Starnge circular artifacts in image

[Astroblagger]

Hello

got out last night for the first time with my atik 460ex and canon 200mm lens, can anyone explain these weird artifacts? they are circular so im thinking it must have something to do with the lens? how can i get rid?

[vlaiv]

No, it is, what I've just learned, called walking noise

It is hot pixel that shifts a bit from one frame to the next creating streak like pattern on image. Depending on the cause of the image shift between frames it can be linear or circular if polar alignment is off.

It is caused by field rotation - same thing happens to people imaging with alt-az mount. If you inspect your frames you will notice that each is a bit rotated compared to previous (maybe less than a degree).

Cure for it is to:

a ) use darks if you are not using them already

b ) use bad/hot pixel map or cosmetic pixel correction when calibrating frames

c ) use sigma clip stacking method instead of usual average method

[Astroblagger]

Hmm strange, my graph in phd looked ok?

[vlaiv]

7 minutes ago, Astroblagger said:

Hmm strange, my graph in phd looked ok?

Yes, it's not related to phd graph at all. Phd graph just shows that phd and mount tracked selected star to correct position. If you have improper polar alignment, guiding is capable of holding guide star spot on, but the rest of the frame will tend to rotate around that selected guide star - think how you can look at Polaris and it does not move but all stars around it follow a circle. Similar thing happens if you have too bad polar alignment and keep single star fixed in field - field will tend to rotate around that star (it will tend to rotate in general because of the differences between RA circles - one that earth does and one that mount does - which are different, but fixing one star in the image into single position will result in rotation of frames - just check each frame and see if there is slight rotation of field between them).

Same thing happens with alt-az mounts - there too system is able to keep center of the image between the frames, but due to very large PA error (it really depends on ones latitude if azimuth is level with ground) frames will rotate around that center.

[Rico]

If you're in the UK did you adjust for the change in daylight saving time? If not ... that may have thrown the alignment off.

[Astroblagger]

Think I did, first time Iv seen this appear in any of my images?