What's the deal with filters?

[OK Apricot]

Specifically for visual observing, what are the benefits of these different types of filters you can get? I feel like I'm missing out on observing potential from what I've read, but I don't understand why? 

Sorry if a silly question, but very keen to spend some money 🤷

[Deadlake]

To help you out what objects of interest do you wish to observe and how dark are the skies you observer under?

[OK Apricot]

Typically around Bortle 7, dropping to 6 on the better nights. That's all I could tell you. I'm a full on sucker for deep sky objects though, particularly galaxies and globular clusters. 

[Louis D]

Filters simply cut out some wavelengths of light to make others more prominent.

For nebula, a good quality OIII filter can make otherwise invisible nebula like the Veil pop into visibility by blocking all light except around the wavelengths where the nebula emits light.

A UHC filter is good for more types of nebula, but doesn't increase contrast as much because it has a wider passband allowing in more light pollution and sky glow.

For both of the above, stick with Astronomik or Lumicon brands for best results.  I've tried cheap OIII filters, and they are basically blue-green filters that don't even pass the proper wavelengths to be useful.  The cheap UHC filters are reportedly better than the cheap OIII filters, but I've never tried one.

A Moon & Skyglow (M&SG) filter uses Neodymium doped glass to cut out certain middle wavelengths of light to accentuate the blue and red ends of the spectrum producing a slightly purple hue.  They tend to be a jack of all trades and can help somewhat with light pollution and as a decent Jupiter/Mars planetary filter.  Don't waste your money on the expensive brands.  Every spectrographic analysis of cheap brands I've seen shows they are all almost exactly the same.  It's one case where you don't have spend a lot to get decent performance.

Variable polarizing filters can be handy on the moon and during solar observing with the proper solar filtration (full aperture or Herschel wedge) to attenuate the light to a level making detail detection easier.

Various yellow filters can be handy to cut unfocused violet light on achromatic refractors when observing bright objects, improving sharpness by leaving only the wavelengths which are properly focused.  Combined with a light green filter, unfocused red light at the other end of the spectrum can also be attenuated on very bright objects like Venus.  Of course, the object ends up being very green.

Various color filters can help bring out specific planetary details, but are generally not thought of as particularly useful or necessary.

[OK Apricot]

Thanks for taking the time - a very useful bit of info there Louis. OIII sounds like a good shout. 

[DrMike]

Hello everyone.

I have a decent set of both 2” and 1.25l” eyepieces, but can anyone recommend a decent set of coloured filters for moon/planetary visual observation? The basic 4 colours, a moon and possibly polarising filters.

I’ve seen several sets on sale, ranging from dirt cheap to ridiculous.

Any advice appreciated.

[johninderby]

I bought a set of standard coloured filter ages ago. Been sitting in a drawer unused for ages as well. I quickly found specialist filters were more useful. 

Some need a moon filter and some don’t. All depends on how your eyes handle bright light. The variable polarising filter is very useful though as you can dim to the right level and comes in handy for other bright subjects such as Venus.

[Don Pensack]

Nebula filters increase contrast by reducing the light from the sky without dimming the nebula.

A narrowband, UHC-type, filter passes hydrogen emission line and the 2 O-III lines in the spectrum and little else.

Without knowing the spectrum of emission of the nebula, this is the 'universal' type of nebula filter.

[Nebula filters do not work on dark nebulae or reflection nebulae]

If the object has little hydrogen emission, but is strong in the O-III lines, narrowing the filter will increase the contrast even more.

 

So, narrowband--all nebulae, and especially the larger hydrogen gas clouds forming stars (M42/43, M8, M20, M17, M16)

O-III for planetary nebulae, supernova remnants, and Wolf-Rayet excitation nebulae.

Brands to look for: Astronomik UHC and O-III, TeleVue Nebustar and O-III, DGM NPB, Lumicon UHC and O-III, Chroma UHC and O-III, ICS UHC and O-III

2nd choice: Orion Ultrablock and O-III

 

For planets, the most effective filter I've used is the Baader Contrast Booster filter.  It is the one I reach for nearly every time.

The Baader Moon & Sky Glow is also good, but a bit less effective on Mars than the CB.

For the Moon, you really only need a filter at low powers--at high powers, the moon is kind of dim.  But a variable polarizing filter is most useful, though a 25% transmission neutral density filter also works well.

[Louis D]

5 hours ago, DrMike said:

I have a decent set of both 2” and 1.25l” eyepieces, but can anyone recommend a decent set of coloured filters for moon/planetary visual observation? The basic 4 colours, a moon and possibly polarising filters.

I’ve seen several sets on sale, ranging from dirt cheap to ridiculous.

If you really want to try some color filters for cheap, order this set from China.  They're a bit cheaper here.  I recently received mine and they seem decent enough, although I've yet to try them out under the stars to verify if they are optically flat and don't introduce reflections.  They arrive individually plastic sleeve wrapped inside the displayed box, which is inside another clear plastic sleeve.  Mine came padded with thin closed cell foam around the entire case that I cut to size to pad the bottom/sides of the case and the underside of the lid to keep the filters from rattling about.  They don't have front side threads, so you can't stack any other filters onto them.  They are also a bit constricted on clear aperture, so not a good choice for widest true field 1.25" eyepieces.  The filter cells are a nice hefty metal, though.

I bought them mostly for the magenta filter which is very hard to find elsewhere.  I verified that it does a good job of passing red and blue while suppressing green by taking an image of a sunlit translucent surface through it and checking the RGB channels for intensities.  I plan to try it on Mars at the next opposition.

