How do filters enhance viewing?

[Roosto]

I find viewing moon without a filter to be OK. How would a filter change what I see? Jupiter, Saturn, etc., how will the filters help and change viewing?

[sgazer]

filters typically pass certain bands of colours. On planets, certain colours of filters will enhance certain markings and bandings on the planets. A moon filter, usually just reduced the brightness of the moon as it's too bright to view in most scopes over about 60-80mm. Ford deep sky visual, filters typically reduce light pollution. An LP filter will pass the light which comes from the nebulas (like Ha, OIII etc) but will block those which come from street lights for example. Thus darkening the background and enhancing the deep sky object. For imaging, the filters can be quite narrow band. A UHC filter will pass quite tight bands of 'nebula colour' and filter out unwanted light, and going to the extreme, you can get a filter to pass a single narrow band, such as Ha (hydrogen alpha) which is what nebula composed of common hydrogen gas emit, which will make the background very dark, whilst bringing out the deep red colour.

[swamp thing]

The moon is no brighter than we see it with our naked eye in ANY scope. If looking at the moon doesn't hurt your eyes unaided then it won't through a scope regardless of aperture.

I find it very easy to stare at the moon for long periods without any problems. Others may differ, but its not brighter through your scope, just bigger.

[Roosto]

filters typically pass certain bands of colours. On planets, certain colours of filters will enhance certain markings and bandings on the planets. A moon filter, usually just reduced the brightness of the moon as it's too bright to view in most scopes over about 60-80mm. Ford deep sky visual, filters typically reduce light pollution. An LP filter will pass the light which comes from the nebulas (like Ha, OIII etc) but will block those which come from street lights for example. Thus darkening the background and enhancing the deep sky object. For imaging, the filters can be quite narrow band. A UHC filter will pass quite tight bands of 'nebula colour' and filter out unwanted light, and going to the extreme, you can get a filter to pass a single narrow band, such as Ha (hydrogen alpha) which is what nebula composed of common hydrogen gas emit, which will make the background very dark, whilst bringing out the deep red colour.

Then it is necessary to have a set of filters to get the most out of your telescope and viewing?

[gkec]

The moon is no brighter than we see it with our naked eye in ANY scope. If looking at the moon doesn't hurt your eyes unaided then it won't through a scope regardless of aperture.

I find it very easy to stare at the moon for long periods without any problems. Others may differ, but its not brighter through your scope, just bigger.

This is obviously not the case. If it were we would not be able to see fainter stars than with the naked eye and inadvertently looking at the Sun with the naked eye would damage your eyes (Apparently some members of a Sun worshiping religion who look at the Sun for long periods of time do damage their eyes).

I suspect that the iris contracts enough to let you look at the moon comfortably. Certainly you can't see much when you step away from the eyepiece.

[acey]

Swamp Thing is right, the surface brightness of the Moon (or any other extended source) seen through a telescope is no greater than when seen by the naked eye - in fact it's always less, because of light loss in the telescope. Faint stars become visible through a telescope because of the darkening of the sky background: the stars themselves are effectively point sources and are therefore not enlarged and dimmed by the telescope, as long as the magnification is not too great. Instead the apparent magnitude of the star is effectively increased.

So it is true that the Moon is no brighter through a telescope than with naked eye - but if you're viewing at night with dark-adapted eyes then that can be bright enough to be uncomfortable. It is also true that if you look at the sun for more than a second or two when it is high in the sky, with naked eye or telescope, then you will probably damage your eye. A Moon filter (or solar filter) cuts glare, therefore making details more perceptible.

Certain types of DSO (emission nebulae such as the Ring Nebula or Crab Nebula) emit a large proportion of their light at certain specific wavelengths. A nebula filter transmits these wavelengths and blocks others. The effect is that the sky seems darkened and the object is more visible. Starlight is dimmed by a nebula filter so they are not generally useful on anything except emission nebulae.

Light pollution filters claim to work by blocking the wavelengths emitted by streetlights. In the past, streetlights were mostly low-pressure sodium (the old orange lights) and this could be effectively blocked. Most streetlights nowadays emit a broader range of wavelength (hence look whiter). A light-pollution filter will have far less effect against that sort of light.

All filters work by blocking light (i.e. making things dimmer) but they enhance contrast by (hopefully) blocking unwanted light and transmitting wanted light. A filter that can tell the difference between white light from a galaxy and white light from man-made sources is impossible, so the best measure against light pollution is to get away from it and go to a dark site, if possible. Even at a dark site, I still find a nebula filter (such as UHC) to be useful on the faintest emission nebulae, though on bright nebulae (e.g. any Messier) the unfiltered view is often just as good, or better.

