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Upper Brightness Limit 6" Reflector?

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cmy's picture
Upper Brightness Limit 6" Reflector?

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,



lmk's picture
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!

Herr_Alien's picture
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.



cmy's picture
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

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