Using Filters

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Paul D
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Using Filters

Unread post by Paul D » Thu Aug 17, 2017 2:05 am

After my presentation at the August meeting I never thought my use of one word would get me so many questions. That word was OIII (Oxygen III filter). I had a few people asking me what filter to use on what object? So I will attempt to give a bit of a guide on filters and there uses. I should note that my descriptions will be for purely the visual observer and not the astro-photographer.

Broad Band Filters

These filters are sold as either Deep-sky filters or LPR filters. There main objective is to add contrast to your to your field of view eliminating the brightness of the sky glow and enhancing the object that you are looking at. This type of filter works well to bring out detail in galaxies. So if your in a light polluted area and you want to see the spiral arms on M51. Well you stand a good chance of seeing them using a Deep-Sky or LPR filter. These filters are a staple for anyone who observes and it also covers the widest range of targets.

Narrow Band Filters

These filters are normally sold as UHC (Ultra High Contrast) or LP-2 (Light Pollution 2) There are a few others which I prefer to put into another category which I will discuss next. This type of filter adds a lot of contrast to very bright nebula and deep sky objects. It really darkens the background and enhances a bright object like M42 (Orion Nebula) for example. This filter wont get as much use as your Deep-Sky or LPR filter but its still a nice filter to have.

Line Filters

These are specialized filters used to observe certain types of objects like emission, reflection and dark nebula. They normally cover a certain band of width of gases and transmits only that type.

OIII (Oxygen III) This is a general purpose filter great for showing nebula like the Crescent Nebula, Veil Nebula, to name a few. Most planetary nebula will pop into view with the aid of an OIII filter and M27, and Helix nebula will look like photos with this filter in place.

Hydrogen Beta This is a very specialized filter used on very few objects. But in most cases the only way you will be able to observe these objects is with this filter. California, Horsehead and North American nebula all require the H-Beta filter to observe. Interesting thing is that the North American Nebula is so large that at a dark site like Stellafane simply holding the filter to your eye and looking up at that part of the sky will reveal it. For all three of these objects a dark sky is a must.

Swan Filter (C2) This again is a specialized, so specialized I think myself and one other person has it in the club. While this filter can be used on nebula it really works best on bringing out detail on comets. If I am having trouble finding a tail on a faint comet this filter will most of the time yield it. I have used this on other object with mixed results but it does not bring out detail like the filters geared for those objects.

General Filters

Polarizing filter Do you like looking at the moon but hate being blinded or losing you dark adaptation? Try a polarizing filter, it reduces the transmission of the moon acting like a pair of polarized sunglasses for your telescope. Besides the noon the only other object I have used this on was Venus.

Solar Film or Specialized Glass Solar Filter First the customary warning, DO NOT look at the sun with anything but approved solar material. Thousand Oaks makes the solar glass filters and Baader makes the solar film. These two companies are the two I trust for solar film or glass. Any other companies be sure to do your homework first before buying. Solar glasses MUST be stamped on them somewhere with ISO 12312-2 to be deemed as safe to use to look at the sun. For your telescopes or binoculars ALWAYS hold your filters to bright light or the sun and inspect for pin holes or scratches before using them on your telescope or binoculars. If you can see light through your filter throw it away and make or buy a new one. You will go blind using a defective filter.

Minus Violet This filter is normally used to correct color on refractors. But I was told to use it on my reflector to observe Jupiter. This filter does amazing things with Jupiter and can bring out the GRS (Great Red Spot) when you are doubting if it is actually in view or not. I was recommended this filter a long time ago by Mark G. and it is something I always use when observing Jupiter.

Color Filters As the name states, these are a series of different color filters from blues to purples, reds to oranges, yellows to greens that enhance the details on all of the planets. I will not go into detail on these unless someone wishes me to.

If anyone has any questions about what filter I use on a certain object or anything about filters at all. Please either post it here or shoot me a private message and I will be happy to help you as best I can.
Last edited by Paul D on Thu Aug 17, 2017 9:17 am, edited 2 times in total.
Paul...

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Stargrrl
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Re: Using Flters

Unread post by Stargrrl » Thu Aug 17, 2017 7:01 am

Thanks Paul, very helpful! Rebekah
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mrgizmo65
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Re: Using Flters

Unread post by mrgizmo65 » Wed Aug 23, 2017 12:09 pm

Paul thanks for posting these tips for improving our viewing help. I do know the use of filters can make a vast improvement. The only thing I did not know is what filter to use for what subject or condition? there are many that will find those tips useful to improve their skills. I did find a chart that B and h photo has in their article buying and using filters. I could not down load it but that is where its. I know the colored filters are sold in sets, can the others be bought as a set??? If you could if you know about eyepieces could you tell us what to"look" for when buying them?? My particular interest is the eyepieces with the green tinted glass. Thank you again for sharing your knowledge. Jerry :) :)
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Paul D
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Re: Using Flters

Unread post by Paul D » Wed Aug 23, 2017 12:35 pm

Jerry, eyepieces have coatings on them. From my understanding all coatings are the same. The different colors of coatings depends on a number of things. First the coating color can change from green to purple sometimes just by looking at it from a different angle. Another factor is how thin or thick the coatings have been placed on the eyepiece itself. Thin coatings usually yield a greenish hue, while a thicker layer of coatings will yield a purplish hue.

The coatings themselves act to prevent internal reflections and light scattering. With thicker coatings ( usually the purple tint ) they are said to actually be able to absorb some of the light reflections. This is why when cleaning your eyepiece its important to do it correctly so you don't damage these coatings. They are at the top of the lens and although they don't damage easily I have seen people damage them in the past cleaning eyepiece like they were window glass.

Color filters are the only filters sold as sets. They are relatively inexpensive to make. The other filters are so specialized most people buy the ones that they want. Most will buy the UHC and the LPR filters for starters. These two give you a wide variety of targets simply because they add different levels of contrast. They may not "bring out" an object like the specialized filters but they will improve the object greatly in most circumstances.
Paul...

16" f/5 Night Sky Truss (Midnight Mistress)
10" f/5 Home built Dob with Parks mirror.
Pre-Meade PST
Celestron Skymaster Binos 25-125x80
Meade Travelview Binos 10x50

See that 16" in the sleek black dress? She is all mine. :)
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mrgizmo65
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Re: Using Filters

Unread post by mrgizmo65 » Wed Aug 23, 2017 8:17 pm

Thank you Paul for taking some of the mystery out of the subject of colored glass is there a book that discusses in general without getting too deep into it, the differences and advantages of the different eyepieces??? I appreciate your response Jerry
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AndyG
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Re: Using Filters

Unread post by AndyG » Sat Sep 02, 2017 2:54 pm

Great info, Paul. I only have one filter that I use regularly for visual observing with my dob: my OIII filter. Works great for the Veil and the Crescent too as we learned at Stellafane.
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DonB
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Re: Using Filters

Unread post by DonB » Sun Sep 03, 2017 1:53 pm

Yes, very nicely presented and essential info for the imager. Thanks

Don
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