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Help with M31 processing...

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Whilst sat back watching the Perseids, the sky was so clear I simply couldn't resist having a go at M31 - I'd had a couple of previous (poor) attempts with the C100ED so gave my 40D its 1st light with a 180mm lens at f4, using ISO 800, 10x2 minute subs plus 10 darks (+bias, but no flats/flat darks - I was simply too tired at the end of the evening and had to remind myself I was actually out Perseid spotting!)

The raw files looked better than anything I've imaged before so I had fairly high hopes that I might be able to get something reasonable from these, especially when I stacked them in DSS and the majority of frames contained more than 2000 stars... However, whilst this is probably my best to date, I still feel that I could get more out of this (processed in CS5, also using Astronomy tools).

Can anyone help me tease more out of this, please?

Just for interest I thought I'd also attach a jpeg of one of the original unprocessed frames where Andromeda got "buzzed" by a satellite! I had to discard this frame, which was a bit annoying really...

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Hi Andy, you have done a great job of getting rid of the light pollution but you have clipped the black point a little too savagely - this is a common problem as we strive to obtain an inky black background for our images but in reality, especially in the UK, the skies are never completely black. Unfortunately, once clipped, the data is lost but I have had a play with your posted data and put my own interpretation on it but you may not like it - but if you do, the details of what I did are listed below!

The image has a blue hue to it so I let PS do the initial colour balance using:-

Image - Adjustments - Auto Colour

I followed this up with Image - Adjustments - Colour Balance and adjusted the Mid tones of the bottom slider to reduce blue and increase yellow. This gave a more 'natural' (to my eye!) colour balance.

Next, I wanted to boost the dust lanes so I used one of my own actions to produce a duplicate layer and sharpen it. This is the PS procedure:-

Use 'save as' to save the image under a different name incorporating the word 'sharp' and then select the original image again. This procedure will give you two versions of the same image on screen at the same time. With the original image selected, produce a duplicate layer by selecting Layer - Duplicate Layer and name this layer 'Sharp'. Select this new layer from the Layers palette and set the blending mode to 'Overlay'. The image will take on a high contrast appearance but this is normal. Select Filter - Other - High Pass and adjust the pixel radius to achieve the amount of sharpening that gives the result you want (try 3.2 as a starter as that is what I used for your image) and click 'OK'. Flatten this image using Layer - Flatten Image, select the flattened image, copy it to the clipboard and paste it on top of the image now named with the word 'sharp'.

Now convert this layer to a mask by selecting Layer - Layer Mask - Hide all and use a suitably sized brush to brush over the dust lanes and, magically, you will see them sharpening.

There is more that can be done but that's a starter - hope you like it!.


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Hi Steve - Thank you VERY VERY much...!

I'll have another crack at this and start again from the master stacked image and have a go at the process you used...

I must admit I was a little conscious that the sky looked simply TOO black - There was a point in the Astronomy tools instructions (which I used to get rid of the majority of the light pollution) where it told me to use the black dropper... Obviously not such a good idea...

I really ought to buy a cilp filter for using the DSLR with lenses to make post-processing a little easier, but it's a case of priorities - that vs a guide cam... Thanks again Steve (and thanks for the dovetail plate description in your book - It works a treat!)

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There was a point in the Astronomy tools instructions (which I used to get rid of the majority of the light pollution) where it told me to use the black dropper... Obviously not such a good idea...

Assuming that the tools you refer to are Noel Carboni's, the black dropper stage is required but you do need to choose the selection point carefully, not too bright an area or the result will go too far and not too dark or not enough LP will be removed. I would be surprised if NC's LP removal tool would clip this much though. Unfortunately not available as a clip filter, I use a Hutech IDAS LP filter and this makes a fantastic difference to my start images.

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I had a quick play with your image yesterday,got called away by the missus (more important things to do :p)and forgot to post it.

Anyway,this is one result using the tools in the history window. How they are used is down to the individual of course and a different result can be achieved every time........ great fun.:)

This was a ten minute job so no work of art,just an example which may be useful.


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Looking forward to seeing your rework..
Yes, I'm very much looking forward to having another go at this, especially now I know (and can see) what CAN be done (by both Steve and Cloudwatcher) even though these reworks were base on an image that I'd probably half ruined before I even posted it... I just wish I'd brought my laptop away with me!

