Planetary filter

[Hammerman]

Originally Posted by janoskiss

Yes, ND and/or crossed polarisers for variable brightness is what you need.

I couldn't agree more. I use my Variable Polarizing filter much more on planets than I do my color filters. The variable ones are nicer than the fixed brightness of a netural density filter because you can adjust it for brightness with different scope/ep combinations, and adjust for your own eye, or personal taste.

[matt]

Thanks Hammerman

The variable polariser is shaping as a very useful option.

[ausastronomer (John Bambury)]

Originally Posted by janoskiss

I certainly don't need filters in my 8" on any of the planets. But when viewing (or showing others) the full Moon at moderate magnifications, the ND filter comes in handy.

Steve,

This is all very true. When showing the moon to inexperienced observers in an 8" or larger scope the brightness does pose a problem to them. The variable polariser does a better job of this than the neutral density filter IMO, although with newcomers losing the extra detail in the ND filter doesn't matter anyway because their eyes aren't accustomed to observing at this low detail level and they also dont really know what they are looking at.

CS-John B

[ausastronomer (John Bambury)]

Originally Posted by matt

Thanks Hammerman

The variable polariser is shaping as a very useful option.

Matt,

It's a far better option than the ND filter IMO if you must have one. However I still see absolutely zero need to reduce the brightness of Jupiter in a 9" scope. If you said you wanted a filter to "enhance" certain surface features I would agree, but I have no issues with the brightness of Jupiter and I have observed it unfiltered in scopes up to 25" and trust me in a 25" scope it's a lot brighter than in a 9"

CS-John B

[matt]

Appreciate that John. Thanks.

I guess we have to keep in mind that everyone's eyes are different and have their own degree of sensitivity, and interaction with the light coming through the eyepiece

I guess in a way I am looking to "enhance" what I'm seeing, by reducing what is for me overhwelming brightness which is robbing me of seeing more detail?

It won't hurt to try a few filters and see how things go

BY the way, this was viewing through my 8" f5 newt.

[janoskiss (Steve H)]

The near full Moon can be uncomfortable for me especially in wide angle eyepieces. I have to keep my gaze fixed very close to the centre of the FOV, otherwise I get blackout with my contracted pupil. When the lunar surface fills the FOV I want to be able to let my gaze wander around the entire field and look at all there is to see as I please. I definitely need a filter like the ND, or high magnification, to be able to do that comfortably (esp. when binoviewing).

Maybe a different design eyepiece would help. Here I'm referring to my experiences with the 19mm Panoptic in a 2x barlow in the 8" Dob, so the Moon just fills the 68 degree FOV.

[ThunderChild (Chris)]

Originally Posted by ausastronomer

When showing the moon to inexperienced observers in an 8" or larger scope the brightness does pose a problem to them ....
CS-John B

Hi John,

Could you ellaborate on this for me? I definitely class myself as a nebie - and I only own a 6" to boot. But looking at a bright moon without a filter leaves my eyes "moon burnt" - where I have a whopping large bright circle burnt into my retinas for a few minutes. It's uncomfortably bright.

I can understand and anticipate that as I gain experience, I will learn to pick out better detail in what I'm seeing. But are you saying that the bright after image from looking at bright lights no longer happens to those with experience?

[janoskiss (Steve H)]

Originally Posted by ThunderChild

I can understand and anticipate that as I gain experience, I will learn to pick out better detail in what I'm seeing. But are you saying that the bright after image from looking at bright lights no longer happens to those with experience?

Yes, by that time you would have burnt out most of your retina so the bright light won't bother you any more.
Nah, only kidding!

If you up the power the image will grow dimmer (as the square of the magnification). At the sort of powers you would use to pick out fine detail, the brightness will not be excessive.

[ThunderChild (Chris)]

Originally Posted by janoskiss

If you up the power the image will grow dimmer (as the square of the magnification). At the sort of powers you would use to pick out fine detail, the brightness will not be excessive.

Hi Steve - yeah I got that. I was just a little confused - that statement just sounded as if the physiology of your eye changes as you gain experience so that it isn't as effected by bright light anymore.

I imagine that if you do stare at an unfiltered full moon too many times your eyesight will be too shot to notice the big white patch in your eyes!

Hang on - just passed 100 posts! That snuck up on me!

