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Dyptorden

Filter advice

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Hi everyone,
There have been some discussions on the forum but I didn't find the exact info I am looking after.
I own a 8" Dob (just recently got it, I am a junior) and I am gathering information regarding the filters I might need.

Moon:I tried looking at it with 25mm and 17mm eyepieces (the only two I got at the moment). It was so bright that it blinded me.. took me few minutes for my eyes to go back to normal. So I found out that a nice filter that might help would be a double-polarized filter to decrease the brightness according to my wish.Also I found out that details could be increased if I use an IR filter and here is the first question...

1.Do I need an IR blocking filter or an IR passing filter? Everywhere I looked people only refer to IR filter but this doesn't help a newbee :)

Another thing is that most of the moon pictures on internet are black/white.So here comes my second question

2.Are there filters that make the moon look black/white, or is it made on the PC in order to increase details?

Regarding DSO's:

3.I keep hearing about H-Alpha, H-Beta and OIII.How does each of these filters influence my view? Do they help me get a colored view of the nebulae?

4.What other filters could help me get better images/details/contrast of DSO's? I've heard of UHC and some say there should be some filters that increase quality when observing from a light polluted environment.

5.Also what is the effect of a filter that blocks UV?

Thank  you very much, and sorry if my questions sound stupid to some of you...

Carl Sagan - The Demon-Haunted World: Science as a Candle in the Dark: "There are naive questions, tedious questions, ill-phrased questions, questions put after inadequate self-criticism. But every question is a cry to understand the world. There is no such thing as a dumb question"

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In an 8" scope a moon filter will likely be useful, the question is which moon filter. I would say try a 30% ND filter, although you may find a 50% better.

Of course the magnification and so image size matters, you collect a fixed amount of light so a bigger image means a dimmer image..

I suspect they are referring to an IR Blocking filter. You sort of half see/sense IR. An IR blocker simply makes a well defined cut off of wavelengths. An IR pass would in it's simplest form be nothing, not even plain glass, simply nothing.

Moon images are processed, people sharpen and add contrast to bring aspects out.

The other filters are narrow band filters, a Ha passes just a small section of the spectrum, as it is small you will likely not see a great deal visually through one, and anything you do see will be in a single colour. Ha would appear red, simply the Ha filter only allows through the red part of the spectrum that correcponds to Ha.

Ha = Red = 656nm

Hb = Green = 486nm

OIII = "Green" = 493 to 500nm

OIII comes from doubly ionised Oxygen and can give rise to 3 or 4 close wavelengths all around the 490-500nm "colour".

No filter adds colour, a Red filter removes the Blue and Green so leaving Red. So if the colour was not there to start with a filter will not make it coloured.

Do not have any but one discussion basically said that the best single filter to consider was a decent OIII filter.

A UV blocker would not do a great deal to visual and likely little to imaging.

An IR block may improve an image on a camera and I have read that one improves the visual aspects of looking at Mars. The eye does not cut off sharply, so IR is sort of registered a bit and blurs things. An IR cut off simply defins things a bit better concerning what gets to the eye. Also in refractors the IR focuses at a different plane. A UV blocker may make the image appear sort of redder or warmer.

Edited by ronin

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Personally I'd say save your money and don't bother with a moon filter but do try and get a UHC filter as there are some nebulae that will look quite a bit better with your scope with such a filter including famous names such as the Veil Nebula which can be practically invisible without a UHC or and O-III filter.

Using a bit more magnfication on the moon will make it easier on the eye and help you pick out the finer details.

I have a 12" scope but I find I don't need a moon filter at all.

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I find that filters aren't necessary viewing the Moon and I don't think that it's possible to damage your eyesight by lunar observing.

Planets are a bit different. Often high haze will do the job of reducing brightness for you. So will reducing the aperture by removing the small cap on the top cover. Ensure the aperture is placed opposite the focuser.

I'd advise a simple SW UHC filter to begin with. They're very useful on planetary nebulae and bright nebulae. I've also used one to great effect on planets when they've been bright.

It might help to tease details out of planets by colour filters, but to me it's very much a matter of personal preference.

Nick.

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cotterless45 my magnification was ~70x, and believe me, I felt the light was blinding me. At higher magnifications the brightness decreases?

John you suggest to buy a UHC or and O-III filter, but as ronin said, the O-III only lets some parts of the spectrum pass, won't that practically decrease the quality?

