Planetary filter

[matt]

While I appreciate there is a wide variety of coloured filters which are very handy for looking at individual and specific planets, can anyone recommend a filter which helps reduce the brightness/glare of an object like Jupiter while significantly increasing contrast and thus detail???

I'm just looking for a filter which helps bring those details out without introducing any of its own colour ... sort of a neutral type filter???

Thanks :)

[Robert_T]

Hi Matt, I have a range of colour filters, but for that purpose I use a light blue filter. It reduces the glare without much apparent change in colour.

cheers,

[matt]

Thanks Rob.

Yeah... I've got coloured filters as well but was hoping there might be something "uncoloured" which does the job???

[Robert_T]

Matt, I suspect that anything that cuts down glare will intro some colour - eg just look at the range of sunglasses. The pale blue is a bit like a good pair of shades that cut glare with very little false colour... I often forget I have it there.

cheers,

[iceman]

Have you tried an ND filter?

[Robert_T]

Ah, good point Mike, do Neutral Density filters have any shading?

[iceman]

Not colour, just darker.. The sort of look like a grey when you look at the filter, like a pair of tinted (without colour) sunglasses.

[janoskiss]

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

[ving]

if you are looking to cut dow glare why not just stop down the aperture?
create an aperture mask and it will do the same job for you...

[janoskiss]

No it will not. If you reduce the aperture, you also reduce the angular resolution of the scope in proportion. A filter dims the view but preserves resolution.

[ving]

oic!

well it works for me :P

[matt]

Yeah, a ND filter sounds about right. So does the crosses polarisers. I like the idea of rotating one of those to vary the amount of light through depending on the subject's brightness.

I may also get myself a light blue filter just to compare, and to have handy in my filter kit. It may offer other qualities not found in the ND and polarising filters???

Definitely don't want to lose resolution due to stopping down the aperture :(

Thanks for all your help. Now, what's the sobbing sound? Oh yeah, that's the sound being made by my credit card :lol:

[Robert_T]

Matt if you do get a colouredfilter as well as the ND make sure it's a pale or light blue - I have this, a blue and a dark blue filter but the latter two introduce significant colouration.

cheers,

[Starkler]

I find that throwing away light merely reduces the eyes ability to see fine details.

I have a vari polariser and used it twice only before deciding that such filters dont help me at all.

[matt]

yeah Rob, a light blue filter, like i said I might buy in my last thread:shrug:

[Robert_T]

don't worry Matt, It's just my old eyes... I can barely see the belts of jupiter without a webcam let alone read other people's posts properly:rofl:

[matt]

:lol: you're right, mate. just having a lend.

Shame about your old eyes. At least you've got adequate optical compensation (overkill) what with the c9.25 and the Mewlon!

[ausastronomer]

The 1st person that has posted the "totally" correct answer IMO. Whatever filter you use will reduce the amount of visible detail. A 9" scope is not large and it doesn't need a glare reducing filter IMO.

If you're going to observe a planet with a Neutral Density filter you may as well save yourself the expense of the filter and just wear your sunglasses at the eyepiece.

One possible option that "has" been mentioned that works reasonably well is the light blue filter that Robert mentioned. The correct "wratten number" for this filter is an 80A light blue. This filter will reduce the glare a little without reducing too much detail. It will enhance some surface features and reduce others. It will however render unnatural colours to all the features.

Another filter that works "reasonably well" is the Sirius Optics "planetary contrast" filter. This filter cuts the glare a little and gives a similar view to the 80A light blue, but doesn't "dampen" the image quite as much as the 80A. Its not a bad option but the filter is a "surface coated" filter and quite delicate. I think Baader also have a planetary filter that probably works ok but I havent tried it.

I actually own lots of filters suitable for planetary observing including 12, 2" Hoya colour filters, a 2" Variable polariser, a Sirius Optics planetary contrast filter and at the end of the day I never use any of them on Jupiter or the moon in my 10" scope, nor in larger scopes for that matter.

CS-John B

[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.

When looking at Mars during last year's oppositions through Darren's 12" at Snake Valley, the ND and/or a polariser filter seemed to help.

[dhumpie]

I find I prefer the views sans filters (even in the 10") although I sometimes use the scratched 80A blue filter that 1 ponder's kindly "donated" to me :)

Darren

[Hammerman]

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]

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]

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:shrug:

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]

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]

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]

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

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]

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. :shrug:

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! :lol:

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

[ausastronomer]

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]

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]

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

[janoskiss]

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]

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]

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.


80A

[matt]

A #82A, thanks Steve. light blue.

And yes, the variable polariser is a single unit.:P

[janoskiss]

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]

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

[matt]

That's the one, John:thumbsup:

Cheers

[ausastronomer]

Gimme a few minutes and I will post a digicam pic of both filters (80A and 82A)so you can all see the difference.

CS-John B

[janoskiss]

I looked up the 80A and the 82A on Bintel's site, and the descriptions are identical with 30% transmission (most likely a mistake). But in the photos, the 82A looks a lot lighter, almost white.

One thing I find annoying with my GSO filters is the black but shiny housing. This has not been a problem when using them on individual eyepieces, but in the binoviewer they produce some nasty glare and reflections and are pretty much unusable on the Moon. :(

[matt]

Well, check other sites for your info, Steve.

Check MyAstroShop's filter guide for details on light transmission %

[ausastronomer]

Here is a picture of the 80A and the 82A filters. They are both described as light blue but as you can see the 82A (on the right) is a lot lighter than the 80A. As to them both having 30% light transmission ? Rubbish !!!!

Both of these filters are 2" (48mm threads) Hoya filters. These are made in Japan and at the top end in terms of quality.

[matt]

John

Where are the pix??? And I think you've made another naming error in your first paragraph, matey:lol:

it's OK. I know what you mean.

Ahhhh there they are (the pix)

they didn't load for me 1st time.

yeah, quite noticeable difference. Good comparison.

[ausastronomer]

All fixed, Don't mind me I am slightly retarded :)

CS-John B