For planets, colour filters may enhance certain details, though on bright targets like Jupiter you might find that a neutral filter (like a Moon filter) works just as well, by cutting glare and scattered light. The biggest killers of planetary detail are air turbulence, over-magnification and optical limitations in the telescope. A filter won't solve any of that.

[gkec]

OK -- just reading this

http://www.rocketmime.com/astronomy/Telescope/SurfaceBrightness.html

I'll get back to you once I've understood it.

[swamp thing]

1362473515[/url'>' post='1854395']

This is obviously not the case.

Okay! If you don't believe me try researching it

.

1362480438[/url'>' post='1854509']

OK -- just reading this

http://www.rocketmim...Brightness.html

I'll get back to you once I've understood it.

Aah! I see you are

Its a common misconception that aperture brings with it brighter images of extended objects.

When what it really brings is bigger image scale (for a given f/ratio), and better resolution, this is how we see more through big scopes.

[John]

The key to seeing fine planetary detail is to prepare your eye properly beforehand. There is a trick involving a piece of while card and a while light torch. You stare at the illuminated card for a short while and this makes your pupil adjust to a state where your eye will perform better on picking out the subtle contrast / details on Jupiter, Saturn etc. It sounds odd but it does work.

The only filters I use are i) a full aperture while light filter for solar viewing and ii) an O-III filter for increasing the contrast of nebulae.

[gkec]

Okay! If you don't believe me try researching it

.

Aah! I see you are

Its a common misconception that aperture brings with it brighter images of extended objects.

When what it really brings is bigger image scale (for a given f/ratio), and better resolution, this is how we see more through big scopes.

OK done the research.

My first thought was, where have all the photons gone?

So from the link

http://www.rocketmime.com/astronomy/Telescope/MinimumMagnification.html

the maximum surface brightness is achieved at the minimum magnification which for a 200mm objective and 7mm exit is about 29x. (200/7)

The surface brightness is constant for a fixed area. So if you magnify an object by 29x then the area of the object is 29² times larger compared to the naked eye object . As the surface brightness is constant, assuming perfect optics and the whole object remains in the FOV then there is 29² times the amount of light. In the case of the sun this is obviously really dangerous as you get all the light funneled into your retina.

An analogy is if one pixel is illuminated on your laptop screen then you would notice it in the dark. If the whole screen were lit then you can read a book with it in the dark. Again the surface brightness is the same but the total brightness is much greater.

As you go beyond the minimum magnification then the surface brightness diminishes but the total brightness stays the same.

[Stu]

If it is not a hanging offence , there is some discussion along similar lines in this thread on CN

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/2599196/page/73/view/collapsed/sb/5/o/all/fpart/1

Stu

[sgazer]

which ever way telescopes work, it can seem too bright to look at the moon through a scope, because it's filling more of your field of view than when not using a scope. A bit like staring at a light bulb a few meters away is ok, but right up to your eye is uncomfortable, even though it's no brighter.

[sgazer]

Then it is necessary to have a set of filters to get the most out of your telescope and viewing?

Depends on the target and your skies. Dark skies might not need filters for DSOs. Imaging generally improves with filters as the long exposures are making it much more sensitive, so you want the back ground as dark as possible. Planets are a bit trial and error and subjective visually, but there are some guides on the net.

[John]

which ever way telescopes work, it can seem too bright to look at the moon through a scope, because it's filling more of your field of view than when not using a scope. A bit like staring at a light bulb a few meters away is ok, but right up to your eye is uncomfortable, even though it's no brighter.

Maybe it varies person to person ?

I've been viewing the moon at 265x and 318x recently with my 12" dobsonian and the brigthness of the view seemed entirely comfortable to me. As I was searching for features that were very challenging to observe I would not want to put another piece of glass, of indeterminate optical quality, between my eye and the image.

Of course I'd not switch straight from the above to searching for a very faint DSO but with the moon in the sky faint DSO's would not be on my priority list for that session anyway.

[michael.h.f.wilkinson]

The main problem with viewing the moon without a filter (which I did for years) is that one eye loses its dark adaptation, while the other doesn't. You then feel half blinded when you withdraw your eye from the scope.

On planets I find filters unhelpful, but for certain DSOs UHC or O-III are a must

[John]

The main problem with viewing the moon without a filter (which I did for years) is that one eye loses its dark adaptation, while the other doesn't. You then feel half blinded when you withdraw your eye from the scope.....

But thats only for a few seconds, or maybe my eyes are odd ? !

[Naemeth]

I must admit, although I don't do much observing of the Moon, it is awfully bright. I always increase the magnification (never usually use my 26mm, it's too bright at 25x, much better at 160x or so) to view. Must be my young eyes... mind you, Jupiter is pretty bright at 25x .

