First DSO attempts (sugestions please!)

[roundycat]

the uncertainty that is in your master dark is additive, ie, it adds to your light frame data. The exposure length of your lights is irrelevant.

The idea of taking darks/bias on a 1:1 basis compared to your lights will get you nowhere fast. If you take one light and one dark it will add over 40% noise to the light. Regardless of the exposure time.

I tend to use 20 of each as this reduces the added noise (uncertainty) to around 2% which is quite manageable when finalising the post processing. Shoot more if you want but you really are in the realm of diminishing returns.

I have posted this link again Astrophotography have a look at the graph at the bottom of the page.

Dennis

[AndyUK]

Thanks again Dennis - I did actually look through all the threads in this post specifically looking for that link... and found / reread it. I think I'm perhaps now getting confused by (possibly) conflicting views(?)

My current "routine" is to take 10 darks just before the beginning of an image run and then 10 right at the end... It's seemed to work okay so far - I just want to try and get a routine process together and obviously I want to make sure it's "right"

[roundycat]

there is no doubt about it, you do get conflicting advice when it comes to image calibration. Often hidden away in the conflicting advice is advice that is just plain wrong.

I went back to your original post and lifted three of your images. All of them show the black point being set to somewhere between 0 and 1. The expression 'black point' is something of a misnomer, it should never be black. Don't go lower than about 10% in brightness or 25 in levels. The info palette in PS will show all this. In general your white point has been moved too much with no attendant movement of the mid point. However, when I looked closely at your pictures there is much evidence of low level noise (not helped by the j-pegging of course) and if that is there after dark or bias subtraction then there is nothing else for it - you need much more exposure.

Your stars are sharp, you may be able to push the exposure towards one minute and that will help a lot. Aside from issues about not being able to cool your DSLR and it getting hotter with longer exposures there should be no real difference between the DSLR exposure and one from a 'pukka' astro camera. Both will build signal quicker than noise and in both cases you get benefits from stacking many exposures.

With your procedure of taking an exposure and then 'resting' for 30s before doing another your darks may not match but you may well have a slight movement between exposures which you can exploit. Try a simple experiment with un-calibrated raw frames. Subtract your master dark from each frame in the normal way, combine and save. Then subtract a bias from the same un-calibrated raws (no darks), combine and save.

Open both master light frames and compare. To get the best of this you must not combine the frames using an algorithm that adds or averages the raw frames. You must use an outlier rejection method such as Median or Sigma Reject. This will help to drive down the noise and should bring about an improvement.

As an aside, there is no need to use flat darks (not dark flats!) if your flat exposure time is fairly short. Not enough thermal pixels will be generated to worry about. Combine as Median and if there are hot pixels either filter them out or use a very small gaussian blur which amounts to the same thing.

Dennis

[AndyUK]

Wow Dennis - Thank you very much for this! I was at first amazed to see how you managed to find out so much about those images (I've never used the info palette!), but I think I can understand why the "black point" is between 0 and 1... it being due to my clumsy attempt to make the sky darker, which of course, light pollution aside, and especially at this time of the year, is simply not true. So by making it aesthetically effectively black, I've almost certainly clipped data.

There's a lot in your post for me to digest (and research) as many of the terms are things I've heard / read about, but not totally understood (). I've printed it out so that I can use it as a reference and try and do as you've said - By the sounds of it, I need to understand a lot more about the various options in DSS (I've been using the default to date)

Your stars are sharp, you may be able to push the exposure towards one minute and that will help a lot

This is something I'm not that confident about at the moment as I have some concerns that my polar alignment isn't accurate enough - I haven't wrapped my head around drift alignment yet and similarly I haven't sorted out PEC training either [i know these are things I really MUST do!], but although I know it's more money (which I haven't got yet!), I see my possible salvation lying in a guide camera (although I know I'll still need to sort out alignment and PEC as well)

I do find it a little frustrating that taking a .tif file (16 bit), before I can do anything with it I have to "downgrade" it to 8 bit as I'm sure I'm losing signal even in that process(?), but if there is a way around this(?) I suspect we're talking about a version of Photoshop CS?)

