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Filters anyone?

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#1 jdjetson

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Posted 10 June 2017 - 02:13 PM

Hi guys,

 

I'm hoping to better understand the purpose of filters.  Can anyone please explain the difference between filters and the best way to use them?  I thought I could just get one but then realized how many different kinds there were. So once again, find myself asking the question here :)

 

My interest would be to get the most out of deep sky targets, assuming I can find them!  

 

Also, would you recommend one brand over another?

 

fyi I have a 4" refractor and live in a "red" zone

 

Thanks for reading my post.  Clear and dark skies everyone!  wavey.gif



#2 The Ardent

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Posted 10 June 2017 - 02:21 PM

I suggest you get these first

http://www.barnesand...ence-dickinson/

http://www.philharrington.net/sw2.htm

#3 havasman

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Posted 10 June 2017 - 02:30 PM

JB,

 

That's a broad topic. I'll talk briefly about nebular filters and then give you some resources.

 

Energy from nebulae reach us at predictable wavelengths. Nebular filters are designed to remove energy NOT at those wavelengths and increase the visibility of nebulae against the newly "cleaned" field. Different nebular filter types (UHC and UHC-types, O-III and H-Beta) have different passbands. Selecting the correct filter whose passband best matches the emission spectrum of a nebula effectively improves the visibility of the nebula.

 

There are two seminal articles on the subject under the resources tab at this site - http://www.prairieastronomyclub.org/#

 

There are MANY forum posts on the subject you can search.

 

Filters work great. Prime season for their use is when the Milky Way brings its wealth of nebulae around into view, like now and for the next 6 months.


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#4 GaryCurran

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Posted 10 June 2017 - 03:03 PM

Filters basically break down into two categories.  Visual filters and Imaging filters.

 

Visual filters are usually some color, very similar to camera filters.  They are used to enhance the color of objects, like planets.  You see the object, and you'll use a filter to add contrast.  Since most of the observing you'll be doing that a filter will have an effect on is going to be planetary or lunar, here's a great article on the basics of VISUAL filters.

 

http://agenaastro.co...ary-filter.html

 

Astroimaging filters are far different.  Since a camera can have a much longer exposure time than the human eye, it can 'perceive' dimmer objects, and also can perceive wavelengths beyond our own capability, such as ultraviolet and infrared.

 

The intent of ALL filters is to block some wavelength of light.  So, if you have a 'red' filter, you simply have a piece of glass that is coated in such a way that it blocks all wavelengths except the red part of the spectrum.  A blue filter blocks all the wavelengths except the blue.  Then, you'll have polarizing filters, which are two filters with special coatings on them that block out a percentage of the light.  A Variable Polarizing filter allows you to change how much light gets blocked out.  Variable polarizers are great for the moon, since you can cut back on the amount of light to your eye.

 

For imaging, you have a basic 'LRGB', or Luminence, Red, Green and Blue filter set.  The RGB set is very basic, in that those three colors can duplicate our visual spectrum.  Red, Green and Blue make up every color we can see, and when combined, produces white.  Yeah, I know that sounds a bit strange, but take my word for it, okay?  :D  It's not the filters themselves, but the combination of those three spectra of light that do it.  This is where the 'Luminence' comes in, it provides for the 'white', in the most basic of concepts.  The LRGB color set is used with monochrome imagers, or devices that can not recognize color.  Color sensors, like in a DSLR, has something called a Bayer Mask over it.  It's a matrix two pixels by two pixels, where two of the pixels are green, and then one each for red and blue.  This mask or matrix over the sensor is how the camera 'sees' color and recreates it.

 

Then, there are specialized imaging filters, the three most popular are Ha, or Hydrogen Alpha, OIII, or Oxygen III, and SII, or Sulfur II.  Each of these filters allows only a very narrow part of the visible light spectrum to pass.  Here's Astronomik's page on their 'Narrowband' filters.