Steve - Yes, it is Neil Carboni's tools I used. I was only thinking earlier today how I should probably have picked a different area so as not to clip the data. I do recall that one of the other actions (I'll need to have another look at it to remind myself, but it might have been DSO enhancement?) seemed to leave "square block" artifacts after it had merged down and I think I may have re-used the black dropper again to get rid of those blocks - This would explain your comment that you wouldn't expect the LP removal to be so savage as all that...

I use a Hutech IDAS LP filter and this makes a fantastic difference to my start images
I guess my same quandary still applies (Guide cam vs Hutech IDAS LP filter) but slightly worse as they'll cost about the same! However, it looks as though whichever I go for first, I'm going to need the other sooner or later. I did enjoy using the 180mm f4 lens straight on the mount though (I guess it's actually nearer 290mm with the 1.6 multipler)... Whilst writing this post, I think I'm changing my mind and should consider getting the Hutech IDAS LP first if I'm going to concentrate on the larger DSO's this year... and maybe get the guidecam next year (or maybe I should see if I can sell my SW102 AZ Synscan :p)
This was a ten minute job so no work of art,just an example which may be useful

Cloudwatcher - I'd hate to see what you'd be able to do with my original frames if this was "just a ten minute job"! Thanks very much for the target...!

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I'm not normally so indecisive (honest!)- I've only really been into imaging for the last 4 months (and only really got back into Stargazing about 2 months before). I know that my main problem is that I'm very impatient and simply hate the fact that I don't have enough money to buy all the things I'd like (or think I need :)). However, as you say, that seems to be the nature of things and this certainly isn't exactly the cheapest hobby :)

So thanks very much for the advice. One of the other things I know I simply MUST do is get a better grip on Photoshop, DSS and Registax, which of course costs nothing but time... and that's something I DO have! Thanks again - Andy

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I simply MUST do is get a better grip on Photoshop, DSS and Registax, which of course costs nothing but time... and that's something I DO have!

Ah yes, I heartily agree with this sentiment - I typically spend as much time post-processing as I do in capturing the data in the first place!

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Cheers CW - That's much better - Thanks! I've had a couple of goes at reworking from scratch, but at the moment I can't seem to get anywhere near the results you and Steve achieved... :D

As you noted

How they are used is down to the individual of course and a different result can be achieved every time........ great fun.:)
I'm not sure about it being "fun" at the moment, but I'm certainly learning a lot about the powers of CS5!

Still, I'm on holiday in Cumbria this week and although I've brought a travel scope with me the weather forecast doesn't look too encouraging (typical, as this is the first time I've been somewhere where there's virtually no light pollution) so hopefully I'll be allowed to wrestle my laptop away from my daughter (and get permission from the wife!) to spend some more time on it...

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.................I'm certainly learning a lot about the powers of CS5!

:D Good man,keep at it............. I'm still struggling with CS3!

Had a bash at your other image with the satellite streak........ well part of it..... which,with a bit of work, could make for an interesting picture.


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Cheers CW - That's actually quite depressing... It looks as though you can get more out of a single frame than I get out of my entire stack :D! Can I ask where you learnt your processing skills? (Do you know if anyone runs courses???)

Steve's book does have a very good explanatory chapter on image processing, but I think what I really need is one of those "dummy" books on Astro Image processing that explains not just how to do something but also what it does... and of course that doesn't exist.

I can understand why (with there being so many potential applications to use out there) but I must admit I'd initially (obviously foolishly!) thought that the tricky bit was going to be getting the frames in the first place... I know I still have some way to go on that as well - a guide cam would certainly come in pretty useful - but I'm certainly not going to give up. However, this is beginning to do my head in! :).

I have also played with an image that I took on the same night through the 100ED - I was going to bin them (as they were only 30s exposures and the poor SNR was very evident), but I was actually quite surprised how much came out. I'll have to keep these to process again later (once I've got the tiniest inkling about what I'm doing!). (Now I'll go back to have another crack at those 180mm shots...)

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That looks a fine image apart from the colour balance,which is a personal thing anyway.

