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Weird artefact with a dslr and 50mm lens
I could use some assistance with a artefact i get after stacking.
I use a dslr (nikon d5100 modified) with a uv/ir cut filter on the front of the lens.
The lens is a Nikkor 50mm manual focus.
When i stack my lights i get circle artefacts i can't get rid of. I use Flats, Bias, Darks.
Can Anyone help me or explain how this artefacts happen?
Oeh now that is interesting. Looks like some kind of interference pattern. Could you upload a single sub with a good stretch to bring out the background? And the flats didn't work well as you can still see quite a few dust spots.
I've uploaded 4 files, a single shot lps corrected, a single shot only transformd from nef to jpeg, a master flat and a stack of the whole session.
You can clearly see that the MF does't work good (dust spots visible), i did also create a stack without any flat at all (to rule out the "bad flats") but the aretfacts are still visible in the stack.
What can i do to remove the cirlce pattern? Or is my lens defect?
Hope you can help me.
Well it's difficult to say, but I clearly see the pattern in your single sub as well, so it's in the data itself. Indeed when the flats don't seem to work, it's better to leave them out as they can introduce problems of their own. I've never seen it to be honest, do you have a photo of your setup as you used it? And what filter is it exactly (make)?
Maybe if you look closely at the filter and lens, do you see smudges or a particular "line" on it, maybe a scratch pattern? Interference patterns can be caused by obstructions in the light path and the behaviour of light itself around them. It might even be caused by parts in the camera, before the sensor.
edit: By looking around a bit, they may very well be Newton rings. They can be caused by glass transitions and such, so maybe try a photo without the UV filter next time.
Thanx for you're reply, it is a simple setup, just a dslr (modified) witf a standard nikkor 50mm lens.
The uv-ir filter is a standard filter, it is the same as used by ZWO.
Looking closely to the lens i can see that the lens is almost 0,5 inch (1 cm) deep in the body. From the outher thread till the lens there is a spiral pattern in the lens body. If i would place the uv-ir filter in front of the lens (like i did) would it be possible that the spiral stucture we see would be a reflection of the spiral sturcture in the lens body reflecting in the uv filter?
There arn't any spiral or scratch visible, also when i take a picture during daytime there arn't any pattern visible.
You probably don't see it in daytime photography, because, as you can see from that single sub, the signal is very low and completely disappears with a much brighter picture. These things haunt you when doing astrophotography and having to deal with faint signals and noise. I'm suspecting the filter actually, it could be it just happens to cause Newton rings. Worth a try, you might even be able to test it also on cloudy nights, just taking subs of clouds basically. 🙂
I've got also a bit better filter i will try what happens with and without the filter.
ps. you also mention it is modified, who did the modification and what was done?
I did it by my self. Only the ir and Moire filters removed.
With my Newton and refractor I've never encounterd any Newtonian ringing pattern. So it must be the new lens.
Ok that's good to know. I hope it sheds some light ( 😉 ) on it!