 

http://www.astronomi...nienfilter.html

 

You would not use these filters for visual use.  First of all, the targets they are designed for are usually faint fuzzies, and secondly, they would remove all of the visible light except for the band of spectra that they are designed for.

By the way, these narrow band filters are also used for monochrome cameras.

 

You didn't specify what you wanted filters for, but I'm assuming visual.  You'll also hear about 'nebula filters' and 'light pollution filters' and those again, fall into the imaging category, not the visual category.

 

For visual, you might consider this set of 6 filters.

 

http://agenaastro.co...filter-set.html

 

For Lunar stuff, I recommend a variable ND, or Neutral Density filter.  This allows you to change the amount of light, anywhere from 5-25% of the incoming light.  In and of themselves, ND filters usually require the removal of the eyepiece, and changing out the filter on the bottom of the eyepiece and then reinserting it. With non-variable ND filters, you need to have one for each transmission level you want, so it makes it more expensive in the long run.  There are variable ND cells that allow you to change the density of the filter, but require you again remove the eyepiece from the diagonal.

 

Meade, and BST, both have variable ND filters that you put the eyepiece into, and then adjust the filter without remove the EP.

 

http://agenaastro.co...ing-filter.html

 

I hope this answers some of your questions. 


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#5 Jon Isaacs

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Posted 10 June 2017 - 03:21 PM

Visual filters are usually some color, very similar to camera filters.  They are used to enhance the color of objects, like planets.  You see the object, and use a filter to add contrast.  Since most of the observing you'll be doing that a filter will have an effect on is going to be planetary or lunar, here's a great article on the basics of VISUAL filters.

 

 

Visually, the O-lll, H-Beta and Narrow Band UHC filters are very effective for enhancing the contrast of many nebulae.  I have a set of planetary filters but rarely use them. I use my deep sky filters the majority of nights.

 

Dick covered the basics, there are two primary emissions, the O-lll and the H-Beta.  The UHC filter essentially‚Äč combines these. The general recommendation is to start with a true UHC filter like the Orion Ultrablock, Lumicon UHC (the first) or the DGM NPB (I believe that's the one).

 

I use the O-lll more frequently than the UHC, those are the two to have. The least used is normally the H-Beta and it's almost always used from dark skies with objects like the Horse head, the California and Barnard's loop, difficult objects.

 

There are a variety of UHC filters, some are broadband, you don't want those.. 

 

The 2017 Filter buyer's guide by Don Pensacola is pinned to the top of the eyepiece forum. It's an excellent resource..

 

Jon


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#6 Alex McConahay

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Posted 10 June 2017 - 03:47 PM

Since we are in the General Astronomy section of the Forum, and not in imaging portion, I will restrict myself to a description of visual filters.

 

To start with, eyes are better at picking up contrast than they are at discerning absolute brightness. So, seeing something does not depend on "absolute brightness" itself so much as whether it is "brighter" or "dimmer" than something else. That is why we can see sunspots, for instance. They are still very hot and bright, but just not as hot and as bright as their surroundings.

 

All filters eliminate some light or another while letting other colors (wavelengths) of light pass. This means that something that glows in the color that is passed is brighter than something whose brightness is blocked. If you do this, you increase contrast. And that is what the eye can see. 

 

SO, if you have red spots on a blue planet, and you use a red filter, the red spots will seem brighter (they are not--in fact they are probably dimmer than without a filter). The blue background will fade to black. Poof. You can see the bright red spots against a black background.

 

Imagine that all around you there is a haze of greenish yellow light. There is in most places. It is caused by light pollution. Now, imaging you have a filter that passes all colors EXCEPT this greenish yellow light. You have yourself a "light pollution" filter. The green-yellow of light pollution drops out, and you see the rest of the colors.

