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Bubble Nebula NGC7635 + artifact?


Tommohawk

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Hi all.

Had some nice sky last night so had a bash at bubble nebula - see below

Autoguiding working nicely, Dec and RA RMS errors <0.2, 13 lights x 8 mins = 104 mins, masterdark and bias. ISO800 Modded 550D Canon.

I was farily pleased with the result, though I will probably try and get some more lights when sky allows. My views are quite obstucted, but this target should be good for months yet.

One odd thing. When processing I noticed a circular dark area, about the same size as the bubble, to the right and above the bubble as it appears in my pics. I compared this to several online images and can't see this anywhere else. I've posted an enhanced version of this below. There is also a short dark "lane" which does appear on other pics.

So... either I've found an exciting hitherto undiscoverd cosmic feature which astronomers will name in my honour, or, more likely, theres a blob of peanut butter or similar on my sensor.

I dont think it's on the mirror, cos otherwise it would be defocussed more... I think. Any ideas?

Bubble Nebula:

Artifact?:

post-33831-0-33651000-1408393311_thumb.p

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Dust Bunny on or near the sensor ... Flat frames will remove it along with the vignetting ....  :smiley:

Hey Steve thanks for quick reply - thats a shame!! The sensor was professionally cleaned a while back. Should this be cleanable? I did flats once and found it a total pain.

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You really do need to do flats, an unfortunate fact of life when it comes to imaging im afraid :)

Its really worth calibrating your images properly before stacking becuase it allows you to get the most out the data you have captured.

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either a dust bunny or what used to happen to my ccd chip is that when i got coldish a little droplet, and i mean little, of water would stick to the chip and stay there, fairly hard to see it was that small, but i would warm it up for 5 mins and it would disappear.

looked just like you image. 

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either a dust bunny or what used to happen to my ccd chip is that when i got coldish a little droplet, and i mean little, of water would stick to the chip and stay there, fairly hard to see it was that small, but i would warm it up for 5 mins and it would disappear.

looked just like you image. 

Yep it's very round for dust I would have thought. Do you agree it must be sensor not mirror/s?

You really do need to do flats, an unfortunate fact of life when it comes to imaging im afraid :)

Its really worth calibrating your images properly before stacking becuase it allows you to get the most out the data you have captured.

Hmmmm - I'm going to have to find a smart way of doing that. Usually by the time I'm done, I'm done... if you see what I mean. And I have to dismantle outside so cant really do next day.

For now, I ought to be able to get set up in the light and use the t shirt trick, but once the clocks go back thats a problem. 

I can't really hold the laptop screen to the OTA - I tried this before and couldnt get properly lined up.

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There is no need to remove the camera at the end of the imaging session, just leave it in place (dont touch the focus) and you can do the flats indoors if you have a dark enough room to do it in (the basement is ideal).

Then, if you leave the camera where it is you can re-use the flats as many times as you wish (or until you get more fluff on the optics).

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The cause of the artefact is certainly not on the sensor but a short distance in front ot it. Dust on the sensor is effectively in focus. It blocks the light from hitting a few pixels and blacksthem out. If a doughnut is created, as here, the source is forward of the sensor on a chip window or filter.

You have to take flats. Everyone does! There is no way round this and the difference it makes is huge.

Olly

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The cause of the artefact is certainly not on the sensor but a short distance in front ot it. Dust on the sensor is effectively in focus. It blocks the light from hitting a few pixels and blacksthem out. If a doughnut is created, as here, the source is forward of the sensor on a chip window or filter.

You have to take flats. Everyone does! There is no way round this and the difference it makes is huge.

Olly

and when you consider just how quick it is to take them compared to the target data, there is no reason not too!

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Sounds like I need to get to grips with flats!

I've tried simply to keep everything super clean, but looks like this will be ongoing problem.

Am thinking that, as suggested, leaving camera attached would be a big help. Do you think its ok to make minor focussing corrections without having to redo flats?

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Sounds like I need to get to grips with flats!

I've tried simply to keep everything super clean, but looks like this will be ongoing problem.

Am thinking that, as suggested, leaving camera attached would be a big help. Do you think its ok to make minor focussing corrections without having to redo flats?

In theory you should redo them. In practice I never do. I use the same flats for months, sometimes, and I use only a luminance flat for all my filters. My cameras stay in place, though. If ever I find it stops working I'll stop doing it but until then I'm happy to save time. You cannot possibly reshoot flats every time you tweak the focus. You'd never get any lights done. Flats are very important and cannot be dispensed with but they don't have to become the subject of religious fetishism! They just measure the characteristics of your light path. The thing is to understand what they do and how they work. You need them but you don't need to complicate them. In fact a good flat that really works even if it shouldn't is better than a bad one that should work but doesn't. I've had plenty of those!

Olly

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  • 2 weeks later...

In theory you should redo them. In practice I never do. I use the same flats for months, sometimes, and I use only a luminance flat for all my filters. My cameras stay in place, though. If ever I find it stops working I'll stop doing it but until then I'm happy to save time. You cannot possibly reshoot flats every time you tweak the focus. You'd never get any lights done. Flats are very important and cannot be dispensed with but they don't have to become the subject of religious fetishism! They just measure the characteristics of your light path. The thing is to understand what they do and how they work. You need them but you don't need to complicate them. In fact a good flat that really works even if it shouldn't is better than a bad one that should work but doesn't. I've had plenty of those!

Olly

Hi Olly

Sorry for slow response - been away. OK I'm going to try and leave the camera attached, so that should mean I can re-use flats. I only make tiny focussing readjustments, and in theory that should smply be to maintain things rather than change anything.  Of course I'm using DSLR which is proabably easier to move out of position than CCD. In fact the t-ring has a small amount or rotation even when fully engaged which is pretty annoying.

Main issue is how to get flats - I like the light box idea but really struggled using 15.6" laptop to cover 200mm scope. I'm thinking maybe very bright room with T-shirt over scope? Or T-shirt and then direct or indirect camera flash?

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