The green is also a nice light shade that blocks basically all red light according to my tests.  It does let some blue through, so it's not chromatically pure green.  I paired it with a #12 Yellow filter with front threads to block that blue light and verified it can produce a nearly pure green color in that configuration.

The red is fairly light as well, and yet has no orange cast at all to it.  I'll have to try it on Mars.

The yellow is a bit pale like a #8 Yellow, but might help cut stray violet light on a fast achromatic refractor.  Since I already had a #12 Yellow filter, it worked out fine to get this one.

The orange is a bit too dark for my tastes.  It works, but I'm not sure what objects it would be useful for.

The blue is a bit too dark as well for me.  It's probably fine, but I prefer a light blue filter for Jupiter.  Less is more in that use case.

Edit: I just noticed you're in the US.  Unfortunately, the seller I bought from is sold out of this set.  Another seller has it here for a bit more than I paid.  I noticed these sets and these sets with slightly different colors are now available at a slightly higher price, and I don't know if the colors are in the same hues and shades as my set, but they appear to have more clear aperture and might have front threads.  The case sure looks slick.

Edited February 11, 2022 by Louis D

[Zermelo]

You might find this site of interest, with the caveat that it's one person's view (and note the equipment that was used):

https://www.prairieastronomyclub.org/filter-performance-comparisons-for-some-common-nebulae/

[Zermelo]

20 hours ago, Louis D said:

I've tried cheap OIII filters, and they are basically blue-green filters that don't even pass the proper wavelengths to be useful

Louis, I'm interested - what would you regard as cheap in this context?

I have an OVL OIII, which I know isn't in the top rank.

The only data I can find is this on the OVL site:

[Louis D]

I have the Zhumell OIII which cost me $10 from Hayneedle back in 2014.  This 2009 review on CN shows why it's barely usable.  Its passband is shifted 10nm to the right of where it should be.  I can verify it makes little difference other than dimming nebula along with the background.  Contrast barely increases compared to a 1990s Lumicon OIII I own.

The review does praise the Zhumell UHC which I wish I had picked up for cheap back then instead of the OIII.  It seems to hit the mark where the OIII missed.

[Louis D]

Here are a couple of references describing color filter use cases.

[DrMike]

Thanks everyone. Plenty of food for thought.

[PeterC65]

Prompted by this post, I've been assessing whether to buy some more colour filters for Lunar / Planetary visual observing. Having decided that I could do with a few more (what a surprise) I've swapped around the filters currently in my filter wheel only to discover that the Astro Essentials W15 won't fit!

It seems that the W15 is 1mm thicker than the W8 which fits fine. These filters are meant to be physically identical!

So despite the extra cost, I will be buying Baader filters. I already have two of these and they fit the filter wheel just fine.

As part of the assessment, and using the link provided earlier by @Louis D, I've tried to compare the Baader colour filters with those from other suppliers (those that conform with the Wratten filter specs which Baader do not). My conclusions are:

• Baader Yellow approximates W8 (not W12 as Baader claim) but with a sharper cut-off shifted about 15nm towards blue and with a pass band light transmission of 99% rather than 90%.
• Baader Orange approximates very closely W21 but with a pass band light transmission of 99% rather than 90%.
• Baader Red approximates very closely W23A (not W25 as Baader claim) but with a pass band light transmission of 99% rather than 90% .
• Baader Green approximates W56 (not W58 as Baader claim) but shifted about 20nm towards blue and with lower peak light transmission (70% vs 80%).
• Baader Light Blue approximates W80A but shifted about 40nm towards red and with lower peak light transmission (70% vs 80%).

My scopes have smaller apertures so I've not looked at the Baader Dark Blue filter.

Edited February 12, 2022 by PeterC65

[Don Pensack]

some major links on filters:

Planetary:

https://www.cloudynights.com/topic/713148-planetary-filter-shootout/

https://www.cloudynights.com/topic/533147-spectroscopic-analysis-comparison-of-planetary-filters/

Nebula:

https://www.cloudynights.com/topic/527199-spectroscopic-analysis-comparison-of-nebula-filters/

https://www.cloudynights.com/topic/598011-nebula-filters-tested-over-a-2-year-period/#entry8205435

 

Comparative charts:

https://searchlight.semrock.com/?sid=a08a1af9-84ee-49d2-959d-153d7e7c0eb8#

Edited February 12, 2022 by Don Pensack

[rwilkey]

On 10/02/2022 at 21:51, OK Apricot said:

Typically around Bortle 7, dropping to 6 on the better nights. That's all I could tell you. I'm a full on sucker for deep sky objects though, particularly galaxies and globular clusters. 

No filters will help you with galaxies and globular clusters, I am afraid, you just need clear skies.  

[kirkster501]

1 minute ago, rwilkey said:

No filters will help you with galaxies and globular clusters, I am afraid, you just need clear skies.  

Yes, I agree.  Whilst it is true that filters can "enhance" certain parts of a view, they do so by removing other parts of the spectrum to enhance whatever that filter is designed for.  To get the light in the first place over and above the background sky then you need clear skies.

[Louis D]

1 hour ago, rwilkey said:

No filters will help you with galaxies and globular clusters, I am afraid, you just need clear skies.  

If you've got enough aperture to resolve a GC at high enough power, light pollution isn't such a big issue.  For instance, M22 is barely visible as a brighter fuzz patch near the Teapot asterism from my backyard at lower powers, but at over 200x in my 8" Dob, it breaks up nicely into teeny tiny pinpoints of light for the most part.  Not all GCs will resolve with only 8" of aperture, though.  In fact, only a few of the closest/largest ones resolve nicely for me.  I need to get a bit further south and seek out a clear view of the southern horizon sometime to see if I can bag Omega Centauri.  It should be just doable from southern Texas IIRC.