Filters for DSO imaging

[MartinB]

Whilst most of this article is still relevant there has been some changes the most significant of which is the switch in street lighting from orange sodium to white LED.  Traditional LP filters which mainly block sodium light are of no benefit in reducing white light pollution and will simply slow down acquisition of signal from your target.  IDAS produce filters which target LED emission wavelengths and have received some promising feedback.

The other exciting development is high quality dual narrowband filters allowing owners of one shot colour cameras to collect OIII and Ha data at the same time.  These are proving highly effective.

MartinB 04/02/2021

 

Some time ago I volunteered to write an article on filters for imaging. Here it is! It doesn't include filters for use in solar system images which is another topic in it's own right.

Filters for DSO imaging

Some people on the forum have recently been asking questions regarding use of filters for DSO imaging. In this article I will cover the principles underlying the use of filters. I will also try to come up with some basic advice regarding choosing between filters that broadly do a similar job.

Filter wars

I am going to do my best to offer dispassionate advice but it is a fraught job recommending a particular make or design of filter. Competition between filter manufacturers is intense, each making claims and counter claims. This battle appears to have spread to retailers who tend to stock specific brands of filter. I think there is quite a lot of politics going on in the world of filter retailing so I will try to tread lightly.

Relevant filters for DSO imaging are: -

IR block

Light pollution

RGB

Narrow band emission line filters

Filter quality

There can be massive price differences between different brands of filter serving an identical purpose. Why is this?

i) the glass - has to be very accurately manufactured. The thickness of the filter should be exactly uniform to avoid optical aberrations. Quality manufacturers will also strive for consistency across a number of filters within their range to ensure they are par focal.

ii) coatings – filters sit very close to the end of the optical path. Therefore any imperfections or dust has far more effect than similar problems on the objective lens. Dust motes are one of the curses of imaging. No matter how carefully you handle your filters they will need cleaning. It is very important that the filter coatings are tough enough to withstand this. The coatings also have to do a good job minimizing light scatter, preventing unwanted light aberrations and maximising contrast.

iii) Optical design – some filters absorb unwanted wavelengths of light whereas others reflect them. Reflecting filters are made by building up layers of optical coatings in an alignment which blocks out unwanted wavelengths through interference. This type of filter is known as dichroic and they are sometimes referred to as interference filters. They have some potential advantages as well as disadvantages including being expensive to manufacture. I will discuss them more later.

Some filters are sold as suitable for imaging and others are for visual use. It is advisable to stick with filters designed for imaging since these are likely to be made to more exacting specification (sadly with a price tag to match).

Since competition is pretty fierce the major filter manufacturers cannot afford to lower their standards. By sticking to reputable brand names such as Astrodon, Astronomik and Baader you should be assuring yourself of high standards of manufacture and good back up in the unlikely event of problems.

IR blocking filters

A good scope for imaging should bring light to focus at a single point. Scopes are designed to achieve this ideal as closely as possible. However the invisible infra red part of the spectrum will not be catered for and this part of the spectrum is not as well focused. Although our eyes can’t detect infra red a CCD chip can. If infra red isn’t blocked your images can be degraded with features such as bloated stars and a loss of resolution. This is much more of an issue in refracting than reflecting designs. If you are imaging with a Newtonian or an SCT you may be better off without IR blocking. A simple IR blocking filter will solve this problem. Some other filters e.g. many RGB filters have IR blocking incorporated whereas many light pollution filters require use of an additional IR filter. NB do not confuse these filters with IR pass filters which are often used in solar system imaging.

The luminance filter in most LRGB imaging sets is a simple IR blocking filter.

Light Pollution Filter

Most streeet lighting emits light in quite discrete parts of the spectrum. Filters have been developed to block out these parts of the spectrum leaving the majority of the spectrum to pass through unhindered.

Although touted as selectively blocking unwanted light they do generally reduce the amount of light getting through to the chip making increased exposure time necessary. If your LP isn’t too bad, especially if imaging high in the sky don’t just assume an LP filter is necessary. There is a trade off between cutting out unwanted skyglow and extended exposures. You should experiment to find out what works best for you in your location.