With your procedure of taking an exposure and then 'resting' for 30s before doing another your darks may not match

I use the same procedure also for taking the darks (30s exposure, 30s rest), which I would hope mean that they do match (or am I maybe missing something here?), but I'll certainly try your experiment to see what improvements can be made - Having had a very quick look at the default DSS settings for registering / stacking, I can see the switches you're talking about, and of course the default setting's I've been using all appear to be set to average... So I'm now very much looking forward to reprocessing some of these!

As an aside, there is no need to use flat darks (not dark flats!)

I laughed when I saw this term in DSS - I thought of someone living in a flat with no lights on!.

(I actually feel "enlightened" now - Thank you very much indeed Dennis...!)

[roundycat]

just a couple of quick comments. I am not a DSS user so cannot offer any advice. One thing I am becoming convinced of and that is Craig Stark is too clever for his own good. I have lost count of the number of things he has changed the name/title of just to be different and it doesn't help. The is a sigma reject routine in DSS which may be called sigma kappa reject but don't bet the house on it.

I take it you are using PS Elements? I know that earlier versions of PS, certainly 6 and 7, could manage 16 bit in Levels and Curves which is where you want it. Once you have done the major early stretching going to 8 bit is not such a huge loss, especially when you are starting out. After all, we don't learn to drive in a Bugatti Veyron. If you can get hold of PS 6 or 7 it will serve you well and they can be had on the cheap. No activation required - say no more. You cannot us elayers in 16 bit with the earlier programs but again, that may not be such a problem.

The problem with camera temperature changing whilst taking lights/darks is that thermal noise, or hot pixels, double (or halve) with every 7 degrees C or so change in temp. If your darks are not an exact match for your lights when you subtract a master dark it will over correct some pixels and under correct others. That causes a lot of low level noise. This is where the sigma reject algorithm comes into its own, especially if used with dithering between exposures. Dithering is merely moving the mount slightly between exposures so the next image falls on the chip a few pixels away from the previous one and so on. When aligned the noise doesn't line up so is rejected. Very cleanly.

I wouldn't worry about taking exposures at different speeds or exposure time. For the targets you are shooting there is no worry about overloading the camera. I doubt if your individual exposures are getting much above the sky background.

As a matter of interest I just looked at an old M31. A 60m R made from 30 120s exposures just made 11,000 adu in the centre and I believe that was at f5 so no need to use short exposures on that one assuming your camera manages at least 14 bit. (16,384 levels).

Dennis

[AndyUK]

Hi Dennis - Thanks very much again... This is all VERY useful info, but it bears out that I obviously have SO much to learn on this subject(!). I know that a lot of people use MaximDL which is something I aspire to at some time (is this what you use?), but thanks very much for trying to help out on DSS (being in an IT environment myself, I know this is problematical at best!).

There's 2 potential options in DSS for stacking that mention kappa sigma, one is kappa sigma, and the other is median kappa sigma - For both there are kappa options which defaults to 2.0 with 5 iterations (whatever that means?!). I'm having a go at re-registering and stacking using median kappa sigma on the latter M81 images as I write (it certainly lengthens the processing time!)

Yes, I currently use PS Elements 7 - I'm familiar with levels, but to add curves it seems to require a separate download (which I'll try and find). I must admit, the cost of CS4 (let alone CS5) looks a bit scary to me - Admittedly not quite as much as a Veyron, but I could buy a very reasonable quide camera for that money...

Thanks also for the explanation on dithering - I understand that now, although I'm sure that there almost certainly will be a certain amount of dithering caused by my imperfect polar alignment and lack of PEC training..!

I'm currently being forced to change my DSLR (as a result of my work forcing me to move to Windows 7) as Canon aren't making a W7 driver for it . However, wishing to maintain use of my existing kit I'm now looking at going for a 2nd hand 40D (which unlike the 300D, shoots @ 14bits) - Perhaps the new laptop move is actually fortuitous (bigger HDD, more memory), although I'm dreading having to download all the software patches... assuming they're available!

