Izar - Epsilon Bootis

[chiltonstar]

Has to be one of the most beautiful double stars in the heavens (IMO)!

Three successive AVIs with 180 Mak, AS1224. 1000 frames each. I always get "propellor" diffraction rings with stacking, whereas by eye they are often complete when the seeing is good. Reasons?

Izar, Sep = 2.9 arcsec, PA = 343, Mags 2.6, 4.8

Chris

[vlaiv]

Huh, that looks like slightly pinched optics?

Camera has linear response, while human eye does not, so you can see whole rings because eye can see both bright and dark parts and form complete image. Camera can also see bright and dark parts, but you have to do non linear stretch to see whole rings.

Next time you go observing, and see the whole rings, try to see if they have equal brightness all the way around or there are three brighter spots on rings.

Also, look at defocused star image and see if you can spot pinched optics signs - maybe primary support need adjustment.

[chiltonstar]

1 hour ago, vlaiv said:

Huh, that looks like slightly pinched optics?

Camera has linear response, while human eye does not, so you can see whole rings because eye can see both bright and dark parts and form complete image. Camera can also see bright and dark parts, but you have to do non linear stretch to see whole rings.

Next time you go observing, and see the whole rings, try to see if they have equal brightness all the way around or there are three brighter spots on rings.

Also, look at defocused star image and see if you can spot pinched optics signs - maybe primary support need adjustment.

Maybe..... But it only appears with average to good seeing. No sign of it with excellent seeing, and it matches quite well the images of seeing effects in CDSA.

Chris

[vlaiv]

2 hours ago, chiltonstar said:

Maybe..... But it only appears with average to good seeing. No sign of it with excellent seeing, and it matches quite well the images of seeing effects in CDSA.

Chris

Individual seeing frames may look like that - broken first ring, but stack of such images should give you more or less uniform round ring, since each frame should have it more or less pronounced and randomly distributed.

You mentioned that you always get them when stacking. It might be due to stacking procedure / algorithm. Examine previous stacks and look at artifact orientation in regards to RA/DEC. If it is always the same, there is high likelihood that it indeed is pinched optics (Mak 180 has fixed vixen dovetail and if using EQ mount, mirror should be always oriented the same in relation to RA/DEC).

As for stacking method, I would use following, but not sure if any software implements that (except MetaGuide, but that is not for stacking): choose/reject frames based on peak / max value - if star profile is too corrupt by seeing it will be smeared out with no distinct central peak. Alignment should be on either max value, or centroid of top X percent pixels (like centroid of top 20% pixels by value - other can be considered as flaring out).

[chiltonstar]

Many thanks for this Vlav - very useful indeed! I think the effect is there on the original AVI, (ie not introduced during stacking).

Another suggestion made to me is that it arises from cooling effects. As the primary has a three point mounting (in the case of my Mak three pairs of bolts), heat "bleeds" through these to the outer back plate as the scope cools creating three cooler patches on the mirror, and thus the distortion of the diffraction ring. This explanation, if feasible, would certainly explain why all sorts of obstructed scopes get the effect and why, at least in my case, it appears when the seeing is average or slightly better, but not when it's excellent.

I am probably being over-OCD about it, and it is true as Vlav says above, a bit of levels stretching solves the problem and gives a more accurate impression, for example:-

Chris

[Arcturus D]

This double star is highly weather dependent. Even slight sky transparency issues reduce the gold and blue stars to 2 white stars.