The few blue/green 'worms' can be removed with the Spot healing brush and I would adjust the Hue/Saturation levels to suit my personal taste............ Oh,and flip it Vertically. :D


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Hi CW - I sense that you did something more than just remove the "worms" and flip the image; You seem to have teased out quite a bit more of the gas cloud than I was able to...!

The colour balance is something else I'm battling with - It's all a bit "trial and error" at the moment, but thanks very much for your (and Steve's) help with this - I THINK I'm beginning to get an idea of the process, but obviously got a VERY long way to go yet. Still, it's certainly a way to wile away a few cloudy nights...

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Hi CW - Another start again from scratch on the prime focus image (with some different DSS settings, and I seem to have somehow got some different colours this time... As both you and Steve noted, colour balance is a bit of a personal thing and this maybe not be to everyone's taste, but I think I'm finished playing with this one now.

If I can get some longer subs (with a guide cam), I'll have another go at M31 again - I've found it quite a challenge but I've learnt a great deal from you and Steve on post-processing and also DSS...

Thanks again! ;)

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I have never imaged with a DSLR so don't know the workflow but when I work on colour balance from mono and OSC CCD cameras I use Levels to see where I am. I scroll between the R G and B channels in Levels and try to get the top left of the graph to the same point in all colours. You can only clip back to move a channel so you need to be careful not to clip back to far too soon.


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Having seen some of the beautiful and detailed images Olly produces you can't fault his advise and dedication to the 'dark art' ;)

I'm more into visual (lack of patience I guess) and tend to opt for the quick fix of images.

Speaking of which Andy,your last image appears on my screen as one of two halves....... the top blue and the bottom green.:mad:

The attached screen save is of a 5 min fix technique that might be useful........ further enhancement is up to individual taste.

Just noticed that the 'screen save' was done before merging the layers so 'merge down' does not appear on history list.:mad:


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Cheers all...

For such a "bright" DSO its a blumming nightmare to process!!! I`ve never had any luck with my attempts.....
I certainly can't (and won't) disagree with that - I could probably spend months on this stack alone and still not get it anywhere near CW's reworks! ;)

CW - Yes, you're right - mine is blue top/green bottom. Work as I might trying to get rid of it, it just wasn't happening for me, so thanks very much for the rework and history - I'll see if I can have just one more go...

I tried using a process I found on the internet where someone had used MaximDL - It was fairly detailed (dumbed down!) and I thought I'd try to emulate/translate it with what I had. The initial image I got out of DSS was MUCH better, but it seemed to leave me with that extra colour gradient on the image... Again, I know I still need a great deal more time playing with CS5 as there's obviously a lot I'm not using which is why may explain why it won't play with me as nicely as CS3 obviously plays with you! :mad:

Olly - Thanks for your advice on levels... I have used the separate RGB channels in levels in one layer (to remove the light pollution as detailed in Steve's book) but I've not tried using it on a limited selection of the image as yet - Although not in CW's latest history list, are you suggesting I need to play with the magic wand tool on selective areas/colours...?

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Okay, this really IS the last time...

Having read Moochers comments and also some other posts mentioning processing difficulties with M31, I've captured 2 more sets of subs (which was quite a challenge trying to get somewhere near the same framing!).

This is the product of a stack of 97 subs - 42x60s (ISO 1600, 20 darks), 25x120s (ISO 1600, 20 darks), and 30x90s (ISO 1000, 19 darks) - Total 137 minutes. I know that the ISO's are pretty high, but I had to select those to compensate for my obviously poor polar alignment (:)), whilst still getting halfway decent exposure times...

I'm still a bit bemused why DSS shows all the frames as having depth = Gray 16 bit (:o), and I'm positive that with more photoshop skills / practice I can still do better than this, but I've come to the conclusion that in order to get a reasonably decent image of this beast I really need a guide camera... and probably a focal reducer to make it easier to frame (both of which will have to, very frustratingly, wait for now :D).

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Thanks CW - I think you're probably being a little kind, but as you said earlier, colour balance is a personal thing... I consider it praise indeed that you "approve"! :o

I've learnt a lot from yours (and Steve's) input on this - Thank you very much...

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