Purkinje effect in visual observing

URSA Astronomical Association, Variable Star Section (Finland) (URSA)
Wed, 06/22/2016 - 19:13

Hello everyone,

I'm interested in the physiological background of making visual observations of variable stars and especially the well-known Purkinje -effect.

As far as I know, the practical consequence of Purkinje effect for visual variable star observer is that red stars appear to grow in brightness if stared for prolonged periods of time and especially when comparing the red variable with non-red comp stars. This is happening because the more red-sensitive cone cells get activated by bright light, and especially if the light is red, they became overstimulated, and that's why the star appears to become brighter. Am I right?

As far as I know, Purkinje can physiologically be explained as the process of dark adaptation in your eyes. During the process, red-sensitive cones get deactivated and more light-sensitive (but not vapable of seeing colors, although the rods are more blue sensitive) rods are activating. In this process, the sensitivity of retina shfts towards bluer light (because blue-sensitive rods are taking over) and eventually there is no perception of colour at all. The in-between state of these modes of seeing is called mesotopic, totally day-time mode is called photopic and night-time mode is called scotopic. If you are in scotopic mode (fully dark adapted) and you stare a red star, you get back into mesotopic mode because the bright, red light excites the cones in the retina and this overstimulation causes the red star grow in brightness. Have I understood this correctly?

Some people also give advise to not observe during twilight or Moonlight, or at least be very cautious about your observations during these conditions, because especially red stars appear too bright in these conditions. This must also have something to do with the Purkinje effect? Why is this happening, can someone explain?

I also know, that the Purkinje effect has something to do with the size of your instrument in relation with the brightness of the star. If you are observing a bright star with large-aperture instrument, there is a risk of over-estimating the brightness of the star because of Purkinje -effect. Is that correct?

So concluding from all above:

a) One should avoid staring red stars for prolonged times when making estimations - this is called 1) the "quick glance" -method. By doing so, your red-sensitive cones don't get so much excited by the red colour. There is also a method known as 2) "out of focus -method", which also helps to get rid of the redness of the star. There is also the third method for getting rid of the Purkinje effect, and this is known as 3) "Method of reduced aperture", where you apply an aperture mask over for your telescope when observing (bright) red stars to reduce the amount of light that can enter your telescope and by doing so, making the star appear fainter.

What is the best method? What works best for you? Is there any published study about these methods and comparing them? Would there be any ideas or suggestions for such study?

b) If one must observe (red stars) during Moonlight or twilight, is there any specific advice?

Do you people have any thoughts or ideas about this topic? I know that this is one of those topics that is frequently popping up, but I think it will be still beneficial for all visual observers and especially beginners.


Juha Ojanperä (OJMA),



American Association of Variable Star Observers (AAVSO)
There is no best method :)


a) I think that there is no best method. But depending on the context you might find yourself using one method or the other.

I usually go for the 2nd method - I constantly defocus my optics (incidentally that's why I don't like individual focusing binoculars). But this works if the star is bright enough so that defocusing the optics doesn't cause the star to vanish entirely.

When the star is dim enough that defocusing doesn't work, then I use the quick look method (the 1st one).

b) I avoid red stars and the Moon combo, and find something else to do: cepheids, eclipsing binaries.


American Association of Variable Star Observers (AAVSO)
The Porkinje Effect and Aperture

"I also know, that the Purkinje effect has something to do with the size of your instrument in relation with the brightness of the star. If you are observing a bright star with large-aperture instrument, there is a risk of over-estimating the brightness of the star because of Purkinje -effect. Is that correct?"

Yes, that is correct. Although a few observers will say that it is not true, after more than 50 years with AAVSO, corresponding with many long-time members and seeing the monthly observation reports of a host of others, I can say with certainty that the effect was very clearly present in the data of the vast majority of observers. 

Personal experimentation in this area consistently indicated a difference of about half a magnitude, sometimes more, between my 12.5" Newtonian and a pair of 10x50 binoculars in concurrent estimates of various red variables when they were in the comfortable range of both. This conclusion was essentially affirmed in the pages of the older observig manuals of the association for years, when they would advise that the most accurate visual estimates of red stars were derived when the variable was about 2 to 4 magnitudes above the instruments visual threshold. Likewise, the manuals suggested not observing red variables in either twilight, or very bright moonlight, both of which produce a degree of enhansement in the apparent brightness of red stars..

J.Bortle   (BRJ)

American Association of Variable Star Observers (AAVSO)
Defocus is the best

I would concur that defocus of stars is the easiest and most effective way to improve accuracy of estimates. Not only is it easier to judge the difference in brightness of two circles, vs. two "pointlike" Airy disks, but it allows you to adjust the brightness of the stars through an infinite range to the optimal appearance to match your scotopic vision, just by focusing. No need to make aperture stops, or buy new telescopes!