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Old 02-06-2016, 09:43 AM
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Atmos (Colin)
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Getting Blue Halos?

So far I have only had three attempts at RGB imaging since getting my new scope and camera and on all three nights I have ended up with bad blue halos around bright stars.

The first time I put it down to high level haze coming over. The second time I put it down to bad seeing (had FWHM of 3.8" in 100s images but 2.2" in another part of the sky with 900s Ha). The halos remained after both running a decon on both green and blue which failed so then a convolution on red and green, also failed. The third time I thought that maybe some faint high level cloud may have come before heavier cloud that came over 20 minutes before I finished the blue that night.

Having a 3/3 strike rate with blue halos I started trying to think of something other than poor seeing being the issue. The first thought was that my scope had the colour correction of a fast doublet (unlikely). I then dismissed this as if this was the case my luminance would also be pretty shocking which it hasn't been. This was further backed up as on Tuesday night I had my DSLR on the scope just to make sure, no blue halos.

This then leaves me to either being the filters but never had the problem when they were on my QHY9, they QHY22 does have a considerably better blue response however. Then I considered that maybe it was a UV correction problem and leakage through the blue filter. I have Astrodon Gen II E Series LRGB and the L&B have the same cutoff at shorter wavelengths.

15 minutes ago I got to thinking, at the moment my QHY22 isn't mounted directly onto the CFW (has a ~10mm universal camera adapter thing). Could it be that the blue filter does not converge correctly when it is considerably further back from the CCD, maybe 20mm as a guess?

I may be gettin the same issue with my OIII filter but not to the same extent. Less to do with halos but I have noticed that I continually get a worse FWHM with the OIII than Ha and SII... Or is this normal?
Last night for instance, a very brief check, guessing maybe 3" in Ha and 3.7" in OIII. Haven't run the numbers yet, just did a PSF in my head

I am planning on correctly mounting the QHY22 to the CFW simply for potential flexure issues but could it be causing problems with blue subs?
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Old 02-06-2016, 10:37 AM
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Shiraz (Ray)
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PRejto had the same sort of halo problem. http://www.iceinspace.com.au/forum/s...d.php?t=126274

You will normally get larger FWHM in blue in most seeing conditions - the seeing varies with lambda^(-0.2) so there is a broadening at short wavelengths. My impression is that it might be a bit worse than the theory suggests - it certainly is for "lucky" planetary imaging.

Last edited by Shiraz; 02-06-2016 at 10:49 AM.
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Old 02-06-2016, 11:35 AM
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Atmos (Colin)
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Quote:
Originally Posted by Shiraz View Post
PRejto had the same sort of halo problem. http://www.iceinspace.com.au/forum/s...d.php?t=126274

You will normally get larger FWHM in blue in most seeing conditions - the seeing varies with lambda^(-0.2) so there is a broadening at short wavelengths. My impression is that it might be a bit worse than the theory suggests - it certainly is for "lucky" planetary imaging.
Thanks Ray, definitely going to have to read it when I get off work
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Old 02-06-2016, 12:09 PM
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Colin, is this the sexy-tuplet or the FC-100?

Also, having almost exclusively used a Tak doublet for MANY of my images (or the Vixen FL102S), you MUST refocus for blue every time - this is why I would shoot R, G and B separately rather than intermixed - an hour on each, focusing each.
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Old 02-06-2016, 02:22 PM
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Lewis, for the sextuplet The FC100 is exclusively for visual, brilliant for it too!

I end up refocusing every filter anyway as I need to refocus every hour regardless.
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Old 03-06-2016, 07:35 AM
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APO by definition means 3 colours in focus at the same point. Its a bit of loose definition in practice as you get manufacturers claiming things to sell their scope.

TEC140 for example lacked some blue correction and this did not really show up until the Sony ICX694 came on the scene which has much better blue sensitivity than the Kodak sensors (for example its 60% QE in O111 which si about the highest of any sensor).

Peter had a lot of trouble with this. I then notice Yuri improved the latest model of the TE140 to improve the blue correction. These are design parameters the optical designer/manufacturer can tweak.

I had an FS152 fluorite doublet. It was extremely sharp visually but it would tend to get some blue halos on bright stars. This is blue out of focus compared to red and green.

Perhaps others can say, but I was not aware simply refocusing the blue filter fixed it. I thought it was simply the optics don't handle the blue wavelengths well and may not focus it well at all. Worth finding out.

Dr Rhor I think his name is posts tests of various scopes. His tests reveal all the weaknesses even from high end scopes. A German site.

Greg.
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Old 03-06-2016, 08:38 AM
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Quote:
Originally Posted by gregbradley View Post
APO by definition means 3 colours in focus at the same point. Its a bit of loose definition in practice as you get manufacturers claiming things to sell their scope.

TEC140 for example lacked some blue correction and this did not really show up until the Sony ICX694 came on the scene which has much better blue sensitivity than the Kodak sensors (for example its 60% QE in O111 which si about the highest of any sensor).

Peter had a lot of trouble with this. I then notice Yuri improved the latest model of the TE140 to improve the blue correction. These are design parameters the optical designer/manufacturer can tweak.

I had an FS152 fluorite doublet. It was extremely sharp visually but it would tend to get some blue halos on bright stars. This is blue out of focus compared to red and green.

Perhaps others can say, but I was not aware simply refocusing the blue filter fixed it. I thought it was simply the optics don't handle the blue wavelengths well and may not focus it well at all. Worth finding out.

Dr Rhor I think his name is posts tests of various scopes. His tests reveal all the weaknesses even from high end scopes. A German site.

Greg.
It appears to be an effect with these incredibly high QE (in blue) and most refractors. I don't think it can be focused out although I will try using FWHM for focusing as opposed to a bahtinov mask and see if that makes a difference.

