Tips for Choosing an Energy Rejection Filter for H-alpha-Observations

The purpose of an Energy Rejection Filter is to prevent as much solar energy as possible from entering the telescope in the first place and thus avoid excessive heating of the etalon of an H-alpha filter.

This is done most effectively by a filter in front of the telescope. At the same time, this prevents the air in the tube from heating up - the telescope remains close to the ambient temperature, and there are no air turbulences inside of the OTA due to temperature differences. If we look at the solar spectrum, it becomes clear that such a filter must primarily block visible light. Ideally, it reflects the solar energy energy instead of absorbing it and and does not heat itself up in the process as well - that's why these filters are called Energy Rejection Filters (ERF for short).

Since it is occasionally asked: A Herschel wedge or a white-light front-filter cannot be used as an ERF because it also darkens the H-alpha line. In H-alpha, the sun is far less bright than in white light, so there would be practically no light left to observe.


The solar spectrum with (red) and without (yellow) the absorption by the Earth's atmosphere. Quelle:

The solar spectrum with (red) and without (yellow) the absorption by the Earth's atmosphere. Quelle:

There are usually three types of filters mentioned, when Energy Rejection Filters are discussed.

The most effective ones are Baader D-ERF, which are dielectric Energy Rejection Filters. The dielectric coating allows steep filter edges, so that only the wavelengths around the H-alpha line pass through. (As a special case, D-ERFs can also be made for other spectral lines such as the calcium line at 393nm, or as in the case of the Baader Triband telescopes, they can be designed so that narrow-band deep-sky photography remains possible despite the energy protection filter).

A D-ERF has got a broad transmission window of 80nm centered around the H-alpha-line. All other wavelengths up to those far in the ultra-violet and infrared parts of the spectrum are blocked before they can reach the inside of the telescope. This way, the H-alpha-filter in the telescope's focuser is not stressed by large amounts of energy.


Transmission spectrum of a Baader D-ERF

Transmission spectrum of a Baader D-ERF


An older filter type are simple ERF filters made of red glass. In practice, there were three different types of these ERF filters made, each with its own disadvantages. The cheapest version was made of Aero Glass, which has been used since the 1980s. They let most of the red light (down to 580 nm) pass through and usually did not have very good optical quality. The filters with better optical quality used Schott RG610 or RG630 (or comparable) glass, which block up to 610 or 630nm respectively, but offer no protection from infrared radiation. The energy load for the etalon is therefore higher.

As third option, sometimes yellow ERF-Filters can be found – but they offer almost no protection against energy, as a look at the solar spectrum shows, and should be avoided at all costs. They are only a little bit better than no ERF at all.

As these Energy Rejecting Filters are placed in front of the objective lens, the demands on the optical precision are the same as for the front lenses, so that they are not cheap.

ERF – behind the front lens

Therefore, it becomes attractive to install the Energy Rejection Filter close to the H-alpha-filter, so that a smaller diameter is sufficient. This option is often discussed, especially in the case of self-builds. It has a few disadvantages, among others:

  • The inside of the telescope can heat up, which can cause seeing problems inside of the telescope. Thus, higher magnifications can't be used, and such a solution becomes mostly interesting for smaller telescopes, which can't magnify that high, anyway
  • If the tracking is not perfect and the sun moves out of the field of view, it can shine on and damage the inside of the telescope – just as when projecting the image of sun

Baader SunDancer II H-alpha Filter

Baader SunDancer II H-alpha Filter (#1363056 , € 3545,-)

For smaller telescopes up to 80mm aperture and slower focal ratios (about f/8 or slower), there are complete systems such as the Baader SunDancer II H-alpha Filter (#1363056 , € 3545,-) , in which the block filter is paced in front of the telecentric and also corporates as an energy protection filter. The Daystar Quark filters also work without an additional ERF up to 80mm telescope aperture. With these H-alpha filters, the sun can also be observed without an expensive D-ERF; but an additional D-ERF can reduce bad seeing inside of the telescope tube if necessary or lets you use the H-alpha filter on larger telescopes, too.

Sometimes – not only on ATM-websites, but also by some dealers – a yellow ERF-front-filter or even a UV/IR-Cut-filter in front of the star diagonal (and thus in in front of the complete H-alpha-Filter-unit) is recommended as Energy Rejection Filter for these small H-alpha-systems. But a look at the solar spectrum shows the problems of this concept: Only a small fraction of the sun's energy is in the UV/IR part of the spectrum at all; while most energy is in the visible part.

The idea probably originates from visual observation of the sun in white light. There, a UV/IR blocking filter makes sense if you are not sure that the solar filter attenuates the visible light and completely blocks the invisible UV/IR radiation, too. For the eye, unfiltered UV/IR radiation is harmful because we do not notice it; for the filter system of an H-alpha filter, it is not a big problem: if it heats up the filter too much, you can see it in the temperature display and the decreasing image quality long before any damage occurs - unlike in the eye.

