Does Mild Astigmatism Affect Photometry?

American Association of Variable Star Observers (AAVSO)
Tue, 12/20/2022 - 20:25

Hello! One of the configurations I am a considering for a new Newtonian telescope is the edge support. The online calculator suggests that there may be mild astigmatism when the scope is pointed at the horizon such that a seasoned observer might notice it. (I suspect that the astigmatism would not be noticeable at 30 degrees and higher though the computer might be able to detect it by analyzing the image.)

    I thought I would ask - does mild to minimal astigmatism affect photometry? Best regards.


You'll likely do photometry…

You'll likely do photometry above 30 degrees, and probably never at the horizon. Also, in your analysis work in VPhot or with other S/W you define the measurement circle size, so keep it large enough to encompass the entire image spot.




As Peter mentions, you can usually increase your measuring aperture size if the star profile has issues, such as astigmatism or coma, and a photometrist rarely goes below 20 degrees altitude (airmass=3), because the atmosphere gets really weird near the horizon, second order extinction becomes really important, and you get differential refraction between the top and bottom of your field of view.  Don't go low unless your science really needs it!

Usually astigmatism only affects out of focus images.  In focus, you don't see it.  Astigmatism is actually handy when trying to get best focus. :)  It is interesting to watch - there is a preferred angle when you are out of focus, the image becomes round at focus, and then the preferred angle shifts 180 degrees when you continue past focus.

Remember for a Newtonian that these exhibit coma, which quickly becomes apparent when you move away from field center.  There is also field curvature, which makes good focus difficult except at field center.  I strongly recommend getting a coma corrector/field flattener for Newtonian configurations.  Astrotech used to have a really cheap coma corrector called the ATCC, which might still be available somewhere.