 

Imagine you have a gas that emits light at a very red wavelength. That gas forms a swirling cloud around a distant star. Put the proper color red filter in the light path, and you see that reddish cloud. But the rest of the colors drop out, and the cloud stands out. That is what a nebula filter does. Different gasses glow at different wavelengths (colors). O3 is bluish cyan. Hydrogen Alpha is bright red. Put a filter that passes the wavelength, of that gas that you are looking for, and you will see structures made largely of that gas. 

 

that's what a filter does.

 

Alex


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#7 chrysalis

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Posted 10 June 2017 - 03:49 PM

Attached File  Filters & Uses.doc   132KB   30 downloads

 

Attached File  Filters Choosing Color.doc   38.5KB   19 downloads

 

And of course a definitive work by David Knisely:

 

Attached File  Filters Performance by Objects.doc   72KB   30 downloads

 

 


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#8 Achernar

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Posted 10 June 2017 - 04:05 PM

Are you interested in color filters which are used mainly for observing the planets, or are you looking into nebula filters to combat light pollution?

 

Taras



#9 vtornado

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Posted 10 June 2017 - 04:12 PM

All excellent advice above.

 

From a person who owns a f/10 100mm achromat in a red zone, most planetary filters are not very useful.

Beginner sets usually have  #15 Deep Yellow, #25 Red, #58 Green, #80 blue

The numbers are wratten number, that describe which frequencies they pass and which the block.

 

In a 4" telescope, dark/medium filter sets block too much light and imho give too much color to the object.

 

However ...  I do like to use a blue #82a filter and a yellow #8.  The light blue has allowed me to see the polar caps on mars easier.

Because it knocks down some of the red of the planet.  A yellow #8, block alot of CA which you will see on the moon or Jupiter in your

scope.  A fringe killer or minus v filter does a better job at blocking the purple but ... it is usually much more expensive than a yellow #8.

 

I do have a narrow and broad band nebula filter.  The narrow band helps a lot on the orion nebula.  Not so much on the dimmer ones.

 

I have heard that some of the nebula filters are really nice in dark skies.

 

Also not that my polar cap viewing was done last year when mars was close.  Now it is too far away to see any detail,

no matter what filter you use.


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#10 kfiscus

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Posted 10 June 2017 - 04:21 PM

The following advice is for the OP and anyone else considering deep sky filters:

 

They do amazing things to emission nebulae IF you are dark adapted.  It's like getting a whole 'nother sky to enjoy.

 

Buy a quality O-III filter or DGM's NPB (Narrow-Pass Band) filter.  Used ones are frequently for sale in the CN classifieds.  DGM sometimes has cosmetic seconds that are a little cheaper.

 

Warning:  If you try a good filter and enjoy what it does for you, you'll end up considering buying 2" versions of your filter(s) for the filter slide or wheel you need.  None of these things are cheap, but they are the best upgrade I've made to my equipment.  Deep sky filters are too effective and too expensive to not be used out of habit.  They should not sit in their cute little cases waiting for you to remember that you have them.  My filters are stored INSIDE my OTA in my filter slide.  They get used every night that I go out.


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#11 Mike W.

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Posted 10 June 2017 - 04:36 PM

try this idea for your filters, works for me

Attached Thumbnails

  • thumbnail_IMG_20170311_100000.jpg

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#12 havasman

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Posted 10 June 2017 - 05:00 PM

 You'll also hear about 'nebula filters' ... and those again, fall into the imaging category, not the visual category.

I do not think that is essentially correct.


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#13 Rob medic

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Posted 10 June 2017 - 05:10 PM

Love the small tackle box idea. Gonna do that! Thanks!

#14 Jon Isaacs

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Posted 10 June 2017 - 07:22 PM

 

 You'll also hear about 'nebula filters' ... and those again, fall into the imaging category, not the visual category.

I do not think that is essentially correct.