In fact this is what I don't understand... best pictures (I am interestend in observation atm) are made with all those 3 filters, H-Alpha, H-Beta and O-III. If H-Alpha only lets part of red pass, then no other color gets to H-Beta or O-III, so do the photographs use all 3?

And about the black/white photos of the moon? How comes that there aren't too many yellow pictures, just as I saw the moon?

Thank you again for your time and patience

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I ought to make it clear I'm I'm referring to visual observation - I don't image.

The UHC and O-III filters (and even more so the H-Beta filters) only allow a narrow bandwidth of light through - which co-incides with the light emitted by the target objects. This has the effect of enhancing the contrast of the object. There are many pieces on the web much better than I can produce on these filters. Here is a good one, for example:

http://www.prairieastronomyclub.org/resources/by-dave-knisely/filter-performance-comparisons-for-some-common-nebulae/

The reason I suggested a UHC (or an O-III but the UHC is slight more versatile) was that they really do make a big difference on some deep sky targets and I was looking to find a filter that would give you a good "win" for your investment :smiley:

They really do work !

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For the Moon, I find a variable polarizing-filter helps greatly. These can be tuned to let from 40% down to 1% of the light coming at you. And these can be useful for other bright objects, such as Venus:

http://www.firstlightoptics.com/moon-neutral-density-filters/variable-polarizing-moon-filter.html

Good catch, John, on the article you linked.

Clear & Not-So-Blinding Skies,

Dave

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Regarding the UHC I found these three filters to be highlighted :

TeleVue - NebuStar
Astronomik - UHC
Baader Planetarium - UHC-S
Which of them do you think would be the best choice?

And regarding moon observation, I've found : Baader Planetarium - Moon & Skyglow Neodymium which is neither a double-polarized nor an UHC, yet some say it makes wonders. What should I believe? Is it worth? Does it act as an UHC... maybe?

Again, thank you!

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The Astronomik UHC would be the best of those I feel. The Baader Moon & Skyglow Neodymium is a different type of filter, much more subtle and more often used when viewing the moon and planets.

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...  It was so bright that it blinded me.. took me few minutes for my eyes to go back to normal. 

This is normal! I find looking at the Moon "blinds" me at night, so I always leave the Moon until the end of a session. Don't panic - you won't fry your retina (as looking at the Sun would).

... At higher magnifications the brightness decreases?

Yup, at higher magnification the image gets dimmer. I think of this as the light being gathered by 'scope being more spread out to give me a larger image. As John suggests, I tend to increase the magnification if the brightness is bothering me (but it doesn't, much).

Regarding UHC and OIII filters and so on - note that these only help with some deep space objects. Things like emission nebulae and planetary nebulae tend to emit very specific frequencies of light. A filter like that can let through those frequencies, while rejecting other frequencies, which increases contrast. Also, those rejected frequencies may be light pollution, so it can help with seeing faint nebulae under conditions where you wouldn't see them otherwise.

So I would describe it as a filter like this decreases brightness, but improves contrast, and therefore the quality of the image.

However, some deep space objects do not emit specific frequencies of light. This includes stars, mainly - and therefore open clusters, globular clusters, and galaxies - and also reflection nebulae. Filters just can't really help with those.

I use an Astronomik OIII filter in my 10" reflector, and while it does dim the stars quite a lot, it really does help at picking out detail in nebulae. Even so, don't expect photographic levels of detail.

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Your standard Sunglasses will work for filtering the light levels from the Moon, and also helps  reduce any local light pollution directly into the eyes? ( and on the subject of protecting your eyes, some folk also use eye patches to prevent you from having to squint your eyes, straining your eye muscles, with one eye open, the other closed,  but that's not for filtering light levels - just comfort. I  don't like to observe with  my left eye alone, but have used the patch )

I have a Moon filter, but hardly ever use it. The best option I have found is to just use the 2" aperture cap, removed from the dust cap. Yes it reduces the glare and  increases the focal ratio, but its fine for my use. Also, thin high level clouds  can  filter the Moon.

If/ when I buy another filter, it will be a colour filter #80A Blue to help my eyes to discern the difference between the belts and the Great Red Spot on Jupiter.

Edited by Charic

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Reducing your aperture to view the moon is not a good idea. By 'stopping' down ones scope you will lose considerable resolution and your scopes ability to pick out fine surface detail.

Secondly, the moons brightness is finite. In other words it gets no brighter regardless of aperture. The brightness of extended objects (the moon, planets, nebulae and galaxies) is determined by exit pupil. 