I've said it before, but this must be THE most expensive hobby to start up, let alone then moving on to the "big boys" stuff, but thanks again Dennis - If there's any visible difference in the re-worked M81, I'll post it up...

[AndyUK]

(Firstly, please forget my comments about curves not being in Elements 7 - I found them under Enhance/Adjust Colour... Sorry!)

I restacked the same files using median kappa sigma and it certainly seems to have produced a different file... For comparison, here are the UNPROCESSED images:

Average stacking settings

Median kappa sigma settings (Kappa = 2, Iterations = 5)

Again, these are UNPROCESSED - All that's been done is a convert to 8 bit (to save as JPG). I apologise in advance for my naivety, but is this what I would expect?

At first look, it would appear that although I may have lost some noise, the amount of detail also appears to have significantly decreased - I did try an initial histogram stretch (moving midrange slider back towards the right of the Histogram), but to start seeing anything at all in either object, the artefacts being created started getting extremely ugly...

Have I done something wrong again, or does this prove that I still need a shedload more subs / exposures (165x30s)? If so, I don't think I'll try doing any more with this for now as I'd probably be flogging a dead horse - Perhaps I should try having another 2 or 3 nights imaging on this before trying again? (or maybe wait until darker nights?!)

[roundycat]

please bear in mind I know absolutely nothing about DSS so I wouldn't dare make suggestions as to how to use it. However, in some cases it may speak the same language as the master program (Maxim) so here goes.

Very likely the '2' value in MKS is the mask width in standard deviations. Number of iterations is probably how many times the algorithm runs through the group of pixel values looking for outliers. I think you can set the iterations to 2, it will be a lot quicker.

Your comment about moving the histogram slider to the right is a bit odd. If we are talking about Levels in PS you should be moving it to the left or 'black' end. Don't move the white point.

I did a quick stretch on the new files and you are right about them being different! I found them very noisy but there is galaxy detail there. I think you are still in the region where the camera noise is equal to the signal it is picking up so separating the two is difficult to say the least.

Next time out take a five minute exposure just to see how bad the trailing is and then work back. A small degree of trailing can be worked out in the processing.

The pictures I have uploaded are just to show the background. Try setting the picture to a smaller size and using less jpeg compression as the jpeg artefacts in both are pretty horrid.

Dennis

[AndyUK]

Hi Dennis - Firstly I just want to say how much I appreciate your efforts (and patience!) in passing on your advice... I'm very conscious that you're probably spending way too much time on helping me out and almost certainly have far better things to do!

No disrespect intended at all to DSS (or its users), but time and time again the addage of you get what you pay for holds true, but of course it is free... As you've suggested, I'll try 2 iterations next time and see what the difference is (and thanks again for the explanation of what this might be doing, even though not a DSS user!)

I'll also have a go at a 5 min sub next time to see how bad the trailing is and see what I might be able to get away with (and I'll be as meticulous as I can with polar alignment). I suppose I should also bite the bullet and have a go at PEC training as well (I have a 12mm reticle and I can always use a barlow).

Re: Histograms and sliders - Sorry, my typing error - I'm moving the midrange slider from the middle to the LEFT, pulling it from the centre:

As the histogram shows (and you noticed!), there really isn't that much data here at all(!) - To be honest (noise aside and Jpg artefacts aside, I'm amazed at the detail that you've managed to pull out!

Try setting the picture to a smaller size and using less jpeg compression

I usually set width to 1024 and when saving to jpeg set the slider to the highest quality I can without it exceeding 2mb (as per forum settings), which I thought was the best I could do... Am I screwing this up too?!

[roundycat]

histogram adjustments look ok but try limiting it to 2.0 and then do it again. Pushing it too far will give you an 'accidental' white point move.

I didn't realise the forum limit was 2MB. I was thinking the big picture was needing a lot of compression to 'fit' it in and it was that causing the artefacts around the stars. Although posting wide fields is often appreciated a cropped down picture showing just the juicy bits will allow you to use less compression and give the picture a better chance. In the early stages of getting used to processing that may be more use than posting a screen sized muriel!