As with Peters testing, having a UV block makes a difference so it think it is less to do with blue correction and just UV correction. I am going to try and see whether 2x2 binning fixed the problem. Worth giving a shot.
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Old 03-06-2016, 09:01 AM
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I found refocusing blue helped a LITTLE - but it does not remove it. ALL the Taks I have had do/did it, EXCEPT the FSQ-106ED (yes, the N's all do it). The FSQ-85 certainly does do it.

FSQ-85 image: http://www.astrobin.com/full/119145/B/

FSQ-106ED image: http://www.astrobin.com/199796/0/

FC-100 (OLD) image: http://www.astrobin.com/full/176758/C/

The last one is worth looking at all the revisions, showing degree of blue flare through this doublet - and the ways I tried mitigating it (depends on the stretch of each channel)
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Old 03-06-2016, 08:41 PM
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My results (TEC140, earlier model) are that focusing (better) doesn't make much difference. It depends on the lens figure. Visual blue can be razor sharp but all higher frequencies will be out of focus. Things are improved if the blue filter cuts closer to the best focus of the lens. For example my Baader filters cut around 380nm, but the TEC140 is figured to be sharp at around 430nm. I saw some slight improvement testing an Astrodon blue that cuts at 400nm, a bit better cutting at 410 with a Hutech IDAS, and quite good with some special filters cutting at 420 and 430nm.

However, after quite a bit of experimenting I have now abandoned all UV block filters. The increased exposure and difficulty of bringing out a good blue in final images seemed to outweigh any advantage I thought I was seeing. In fact the disadvantage of bloated blue seems to be very target dependent and often was not seen in the final image even though the blue subs didn't look too swift.

I'm just about to start processing my first image taken with the new Astronomik filters that claim to be designed exactly for refractors with this blue halo issue. The higher cut blue is clear from the attached photo. It looks to be about 420nm. Interestingly I have not noticed that I need longer subframe integration in blue as I did using 2" UV block filters in front of my Baader blue filter.

I'm most curious to see how these work in my system!

http://www.astronomik.com/en/photogr...r-filters.html

Peter
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Old 03-06-2016, 09:08 PM
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Quote:
Originally Posted by LewisM View Post
I found refocusing blue helped a LITTLE - but it does not remove it. ALL the Taks I have had do/did it, EXCEPT the FSQ-106ED (yes, the N's all do it). The FSQ-85 certainly does do it.

FSQ-85 image: http://www.astrobin.com/full/119145/B/

FSQ-106ED image: http://www.astrobin.com/199796/0/

FC-100 (OLD) image: http://www.astrobin.com/full/176758/C/

The last one is worth looking at all the revisions, showing degree of blue flare through this doublet - and the ways I tried mitigating it (depends on the stretch of each channel)
As with the issue that Peter was having with is Tec, it appears to be less to do with focus (I check focus every time I change filters anyway) and more to do with the very high UV/Blue QE and low read noise. The stars themselves are not technically bloating, they're picking up an extended shorter wavelength glow.

Quote:
Originally Posted by PRejto View Post
My results (TEC140, earlier model) are that focusing (better) doesn't make much difference. It depends on the lens figure. Visual blue can be razor sharp but all higher frequencies will be out of focus. Things are improved if the blue filter cuts closer to the best focus of the lens. For example my Baader filters cut around 380nm, but the TEC140 is figured to be sharp at around 430nm. I saw some slight improvement testing an Astrodon blue that cuts at 400nm, a bit better cutting at 410 with a Hutech IDAS, and quite good with some special filters cutting at 420 and 430nm.

However, after quite a bit of experimenting I have now abandoned all UV block filters. The increased exposure and difficulty of bringing out a good blue in final images seemed to outweigh any advantage I thought I was seeing. In fact the disadvantage of bloated blue seems to be very target dependent and often was not seen in the final image even though the blue subs didn't look too swift.

I'm just about to start processing my first image taken with the new Astronomik filters that claim to be designed exactly for refractors with this blue halo issue. The higher cut blue is clear from the attached photo. It looks to be about 420nm. Interestingly I have not noticed that I need longer subframe integration in blue as I did using 2" UV block filters in front of my Baader blue filter.

I'm most curious to see how these work in my system!

http://www.astronomik.com/en/photogr...r-filters.html

Peter
I'll definitely be interested in hearing how this works out for you Peter! How thick are their filters? The Astrodons that I have are 3mm, realistically I'd only be replacing the B if it is worth it.
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Old 04-06-2016, 01:29 PM
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The Astronomik is 1mm thick. If you replace just the blue you will lose parfocality (if there is such a word!). Also perhaps the way blue/green filters overlap or don't overlap may change the way cyan is reproduced. Who knows what you'd get mixing two brands of filters?

Peter
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Old 04-06-2016, 02:27 PM
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Atmos (Colin)
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Originally Posted by PRejto View Post
The Astronomik is 1mm thick. If you replace just the blue you will lose parfocality (if there is such a word!). Also perhaps the way blue/green filters overlap or don't overlap may change the way cyan is reproduced. Who knows what you'd get mixing two brands of filters?

Peter
My blue is not quite parfocal anyway, the filter is but it rarely is truly parfocal in most refractors. Just means I'd have to refocus which I do anyway.

The difference between the blue cutoff on the redder end appear to possibly be about 5nm (520v515) by eyeballing the graphs. The major difference in the blue appear to be the UV cutoff.

Both greens appear to have the same lower end but the Astrodons are narrower (end cut of 565? vs 580 of the Astronomik). The reds start at ~595 (Astronomik) against the 615 (Astrodon).

Changing the blue filter just cuts more of the UV. The biggest changes come from the green and red where Astrodon has a much larger gap (light pollution).
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