slightly pointy tops to stars

[sonnymoon]

So this may appear a little picky, which I'm not usually, but my stars seem to have a slightly pointed top to them. Any ideas what could be causing this? 

 setup is a skywatcher ED80 ds pro with flattener, zwo 1600 mm and Avalon uno mount. This crop is of stars around M81, quite high in the sky and is a final image with around 3 hours of LRGB frames. It was taken in France under dark skies with what seemed to be good seeing and a good PHD graph (can't remember the numbers exactly) but probably around 0.5 RMS. 4 minute exposures, -15 degrees c. I've noticed this slight elongation on more than one occasion and it would be good to get to the bottom of it. Thanks.

 

 

 

Edited June 18, 2019 by sonnymoon
better picture to upload

[newbie alert]

Are all the stars like it or is it a certain part of the frame..

[ollypenrice]

Could be slightly pinched optics.

Olly

[jock1958]

Have you carried out a star test and checked the airy disks inside and outside of focus? If your collimation is good your airy disks should be perfectly circular.

[emyliano2000]

I have the same problem on my AT106LE. Was wondering if it could be pinched optics. It's mostly on the left, more towards the bottom left. I'll try to do a star test tonight.

Emil 

[jock1958]

On 21/06/2019 at 09:04, emyliano2000 said:

I have the same problem on my AT106LE. Was wondering if it could be pinched optics. It's mostly on the left, more towards the bottom left. I'll try to do a star test tonight.

Emil 

Star test is the quickest and easiest way to check collimation on a frac.

My Tal 100rs was out of collimation (very obvious in a star test) I managed to improve things over a long period of time using a Cheshire eyepiece, Laser collimator and lots of faffing around but it’s never been 💯 

[emyliano2000]

21 minutes ago, jock1958 said:

Star test is the quickest and easiest way to check collimation on a frac.

My Tal 100rs was out of collimation (very obvious in a star test) I managed to improve things over a long period of time using a Cheshire eyepiece, Laser collimator and lots of faffing around but it’s never been 💯 

I've done a star test last night. Defocused vega in the center of the frame, measured the circles and everything looks great. Moved the defocused star to where the problem is (left and mostly bottom left) and the defocused star is still round but the circles are not concentric.

Today I decided to take the moonlite off it and check what's happening and discovered the flange wasn't screwed in properly. Tonight, I'll do another star test and I have a feeling that everything will be ok. 🤞🤞🤞

Emil

[jock1958]

4 hours ago, emyliano2000 said:

I've done a star test last night. Defocused vega in the center of the frame, measured the circles and everything looks great. Moved the defocused star to where the problem is (left and mostly bottom left) and the defocused star is still round but the circles are not concentric.

Today I decided to take the moonlite off it and check what's happening and discovered the flange wasn't screwed in properly. Tonight, I'll do another star test and I have a feeling that everything will be ok. 🤞🤞🤞

Emil

Not familiar with your type of telescope but I assume the moonlite is the focuser and anything that’s not properly assembled could be causing your problem.

Hope you get it sorted 👍

[emyliano2000]

12 hours ago, jock1958 said:

Not familiar with your type of telescope but I assume the moonlite is the focuser and anything that’s not properly assembled could be causing your problem.

Hope you get it sorted 👍

Yes, the moonlite is the focuser on my AT106LE and the last night test went great. My problem is finally solved.

Emil

[sonnymoon]

Thanks for your replies. I didn't have notification on and so thought there weren't any......

I don't know much about pinched optics, but am I right in thinking that its worse in cold weather? The temperature was close to 0 deg in the image above. When I took my setup to France a month ago it was very hot and didnt fall much below 20 deg on many nights, but the starshapes were better without the distortion noted above.

So it was the cold?