Odd star patterns

Acquisition Deep Sky Help Needed Artifacts
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Dunk avatar
Dunk avatar
Hi all,

Total newbie here - hoping for a bit of help/advice:

I'm noticing the stars in all of my images have a small triangular "notch" cutout in the bottom of them (see crop from an image below - I realise my stars are also a little over-saturated) - can anyone tell me what may be causing this?

I am thinking maybe tracking issues - but they appear in all my images in the same place regardless of when I took them (so my polar alignment would be slightly different each night) which confuses me why they are always in exactly the same spot..?

Thx in advance for any help/suggestions.

D.

Engaging
dkamen avatar
dkamen avatar
Notice also the other two slightly smaller ones at exactly 120 degrees apart from each other.

And the multiple much smaller ones  all around the star disk.

Please see the discussion here:

https://www.astrobin.com/forum/c/astrophotography/equipment/pinched-optics-wo-zenithstar-73ii/?page=1#post-34033 

I would say don't worry too much about them as they are not really an issue with the DSOs, only with bright stars.
Lynn K avatar
Lynn K avatar
It not only the bottom but at 120 degrees  in three spots.  It has nothing to do with tracking or polar alignment.  It is an optical issue.  As Hans states it may be pinched optics.  However, pinched refractor issues result in a diamond or square shaped star.  You did not mention the type of scope used.  Some time spacers holding the glass optics apart can protrude into the optical path and cause this type of issue.  I had that issue years ago with a WO 80mm. 

Lynn K.
Dunk avatar
Dunk avatar
thx all.

It's a WO 71GT, and a quick search shows I may not be the only one with this issue.

Have loosened the lens-holding screws a (very) tiny amount per above advice and will see how it goes.

(I did also have the dew-shield retaining thumb-screw on quite tight which may have also been contributing to the problem - have loosened that a little too).

Thanks again,

D.
FiZzZ avatar
FiZzZ avatar
Looks like the retaining clips (120 deg) and the focuser tube too much in (casting a shadow on the primary mirror) …
Pinched optics usually give a triangle/diamond shape (you can check my collection specifically dedicated to pinched optics smile)) )

which kind of telescope do you use ?
Dunk avatar
Dunk avatar
Hi FiZzZ - its a William Optics 71GT - no mirrors here!
Geoff avatar
Geoff avatar
This has been an issue with many refractors over the years. Most likely diffraction from spacers or slight rough bits on the lens cell. It has probably always existed but only became noticeable with the advent of ccd imaging.
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