This means that an 2" f/4 scope with a 20mm eyepiece in the focuser will deliver an image of the moon with identical surface brightness as a 20" f/4 scope using a 20mm eyepiece. The difference will be the image scale at the eyepiece.

In fact no telescope shows the moon with greater surface brightness than as what can be seen with the naked eye alone. What a telescope does is make it bigger. 

Try observing the moon in daylight next opportunity. You'll notice it doesn't hurt your eyes at all. This is because your eyes are adapted for brightness. Remember the moon gets no brighter at night.

If using your scope in daylight, BE CAREFUL, make sure it stays well away from the sun. As the sun will cause permanent blindness if it strays into the eyepiece.

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Steve......you say its not a good idea? however large telescopes gather too much light, especially when just looking at the Moon. Reducing the aperture of a larger telescope  will reduce the brightness and glare, reducing irradiation. You can also  remove diffraction spikes, improve contrast, and  reduce  image-blurring effects of atmospheric turbulence, sharpening the image!


A Moon filter will work, but maintains difraction spikes and wont reduce the effects of atmospherics?


An aperture mask reduces the resolution of a large scope to that of an unobstructed refractor of  the same aperture size. 

For the Moon alone, I still maintain that using the 2" aperture cap is an  ok short term fix.

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I'd use sunglasses or nothing for the moon, and save for a UHC/OIII, any of those will allow you to see the Veil, Pacman, etc.. but from a dark spot, of course. 

Flocking your telescope should help with bright objects, I think.

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I don't use a moon filter either! :D

These are not the answers you were looking for. ;)

If the glare really is bothering you, then you could always try a Variable Polarizing Moon filter and then you'd be able to adjust to a more comfortable brightness to suit your needs.

Early doors yet, as I haven't had the time (or clear nights), or variety of objects to give it a good running in yet (and I've nothing to compare it to!), but the ES 2" UHC is also available and has improved my views of the nebulae I've observed with it so far.

But, it's important to remember with a UHC filter, that it won't brighten or clarify the image at all, it simply removes or reduces other wavelengths of light that would otherwise make the object difficult to see.

Like yourself I have an 8" dob and we are very much into fuzzy blob territory here, all the UHC filter will do is make difficult to spot objects easier to identify, it won't make them brighter or sharper, you need dark transparent skies for that (and/or a bigger dob!). :)

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Steve......you say its not a good idea? however large telescopes gather too much light, especially when just looking at the Moon. 

let me to explain.

I have two scopes and have actually done this.

I set up my 10" f/4.5 next to my 20" f/4.5 and use an eyepiece that provides minimum power in both scopes I now get the brightest image of the moon that is possible in both scopes.

The minimum power in my 10" will be half that of the 20"

The surface brightness of the moon is identical in both scopes. The image scale is not. In the 20" the image of the moon is twice the size.

In order for my 20" to make the larger image of the moon at the identical surface brightness to the 10" it needs to collect four times as much light as the smaller scope. Luckily my 20" scope does gather four times as much light as my 10". This is where the extra light goes. Big scopes do not collect too much light they make bigger images. 

Trust me charic the moon is no more uncomfortable to look at through my 20" Dob as through your 8" one.

Once you start to see the real benefits of aperture you understand why it rules. You don't so much get a boost from the light gathered as the enormous shove of the image scale. Eg something like M42 fills an entire 82˚ AFOV eyepiece at minimum power in my 20" this means its huge and as bright as it can possibly be. This is why one can see incredible detail in objects as the big scope doesn't lose light as it pushes your nose right up inside them. It kinda grabs small objects and rams them straight  in your face.

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Excellent explanation.

I have distant aspirations for a larger 'scope, but I very much doubt it'll ever be a 20" Swampthing! :D

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As with all objects the moon can never be brighter than with naked eyes.

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If you take a photo of the moon you will find the camera thinks the moon is one very bright object and will want to use a fast shutter speed even at f/12.

And yet the moon is about half a million times dimmer than the sun.

Looking at the moon through the telescope with dark adapted eyes can be a a bit of a shock but they will quickly close their pupils down for you.

For a camera attached to it the extra light is something of an advantage because now you can play with the gain curve to adjust the highlight/shadow contrast.

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I use all kinds of filters, I am still experimenting but the only one I am currently looking to sell is my LP filter as it is covered by other fillters I have.

Ref narrow band filters, then a UHC gives the best general results although the UHC-S from baader seems to be a far more general filter with quite a wide bandpass compared to others. 

I like a yellow filter for the moon, have tried a lot of different combinations and yellow seems best to me

steve

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