Dennis

[Psychobilly]

Doesn't Craig Stark Develop Nebulosity and PHD guide as well as several other very useful astro utils ... AFAIK he's not "involved" with Deep Sky Stacker...

Billy....

[roundycat]

sorry for making this mistake yet again. I still don't have much time for C Stark's nomenclature but he has nothing to do with DSS, probably doesn't even use it. The guilty party this time is Luc Coiffier, author of DSS; he who also chooses to use his own naming of various features rather than go with the flow and make it easier for beginners to read from one program to another.

Dennis

[AndyUK]

Hi Dennis - Thanks for advice on not pushing past 2 on each level layer. Although it doesn't appear to be doing anything on screen, of course it must be doing something, so a repeat of the task on a different layer can only give me greater control. I'll try that next time.

Re: posting images - To be honest, I've had a few problems with it... Sometimes I get the whole page pics (as above), other times I somehow manage to get it displayed as an attachment, sometimes both and sometimes as a thumbnail (which is totally useless, but I know how to avoid it now). I'll try and crop down in future to the object itself.

Is it me or does the noise on the images follow a (roughly) similar pattern? I know you've mentioned that this might be due to jpeg compression, but might it also be an artifact of light pollution, or even the camera itself(?). I see the same pattern on all frames (if pushing really hard on levels), and I haven't been able to take any flats yet...

[dph1nm]

The idea of taking darks/bias on a 1:1 basis compared to your lights will get you nowhere fast. If you take one light and one dark it will add over 40% noise to the light. Regardless of the exposure time.

Indeed - but what people tend to forget is that 20 lights and 20 darks will have just the same effect, so 'rules' like 'take 20 darks' are meaningless, because the optimum number depends on how many lights you took (ditto for biases).

However, it does all depend on what your dominant source of noise is when you add all your lights togther. You may be completely dominated by the background sky noise, in which case darks might be irrelevant. For my 1000D, however, it is certainly the bias which dominates when you add lots of 30sec-1min exposures together.

NigelM

[roundycat]

Andy, not sure what you mean by bringing layers into it. You do the levels shifts on the background layer, no other layers are involved.

Noise in the image is complex but simplifying greatly you only need worry about thermal noise and bias. Read noise does come into it especially if you are only recording a small signal due to short exposures. I have no idea what level of read noise DSLR's produce, I can only surmise it is higher than an astro camera due to lack of cooling and, probably, lack of double co-related sampling. Lengthen your exposures until they produce signal well above whatever read noise your camera produces and you don't have to worry about it. Thermal noise can be taken out by a matching master dark or by the use of a hot pixel filter or bad pixel map, basically variations on the same theme. (if you have the right software, don't know about DSS).

Light pollution doesn't produce noise in the strictest sense of the word. It is part of the signal, albeit part you do not want. Jpeg compression produces 8x8 pixel blocks which you can see easily if you raise the on screen magnification of the picture to 7-800%. It is not normally an issue.

The camera produces most of the noise, bias, which is always there in every frame you take and thermal noise which is proportional to exposure length and chip temperature. A decent dark should take care of it all but decent darks are not really available if you do not use set point cooling. Dropping the temp a fixed amount below ambient as is done in the cheaper cameras can still give you a problem with darks not matching.

The pattern you see is likely bias, I think that is what we are seeing in the pic of M81 I re-posted. Lots of vertical lines that look a bit like the weave in a piece of cloth. Overlaid on this could be what is erroneously called fixed pattern noise which is usually dealt with by flat fielding. all these noise patterns can be very obvious and annoying if you have little signal in relation to the noise.

One other thing, which may be minor is the quantisation noise due to the camera working below 16bit. If the camera is limited to 12 or 14 bit and that is further reduced by pushing the ISO setting higher then quantisation noise will become more of a problem. Look for it in the form of posterisation of the signal.

