Teardrop shaped stars - APM 107/700

[PadrePeace]

Equipment: APM 107/700 triplet with a 3” Feathertouch focuser, Atik 460EX mono, ZWO EFW, and for these images an Astrodon Lum 1.25” filter was used. Both HEQ5Pro and AZEQ6 GT mounts with steady guiding have both been tried under the scope with no change to the star shape. It occurs whether or not I’m guiding, and also with flattener/reducers in or out of the optical path. 

What’s telling is that the shape appears well formed on 10sec to 600sec exposures so I have discarded trailing. 

I’ve uploaded two star test shots and some raw Lum images taken at both low and high elevations to discount droop. 

Can anyone shed light on what will be causing this star shape across my images and how I might fix it? 

I've also added jpegs if these are more suitable than the original .fit files also loaded here.

 

L_Lum_Flame_Bin1x1_120s__-10C_2020-03-02_21-44-34.fit

L_Lum_Star Test focus_Bin1x1_1s__-10C_2020-02-20_21-49-36.fit L_Lum_Star Test focus_Bin1x1_1s__-10C_2020-02-20_21-50-51.fit L_Lum_M109_Bin1x1_180s__-10C_2020-03-02_22-29-37.fit

Edited March 9, 2020 by PadrePeace
jpeg images added

[Adam J]

2 hours ago, PadrePeace said:

Equipment: APM 107/700 triplet with a 3” Feathertouch focuser, Atik 460EX mono, ZWO EFW, and for these images an Astrodon Lum 1.25” filter was used. Both HEQ5Pro and AZEQ6 GT mounts with steady guiding have both been tried under the scope with no change to the star shape. It occurs whether or not I’m guiding, and also with flattener/reducers in or out of the optical path.

 

Glad you posted this mate. Ill kick off with my interpretation.

1) I can see the star is pear shaped pointing downwards in the image.

2) You dont mention it but its very important to note that from memory rotating the focuser resulted in the distortion rotating relative to the camera and remaining fixed relative to the object being imaged. For me that rules out sensor tilt in the camera as a cause. I also know you have confirmed that the focuser is aligned using a laser colimator.

3) Its only visible when viewing the image at full size, so not the worst I have ever seen (some horrific star issues get posed on here) but still something is not right for a scope in that price range. I dont think that the flame is the best example of this as poor seeing has disguised the issue a little.  The better example is M109 where you can see something is wrong even with the smallest stars.

4) I dont see anything obvious in the star test, although it might be worth re-doing with the star less defocused and with shorter exposures. I have seen some people use a guide camera and then stack the results as you would a planetary image to resolve more detail in the rings. Seeing can also effect the result.

You are aiming for something more like this (my Esprit 100):

Using a shorter exposure and in better seeing shows many more rings in more detail making it easier to judge collimation, 10 rings are visible or more. Although this is still probably more de-focused than is ideal. Something like a 0.05s exposure from my ASI1600mm pro, not sure what the min exposure is for your ATIK460EX, hence maybe use a guide camera.

On the whole, from the shape I think it might be a case of pinched optics. But as you know I dont have sufficient experience of refactors  yet to be sure or to rule out something like coma or a colimation issue. So hopefully someone else will be able to comment.

Adam

 

Edited March 10, 2020 by Adam J