 

 

:waytogo:

 

Nebula filters are very effective visually.  Without the O-III, H-Beta and UHC filters, few would ever have seen the Horsehead Nebula, California Nebula, many facets of the Veil.  In this hobby, most improvements are subtle, planetary filters might help a little.  The difference between $50 eyepiece and a $300 eyepiece, you pretty much see the same thing in both eyepieces, one is just more perfect than the other.. 

 

But these filters dramatically increase the contrast of deep sky objects.. I have seen the Veil from my red-zone backyard in a 4 inch refractor. I see a detailed view of the Veil from my backyard in the 13.1 inch.  This would be impossible without a filter. The view with the filter from my back yard is about like it is without a filter from a dark sky site 3 whose sky is about 3 magnitude darker.. That's an improvement in contrast of 10 to 15 times.. 

 

Jon


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#15 jdjetson

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Posted 10 June 2017 - 10:12 PM

Wow, great advice and information.  Thanks for giving so much detail and links to other resources.  I had a feeling this topic was more involved than I originally thought.  

 

Once again I really do appreciate the help from CN members.



#16 View2

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Posted 10 June 2017 - 10:53 PM

The orion ultrablock is actually a pretty good one. I couldn't see the veil nebulae without it, and with it in it was very obvious. It is a pretty good filter in that there are many nebulae it will work on, and of course some that it will not.
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#17 M57Guy

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Posted 11 June 2017 - 01:27 AM

adding my 2 cents, boiled down (cliff note style) to the expert advice above.....

 

UHC as a starter DSO filter with an O III as the follow-up

 

in for a pound, in for a penny...

 

presuming your refractor is an achromat.... a #8 (light yellow) is an inexpensive way to reduce the purplish chromatic aberration from bright stars and planets

 

#82A (light blue) is a good choice for additional contrast on Jupiter and Saturn (many colored options here but this would be my recommendation as a starting point thought a 4" refractor)

 

#21 (orange) is great for additional contrast of lunar craters or a ND 25 as an alternate

 

careful not to catch filter fever......



#18 Rob medic

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Posted 11 June 2017 - 02:17 AM

82a light blue is probably the filter I use the most for planetary. UHC and OIII are definitely my main nebula.

#19 chrysalis

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Posted 11 June 2017 - 05:12 AM

All the above considered, for my eyes, only Mars really improves significantly upon use of red, red-orange, or magenta filters, which increase the contrast of surface features against the reddish planet surface. Next is Mars through blue filter for polar caps. Beyond these, my eyes do not perceive any significant improvement on other planets with other filters.

 

Narrow pass band and OIII filters do work wonders on emission nebulae! A fine example is NGC7293 (Helix Nebula): it rides in light-polluted skies to my southern skies here and is INVISIBLE without a filter; however, it is immediately obvious once I interpose a DGM NPB or OIII filter.


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#20 Mike W.

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Posted 11 June 2017 - 07:26 AM

DGM's are the most recommended of the manufactured filters for nebulae.

A few spectra tests have also shown them to be a head above the rest.

My pocket book has shown that they are the result of testing against several other manufactured nebulae filter lines.

 

Orion seems to be the only manufacture to offer the Magenta other than Brandon,but Brandon has a different thread pitch so you need an adapter to use their's on any other eyepiece than Brandon, and Orion's is only available in their large color filter kit.

Hence, the one color filter I don't own,,,,,,,,mad.gif

 

There is the Bandmate & Mars filters which claim to be band-pass filters for planetary viewing.

 

I've had little opportunity to try them, so far it's not encouraging news for them.

#82A light blue and #8 light yellow-green for the planets.

I ordered a GSO #23 light red but beside my #25 red there seems to be no difference in day light or at night.

