Polarising Filter

[StephenRH (Stephen)]

I am getting a decent sized Dob made by Peter Read, SDM Telescopes, VIC, Australia. 28" f/3.3. I will be using it some of the time to show school students and parents the Moon and Planets. This telescope will have a pretty strong light grasp. Do I need to get a Polarising Filter to cut down the light when showing them the Moon?

I would really appreciate hearing back for anyone who has biggish Dob.

I will post photos when the scope arrives.

Thanks

Stephen
Pukemaru Observatory
341 Snelgar Road, RD 2
Kaitaia, New Zealand 0482
+173° 20' 07.67" -35° 05' 39.52"

[Atmos (Colin)]

I found I needed it with my 16" dob so I would say definitely for a much larger one.

[raymo]

Many people use one with a 6 or 8" scope, so I would say that its a
no brainer.
raymo

[janoskiss (Steve H)]

For public viewing nights a brightness reducing filter is useful. Crossed polarisers are good but an ND filter is easier to work with. ND's are also more readily available in 2" format.

It's worth noting that the surface brightness of the Moon's image and the amount of Moonlight hitting the retina per unit area depends only on the exit pupil and not on the aperture of the scope. Of course, for a given exit pupil, a bigger scope will yield a larger image. A 28" is big enough to just about fill one's field of view with the face of the Moon at the full brightness of 7mm exit pupil.

Some folks get alarmed about eye damage when looking at the Moon through a large telescope. But this can never happen. Even at max, ~7mm, exit pupil, Moonlight through any scope is not as bright as clear sky in daylight. It's just the contrast between the Moon and the darkness of night that gets you. For serious Moon observing you can pick up more detail without a filter. Downside of course is that you won't be able to observe much else for several minutes afterwards.

[StephenRH (Stephen)]

Thanks to those who have replied. Much appreciated. I will get one. I see Amazon is selling these in the 2" size. The telescope will have filter slider and I will be able dot screw it in.
Interesting comment about not being able to look at anything else for a while. It's bad enough just looking at Jupiter with my 12", let alone a Full Moon.
Thanks again
Stephen

[raymo]

I personally have never used a filter on the moon with any size scope.
I just look through the eyepiece from about 150mm away, and move closer over a period of around 10secs, by which time my eye is quite comfortable.
raymo

[Renato1 (Renato)]

A polarising filter alone won't cut the light down enough when viewing the moon to make it comfortable. Two polarising filters rotated around to get maximum light cut-out will be better - but when I've tried it, it made the image a tad mushier than that with a single equivalent Neutral Density filter.

That said, I've never looked at the moon in a 28" telescope. You may well need two dark ND filters.
Regards,
Renato

[StephenRH (Stephen)]

Thanks, Renato. I have been looking at the Orion 5562 2-Inch Variable Polarizing Filter which does exactly what you say. I do have a ND Filter so I might just try that before I get the polarising filter.
One thing I know - I won't try looking at 16th magnitude stars just after I have looked at the Moon!

Kind regards
Stephen

Originally Posted by Renato1

Two polarising filters rotated around to get maximum light cut-out will be better - but when I've tried it, it made the image a tad mushier than that with a single equivalent Neutral Density filter.
Renato