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Do we need to worry about star image spikes?

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Do we need to worry about star image spikes?

I am looking into using a reflector telescope to do aperture photometry but am concerned that the image might show star spikes, caused by the secondary mirror support, which might extend beyond the measurement aperture into the gap and sky aperture. Are these spikes common and if so how are they taken into account when determining aperture sizing and analysis?







Roger Pieri

Hello Gary,

sorry for that late answer ! I am also using a Newton and never had such problem. The photon count involved in the spikes is very small and do not significantly change the result if missed. Then the count of the spikes being involved is the same fraction / ratio of each star total count. By the way, if missed, this does not impact the count ratios between target and comparison, and indead the magnitude calculation of the target. We shall use the same aperture for all stars of a given image. In fact in most of our instruments the PSF somewhat changes depending the position in the image, then we should select a photometric aperture size large enough to get all the photons of each star being involved. But in any case the spikes are not considered and usually are not visible / detectable at all into the star image. 

If you see the spikes that means you are strongly saturating. Saturation of the sensor or the AD converter is probably the most common cause of error in photometry, it should be checked before each observing session, each involved star, and  more often if you observe in a situation the extinction is decreasing.

Clear Skies !


Eric Dose
Eric Dose's picture
Image spikes - probably low risk

Aperture-measuring algorithms of several popular programs effectively ignore outlier aperture pixels. Sigma-reject and median aggregating functions would both reduce the effect of any spikes into the aperture, and quite dramatically.

And this is before one compares comps and target apertures, which as mentioned above, should indeed cancel most of any lingering effect. I would be more concerned about interfering spikes from some extremely bright star nearby than about spikes from your target or comp stars themselves.

Good to know.

Thank you Roger and Eric for the replies. Good news the spikes are a sign of saturation which is checked for and that the software will ignore the outliers. It is a good day because I just learned more! Much appreciated.


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