[ausastronomer (John Bambury)]

Hi Thunder,

You need to appreciate that the moon's surface is very rough, about the colour of bitumen and is transmitting reflected light only. The light intensity of the Sun is about 30,000 times that of the Full Moon. The moon appears so bright, because everything else is so dark. You will not damage your eyes looking at the moon. The moon when observed in its brighter, waxing gibbous phases, in a scope with an aperture over 4", appears very bright. The larger the aperture of the scope the brighter it appears. When you look away from the scope after observing the moon for a while you get the "ring of fire", this is a transition thing only. Three things happen when you look away, your eye's pupil is rapidly dilating, the chemicals which assist with dark adaptation are flowing rapidly (photopigment regeneration) and your eye is changing the visual receptors that it uses for vision from Cones to Rods. It's exactly the opposite effect to opening the curtain in a dark room with bright sunlight outside or turning a bright light on when you have been sleeping. Its unpleasant for a few seconds but no damage results. Countless thousands of people, including myself, have observed the moon unfiltered for decades without issue.

When observing the moon alone I always observe it unfiltered, it takes a little getting used to but you will get there. Throw in a few F's and B's as the light hits and this helps you get through those 1st few seconds
When I have inexperienced observers with me, like my 9yr old son, or my wife, or at school viewing sessions etc, I use a variable polarising filter on the moon for the comfort of the other observers.

There are a couple of things you can do to make it easier for you to get used to observing the moon unfiltered (and bright planets for that matter).

1. Turn some outside lights on. You only need to have dark skies to observe dim targets, not bright targets. Don't turn lights on that are likely to cause troublesome reflections in the telescope, but enough to increase the exterior brightness levels so that you "don't" get dark adapted.

2. Increase the magnification. As you increase the magnification, the exit pupil reduces, which dims the target.

Ultimately visual astronomy is all about extracting as much detail as possible from the chosen target, to do this you need to maximise the number of photons that reach the eye. Don't waste them, they are precious

CS-John B

[ausastronomer (John Bambury)]

Originally Posted by ThunderChild

Hi John,

I can understand and anticipate that as I gain experience, I will learn to pick out better detail in what I'm seeing. But are you saying that the bright after image from looking at bright lights no longer happens to those with experience?

You always get it but you get a little more used to it, its not pleasant for a minute or two but harmless.

CS-John B

[ThunderChild (Chris)]

Thanks John. I guess I'll get there eventually!

[janoskiss (Steve H)]

Yes, when you think about it the Moon is not that bright. It is not much brighter than the sky during the day, and it certainly does no harm to look at the daytime sky with a telescope. (And if you are observing the Moon during the day with a scope, a single optimally aligned polariser will boost contrast considerably by cutting down on scattered sunlight more than on direct moonlight. Just rotate EP in focuser to align the polarising angle for max contrast.)

[matt]

Got a couple of polarisers on the way. Gonna try the crossed polarisers thing and see how that goes.

Am pretty sure I'll also grab a light blue 82a

[Hammerman]

Originally Posted by ausastronomer

Matt,

It's a far better option than the ND filter IMO if you must have one. However I still see absolutely zero need to reduce the brightness of Jupiter in a 9" scope. If you said you wanted a filter to "enhance" certain surface features I would agree, but I have no issues with the brightness of Jupiter and I have observed it unfiltered in scopes up to 25" and trust me in a 25" scope it's a lot brighter than in a 9"

CS-John B

Normally you are correct that you wouldn't want to decrease the brightness in a smaller scope. It's just at times under certain seeing conditions if you reduce the brightness a touch (which is why I suggest the VP instead of the ND) you can bring out a little bit more detail. I've noticed it on both Mars and Jupiter, but noticed no difference on Saturn (yet). Keep in mind that all of our eyes are a bit different too. What works for some of us might not work for all of us.

[janoskiss (Steve H)]

Originally Posted by matt

Got a couple of polarisers on the way. Gonna try the crossed polarisers thing and see how that goes.

I have never seen one in the flesh but I believe the variable transmission crossed polarisers are sold as a single unit. You can do the same thing with two polarisers yourself of course but it will be more fiddly.

Am pretty sure I'll also grab a light blue 82a

80A

[matt]

A #82A, thanks Steve. light blue.

And yes, the variable polariser is a single unit.

[janoskiss (Steve H)]

I've got the wrong one then. Mine's says 80A.

[matt]

yep. you're wrong.

80A is blue but i'm getting the light blue (higher light transmission)

[ausastronomer (John Bambury)]

Originally Posted by matt

Got a couple of polarisers on the way. Gonna try the crossed polarisers thing and see how that goes.

Am pretty sure I'll also grab a light blue 82a

Matt,

The 82A is the one you need and you have correctly quoted above. I actually gave you the wrong number the other day when I mentioned the 80A. I own the 80A and the 82A and I mixed myself up. The 80A although its called "light blue" is actually quite dark compared to the 82A. The 82A is very pale blue and does a good job on Jupiter. It also helps with the Martian polar caps.

CS-John B