Blownout core how can I resolve this ?

[StarRaver]

Hello all, recently i posted a couple of pics from testing my new setup of Andromida and the dumbell nebula, As im new to imaging i was wondering if there is anyway of capturing Andromida with out the core being blown out as the core is so bright and the dust lanes so faint ?

Do I need to take two different exposures a lower iso or shorter exposure for the core and longer exposure for the dust lanes or is it all down to the processing techniques after you capture your subs ?

I have had a look at and 45 hours seems like a lot of subs and work for a complete begginer At the moment processing isnt my strong point so I still have a far way to go in that field

any help and advise would greatly be appriciated thank you

[andrewluck]

Non-linear stretching during processing is the the key to this. You want to boost the mid-tones without overblowing the highlights. Lots of information available on the web on this. This is a site that really helped me when I was starting out: http://starizona.com/acb/ccd/ccd.aspx

Andrew

[StarRaver]

thank you the link looks like it could be very helpfull, im quessing this applys to all imaging and not just ccd as im using a dslr

[Bizibilder]

You are right about taking a different exposure to get the core of an object like M32. Once the image is "blown" - ie all the pixels are showing pure white - there is no amount of processing that will retrieve the situation. For any object with a high dynamic range (posh name for it) you really need to take two or more sets of images with different exposure times and combine them in you chosen processing software.

[StarRaver]

Thank you for that, is there any set exposure lengths for the core im quessing a low iso and around 20 or 30 seconds or shorter compared to 1 or 2 mins for the dust lanes and will dss automaticly sort it all out in processing ?

[ian_bird]

Hi

I'm one of the guys who collaborated on the 45 hour M31 (Well - the first 20 hours of it). We were using 5 minute subs in LRGB and 15 minute subs in HaII. I think this is why CCDs are so good for DSO imaging as they have such a high dynamic range. Then it's all down to Olly's silky smooth image processing and multiple layers in Photoshop.

But Olly's your man to comment on this. Not sure how you would achieve a similar result with a DSLR.

Regards

Ian

[Bizibilder]

If you are using 2 min exposures then maybe go as short as 20-30 seconds for the core. You will need to take quite a few and stack them. I've had a look at your M32 and it is very good for so few subs. You will need to take many more darks - at least 30 - but this is not a big problem as you can take them during the day (or on cloudy nights) by just putting the lens cap on and taking a note of the temperature (take a set every 5° or so - do it over time) so you build up a library of dark frames - decide on your favourite exposure times and take a set for each. I set my camera up in the garage and use a remote timer - just set it going and let it click away all evening. If you are using DSS then as bias data is contained within the darks you don't need them.

You have some vignetting around the edges of your pictures (dark edges and corners). This can be removed by taking "flat frames" for which you will need some sort of evenly illuminated light source, some folk use a computer screen set to white (open notebook or wordpad and set it to full screen), others make up a lightbox using bulbs or white LED's - these only cost a few pounds to make yourself. My own one is here: http://stargazerslou...box bizibilder and it works well (even if I say so myself!). Just put it over the end of the scope without adjusting anything, especially the focus, set the DSLR to Av setting and take around 30 pictures. You have to take light frames every time as they only work if the camera / optical train / focus are exactly the same as your light frames. Hope this helps.

[StarRaver]

Thank you Roger, I have been looking at el panels but they are quite hard to find in white ( for me at least ) and not to sure how to power them.

Hopefully when the cloud has gone and we get a clear night I will have a bit more time to capture more subs at different exposures and try using flats with the laptop screen for the time being.

As im still new to this all and have only had my mount and scope a few days every little extra bit of information help. My mind has been in overdrive ever since taking up this hobby from all the books and research on the net and yet still have to learn about processing the final image.

Its nice to know there is a place to turn to when help is needed thank you all

[Gina]

I just use a white T shirt over the dew shield and take flats in the daytime using skylight. Works for me

[ollypenrice]

In the case of M31, and other challenges like Alnitak, I have found short subs to be entirely ineffective with CCD. I've taken them but been unable to use them, so far. They work superbly on the Trapezium in M42 though.

Instead I'd go with Andreluck's advice and make dedicated stretches of your full data using a core control curve like this. The demo is on Alnitak but I used the same system on M31.

The idea of this curve is to get the background up but keep the bright signal down. As you can see, Alnitak is perfectly well resolved in full length subs but given the same stretch as the rest of the image it turns into a supernova impersonator! So it needs its own stretch.

Once your core is looking reasonable place it as a layer underneath and, using a low opacity eraser well feathered, gently and iteratively remove the top.

I think the shortcoming of short subs is that they don't blow the bright stuff, which is fine, but they don't go deep enough to pick up the buried faint stuff - which is not fine! Now it may be that short subs work better with DSLRs because they don't have the dynamic range of CCD but before you shoot any short subs have a good look at what a core control curve can do for your existing data. Also check out on the net what the best imagers can do in terms of getting into the core of M31. I think you'll find that 'not much' is the result of that search! Some do pull out some funny patches of data but at a cost to the overall look of the image. The truth is that Andromeda has a very bright core.

Olly

PS, The rework I did of the core of M101 also used the full length (15 minute) Luminance subs. The data is in there. Shorts would not have helped here.

http://stargazerslounge.com/topic/166918-m101-new-core/

[StarRaver]

Thank you that olly I think I will have a try at that later on if the weather changes for the worst, like I said processing isn't my strong point at the moment and all im using is levels and curves ( doubt that im using the right either ) so its all trial and error at the moment

[StarRaver]

Sorry about half spelt and missing words phones don't make good keyboards lol

[Tim]

Start simple. Get more data, and make it better quality. Make sure your focus is perfect, make sure you throw away any dubious subs, get as much data as you can muster on a target. Take matching darks, and especially flats and bias frames. If you can make the data you have to play with as good as it can be, you will be off to a better start.

Then, spend a while just playing in a decent image processing software. Read up on techniques, and watch video tutorials on how to get the best results.

You have started with a doozy really, M31 is notoriously difficult to process, it has a huge dynamic range from extremely bright to almost invisible. I have images here that get detail in the dust lanes right to the centre almost of the disc, but bringing it out and making it look natural is nigh on impossible, as Olly says, the centre of M31 is extremely bright.

At any rate, keep asking questions, it's the way to learn

Tim

[ollypenrice]

Yup, I think it would be worth getting all the world's imagers together to talk about two things, the core of M31 and Allnitak! I might have something to contribute on Alnitak but on M31 I'm about stuffed!

Olly