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title: Star Shape Diagnoses Procedure

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# Star Shape Diagnoses Procedure

The following is a streamlined procedure distilled from [Mike Lockwood's instructions](http://www.loptics.com/articles/starshape/starshape.html). Please refer to the original article if you can't fix a problem with just the steps in this summary, don't feel that you understand any of the steps well enough, or just want to learn about the whole issue in more detail.

Assumption: We have a properly designed mirror cell (as mentioned in the Fifth condition of the original article).

1. Satisfy the initial conditions: 
    1. Put the star in the center of the field of view and keep it there. 
    2. Check if your own eyes have astigmatism by tilting your head - distortion from your eyes will rotate with your head.  
    3. Use high enough magnification so your eye's pupil is not blocking the light cone.
    4. Let mirror cool down enough.
2. Perform a star test by defocusing inside and outside of focus, and identify problems by observing the shape of the Airy disc: 
    * If the defocused star has a comet-like tail that always points away from the center and remains on the same side of the star both inside and outside of focus, this indicates collimation error and coma.
        * Correction: Adjust the primary mirror collimation to move the star towards the coma tail until the star appears symmetrical when defocused. 
    * If you observe any other aberrations or distortions, including the ones looking like astigmatism or triangular star shape described below, they could still be caused by temperature differences in the mirror. To be sure:
        * Allow the mirror to cool down for a longer period (30-60 minutes or more) to reach thermal equilibrium.
        * If the issue persists, consider improving the placement of cooling fans to promote even cooling (refer to the original article for more information). 
    * If the defocused star looks oval and the oval rotates 90° when going from inside to outside focus, this indicates astigmatism.
        * Correction: Rotate the primary mirror. 
            * If the astigmatism rotates with it, the mirror itself is the issue. 
            * If not, it may be caused by the focuser, corrector lens,  mirror cell or the secondary mirror. Investigate and adjust accordingly (see original article and Remark 1. below about the secondary). 
    * If the defocused star has a triangular shape that appears to rotate or flip when going from inside to outside focus, this suggests an optic is being pinched.
        * Correction: Inspect the primary mirror clips to ensure they are not too tight. The mirror should be a little loose. If the triangular shape rotates with the mirror, it was likely pinched during manufacture. If not, it's probably a mounting issue. Adjust the mirror cell and clips as needed.
3. Recheck the star test after making any corrections (again, after allowing sufficient cooling time) to ensure the issues have been resolved.
4. If any problems persist, consider further testing or consultation with the manufacturer.

For more information and details about any of the above points/steps, and more related information, refer to the [original article](http://www.loptics.com/articles/starshape/starshape.html).

## Remarks

#### 1. AaronH [noted on Cloudynights](https://www.cloudynights.com/topic/915630-feedback-request-for-a-streamlined-version-of-mike-lockwood‘s-instructions-to-diagnose-star-shape-issues/?p=13354264): 
> If the astigmatism doesn't rotate with the primary, it can also be caused by astigmatism in the secondary.
> 
> This can be caused by an overly-tight mounting of the secondary, which is quite common with mass-produced Newts that bond the back of the secondary to the mirror stalk using adhesive across the entire mounting surface. In some cases, the mirror stalk's plate is as wide as the mirror itself, meaning it is bonded to the stalk across its entire rear surface. This can result in astigmatism (that varies with changes in temperature).