Comet shaped stars / stars with tails

Hi,
This looks like a 'tilt' problem - something in the image train is not orthogonally perfect. I've had this issue with some manufacturers' "2-inch" accessories that aren't perfectly milled. If this is the case with anything in the train, a winding or two of masking tape can tighten things up.
 - - Steve

Over the span of 30 minutes it corrects itself to some degree with further leads me to believe that it isn't something mechanical.

Guiding performance? Bad polar alignment?

Mentioned in the original post, guiding is under 0.8"RMS and my PA is within 0.9 arc min according to PHD2

Try to solve the image to find out the angle of the tails.

As mentioned in my other reply:

My main theory is that since I live in the city and the scope is on a second story balcony and that the "tail" is completely vertical compared to the ground, it must have something top do with hot air from perhaps the apartment under mine (which would explain why it only every now and again)

What kind of telescope is this?  It is very difficult to diagnose anything without detailed information about how the data was taken.

John

Hmm, I see you are on a second story balcony. Maybe it could be vibrations?  Not sure how your building or balcony is built.  Even a solid concrete structure will transmit some vibrations which could affect your image?

The scope is an 8" ONTC from Teleskop-Service with their TSRAP focuser, TSGPU and 294MM. I use vibration pads on the balcony (which is an old concrete building).

Looks like my theory wasn't far off, thanks for everyone who pitched in!

I am currently building my own observatory so I assume this may not be as big of an issue in the future, will still look into getting a fan for the primary.

That aberration field is not due to image tilt.  Image tilt will introduce out of focus star images on either side of a line through the image.  The shape of the stars is also not consistent with coma.

This looks like it may be due to a thermal effect but I'm not convinced that it isn't due to a mechanically distorted component that may change with temperature.  Have you defocused a bright star in the center of the field to see what it looks like?  (Your image above is too far out of focus and way too over-exposed to see what's going on).  If this is due to a thermal plume, you'll see it right a way as a moving plum of air in front of the defocused star image.  You'll also be able to tell if the mirror is being distorted in some way.  That will look more stable–only changing with the overall seeing.

I recommend star-testing this scope to better understand what's happening.   Your "collimation image" showing the shadow-donut does not display a properly aligned Newtonian, but that depends a little on how the secondary is sized.  A properly aligned Newtonian should have a secondary that is slightly de-centered in the entrance pupil.  Yours looks well centered and that can indicate that you may have some coma; but again, I don't think that's what is causing the comet shaped stars.

One way to ensure proper alignment is to use SkyWave.  It can tell you pretty accurately what the wavefront looks like and it can provide information on secondary de-centering.   It can also help to dial out any alignment induced wavefront errors.

John

Same problem with a Sky-Watcher Esprit 120.  Tonight I've been trying to debug things and have concluded there isn't any issue with tilt or collimation.  

Aldebaran out of focus, 500x500 px crop in center of frame.  So it's a little off the center but best I can tell it looks good


Abreaction inspector on the same field, 2 seconds with L filter.  Star tails are wider than original post.


Tilt, as far as I can gauge, is manageable.  Last tilt corner measurements tonight after dialing it in (which was done before the above images were taken):


Scope had been outside for many hours by the time I took these.  I don't see similar results with longer focal length (reduced C8) and it doesn't matter when in the sky I point.  So different temps from various surrounding materials don't appear to be a problem.

Other equipment to note:  EQ6-R Pro and ASI2600MM.

andrea tasselli:
For a refractor this kind of aberration can be due to a tilted optical element. Triplet refractors and spaced doublet are aplanatic systems and shouldn't show any coma whatsoever.

Does that mean adjustment of the lens tilt?  Or is it something more subtle and less a DIY fix, such as lens elements not aligned to each other?  Would love to get this working back to 100%, thanks for any tips

@jewzaam Maybe focuser sag.  I've seen it with Esprit scopes before.