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M31 and subs question

swag72

By swag72
in Imaging - Discussion

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swag72

swag72

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I have taken the following subs of M31 using my 5D2 and 300mm at f4.5, using my HEQ5 goto.

Lights - 25x91s, 39x44s, 28x60s and 2x120s

Darks - 4 x 91s, 28x44s and 9x60s.

I haven't taken flats - Still trying to work on those!!

So I stacked the 1hr 38mins worth of images into DSS and what I got out was quite a little underwhelming. Is this because I need loads more data? What would you suggest I should get on the next cloudless night?

Also can you point me in the direction of how to take flats - REALLY EASY method please!! For some reason, flats scare the living daylights out of me!

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johnb

johnb

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Im no expert but often find im not inpressed when I look at the DSS Output, however once i then open the file in PhotoShop and start the Levels and Curves process all the data seems to appear very nicley. Assume you post processing ?

John B

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andrew s

andrew s

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You should have enough data for a reasonable image. What post orocessing are you doing after DSS? The arttached was my first DS image with a Epsilon 180 with 20 30sec subs (no flats or darks) I stacked in DSS the imported into PS CS3 using FITS Lib and a log streach then sone curves in PS.

Andrew

post-21962-133877513522_thumb.jpg

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swag72

swag72

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The DSS image was quite flat, so I wasn't too bothered as I know what can be done in PP. But even curves, levels etc just left the whole think looking like a soft mass. No definition or anything. Very disappointing. Perhaps though I just need more?

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Uplooker

Uplooker

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When you saved the output from DSS did you "Save with adjustments embeded" (or words to that effect) makes a significant difference. After that you are into working / playing around in Photoshop or similar for hours (weeks in my case) trying to extract the detail you have actually captured.

Ian

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JCJC

JCJC

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For flats with DSLR and lens, maximise notepad on a computer/laptop screen, point the DSLR at it and put it on aperture priority mode, with the same ISO and F number as you took the lights. If the exposure time keeps changing, put the camera back to manual and set the F number, ISO and exposure to the same as the aperture priority thought it should be. Take maybe 20 and whack them into DSS and see what happens.

This method has always worked ok for me.

Jordan

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RobH

RobH

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Hello Sara.

With all types of calibration frames, take at least 20. If you don't do this, you'll end up adding noise to your image.

Calibration frames are no different to your lights (subs) in this respect. All of them contain a random noise element, which will be removed by the right stacking process if you have plenty, but just 4 darks for example isn't enough.

Cheers

Rob

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Tim

Tim

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To get the best results from a mix if exposure lengths in DSS, use the entropy weighted stacking method.

Personally I prefer to find the maximum useful exposure length and then work at that exclusively, possibly taking a shorter sub sequence later on if needed for bright cores etc.

If you can stack up the subs you have, using the entropy setting, and then do nothing to the 32bit autosave file that DSS will give you, we can run the result through various software we have to see what is there. You'll need to upload the autosave.tiff somewhere free, it'll be quite big.

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swag72

swag72

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To get the best results from a mix if exposure lengths in DSS, use the entropy weighted stacking method.

Personally I prefer to find the maximum useful exposure length and then work at that exclusively, possibly taking a shorter sub sequence later on if needed for bright cores etc.

I stacked using the kappa sigma option as I had read that over 40 subs, that is the best one to use :( I'll try again with the entrophy weighted method. Is there an idiots guide of which method to use for what mixture of subs?

How do you determine the maximum useful exposure length? I am just taking random times as I know no better. To find the most useful exposure, what would I be wanting to see when I preview the picture?

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swag72

swag72

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Back to it again tonight - I don't think I realise how lucky I am over here in Spain! So set up for more subs - But what would be the best exposure time and how do I determine that?

I am currently taking 40 x 120s, so that should take my total well over the 3 hour mark. Do I get more shorter subs as well? Keep mixing it all up and just throw the subs into DSS?

Would much appreciate your help on this one.

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RobH

RobH

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Hello Sara.

At 120s, are your subs at all burned out in the cores?

With M31 and my very sensitive Atik 16HR, the core isn't burned out.

If it is on yours, shoot about 15 minutes worth of 1 minute subs to use for the very inner core. When dealing with bright subjects, you don't need lots and lots as you're not worried about the signal to noise ratio in the same way as you are when trying to stretch faint data.

You may fine that you don't need darks for the same reason.

If you can get longer subs, I would grab some as they will allow you to go much deeper with the faint outer areas.

Cheers

Rob

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swag72

swag72

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Hi Rob, yes I am getting out a burnt core at 120s, so will go for some shorter subs as well. Currently do have a number of 30s subs, but will get some more. Also, wondering if you can shed any light on why I seem to be loosing so much colour in the end image?

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RobH

RobH

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Hello Sara

Are the cores burned out on the individual raw images, before you've done anything at all in terms of stretching?

It could be that they are being burned out during that stage, as it is very easy to do on high dynamic range targets like this. Have a look at the 'curves and levels' info on my site under the tutorials section.

Regarding the colour, I opened the image up in photoshop, but the jpeg on the site isn't of good enough quality to really see.

Could you send me the full resolution TIFF and I'll have a look.

My email address is on my site.

One thing to remember is that when you see lots of colour in the spiral arms, you're normally looking at an image that has had a lot of exposure, and often pretty long exposures to go deep on the arms. The colour is very subtle, so needs a deep image to really allow it to be brought to the fore.

Cheers

Rob

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