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Old 10-06-2016, 12:42 PM
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PRejto (Peter)
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Blue Filter Issue

My first attempt to use new Astronomik filters.....

The R and G mean stacks look totally normal (ca 80 x 5 min subs).

The blue looks terrible and when highly stretched shows what looks like 4 off-centre large overlapping circles. The blue flat doesn't show the circles. Just in case it was the flat I stacked without using the flat and got the same result.

My setup for this did not include the usual ONAG. It was TEC140/SX FW/Trius.

Any ideas?

Thanks!

Peter
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  #2  
Old 11-06-2016, 09:14 AM
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Its either dust on the filter or a defect in the coating.

Try cleaning it with a photographic cloth and using a bright torch illuminating the filter at an angle to show up any dust particles and see if it changes anything.

Greg.
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Old 11-06-2016, 06:16 PM
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Hi Greg,

Thanks for your input.

What I don't get is why this doesn't show up at all in the flat. If it's an obstruction shouldn't it? The other odd thing is that both my green and blue flats (but not red) show a darker patch slightly off centre. You can see it in the flat I posted. That ought to not be seen with a refractor, right?

Peter
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Old 12-06-2016, 05:39 PM
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Hi Peter.

Does the below link help as i see you have what looks like a hotspot in your flat... looks like its in your final image to.
http://www.sbig.com/about-us/blog/fl...he-ugly-truth/

Josh
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Old 13-06-2016, 08:17 AM
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Quote:
Originally Posted by Joshua Bunn View Post
Hi Peter.

Does the below link help as i see you have what looks like a hotspot in your flat... looks like its in your final image to.
http://www.sbig.com/about-us/blog/fl...he-ugly-truth/

Josh

Thanks Josh! That is a most illuminating article. I'm pretty sure this is my problem. I speculate now that my blue images are showing this effect because the contrast is lowest and light pollution is now a greater % of the image. That + IR reflection off the tubes in the imaging train. I guess I need to wonder if these new blue filters are actually blocking IR sufficiently.

Peter
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Old 13-06-2016, 08:49 AM
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Interesting article. This problem only started when you changed filters to Astronomik right? What were you using before -Astrodons?

So if this is the case that there are IR reflections inside your scope from the internal painting or the adapters then the earlier filters were handling it better? Better IR blocking?

Getting a good flat I find to be somewhat of a trial and error and I tend to check the workability of the flats before I think I've got all my calibration data.

Some scopes are hard to flat field, others quite easy. Usually an APO is the easiest of all.

These days with 2 mirrors and a corrector system which is popular on almost any compound scope flats are tricky. Too bright. too dim, darks not matching really well, do you subtract a dark flat from the flat or not, do you use a bias to subtract from the flat when making the master flat or when you apply them? All these factors seem to have an influence so I trial and error them to get the best result.

Greg.
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Old 13-06-2016, 11:45 AM
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My ONAG went back to the USA for adjustment so my setup was not my usual. It was quite simple consisting of only a filterwheel and camera. I went back and looked at flats from the ONAG setup and see only the smallest hint of central darkening. But, that was with different filters. When I get a chance Iwill take more flats with the new filter but my usual setup. This will provide a better comparison.

I will need to process out the errors in the lousy looking blue combined stack as I've broken down the setup I had capturing that data. Live and learn! I took flats, looked at them and thought they were OK. Greg,your advice to check when going along is very good advice.

But, if it's IR stray light why isn't blue blocking this? So perhaps it isn't IR but blue light reflecting off something? But then I would expect my LED light box to transmit enough blue to see the refection.

I wonder if it's possible to take flats using just the night sky? i.e., long exposures using light pollution as the light source and then process out the stars? Is that too far fetched? I imagine gradients would be a quite large issue.

Peter

EDIT: I see this is not an original idea: http://www.cloudynights.com/topic/46...the-night-sky/
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Old 13-06-2016, 11:50 AM
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Quote:
Originally Posted by PRejto
I wonder if it's possible to take flats using just the night sky? i.e., long exposures using light pollution as the light source and then process out the stars? Is that too far fetched? I imagine gradients would be a quite large issue.
Some professional observatories have used this in the past, not sure if they still do. The idea is that thousands of images from all across the sky are used, sigma clipped so that the resulting stack has nothing but the sky flat. This works well if you do a lot of LRGB imaging with star fields, not so much if you do a lot of nebulae shots as there isn't ever much in the way of true background in this case.
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Old 13-06-2016, 11:58 AM
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Peter,

I find I get good flats doing either of these methods:

1. A whitish cloth cover over the end of my telescope. In my case my wife made me a whitish elasticised cover to cover the ends of both my CDK and RHA scopes. I use these when doing flats to diffuse the light and prevent stars showing up.

2. I take flats at dusk and take 6 of each filter and binning I am going to use.
I take flat darks meaning a dark same exposure lengths as the flat. So I try to use the same exposure time for each to keep that simple. 4 seconds is what is working for me for all filters and narrowband. I usually expose to about 22,000 to 29,000 ADU. Too bright and they can overcorrect and the vignetted areas go lighter than the rest of the image. Too dim and the vignetting is not entirely removed.

3. If I don't take dusk flats I take them the next day with my rolloff roof closed and the white cover on and the scope pointing to a neutrally lit part of the interior wall. At my dark site that is the grey underside of colorbond sheeting and at my home observatory that is flat black painted insulation.

4. When creating a master flat I opt with CCDstack to not subtract a bias at that point. I subtract a bias or a flat dark at the time of applying the flat.
I have found a slight mismatch with the bias or flat dark on some scopes can wreck the flat process.

I set my Proline to fast downloads as time is of the essence in falling light at dusk and you have to be fast and organised. I name the file set up the save as box and binning and next filter in advance whilst the current flat is being taken to save time when doing dusk flats.

Both of these work. I also subtract a master bias from my darks and use exact same exposure length and temperature darks usually about 16 of them to form a master.

The CDK has been touchy with flats in the past less so now.

I have never used the flat illumination panels or light boxes. But many do and are happy with them. I think you'd have to make sure they were very even in illumination and quite broadband in their light as CCDs have different sensitivities to different wavelengths.

Greg.
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Old 13-06-2016, 01:53 PM
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Greg,

Thanks for the tips and tricks you employ.

The point I took away from the article at SBIG is that dusk/dawn flats do not contain the light pollution frequencies and neither does a light box.

I've got an idea floating around in my mind. I wonder what you think! It seems to me that my blue frames contain information about the object being imaged as well as the problems in the optical system. I have green and red data that can be flatted without problem leaving just the object being imaged. So what if I took a flatted green image, normalized it to a blue image and subtracted the images from each other? Might that leave me with just the flat field?

Peter
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Old 13-06-2016, 03:06 PM
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Very definitely any light pollution makes flat fielding quite a bit more difficult.

Both my sites are relatively dark, my dark site is dark dark and my home site has semi rural skies so relatively dark especially to the west. It seems dark until I go to my dark site and then the difference is obvious. A real dark site makes the stars look fiery.

You can try sky flats. I think you simply take more flats with no cover on the scope and then median combine to get rid of the stars. Make sure you dither a lot between individual flats taken. Now that's all the wavelengths you are imaging in.


Greg.
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Old 01-07-2016, 11:11 AM
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This looks more like a defect of the filter than anything else. If you consider you are getting no issues from the red and green it is highly likely the filter is at fault.
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