[AndyWB]

I found looking at the full moon the other day that when I looked away from the scope I had a large purple after image, like I'd been staring at a bright bulb for too long. To me, that's too bright; it worried me. In the end, I did what Jonathan said - upped the magnification. Perhaps that's why you've not had any problems, John?

[michael.h.f.wilkinson]

But thats only for a few seconds, or maybe my eyes are odd ? !

It is longer than I like. There is still a difference after a few tens of seconds. I must say cheap moon filters (those nasty green plastic affairs) are a waste, worse than useless, as they degrade the image. I now have a good one from Orion Optics. I like to use 119x on the moon (especially when others also want to look) so I can still get the whole disk in the FOV of the Nagler 17mm. A moon filter is handy under those conditions

[John]

I found looking at the full moon the other day that when I looked away from the scope I had a large purple after image, like I'd been staring at a bright bulb for too long. To me, that's too bright; it worried me. In the end, I did what Jonathan said - upped the magnification. Perhaps that's why you've not had any problems, John?

Yes, that could be it. I tend to use high magnification to view the moon because the features I enjoy seeking out are generally small. I do occasionally take a look at the whole disk and it's fascinating to see the various textures and colours across the surface of the highland and maria areas.

My general observing approach is to try not to use any thing in the optical train that does not need to be there. That goes for deep sky filtration too - I only tend to use my O-III filter on objects where it makes a really significant difference to discerning the target object.

Perhaps I'll try and use my refractors "straight through" as well, as the Japanese tend to prefer - that would eliminate the need for a diagonal !. My back would suffer though, I suspect

I'm sure there are many though who prefer to use filters much more than I do and thats great

I'm just a little concerned when newcomers to the hobby get the feeling that filters are a mandatory part of using the scope and order a set as one their first accessory purchases.

I do agree with Michael though that anything that goes in the optical train should be of decent optical quality.

[Naemeth]

Yes, that could be it. I tend to use high magnification to view the moon because the features I enjoy seeking out are generally small. I do occasionally take a look at the whole disk and it's fascinating to see the various textures and colours across the surface of the highland and maria areas.

My general observing approach is to try not to use any thing in the optical train that does not need to be there. That goes for deep sky filtration too - I only tend to use my O-III filter on objects where it makes a really significant difference to discerning the target object.

Perhaps I'll try and use my refractors "straight through" as well, as the Japanese tend to prefer - that would eliminate the need for a diagonal !. My back would suffer though, I suspect

I'm sure there are many though who prefer to use filters much more than I do and thats great

I'm just a little concerned when newcomers to the hobby get the feeling that filters are a mandatory part of using the scope and order a set as one their first accessory purchases.

I do agree with Michael though that anything that goes in the optical train should be of decent optical quality.

The light loss is minimal . At 1% if you have 99% reflectivity .

[chiltonstar]

Yes, that could be it. I tend to use high magnification to view the moon because the features I enjoy seeking out are generally small. I do occasionally take a look at the whole disk and it's fascinating to see the various textures and colours across the surface of the highland and maria areas.

My general observing approach is to try not to use any thing in the optical train that does not need to be there. That goes for deep sky filtration too - I only tend to use my O-III filter on objects where it makes a really significant difference to discerning the target object.

Perhaps I'll try and use my refractors "straight through" as well, as the Japanese tend to prefer - that would eliminate the need for a diagonal !. My back would suffer though, I suspect

I'm sure there are many though who prefer to use filters much more than I do and thats great

I'm just a little concerned when newcomers to the hobby get the feeling that filters are a mandatory part of using the scope and order a set as one their first accessory purchases.

I do agree with Michael though that anything that goes in the optical train should be of decent optical quality.

I've had visual migraines a few times from looking at the moon unfiltered at low mag, maybe from looking with one eye only as much as the intrinsic brightness.

When viewing Mars and Saturn with my long focus frac, I've removed the diagonal on occasions and used older style eyepieces (less optical surfaces) - it does give better views, but doesn't work on objects near the zenith though (the neck won't take it)!

All the cheaper filters I've used have been uncoated (eg SW LP 2" filter), and all have given reflections/ghost images with a bright object in the FoV.

Chris

[John]

Oh, well I must be unusual then

[skir]

From my personal point of view I have found that even at higher mags I come away from the ep half blinded by the glare of the moon and sort of stumble around for a few seconds before my eyesight recovers which is a bit disconcerting. Therefore I will probably be buying some sort of filter. My choices would seem to be baader neodymium, sw variable polarising moon filter , or a sw light pollution filter . I'm undecided as yet which one to go for.

Sent from my Nexus 7 using Tapatalk 2

[m sutton]

Surely, if I point my 150mm reflector at the moon, then I am getting 150mm - 1% worth of light, squeezed together into 8mm of aging human pupil? Same principal as burning stuff with a magnifying glass?

Whatever - I can't do it without a filter - it really hurts!