Dennis

[AndyUK]

Hi Dennis - I tend to use layers as a matter of course for each processing action... It's just the way I work. When I'm happy with the action in one layer, I then move onto the next action (layer)... and when I feel happy with all the layers, I then flatten them all down it into the background layer right at the end - I just find it easier that way.

From your (greatly simplified - thankfully!) explanation, I can now see why you've suggested I lengthen the exposures as much as possible... However, of course this is seriously going to show up a) my poor polar alignment and the tracking of the mount, but I'll certainly give it a go... if nothing else to see the effect it has on the noise (and to show how bad my setup is!). And then I need to design a mechanism for taking flats...

Although I know it's no substitute for alignment / PEC training, and others on this forum achieve excellent results unguided, this is convincing me more than ever that I need to invest in a guidecam...

Thanks again Dennis - You're advice, explanations and assistance are greatly appreciated!

[AndyUK]

Hi Dennis - I took your advice and did some experimenting with exposure times last night (up to 5 mins). The results were very interesting and I think showed the following:

1. Poor alignment (up-down drift?)

2. Lack of PEC training (left-right drift?)

3. Vignetting (I'd never seen this before - need a light box for flats!)

4. Increased signal - The signal range for the 5min is between 44-106, whereas the signal for the 1min is between 4-44 (and the 30s exposures are between 0 and c. 25)

5. Light pollution(!)

Again, thanks for all the advice, suggestions and comments - I feel as if I'm beginning to start down the right track now, although it's certainly shown up my lack of skills (alignment) and the need for me to get out and get the mount PEC trained... and buy a light box... and seriously look into getting a guide camera if I want to do exposures of more than a 90s...!

Aside from getting the results of this experiment, I did also see my first NLC's (which was a pity, as of course my camera was otherwise engaged!) and also saw what I think may have been some of the Bootid meteor shower... All in all, a good night!

[roundycat]

Hi Andy, some interesting results there. I have had a fiddle with your lower picture and re-posted below. First off, there is no light pollution worth worrying about. The purple sky isn't very nice but a few seconds in PS sorted it out. I balanced it, darkened it, upped the saturation and then re-balanced the background to be a dark blue. There was a slight gradient from bottom to top but that was quite even and responded to a gradient quick mask selection fix in PS. There seemed very little evidenece that you need flats.

I am a bit puzzled by the 'tick' shape of the stars. It almost has the classic shape of a mount that has been kicked mid-exposure. If the polar alignment was out by a big enough margin the stars would trail at some angle across the picture. Superimposed on that trail would be the PE if it was big enough to show. Either way the star trails would basically be a straight (but wiggly) line.

Intermittent clouds prevent you from serious imaging but do allow playing around. I would practice drift aligning and work on a quick and repeatable mechanical alignment for your mount when you set it up. I used to do this and with marks on the ground, a permanently adjusted tripod (for leg length and level) and a laser on top pointed at a mark on the wall I could set up and polar align from scratch in 30m or so.

Once you are near the mark and understand your mount drifting only takes 5-10 minutes max.

What area of the sky was this? Why not try something bright such as the NA neb near Deneb or the area around Sadr?

Dennis

[AndyUK]

Thanks again Dennis... I've now discovered I need to learn to try and master Photoshop as well - I'm amazed what you can get out of such a cr*p photograph!

I assumed that there would be light pollution as the direction of the shot was to the NE (over Bath), and even to the eye it did seem quite light towards the ground in that direction (perhaps that's the gradient you mentioned?).

Re star tick shapes, I'm pretty sure that nothing touched the mount - I guess I'll have to have another go at a few 5min shots and see if it's replicated (admittedly I didn't see this in the 4/3/2/1 min shots, only an up-down shift). I must admit though that I don't totally trust the skywatcher EQ5 synscan upgrade on the CG5 mount as the synscan cog only fits against the top half of the drive wheel on the CG5 mount - I'm hoping I can find someone to modify it and take a few thou off the length of the synscan cog spindle so that it fits better.

