Upper Brightness Limit 6" Reflector?

American Association of Variable Star Observers (AAVSO)
Wed, 02/18/2015 - 15:57

Hi Everyone,

Although I have done quite a bit of binocular and CCD (V) variable star observation, I haven't yet done any telescopic estimating. I have a 6" reflector on a heavy alt-az mount and I also intend to mount a small widefield 66mm refractor along side it to so I can cover both telescopic and binocular variables in one set up. I was wondering at what point a star would be considered "too bright" for the 6" scope? Would I be right in thinking it would be around mag. 8? I observe under Bortle 6/7 skies and I will be mostly observing legacy stars.

Many thanks,



American Association of Variable Star Observers (AAVSO)
Bigger scope is better

Hi Mick, I believe that there is no real upper limit of brightness, though in practice you will find yourself limited by finding suitable bright comp stars within or near the narrower field of view of a bigger scope. So, its more of an issue of the focal length and what low power, wide field eyepices you have available.

Also, you can always defocus the stars if they are very bright, and that solves the issue entirely. I have estimated 7th magnitude stars using my 20" reflector and a very low power eyepiece!

Actually for the light polluted skies you have there, I would recommend trading in the 6 incher for at least a 10 inch Dob style scope. Those are quite inexpensive on the used market, and will provide you a full magnitude gain in visibility through the background "noise".

Mike LMK


Small is beautiful


. . . in this case anyway.  I am a visual observer with a 180 mm f/6 Mak-Newt and a nice pair of 7x50 binoculars.  In my light polluted skies, I struggle to reliably see anything dimmer than about 7.8 with the binoculars, so in the regime 7.5 - 8.0 I almost always turn to my telescope.  With a 24 mm Panoptic, that gives me a sufficient field of view to see my target and comp stars without problems.

Certainly, you could go much brighter than 7.5 with my telescope - which equates to brighter than 7 with your 6 inch scope.  But at some point you have to ask yourself whether it is worth the effort to slew the beast over to that 6th magnitude variable when all you have to do is raise the binoculars, and voilla!

American Association of Variable Star Observers (AAVSO)
Mag 8 - maybe 8.5 - might be it.

Bear in mind that this is quite subjective :).

From a similar sky (NELM 5), I find that estimating stars brighter than mag 8 is difficult in a 4 inch scope, because of having to move the scope by more than what I feel comfortable with. And mind you, the focal length is small (500mm) and with a basic 32mm plossl I get about 3 degrees TFOV.

I found that mag 8 is also a bit difficult to estimate using 10x50 binoculars, so that's my usual threshold.

Now, this doesn't take into accound sky transparency, which does vary by as much as half of magnitude. This will influence your choice between the 66mm and the 6 inch.

I usually opt for the instrument that provides the largest TFOV while still keeping the variable at about one magnitude above what I can still detect.

Just as a side note, Ira Bowen came up with a pretty accurate relation to determine the maximum magnitude one can detect in an instrument:

m = NELM + 2.5*log10(G) + 2.5*log(Dcm)

where NELM - naked eye limiting magnitude, G - the magnification, Dcm - objective diameter in centimeters (6 inch being about 15 centimeters).

Hope it makes sense.



American Association of Variable Star Observers (AAVSO)
Thanks for the info!

Many thanks for your replies. As everyone's circumstances are different, I guess there are no hard and fast rules - especially if it is possible to successfully observe mag 7 stars with a 20" scope! Thanks again.Clear skies,Mick

American Association of Variable Star Observers (AAVSO)
Upper brightness limit

Great question!  I often switch among various instruments for observing, depending on sky conditions, target brightness, etc. and indeed have found it difficult to make reliable estimates when I use too much aperture on a given target.  One of the AAVSO documents suggests that a good rule of thumb is targets within 2-4 magnitudes of the limiting reach of an instrument and, while that was doubtless intended as a lower limit, I have also found it useful as a very rough upper limit - sort of a "sweet spot."  When I go "too bright," I find that it takes me longer to make a confident estimate, and - worse - I often find that my estimate is different than when using a smaller instrument on the same target immediately before or after.  That should come as no suprise, since it is an inevitable consequence of the nonlinear (and non-loglinear) response of the human eye to different brightnesses.  Some ranges of our visual response are more nearly linear than others, according to the literature and to personal experience.  While I can't confirm a specific upper limit for your scope, my own experience suggests that targets much brighter than about six magnitudes above an instrument's faintest reach would be difficult to estimate reliably, though consistent effort may extend that range somewhat.  I personally would not trust anyone's visual estimate of a 4th-magnitude star made with a 16-inch telescope, for example. 

Svensk Amator Astronomisk Forening, variabelsektionen (Sweden) (SAAF)

My experience with my instruments is also that there is an upper limit beyond which estimates are not reliable. I do not have a complete exhaustive study, just my empirical observations with my instruments, I consider the following magnitudes to be the brighter limits for reliable estimates:

102 mm (4") refractor: ~ 6 mag

200 mm (8") reflector: ~ 8 mag

It would be interesting to read what other observers think about the matter and what your experiences are.

Hernán [DHEB]

American Association of Variable Star Observers (AAVSO)
Upper Limit

This is interesting since with my 6" F/8 newtonian, if the  variable is around  7 to 7.5 mag  I normally switch over to my 8X50 finder and a "b" chart to make the estimate. 

Steve TST

American Association of Variable Star Observers (AAVSO)
I think what we should aim

I think what we should aim for is consistency, though with some vars that is not always possible. I remember a few years ago following R Vir down to 10.6 with a 7*50 finder, at which point I stopped following it because using a 14" mirror on an 11m red star would not be the same as with the finder (where the light levels would not be high enough to enable the Purkinje effect to rear its head). CH Cyg is another difficult one that needs a finder to take in the distant brighter stars on such occasions when it is having a max.

American Association of Variable Star Observers (AAVSO)
"Upper" and "Lower"

As I read the comments I wonder if the terms "upper" and "lower" are used consistently in amateur astronomy to mean "brighter" and "fainter?" I see a compaison to students asking me about "bigger f-stops." Do they mean bigger numbers such as f22 or smaller numbers such as f2 being bigger? As magnitude numbers get bigger do they reach an upper limit or a lower limit, rather than a brighter limit and a fainter limit?



American Association of Variable Star Observers (AAVSO)
Aperture masks

FWIW, I use a pair of cardboard aperture masks to reduce my binos' 70 mm. (2.75 ") aperture to a mere 35 mm. (1.38 ") in order to cover magnitude 5-7.

I've noticed a few times that estimations with the full 70 mm. can be quite different from their 35 mm. counterpart (e. g. UU Aurigae).