and you will see an improvement, but in practice for these faint targets you really want to be getting as many 5 or 10 minute subs as you can.

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Sorry but increasing the ISO won't get you any more data. See here: http://www.blackwaterskies.co.uk/2014/01/do-high-isos-make-dslrs-more-sensitive.html

You really want more subs and longer exposures, so guiding is the way to go. In theory you could take shedloads of 30 second subs and you will see an improvement, but in practice for these faint targets you really want to be getting as many 5 or 10 minute subs as you can.

True but the tricky thing is working out what the unity gain is for your camera. So far as I know it's normally between ISO 200-400 on a DSLR but varies a lot between manufacturers. Below this and you can be losing data as the camera is simulating being less sensitive than it really is.

TSED70Q, iOptron Smart EQ pro, ASI-120MM, Finepix S5 pro.

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You don't need longer subs - you just need more total exposure.

This is 160 x 30s subs with an 8.5 inch scope

ngc6946_600x400_6_10_12_11_2010.jpg

NigelM

Not so.

You were imaging at f6. The OP was imaging at f10.

Your setup will be capturing well over twice the amount of data in the same time as the OPs. Imaging at f10 is sloooooow.

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You don't need longer subs - you just need more total exposure.

This is 160 x 30s subs with an 8.5 inch scope

NigelM

Well no, and with respect - had you taken 9 x 10min subs instead of the 160 x 30sec subs it would have resulted in a nice image, with the signal well above the background noise and which didn't require the bejeezus stretching out of it to see the result. What you have there is a very nice 30sec exposure, but a not-so-nice 10min exposure will win every time.

ChrisH

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Is there actually a downside to imaging at say iso800 though? If as you say it just amplifies the data, are you actually losing anything? I compare it to an RF receiver -- while filtering at the IF is better, filtering on the output can still be useful to shape the signal depending on the intended usage.

Sent from my iPhone using Tapatalk

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Is there actually a downside to imaging at say iso800 though? If as you say it just amplifies the data, are you actually losing anything? I compare it to an RF receiver -- while filtering at the IF is better, filtering on the output can still be useful to shape the signal depending on the intended usage.

Sent from my iPhone using Tapatalk

That really depends on the camera and how well the electronics control the readout noise. My old 350D will not work well at such high ISOs without producing excessive noise, my new 70D will work at that (and higher) ISO yet still have low noise. You get what you pay for in this respect and no definitive rule can be offered.

ChrisH

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The problem is you amplify the noise as well as the signal.

Also some cameras lose dynamic range at high ISO, I know mine does.

TSED70Q, iOptron Smart EQ pro, ASI-120MM, Finepix S5 pro.

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If you follow the link about to my post, there is a link in there to Craig Stark's article profiling DSLR performance, where he looks at the kind of questions you are asking (albeit for a single and relatively old Canon model).  It's a bit technical but if you follow it you can perform you own tests on a camera and figure out what the optimum ISO is in terms of dynamic range and read noise performance.  It will vary from model to model and I am wary of giving definitive advice as to what is best.

Personal experience is that somewhere in the ISO 200-800 range is about right, but others swear by ISO1600 and can produce nice looking images to back it up.  As others have said, the downside of a really low (below unity gain) ISO is that you end up putting different voltages in to the same DN so lose detail.  The downside of high ISOs is similar in that with really bright (but different) parts of the image may end up getting mapped to the maximum DN value for the format (whilst all RAWs from Canons will be in a 16 bit data format, the actual output from the ADU will only be 12 or 14 bit depending on the model).  Effectively you are limiting the maximum exposure time on a target before you white out parts of the image.  This can cause you to (effectively) saturate the cores of bright stars and lose colour unnecessarily.

Anyway my original reason for bringing it up wasn't actually this debate but to make it clear that increasing the ISO won't get you any more information in the subs - it might feel like it does because they look brighter off the bat but you'd be deluding yourself.

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Sorry, still trying to get my head around this.  In particular I am trying to figure out why the Iris Nebula is such an issue, when I can get the Dumbell so easily?  Especially as the Dumbbell is Mag 7.3 which is slightly less than the 7.1 of Iris.

Is is simply that Iris has a bright core, then low luminosity around it whereas the Dumbbell is pretty even?

Apart from the Dumbbell is there anything people would recomend I stand a shot at with F10?  Looking at star maps I can see the Elephant Trunk and North America Nebula (both of which I think are out due to size?) then the ponly slight chance may be Pacman Nebula?

I was really hoping for a Galaxy or a Nebula, but all the galaxies are quite low at the moment (Whirlpool creeps over houses to the west now, and Pinwheel only just comes over houses to the east.  Are my only viable targets for tonight really the Dumbbell, and the various clusters?

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Sorry, still trying to get my head around this.  In particular I am trying to figure out why the Iris Nebula is such an issue, when I can get the Dumbell so easily?  Especially as the Dumbbell is Mag 7.3 which is slightly less than the 7.1 of Iris.

Is is simply that Iris has a bright core, then low luminosity around it whereas the Dumbbell is pretty even?

Apart from the Dumbbell is there anything people would recomend I stand a shot at with F10?  Looking at star maps I can see the Elephant Trunk and North America Nebula (both of which I think are out due to size?) then the ponly slight chance may be Pacman Nebula?

I was really hoping for a Galaxy or a Nebula, but all the galaxies are quite low at the moment (Whirlpool creeps over houses to the west now, and Pinwheel only just comes over houses to the east.  Are my only viable targets for tonight really the Dumbbell, and the various clusters?

Exactly so - the core of the Iris neb is very bright and the surrounding gas and dust clouds quite dim, but the magnitude given is an average over the area the object covers. When processing you have a hard job controling the brightness of the core to reveal the outlying areas.

With just 30sec exposures available to you at f/10 and a long focal length it is very difficult to choose a target I think you might have some success with. Perhaps the brightest sections of the Veil Neb (Network neb, E. Veil) will work for you, they show up well in DSLRs because of their particular colour combination. Worth a try - and also worth pushing the exposure to a minute?

ChrisH

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Excuse my processing, still working on this image.

This M31 is 45x300sec f4.5 ISO 800 and I still need some more.

Ideally I would have prefered longer exposures, 600secs would be better but this would push the data way to far right.

post-30455-0-58899500-1408773796.jpg

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coming along very nicely Mike!

Thanks Pete.

I thought taking good quality images was hard but the processing is a different ball game.

If it was'nt for some poor seeing a few nights a go I would have had over 5 hours on M31, but thats imaging.

I can double my data quite quickly now I have twin setup.

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