Thanks very much for the tips on ensuring better alignment - At the moment it's a start from scratch each time as I'm currently trying to do all this on the lawn in the back garden (not exactly ideal, especially as it's floodlit at night by 2 sodium streelights!). Ideally it would indeed be better to have 3 small slabs permanently laid in the garden, with drill holes for each leg and marks on 2 of the tripod legs for the leg lengths - We do curenty have some slabs laid into the lawn but not in the "correct" places, so I'll have to somehow devise a business case to lay them (or relay those) in the garden to disguise their real purpose!

Having had a quick check on Solarium, I believe that I was using NGC 7788 and/or NGC 7890 (just above Caph) - If the single shot hadn't have so awful I might have been tempted to take quite a few more and stack them as I'm sure there's a lot of stars in there (and your reworking seems to show that there's actually quite a bit of colour in the various stars as well).

However, I will have another crack at this experiment to see if the "tick" is still evident and also use the NA neb as the target... and also have a go at mastering drift alignment. I don't currently have a laser pen, but they're not that expensive, and of course I can always use it for pointing stars / objects out to my wife...

Although I now know I can get "an" image with multiple 30s exposures, it's very easy to see the difference beteen those and one 5 times longer - I'm now convinced that this is the way to go...

Thanks again Dennis...!

[OzDave]

Hi, I've been reading the latest posts here with some interest as I went through a lot of this trouble recently too, but seem to have overcome a lot of early problems.

The best things I have done to improve the quality of images so far have been:

1. Start guiding - I got a 6x30 finderscope and took off the eyepiece and replaced it with a PVC pipe offcut that happened to fit. I can now push-fit a 1.25" guide camera into the finder and using PHD along with a gizmo I built called Astrogene 1000 USB-ST4 guide port controller (google it) I can now shoot subs much longer than the 30 seconds I was managing before guiding.

2. Get a Light Pollution filter. I bought the Baader Neodymium one from FLO and after starting to use that, the noise level has really reduced significantly.

3. Started doing flats. Instead of building a light box, I have positioned my PC a short distance in front of the scope and then I slew the scope to point directly as the screen. I open a program that displays a large white screen (like a blank NOTEPAD document or WORD document or something) and maximise it. I then take my flats and flat darks from the PC screen. Using flats has eliminated the vignetting I had been getting slightly.

4. Take longer subs than 30s. Once I started doing this, I realised that 2mins as ISO400 was working quite well. The longer the better really, but you need to get guiding going first. The longer subs capture more photons and help bring out the detail in objects like M81 and M31 where currently you're not getting anything much apart from the core. With 30s subs, you get a lot of noise and not many "useful" photons so that the thing you want to image is mostly lost in the noise and kind of gets calibrated out, except for the bright core.

Oh, one other thing - someone mentioned you don't need flat darks because the exposure time of the flats is very short. That is true, except the flat darks will also remove the bias from the flat, which you will want to do. Therefore, since taking flat darks takes almost no time due to short exposures, I do them so my flats are properly calibrated.

So now I basically take my series of light images, slew round to take the flats and flat darks from my PC screen, then slew back for more lights on a different target if I have time. Otherwise I start packing up, but I remove the camera first, put the lens cap on and just leave it connected to the PC to automatically shoot a series of darks (I get the PC to control the camera for all of the lengthy exposures, so I can just leave the kit to image away while I sit inside and wait). I try to get as many darks as I have lights. The more you take, the better your calibration will be.

Hope this helps.

David

[AndyUK]

Hi Dave - Firstly, thanks ever so much for your advice... I must admit, I'd come to the conclusion myself that guiding's the key and also that I need a light pollution filter - It just comes down to money/priorities, doesn't it?!

I WAS saving up for an ATIK 16ic... but due to a) my work "upgrading" my laptop to W7 and Canon stating they won't be producing a W7 driver for my 300D , I've now bought a new camera - It's very nice (40D) but it has knocked me back...

Someone's mentioned finder-guiders to me previously, and although I'm not 100% sure my DIY skills are up to it(?), it does appear (to me) to be the simplest/cheapest route, if only I can do it... . Although I know it's not sensitive enough, I've thought of trying to use my Toucam 840k and see if that might work with PHD...

