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IC1396 with Nikon lens

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#1 GiorgioF

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Posted 13 December 2019 - 12:15 AM

Cheers.

Here 5 hours of Ha for this nebula.

While quite satisfied by the nebulosity is a pain for me about the stars. The corners are showing all the weak part of the setup.

Looking for advice if someone is using a similar setup.

ASI1600mm, baader Ha 7nm, light of 300" and 600" (different sky conditions in different nights).

Lens Nikon 80-200 f2,8 closed to f5,6.

Process all in Pixinsight.

Thanks for your help

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#2 v3ngence

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Posted 13 December 2019 - 01:18 AM

Did you try the lens correction in Photoshop? It has presets for specific lens models, they aren't quite perfect but it's usually a noticeable improvement!


Edited by v3ngence, 13 December 2019 - 01:18 AM.


#3 GiorgioF

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Posted 13 December 2019 - 03:40 AM

Did you try the lens correction in Photoshop? It has presets for specific lens models, they aren't quite perfect but it's usually a noticeable improvement!

Thanks so much for the suggestion for which I will give a try.
The main issue is how to avoid the repeating in next sessions.

#4 Starblind

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Posted 13 December 2019 - 03:41 PM

I don't use that lens, I have a 300mm F4, 70-200 F2.8 and a Kenko/Miltol 200mm F4.  I use the ZWO ASI183MM Pro with the ZWO Nikon adapter.  Primes tend to work better (if you can afford them).  I'm curious, why stop down that far?  F4 should be more than enough to get a decent flat field.  I never go beyond that.  This was a test shot using the Kenko, 30 exposures at 180s each through an Astronomik NB filter.  A lens correction in Lightroom or Photoshop would help, but that image seems a bit worse than normal in the corners, which leads me to believe the spacing isn't quite as accurate as it should be.  The image blow is barely cropped, LR corrected for the 200mm lens used.  Also, since the majority of the stars in the top right are elongated, it looks like youi have some image tilt in there as well.

 

Bob.

 

North American NebulA - 8-1-2019

Edited by Starblind, 13 December 2019 - 03:47 PM.


#5 GiorgioF

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Posted 14 December 2019 - 03:08 AM

I don't use that lens, I have a 300mm F4, 70-200 F2.8 and a Kenko/Miltol 200mm F4. I use the ZWO ASI183MM Pro with the ZWO Nikon adapter. Primes tend to work better (if you can afford them). I'm curious, why stop down that far? F4 should be more than enough to get a decent flat field. I never go beyond that. This was a test shot using the Kenko, 30 exposures at 180s each through an Astronomik NB filter. A lens correction in Lightroom or Photoshop would help, but that image seems a bit worse than normal in the corners, which leads me to believe the spacing isn't quite as accurate as it should be. The image blow is barely cropped, LR corrected for the 200mm lens used. Also, since the majority of the stars in the top right are elongated, it looks like youi have some image tilt in there as well.

Bob.

Thank you Bob for the sharing.
About spacing I'm sure it's correct. 6.5 of ASI camera backfocus + 20 filter wheel + 20 of zwo adapter give the 46,5mm for Nikon lenses.
The tilt you mentioned make me thinking.
Yes, the image is cropped due to a) the black borders of stacking due to different night work and b) to eliminate the worst of the stars.
Looking at one original uncropped frame, one of the corners is more evident in elongation thus, your assumption may be correct. I must check carefully for tilting. aside a try and error way, there is any "real time" way to accomplish this check? I may need to verify maybe in ccdinspector software.
The f5,6 was to use only the centre of the lenses as it seems the best based on review. But surely, looking at result, after careful tilt check, I will try also with what you suggest.
Thank you again.

#6 Starblind

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Posted 14 December 2019 - 09:32 AM

I had the same issue with image tilt using the exact same setup you have listed.Top right corner (or bottom left after rotating the image).  It is difficult to see in real time or during the day.  To fix mine, I took some subs, rotated 180 degrees and took some more, stacked each set and compared.  The tilt showed up in the same place as I suspected.  This was on the 70-200 AND the 300, but not the F4 Kenko which has a screw  connector for a T2 Nikon adapter.  The only difference was the Kenko is so light I didn't use the camera ring on the 183MM Pro, just the tripod foot on the lens.  Getting the camera ring correctly lined up vertically and horizontally is a pain.  If it isn't right it introduces stress on the image train and tilt shows up.  I still have yet to fix that issue entirely.  The other issue was the connection from the camera to the filter wheel in my case.  That was where the problem lies with the other two lenses.  To fix it (and since I rarely disassemble the setup) I used one of the spacers provided and put that between the camera and filter wheel (I tried a couple of them, but the thinnest worked in the end) because on close inspection the camera isn't perfectly flush with the filter wheel when screwed snugly together.  That fixed it for me, although it still isn't perfect, but pixel peeping only shows the problem in most cases.  In the majority of Nikon lenses, there is enough play either side of infinity to focus, even with the additional spacer(s) adding to the 46.5 mm specified distance.  That fixed the mono camera, but I still have a similar issue with the 183MC Pro and the Nikon adapter, it isn't perfectly lined up in the sensor place (perpendicular to the lens).  Manufacturing tolerances?  I may well have to get a tilt adjuster if I can't fix it.  This whole AP hobby is just one trial after another sometimes.  I prefer to use lenses because I do a lot of photography so they serve dual purpose.  I wish I had bought the bigger filter wheel and bigger filter, then it wouldn't be connected directly to the camera, but hindsight is always 20/20.

 

Bob.




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