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1st effort at the Rosette


madjohn
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Hi folks,

Haven't posted for a while because of the blumming awful weather (is it ever going to settle down and clear?:)) but weekend before last I managed a few hours between the haze.

I've never tried the Rosette before, its quite a large target but my 80mm Orion framed it nicely.

Been struggling with vignetting with this one, I may have to take Steve Steppenwolfs advice:icon_salut: and buy a electroluminescent panel to start doing flats with. (never done flats before with any of my previous images)

46 X 5 min subs at ISO 800, darks and bias subtracted.

Orion ED80 with my WO 0.8 FR, modded Canon 450D, Skywatcher light pollution filter.

I'm having a much easier time of processing my images using the skywatcher filter instead of my EOS clip one, any body else notice this?

If the skies clear soon I may add some more subs to try to get the image smoother, but pleased with this effort as I'm using a DSLR instead of a CCD.

John

post-15189-13387751914_thumb.jpg

Edited by madjohn
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Thats a great shot good star colour...does look like you were still suffering with the mist....a good flat is essential, but depending on how bad the vignetting is you may still have problems in the corners, the key is a well exposed flat with plenty of signal in the corners.

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Very nice image. I did my first flats by just pointing the scope at a plain painted wall during the day. They helped reduce the vignetting effect in the corners. I then tried a 'white' computer screen (just created a white background in paint), which also worked. Finally I made an electroluminescent screen which I now use outdoors during a session.

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Thanks for all the abvice guys, looks like a flats panel is in the offing!

John

Good job on the Rosette ... get a EL panel form fleabay, I bought an A4 one for Taiwan for £30 delivered... works a treat.:)

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If that's your first, then you have a good future in Imaging.

That's a very fine result.

I echo your words re the weather MJ. Totally depressing even for non astronomers. Even Wildfowl are fed up with it.:)

Ron.

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That's a lovely Rosette John. You will be amazed at how much easier images are to process when they have been properly calibrated with a flat. However, if you want to ruin an image there is not better way than applying a mediocre flat. The number of ways to mess up a flat is impressively high! I've found an EL panel is far and away the easiest and most reliable way of getting quality flats so go for it!

You mentioned calibrating with darks and bias. If your darks were the same exposure length as your light frames you shouldn't use bias frames as well because you will be subtracting the bias twice. If you are scaling darks (taken with shorter exposures) you need to subtrack the bias from the darks, scale them and then do both a bias and a dark frame subtraction. If this is what you were doing then sorry to have mentioned it :)

Where does the Skylight filter fit in the imaging train?

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Hi Martin, thanks for chiming in.

Thanks for that imaging train of thought, I have been subtracting equal length darks from my lights, and including my bias so I have been doing it all wrong according to you!:)

Scaling? haven't come across that term before, the bias frames are very short exposures, the fastest the camera can manage, how much shorter has the dark frames got to be from the lights if your using bias frames?

Is there a thread somewhere I can read up about this?

My Skywatcher light pollution filter is a 2" screw in type which is fitted to the nosepiece of my WO 0.8 FR

Cheers John

Edited by madjohn
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If your darks are the same length as your lights you don't need to perform bias subtraction. It is possible to use shorter dark frames than your light frame. If you had 6 minute lights you might decide to get 3 minute darks and use these. To do this you have to bias subtract the darks then double the dark value x2 (scaling) then subtract both the bias and the scaled dark from the light. You have to remove the bias before scaling otherwise that will be scaled as well. Fortunately you don't need to know any more if you are using software like deep sky stacker because that will sort everything out for you. So if you put bias and dark frames into the mix it would only use the bias if scaling the darks was necessary. Very clever! When you get onto flats you should either bias or dark subtract these as well. Again deep sky stacker, maxim and some other programmes can handle all this for you automatically

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Thanks again Martin,

I am using Deep Sky Stacker, (wonderful simple programe!) and I wasn't aware how much it does 'behind the scenes'as it were, so even if I entered the bias it only uses them depending on the darks and lights taken....clever! and a relief that it simplifies the whole process.

So when I start taking flats ( I know the histogram has to be about one third away from the left side) DSS will automatically calibrate then from the darks and (maybe) bias frames too?

Cheers John

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For the flats you can either just use the bias frames or, you can do another set of darks of the same exposure length as the flats. The latter is a bit obsessive and probably only worth doing if you have regulated cooling and a permanent set up.

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