However, seeing all these lovely widefield shots about at the moment, I tried some shots around Deneb with a 20mm lens @f4.5... and basically got nowhere near the results I thought I might get - Although the 300D was unmodded, I was hoping to get at least some fuzzies showing in the view (and maybe even a little of the milky way) but not a thing showed - Just noise.

Having looked at other widefield shots, I see that some are achieved with as little as 5 x 5mins - Although I tried some 5 min shots, the sky was virtually washed out (well, light pink actually!). I could have maybe got away with 3 mins but foolishly ramped back to 90s... which was way too little. As you say, with short exposures, the SNR is way too low.

I've attached a couple of shots anyway - the 1st is my poorly processed (but best) attempt and the second is an unprocessed single shot. I suppose it does show how much stacking brings out, but I seem to be really battling with noise...

Unfortunately I haven't been able to get out since the camera arrived as I'm having to get my timer modified (different connection - Thank God for Andy Ellis @Astronmiser!), on top of which I've had to reinstall all the various apps on the new laptop - Hopefully I'll be up and running again in a week or so. However, I really wish I could get across to the Salisbury star party in a month's time to see how others approach this - Maybe I can drop in at the weekend... if my wife will let me(?)

[OzDave]

One other thing I forgot to mention, I used to think it would be ok to just image objects wherever they happened to be in the sky. If I felt like shooting the Ring Nebula when it was at 45 degrees altitude, I would.

What I've since discovered is that targeting objects near the zenith makes for much better shots. The noise is MUCH less and the quality therefore improves dramatically.

To illustrate this, have a look at a couple of recent posts of mine:

Ring Nebula

This one was shot with light pollution filter at ISO 400 when the object was right overhead. The noise level is very low and the image turned out pretty well I think.

Bode's & Cigar Galaxies

This one was shot with no light pollution filter at ISO1600 when the objects were low down in the north-west. It was also VERY windy and I was testing a new focal reducer which is the reason for the vignetting. But you can see a MASSIVE difference in noise level. Part of that is using ISO1600, but mostly its due to the light pollution and low altitude of the object.

Dave

[roundycat]

Dave, your example of M81/2 is not really a fair comparison with M57. I take your point about ISO but the shaking due to the wind is very evident and will completely destroy any chance of getting clean signal. There is no chance for the signal to build up in any frame with that much movement; you simply cannot compare the two.

Dennis

[AndyUK]

Cheers Dave - I know what you mean - The shots I got of Bodes were pretty much at the Zenith, and my shots of M57 a few weeks back were also relatively fairly high up to, whereas M31 and the Double cluster were pretty low down (esp M31, rising over the neighbours garage beyond which lie the lights of Bath). I know I could wait until later in the year when they're higher up, but I'm trying to get as much practice as I can now before the nights really start to draw in.

For me, my earliest attempts (towards the top of this thread) were a bit of a compromise as taking such short exposures necessitated (I thought) a larger number of subs and the higher up objects, whilst starting out okay, after a couple of hours the camera became in danger of crashing with the tripod leg. Perhaps at this time of the year it would make more sense for me to try and select a couple of objects in darkest sky locations (one early, one late) and then stack the images together after 2 or more sessions... With an object being closer to the celestial pole I might even get away with slightly longer exposures (even unguided).

However, to do that I need to learn how to stack images from different nights (with their associated darks, flat darks, lights etc), as that way I won't feel pressurised to concentrate on one object for the whole nights session with it potentially starting low down (in the murk) before then rising into the darkness. I've had a look in DSS but can't see how to do it yet (if it's possible?), so I'm also trying to trial Maxim DL... but I appear to be having Windows 7 installation issues , which I'm having to throw at their support desk. If Maxim DL facilitates this (and I can get on with it's GUI) this might be the way for me to go (maybe...)

[MikeWilson]

I've reprocessed your M31 in Photoshop. I enlarged image, applied five iterations of curves and saved.

I think you'd get a fairly good result reprocessing the original