Astronomik CLS – this is a very popular filter and works very well. I used it for a long time on it’s own believing it was IR blocking. Actually it wasn’t although I have always been very happy with it! It can be used on it’s own but you might want to add an Ir blocking filter. I believe the latest CLS filters are IR blocking

Hutech IDAS – like the Astronomik this is an interference filter of very high quality. It’s most important characteristics is that it is probably the most colour neutral LP filter around. This means that it is an excellent choice for DSLRs and one shot colour imaging. It will still reduce transmission of light requiring longer exposures to achieve the same non filtered signal. You will still need significantly longer exposure times – up to double compared with no filtration. Newer versions are IR blocking It is an expensive filter especially the 2”

Baader Neodymium – this is an absorption filter and is worth considering if the cost of the above 2 make your eyes water. The neodymium conveniently has light absorption properties that make it an effective LP filter. It is also touted as having an effect on reducing interference from moonlight. I have no idea how it does this or how well it works. The latest version, at least in 1.25” form, is IR blocking.

UHC filters can be used to reduce LP but they are really nebula filters particularly allowing passage of nebula emission lines. They may work ok on emission targets but aren’t designed as LP filters.

There are other LP filters around. If you have the chance to buy one make sure it is designed with imaging in mind.

LRGB

To produce a colour image using a monochrome camera it is necessary to take separate exposures using 3 colours of filter – normally the 3 primary colours red, green and blue. Added to this is a plain IR blocking filter which is used to take a luminance image. This is combined with the combined RGB image to greatly enhance detail.

These filters should be purchased as a complete set since they are designed to be par focal. In other words when you change between filters no adjustment to focus should be necessary. The better quality filters should have their band passes matched so that they give a smooth over all colour coverage. Interference filters allow this to be achieved very accurately but absorption filters can also be used.

Astrodon filters have carved a reputation as being the Rolls Royce filter of choice for the Takahashi and AstroPhysics brigade. Very slightly cheaper are the Astronomik LRGB filters. They are interfernce filters often referred to as type II. All of them block IR so there is no need for a separate IR block.

Baader produce a significantly cheaper LRGB filter set. These are absorptive rather than interference filters. They do let through slightly less light than the more expensive interference filters and but I suspect the differences aren’t huge. One advantage of absorptive filters is that they generate far fewer internal reflections. This can reduce the risk of large, unnatural looking star halos which you do occasionally see with interference filters. These Baader RGB filters aren’t IR blocking so you need an additional IR blocking filter on the nose piece of your filter wheel.

Baader have now brought out an LRGB© filters which Baader claim incorporate blocking of sodium and mercury street lighting and are virtually reflection free. The set has both IR and plain luminence filter (the plain filter is necessary to retain par focality when changing between filters). It will be interesting to see how they perform

Narrow Band Emission Line Filters

Electrons are excitable things! They whir around the nucleus but sometimes decide to move down to a lower energy orbit. When they do this energy is released in a very specific wavelength. This is what gives a flame it’s colour. The wavelength of this light differs from element to element and also depends on the orbital shift of the electron. The most common matter in the universe is hydrogen and it glows! The most common “glow” is a deep red. This is known as hydrogen alpha and has a very specific wavelength of 656.281nm. The dominant light from the glowing emission nebulae is hydrogen alpha but there is often OIII, SII, hydrogen beta and other wavelengths in there as well. Planetary nebulae usually have a very strong OIII component.

Narrow band filters are designed to block out all but a very narrow band of light around the desired wavelength. By doing this nearly all extraneous light including light pollution and moonlight is eliminated. Imaging with an Ha filter greatly improves the resolution and contrast of fine nebular detail. Much longer exposures are possible than with narrow band. In the UK with our light polluted skies narrow band filters, particularly Ha are now very popular. Spectacular images can be produced using Ha as both the luminance channel and to enhance the red colour channel – HaHaRGB.

Narrow band filters are normally dichroic and, not surprisingly, are more expensive than LRGB filters. Their band width is normally specified in nanometers and relates range of light that can pass through. A 13nm Ha filter will allow light between approximately 650 and 663nm to pass through. Some filters have narrower band passes down to less than 4nm.

The narrower the band pass generally the greater the expense! There is an important constraint on how narrow the band pass should be which relates to the angle of the cone of light hitting the filter. The faster the scope the less accurate the interference filter is at passing the correct wavelength. The bandpass “shifts”. This might mean that a 7nm filter working with an F3 scope might only be letting a portion of Ha through whilst allowing through unwanted light. The wider band pass of 13nm is more tolerant of the bandpass shift but will still be affected by very fast systems such as Tak Epsilons and Hyperstar set-ups. Baader have recently introduced a “broad” narrow band Ha filter at 32nm which Hyperstar users are finding very interesting!

Conclusion

The correct use of filters can greatly enhance the quality of the imaging you can take. They are of major value in our light polluted conditions and narrow band filters are transforming our abilty to take stunning shots of emission nebulae in a way that couldn’t be imagined just a few years ago. Suddenly everyone can have a mini Hubble in their back garden!

