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Vignetting difficult to get rid of


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Hi folks, me again.

Took some images of Cigar and Bode Galaxy last night - see attached.  Pleased with the result apart from the vignette that remained.

20X120s subs at ISO1600. similar number of darks and flats (however I mistakenly, perhaps, took the flats at ISO200

I stacked all in DSS and stretched the image in PS. However, lots of Vignette remains on the left of the image.

Kit used was my 8SE Celestron with f/6.3 reducer and a moded Canon 1100D. Plenty of Moonlight about

Anyone offer reasons and remedies to get rid of the vignette??

Thanks for your advice

Alec

post-36789-0-31929900-1425821410_thumb.g

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Maybe your optical chain is sagging a bit? My low profile focuser on my Newt has this issue. Try making things in this chain as straight and tight as you can get them. Any movement or sagging from weight is bad. If all else fails you can use a gradient exterminator program to even things out some.

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Hi Leveye

Thanks for that. I am relatively new to this Photography lark so any help is very welcome. On the issue of optical train I wonder whether my dew shield could also affect things. I have a home made one that I merely line up by eye when sliding it on the end of the scope. It overhangs the end of the scope by about 250mm. If it wasn't aligned too well, would this cause vignette?

Regards

Alec

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It doesn't look as if your flats were 'right.' They can be devils, too. Key pointers;

- If your flats don't look something like a bright centre with darker corners they won't be right. They shouldn't show a general gradient from side to side or top to bottom either.

- They should be exposed so that the histogram peak lies between a third and two thirds of the way to the right. I prefer one third.

- They must be calibrated using dark frames but a master bias (or set of bias frames) makes a perfectly good dark for flats. (You don't need to match expoure times perfectly at these short exposures.)

Olly

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Hi Olly

Have attached a Flat for comments.

Hmm, well that looks OK to me. Was it calibrated by the subtraction of either dedicated darks or by a master bias? This is very important. And how bright is it? ie Where is the histogram peak? You can't tell by looking at a screen stretch.

Olly

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Hi folks

The histogram was towards the centre certainly no more. As I said I took them at ISO 200 rather that 1600 where the lights and darks were.

Olly: not sure what you mean by ""Was it calibrated by the subtraction of either dedicated darks or by a master bias?"". I simply added all the subs, darks and flats to DSS and set it going. 

Alec

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Not sure but don't think it is a flats problem.

Looking at the Sky X for around 21:45 last night puts the almost full moon in an almost exact horizontal line to the right of your image at a distance of 78 deg.

You can see that the image is darker to the right where the corrector of the 8SE would be partially shaded by the raised lip at the front of the scope and gets progressively brighter to the left side where more moon light can strike the corrector and enter the top of the tube.

You can also see in the centre of the image a brighter circle surrounded by a darker annulus ring which corresponds pretty well with the outline of the corrector mounted secondary obstruction.

So I'm thinking at some stage in your imaging run moonlight was falling across the front corrector from right to left.

You would need to look at, and stretch a few of your light subs only, if moonlight was the culprit then the same pattern will be visible in the light frames and a flat will not be able to remove that.

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The above is a good point from Oddsocks.

DSS asks you for flats and also for 'darks for flats' if I remember correctly. (I don't use it.) You can use a master bias or a set of bias frames in this role but calibrating flats with darks is vital. As I say, you can shoot dedicated darks for flats but I wouldn't bother. Just use a master bias. The statistical difference is zilch.

Olly

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Just out of interest I put your posted image in PixInsight, just to see the shape of the DBE background mask that it produced.

It was not worth spending too much time with the output image as the source is lifted off this site as a PNG and the data will be compressed however with DBE used and a little noise reduction your image does clean up well considering.

If you spend a little time with the source data in PI you would be able to salvage something.

DBE mask:

 

 

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Hi Oddsocks

Incredible what you guys can deduce from an image. As I said earlier I am pretty new to all this and some of it is difficult to understand - however that is the challenge. I mentioned earlier in the post that I use a home made dew shield. Once fixed to the scope it extends it by around 200mm. at this length would it not prevent the moonshine getting into the scope as you suggest?

Not used Pixinsight. I much appreciate you guys taking time to provide insights as to what might be going on.

Just to be clear on the point about bias frames. When using DSS, is there a requirement to add, Dark, Flat and Bias frames into the pot?

Alec

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Hi Oddsocks

Did as you suggested and stretched a light sub. See attached. Clearly more light getting into the right of the image, which presumably the flat frames would get rid of.  Interesting how the light can get into the scope tube with that length of dew shield in place?

Alec

post-36789-0-91243300-1425848879_thumb.g

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Have a look into how to do masks to remove light pollution and light gradients. There are some good tutorials out there that will substantially improve the quality of your end result.

I have had similar issue (especially when stretching) when you get that nasty gradient, with masks it clears up most of it.

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When using DSS, is there a requirement to add, Dark, Flat and Bias frames into the pot?

Were the darks at the same ISO as the flats? If not, I suspect DSS might not use them for the flats. You must remove the bias from the flats (and the lights) or they will not work properly - darks include the bias, so using darks insted of bias should in principle work (people who are fussy do take separate flat-darks).  But I would always have a set at the right ISO.

NigelM

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Hi NigelM

The Darks were at iso1600, same as the subs. The flats were taken at iso200. I read somewhere to keep the iso low for Flats rather than keeping them at the same iso as the darks and subs, is this wrong?

 Can you explain what are "flat-darks"?

Hi Aesier

Can you post me a link to a good tutorial for doing this?

Thanks

Alec

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The Darks were at iso1600, same as the subs. The flats were taken at iso200. I read somewhere to keep the iso low for Flats rather than keeping them at the same iso as the darks and subs, is this wrong? Can you explain what are "flat-darks"?

In which case I think you will need bias or flat-darks at ISO200. Flat-darks are just darks taken with the same exposure time (and ISO) as the flats, which is usually much less than the lights. In fact given how short flat exposures usually are, most folks just use bias frames instead, as there is very little time for any significant dark signal to build up. I would just take some 1/4000 shots (or whatever your fastest shutter speed is) at ISO200 in the dark with the lens cap on to use as bias. What I am not clear about is whether if you feed these into DSS along with everything else it will do the right thing!

NigelM

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Hi Alexxx

Would GradientXTerminator have got rid of the v bad gradient in my M81/82 image. I looked at purchasing the plug-in but decided to simply carry on using what I had in Powershop.

Thanks NigelM I will take bias frames tonight and sky permitting repeat the imaging of M81/82 before the Moon comes up

Alec

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Flats won't touch gradients which mainly come from the sky itself. Even from a very dark site I get gradients, notably in colour where the eye is very sensitive to the slightest change or drift in colour balance.

Why gradients appear is a bit of a mystery in some cases, but they do.

Olly

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Hi Alexx

Took your advice on GradientXTerminator BUT:

Following feedback I have downloaded GradientXTerminator on a trial basis. It all work very well, gets rid of the gradient, but I have 1 major problem, I can't save the finished image. When I do a save, it only saves the part of the image that was contained within the lasso tool. Any ideas? I have followed the instructions, including various tutorials. My last action is to "Select -deselect" or CTRL D which should clear all the actions on the image.

I am using Photoshop CS2 32bit, GradientXTerminator 32bit and windows8.1 64bit

Any ideas anyone?

Alec

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