A simple UV/IR filter or a yellow filter is thus a bad choice as the only Energy Rejection Filter, as it lets most of the solar energy passes through it.

If you want to use a filter as Energy Rejection Filter close to the H-alpha-filter, you should use a narrow bandpass filter like a hard-coated Baader H-alpha 1¼" CCD-Filter (35nm) #2458381, or a filter with an even narrower transmission window. It should make no difference if it is a 10nm-filter or even narrower. A soft-coated filter (that is a filter with a standard coating) will quickly age and bleach so close to the focal point of telescopes with f/6-f/8. Especially if you want to use a simple red filter, you should use it behind the telecentric system (so that the energy is ditributed onto a larger diameter) and never with telescopes larger than 80mm aperture. A narrowband-filter will profit from this placement, too.

With self-made setups (such as modified PSTs), you must make absolutely sure that all filters fit together. Baader D-ERFs are heat-reducing pre-filters for H-alpha observation with SolarSpectrum filters that have an additional IR block coating. Especially with modified PSTs, please note that the IR blocking is located on the front lens of some models and is thus removed during a conversion - then an additional energy protection is necessary, which is not provided by a standard UV/IR filter. We cannot give any tips on these conversions, as we do not know the exact construction of other manufacturers.

The purpose of an ERF is to protect the expensive etalon from heating up beyond its operating temperature (and in no case beyond the highest storage temperature). If the operating temperature is exceeded, the central wavelength shifts away from the H-alpha line. An electronic temperature control with cooling counteracts this as far as possible. In earlier times, when 4" f/15 telescopes were used for H-alpha observation and no ERF filters could be used, the telescopes were simply stopped down to f/30 - and if the filter became too warm, the objective was covered until the filter had cooled down again. These observation breaks are still a good idea today – if the filter gets too hot, it will be damaged beyond repair.


8 thoughts on “Tips for Choosing an Energy Rejection Filter for H-alpha-Observations”

  • Marion Hochuli
    Marion Hochuli May 2, 2022 at 2:05 am

    I have a Lumicon Solar Prominence (H-alpha) unit from at least 20 years ago. I've taken good care of this setup. I'm wondering if my red Energy Rejection Filter still blocks all of the IR and UV. Is there a way to have the filter tested? If so by whom?

    • Team Baader Planetarium
      Team Baader Planetarium May 2, 2022 at 9:41 am

      Your ERF filter is most likely a simple Schott RG 630 or similar longpass filter, because 20 years ago there were no ERF available with coatings that also blocked IR and UV, like our modern D-ERF.
      Therefore we recommend to switch to a modern DERF. The DERF are safer for the Halpha filter because they keep heat and UV out of the telescop. And you will experience a much better contrast in the image

  • Michael

    Hi. May I know do we use an ERF for white light imaging? I am currently using a Lunt solar wedge on a 9 inch (228mm) refractor. I wonder if using an ERF will help to reduce the heat load, even though I can't afford a big ERF at the moment. Thank you.

    • Team Baader Planetarium
      Team Baader Planetarium February 28, 2023 at 12:12 pm

      Yes you can use the DERF also for whitelight imaging – but please note that with color sensors your image will be red! Because the DERF has only transmission in the red part of the spectrum.
      Furthermore you cannot use the DERF in combination with SolarContinuum filter because the DERF has no transmission in the green part of the spectrum. Without SolarContinuum filter however, photos will have less contrast

  • Fernando Ferrari
    Fernando Ferrari July 9, 2023 at 9:39 am

    Hi. I have a 127 Mak and I would like to use it for Sun Calcium-K (or H?) imaging. Would a tri-band ERF work as front mounted ERF? Do you guys sell this filter separately? Thank you.

    • Team Baader Planetarium
      Team Baader Planetarium October 5, 2023 at 9:43 am

      Our Triband-SCT telescope undergo an elaborate coating process. They are only sold as full telescopes in sizes 8" / 9.25" and 11". You can't purchase a single "Triband Filter" for another telescope.

  • Lars

    Hi. I have a 7" f/9 Meade refractor and would like to use my Daystar Quark chromosphere filter on it.

    Will your 2" H-alpha filter placed in front of the diagonal be enough as ERF-filter on this telescope?

    • Team Baader Planetarium
      Team Baader Planetarium October 5, 2023 at 9:41 am

      Our CCD or CMOS or Langpass filters are all designed for use for visual and/or photographic use at nightsky objects. They are NOT designed for being placed in the beam of sunlight inside of a solar telescope! The filter glass is too thin and its too tightely clamped so it will burst when it gets warm. Furthermore these nightsky filters do not follow the specifications of Halpha solar filter manufacturers for use as prefilters with their filters. We can only strongly advise and seriously warn against using filters designed for night use on solar telescopes.

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