There is fierce competition in the market place which is good news for us. With the likes of Baader, Astronomik and Astrodon around you can be sure of good quality. The difficulty can sometimes be reconciling the price tag with the size of those little pieces of glass. If you get quality filters they will be every bit as precious to you as fancy eyepieces!

[narrowbandpaul]

very good martin...

isnt the saying..."a filter is worth a 1000 words?"

[FLO]

Nicely done Martin

[Psychobilly]

Nice One Martin...

Billy....

[Sam]

thanks Martin! very useful and very understandable!

[Kai]

Well put Martin

I have just recieved a set of the new Baaders , as you say i am eager to seem how they perform.

[beamish]

Thanks for taking the time to explain a few things Martin !

Karlo

[viperwhite33]

hello, i have been trying to image m42 with a nexstar6se, 6.3 focal reducer and meade ccd camera. if i do a 10 of 15 second exposure i get good nebulae and color but the sars are too bright they join up as a blur, do i need a filter to filter out the star light allowing me to do a longer exposure, and then add the shorter exposure on to it with only stars on it. any help really appreciated. see image of probs im having.

[viperwhite33]

sorry i dont even know how to pu a pic up on this forum. im confused.

[NickK]

It's probably worth explaining why they have odd names - Oxygen is O2 right? Well the 2 here is chemical bonding, not emission, but in it's native earth state however in the vacuum of space, the atoms can perform state changes that can only occur in vacuum we can't recreate here on earth. This is when "forbidden lines" appear where the atoms in the nebulae are excited to states we can't get here.. so OIII appears for example!

Edited January 22, 2012 by NickK

[narrowbandpaul]

Hi Nick,

just for clarification as to the nomenclature used in emission line imaging.

The number after the element referes to the state of ionisation. For O[iII], the oxygen atom is missing 2 electrons. In S[iI], the sulphur atom is missing just one electron.

The square brackets denote a forbidden line. This is a state which is very long lived, which can only exist in low pressure/density environments like space.

The Ha line for example is an allowed transition (from the 3rd energy level to the second).

Cheers

[Chris]

Thanks Martin:), very useful indeed! I'll be off now to investigate the Baader Neodymium further:)

[MidnightSun]

Thx for that....very understandable and useful.

[icrusoe]

Excellent description! Concise and informative - you've helped my purchasing decisions

[russellhq]

Hope you don't mind me asking but I use a Newtonian with an 1100d unmodded for imaging combined with a Baader Neodymium filter but often wondered if I would benefit from a UHC filter from my light polluted site?

[emadmoussa]

Thank you for this article. It's very informative!

[astrosathya]

Hello Russell,

I use a Canon 550D and a 2" UHC with my Explorer-200PDS. It works wonderfully in cutting out many unwanted lines. Light pollution is also suppressed to a very large extent. The filter does give a green hue to the entire image but color balancing gets rid of all that.

Take a look at my image.

[roarke80]

Thanks for this post, very useful!

[adamphillips]

are narrowband filters ok to use with a dslr? I have a canon 5d mark ii. orion has a set of all three on sale for 200 dollars I think I want it

[Thalestris24]

are narrowband filters ok to use with a dslr? I have a canon 5d mark ii. orion has a set of all three on sale for 200 dollars I think I want it

Hi Adam

In principal you can but it's not very practical I'm afraid. They are really for use with monochrome ccd's with filter wheels. You might be able to attach one filter at a time but that would be very fiddly. Even then you'd really need dark skies as you'd require long exposures - the narrowband filters seriously cut the amount of light reaching the sensor. 

Um, as this is a pinned topic you may not get more replies as members don't tend to scour them for new posts. Better to create a new topic

Louise

[WebSentinel]

I've just started out in AP and I've started looking at LP filters as my Eastern skies are somewhat light polluted (thanks to the docks) - this article has been the perfect primer for me. 

Thank you very much Martin!

[wxsatuser]

Depends on the light source, for sodium here.

The best IMO are the Hutech ones, pricey but they are very good, I have this one

http://www.firstlightoptics.com/light-pollution-reduction-imaging/idas-p2-light-pollution-suppression-filter.html

If your using a dslr also consider the Astronomik CLS clip, very good as well.

http://www.modernastronomy.com/filtersAstronomik.htm

[BlabyStarGazer]

Thank you Martin, a very interesting and understandable concise explanation. As a newcomer to DSO imaging, for me it has served to confirm that at last I'm starting out in the right direction.  If only I had seen this post 12 months ago!

[Zadoq]

Thanks for taking the time to write that post. Very useful

[gonzostar]

Hi 

Martin thanks for all your information. Very helpful

Dean