Issues with Image circular pattern and strange stars (noob) - Forums

Topic starter

May 6, 2020

Hi All,

I have uploaded an image of the Orion Nebula. It has a circular pattern in the image. I don't believe it should be there so I think I may have stuffed up the flats.

The other issue I see is there are 4-5 stars in there that are bright and look bow-tie shaped. The most prominent is in the bottom left hand corner. I'm not sure if this is natural, whether its a lens or focus issue, or decelerating approaching visitors.

https://astrob.in/f0ycqq/0/

If someone can point me in the right direction it'll save me a lot of floundering around.

Cheers

Steve

pieter

May 6, 2020

Hi steve apart from the problems it is a nice pic.
Imo the circular pattern points to bad flats. How did you do them.
The bowtie pattern is something i also have. As for i understand it it might have to do something with stress somewhere in the lens. Was it wel climatised to its surroundings?

Marc Agostini

May 6, 2020

Hi Steve,

I would guess the concentric bowtie pattern is an typical artifact introduced by the optics; You can also find it on this imagewhich was taken with the same lens. I have seen similar artifacts when using fast DSLR-Lenses. The only way to get rid of it is stopping down, which causes longer exposure times and will also most likely introduce a new artifact where you can see one spike per aperture blade arround bright stars, so its a choice of what you like better. I find the bowties quite charming, personally. You will find these even on top-notch fast refractors, like the Vixen VSD 100: https://www.astrobin.com/12oxvp/0/?nc=all

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Steve Steele

Topic starter

May 6, 2020

pieter:
The bowtie pattern is something i also have. As for i understand it it might have to do something with stress somewhere in the lens. Was it wel climatised to its surroundings?

Thanks Pieter, It wasn't that cold outside (maybe 8 degrees C cooler than inside) but I did pretty much take the less out of the house and start shooting within a couple of minutes.

I'll try acclimatizing it next time and see if it makes a difference.

Thanks for the advice.

Steve Steele

Topic starter

May 6, 2020

Marc Agostini:
find the bowties quite charming, personally

I liked them, however I don't know enough to know if it's supposed to look like that. They do exist in even the individual frames so I do know they are not a stacking artifact. After the frame was plate solved I did think they seemed to be mostly starts in very close proximity e.g β Ori/Rigel

I don't have a tracker yet so I'm only using very short exposures.

Thanks for helping out.

Gabriel R. Santos (grsotnas)

May 6, 2020

Hello Steve! Second Pieter and Marc. That's a very nice image, despite the small problems.

For the circular pattern, I'd think that flat field correction did not work properly. Flats must have a very uniform illumination, and must be calibrated with flat darks or bias frames - depending on the software the way you go about calibration is different, but the principle is the same.

For the bright stars near the edges - that is a characteristic of fast lenses and DSLR cameras, caused by diffraction of the mirror box (in the case of a DSLR), or perhaps some other physical barriers inside the camera body. If you reduce the focal-ratio (larger f-number), the light cone will enter at smaller angles, and the issue should go away. Obviously, you lose signal that way, but it might be an option if star shapes are the primary consideration (I believe that around f/2.8 the issues should be gone).

Best regards
Gabriel

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Rich Sky

May 6, 2020

Hard to help you without understanding what equipment you are using.
bow tie stars is due to some obstruction in your optical train.

Are you using off-axis guider?
You also have some issues with guiding?

Clear skies

Steve Steele

Topic starter

May 6, 2020

For the bright stars near the edges - that is a characteristic of fast lenses and DSLR cameras, caused by diffraction of the mirror box (in the case of a DSLR), or perhaps some other physical barriers inside the camera body.

Thanks Gabriel.
I'm going to take some more flats and try again with the same shots.

The camera is a Sony A7II Mirrorless Camera, so no physical barriers inside the camera body. I'll clean the front of the lens and sensor and give it another go on the next clear night.

I must admit I'm not too keen on stopping down, I don't have a tracker yet, too many images when I'm limited to 2 seconds exposure.

Genuinely appreciate the assistance.

Well Written Respectful

Steve Steele

Topic starter

May 6, 2020

Rich Sky:
Are you using off-axis guider?You also have some issues with guiding?

Hi Rich Sky,

That's all way too fancy for me at the minute, just a Sony A7III and a 135mm Sony Lens mounted on a stationary tripod.

Cheers

Steve

Steve Steele

Topic starter

May 6, 2020

Hi All,

I took some new flats ( this time at the same ISO as the images ) and tried again seems to have worked much better.

https://astrob.in/f0ycqq/B/

I'm still not happy with the image, next clear night I'll try again, just needs more time I think.

Thanks again everyone for input.

Steve

chefjedidiah

May 7, 2020

What f-stop did you use. We all want the fastest possible, but normal daytime photography lenses are usually optimized to a certain focal ratio. Quite often its 2 or three below the fastest. The bow ties could be the diffraction pattern that is caused by the diaphragm in the aperture of the lens. If your native ratio is 2.4 try 3 or one slower. It will move the vanes in the aperture slightly and could cure it. Like others have said, the circular pattern is bad flats.

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Die Launische Diva

May 7, 2020

Steve Steele:
Hi All,I took some new flats ( this time at the same ISO as the images ) and tried again seems to have worked much better.

https://astrob.in/f0ycqq/B/

I'm still not happy with the image, next clear night I'll try again, just needs more time I think.

Thanks again everyone for input.

Steve

Why you are not happy? The lens seems good enough. Is your image untracked? If yes, please invest on a tracker, it will make you a lot happier

Steve Steele

Topic starter

May 7, 2020

Die Launische Diva:
Why you are not happy? The lens seems good enough. Is your image untracked? If yes, please invest on a tracker, it will make you a lot happier

I’m happy with the lens, i just need more practice.

I don’t have a tracker yet, (this image was done with 100 x 2 second shots) it’s probably my next purchase.

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