 

The variable polarizing filter set is an excellent lunar filter, if separated into two filters, one can be installed on the eyepiece nose barrel, and the other into the nose barrel of your diagonal, (if you are viewing with an SCT or refractor), then just spin the eyepiece until the desired amount of contrast is attained for viewing that area of the moon and in other areas you can again change the contrast without ever removing your eyepiece, a very cool visual aid. Now add a zoom type eyepiece to the lunar experience waytogo.gif

I'm thinking the only way to duplicate this on a Newtonian OTA is with the use a filter wheel or slide because you don't have a second nose to place the other half of the variable polarizing filter set.

I'm still working on that, anybody got a way they use with their reflector scopes other than placing the other half of the set on top of the eyepiece?



#21 Jon Isaacs

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Posted 11 June 2017 - 07:46 AM

DGM's are the most recommended of the manufactured filters for nebulae.
A few spectra tests have also shown them to be a head above the rest.

 

I have seen some test results.  I haven't seen anything that suggests that the DGM filters are "head and shoulder's" above the rest. There are a number of very effective filters to choose from.  If there were one brand that was most recommended, in my experience, that could be the Lumicon's.  They were the first and still considered by many the best.

 

The variable polarizing filter set is an excellent lunar filter, if separated into two filters, one can be installed on the eyepiece nose barrel, and the other into the nose barrel of your diagonal, (if you are viewing with an SCT or refractor), then just spin the eyepiece until the desired amount of contrast is attained for viewing that area of the moon and in other areas you can again change the contrast without ever removing your eyepiece, a very cool visual aid.

 

 

 

A variable polarizing filter does not change the contrast, it changes the brightness.  Other filters can change the contrast because they selectively filter specific portions of the spectrum.  When viewing the moon, no filters, I let my eye adapt to the brighter image.. Dark adaptation is a plus viewing the deep sky, a minus viewing the planets.  

 

I'm thinking the only way to duplicate this on a Newtonian OTA is with the use a filter wheel or slide because you don't have a second nose to place the other half of the variable polarizing filter set.
I'm still working on that, anybody got a way they use with their reflector scopes other than placing the other half of the set on top of the eyepiece?

 

 

If you have a 1.25 inch GSO Barlow, the Barlow section in front can be removed and used as a 1.5x Barlow.  But one can also thread a filter to where the Barlow optics had been.  That provides you with the second set of filter threads, the only question is whether there is enough inward focuser travel.

 

Jon


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#22 Mike W.

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Posted 11 June 2017 - 08:32 AM

Morning Jon, I've got an SW 10 f4.7 truss, the SW's are sold with a low profile Crayford that allows for use of extended light path components, about 50mms of it.

So like a filter slide/wheel, focal extender/barlow, or a CC.

The barlow nose piece idea you suggested, excellent, thank you!smile.gif


Edited by Mike W., 11 June 2017 - 08:33 AM.

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#23 Hesiod

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Posted 11 June 2017 - 08:35 AM

Others have already covered the nebular filters (an "UCH-like" filter is a very useful tool to have, but in my opinion it is a worth purchase only if you can/want sometimes go to darker skies); as for planetary filters, the so-called "Moon and SKyglow" is a quite versatile contrast filter, good on Jupiter, Mars and even on the Moon.

I found it more useful with reflectors or Cats than with my 100/1000 achro, but this is just my opinion.

If you plan to get Wratten filters, I suggest stronlgy to get also a filterwheel, to switch quickly between them.



#24 REC

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Posted 11 June 2017 - 10:04 AM

Our resident filter guru, David Knisely will chirp in here at some point. He has compiled a whole list of DSO's and what filter works best with them.

 

Everybody's first choice is a narrowband filter like the UHC type mentioned above by many.

 

I have a 4" Achro as my grab scope and use the UHC type for many nebula in low-medium power eyepieces depending on the object.

 

For Jupiter right now, inexpensive 80A works well and for more money, Baader moon&skyglow.


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#25 dmgriff

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Posted 12 June 2017 - 12:14 PM

Lumicon Filter Guide pdf printable file

 

http://www.lumicon.c...erspec_prnt.pdf

 

Good viewing,

 

Dave




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