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dark star

Owl nebula O111 filter DSS settings

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Hi, can anyone commend what settings to use in DSS for images of the Owl Nebula taken with a 2 inch O111 filter and unmodified Canon DSLR camera?

The individual RAW pictures look reasonably OK. I can't see any star trails, although in some pictures the stars are a bit blobby. However, when I stack in DSS the stars look they are trailing. My longest exposure was 10 minutes, which probably my guiding wasn't up to, even so the stars don't look too bad. I used a Badder MPCC coma corrector and 250mm f4.8 reflector. My processed pictures look worse than the originals! I am still very pleased to have captured this for the first time (and seen it visually too)!

Thanks for any advice,

David

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I attach a link to the Owl Nebula stacked in DSS in case anyone would like to try processing it. Why the stars are trailing when they don't seem to be in the original picrures I don't know. I am not sure if the originals are not good, am I messing up the settings in DSS or are my processing skills at fault. So far I have not managed to get anything even slightly OK out of the data.

Any advice gratefully received'

David

https://dl.dropbox.com/u/82184859/Owl%20nebula%20dss%2021.TIF

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Few things to try:

  • Check your alignment method is still set to "Automatic".
  • Maybe reduce the star detection threshold; how many stars is DSS detecting for each image?
  • Select your sub with best star shapes as "reference frame"
  • Re-register all images before stacking

Hope it helps!

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Had a quick bash at pulling out some detail (without much success)........it looks rather green.......It's the Owl,David..... but not as we know it. :grin:

post-849-0-76290900-1365424851_thumb.jpg

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Here is a link to one of my RAW files, 300 second exposure. I hope somebody who knows what they are doing can let me know if the problem is in the original image! I am using an Astronomik 2 inch visual O111 filter, however the advert I read said it was also good for imaging with an unmodified DSLR.

I wasn't planning on trying to photograph the Owl Nebula, I was just so amazed to be able to see it visually, if only as faint fuzzy, that I got carried away and tried ti image it! I have very little idea what I am doing!

However, I thought I had managed to get guiding going reasonably OK, using my DMK41 camera and a finder guider. I would be grateful for any opinions on this.

I assume the green is due to the filter?

Thanks for the advice about the DSS settings, I will try running everything through again. Probably I need a lot more data with an O111 filter, I only have around 30 minutes altogether. To be honest, I don't think I have the patience to be a good imager, 30 minutes seemed an awful long time!

https://dl.dropbox.com/u/82184859/l_12.cr2

David

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Hi Cloudwatcher, I am very curious as to how you got the stars to look OK, they look a hundred times better than I have managed to get them! I agree the Owl Nebula looks very strange!

With a star detection threshold of 10% DSS tells me it is detecting 77 stars, but I can't tell if this is in one image or all of the stacked images together!

David

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Hi David,

There's something a little awry with the individual sub you have posted. The stars look like they have moved around a lot and there are 4 duplicates of each one. I doubt that DSS would have been able to detect the stars and align the frames well enough if all the subs were like this.

The stars in CloudWatcher's image look like hot pixels to me. This tells me that the images have indeed not been aligned to each other. In fact, looking at your stacked tiff, the stars have all but vanished due to the stacking not aligning them together. Try stacking just the shorter subs - also, enable the "median filter" for star detection and let us know how many stars DSS then detects for your subs. (I fear that it's actually detecting those hot pixels).

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Hi, would the stars moving around in a 300 second sub be due to bad guiding? It seems like I can stop worrying about processing too much at the moment and concentrate on getting my original images better.

If it is due to bad guiding I will try to get better polar aligned. Could the problems with stars be due to bad polar alignment and PHD having o correct too much when guiding?

Once I am sure polar alignment is OK I will see if there is still a problem. If so, I will have to investigate things like flexure with the guide scope,

I will try processing some shorter subs with darks and see what happens. Also, as I am on holiday this week, finally take some flats!

Thanks for all the advice.

David

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I just had a thought, can DSS stack exposures of different lengths without any problem, from a Canon camera? I assume it can, but wanted to make sure!

David

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Yeah the problem would be due to bad guiding. PHD shows you a guide graph that allows you to follow how it's doing so that's worth a look. Polar alignment is very important but there are many possible reasons for guiding issues; take a look at any one of the hundreds of threads on the subject! :)

DSS can stack subs of different length... I'm not sure exactly how it works, but it does it in steps.

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Hi, I will concentrate on getting my guiding better..

The other thing that bothers me is how few stars are in my original subs, is this normal with an O111 filter and DSLR camera?

David

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The OIII filter will cut out some of the light, yes, but I think you'll find that many more stars become visible in your subs once the tracking / focus are nailed. Personally, I'd try imaging the owl without the filter but I'm interested to see what you do get with it.

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Hi, my focus started out good, I use the FWHM focus aid in APT. However, I start to suspect possible sag where my camera connects to the focuser, something else to investigate! Maybe the camera wasn't square to the focuser the whole time?

The problem with imaging the Owl Nebula without the O111 filter is I get horrendous light pollution. I have CLS clip fllter I could try instead. Given the usual English weather and the fact I need a lot of practice on this I hope to get something better in a few months!

David

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