The Importance of Using Filters

[astroavani]

Even a great camera and a great telescope, nothing can do against a bad seeing. But there is one factor, which allows an experienced photographer to work around this problem.
This factor are the filters, well used, are fundamental in the most diverse fields of photography. In high resolution Lunar and planetary photography, they can simply save a session that would simply be lost.
We all know that the smaller the wavelength, the higher the resolution. From this it can be deduced, that the Infrared having long waves, will have a resolution much smaller than the green or blue, which has shorter waves for example.
Therefore, we should only use the short-wave filters in high-resolution photography, but here is the crucial factor we mentioned above, seeing.
If seeing is good, everything points to using a green filter, or even blue, on a photo that will be converted to black and white. However as most of the time the seeing is from medium to bad the use of a red filter or even Infrared can simply save the job. This is because long waves are less affected by turbulence or bad seeing than short waves. In the case of the attached photo, we have an evident proof of this affirmation, I estimate that the seeing should be around 2/5, 1 being considered lousy, and 5 would be excellent. When we look at the photos carefully, we can see the continuous gain in terms of resolution, of the wave filters getting longer.
In the end, everything depends on the experience of the photographer, knowing how to analyze the moment of capture and thus use the one that is the most appropriate filter.
Photos taken at intervals of 1 minute from one to another, for each photo 2500 frames were captured and 345 stacked. All stacked in AS! 2 with 50% sharpned. No further processing was done, trying to keep them as homogeneous as possible.
https://www.astrobin.com/full/382824/0/?nc=

[MilwaukeeLion]

Nice examples, I prefer red filter from my rgb imaging set over 685 nm IR pass on moon.  IR pass gives moon kind of a pronounced plastic sheen, most nights red filter out preforms it for detail with my equip/conditions.  Not easy to eliminate that sheen in post, adding any multiscale contrast and image ends up too bright/lose detail.  Don't have that problem in post with red filter, the multiscale contrast seems to bring out more detail.

[vlaiv]

Have you tried Solar continuum filter from baader for this purpose?

I think it should give very good results for lunar. It is still in green part of spectrum, but it has much narrower pass band compared to above filters.

[MilwaukeeLion]

5 hours ago, vlaiv said:

Have you tried Solar continuum filter from baader for this purpose?

I think it should give very good results for lunar. It is still in green part of spectrum, but it has much narrower pass band compared to above filters.

Will try that next clear night.

[Moonshane]

I see what you are saying but most people would be delighted with any of these!

[Ruud]

The results speak for themselves. In blue you get smaller airy disks, but if the seeing is much worse in blue, that advantage is lost.

I wonder, are atmospheric conditions ever so that the seeing is better in other wavelengths than infrared?

[vlaiv]

It's all about refraction of the light, after all this is why there is seeing after all.

This image explains it pretty much completely:

Angle of refraction depends on wavelength - red gets bent the least, while blue part gets bent the most. It shows another one important thing - if you have single wavelength of light all of it is going to be bent to same degree, but if your filter passes wide band with different wavelengths - each of those will be bent by different amount, thus creating additional blur - this is why narrow band filters should work better than broad band.

There are couple of things to balance for best performance when using narrow band filter:

1. center wavelength - as we have seen, atmosphere has the greatest influence in blue and the least in red and IR part of spectrum

2. Airy disk size - again depends on wavelength but in reversed way compared to atmospheric blur - blue has advantage over red part of spectrum

3. QE of sensor also depends on wavelength - here green part of spectrum has edge as most sensors have peak QE in this part and it goes down as you move towards blue and red. Higher QE allows for shorter exposures which is important to achieve coherence time to freeze the seeing.

4. Width of band pass of filter - wider the filter band pass, more different wavelengths get thru and each of those will have different angle of refraction in atmosphere, but also more signal will get thru allowing for better SNR and shorter exposure

I don't have a clue how much each of these is important in comparison to others. Baader Solar continuum filter is just about in the middle for most things - so I guess it should work fine for this purpose. CWL is 540nm, so good balance between airy disk size and atmospheric influence and in region where most sensor peak in their QE. It has FWHM of around 10-12nm, so neither very narrow (like 3 or 7nm nebula narrow band filters in OIII or Ha) but still narrow enough to be considered narrow band filter.

If one does not have Solar Continuum but has common NB filters like OIII and Ha, I'm sure these would be very useful for moon capture as well. Indeed, I think it would be interesting to see comparison between these two, similar to above one - it would certainly be indicative of what is more important - airy disk size or angle of refraction as blur due to variety of wavelengths would be almost taken out of equation.

[astroavani]

15 hours ago, vlaiv said:

Já experimentou o filtro de continuidade solar da Baader para este fim?

Eu acho que deveria dar resultados muito bons para a lunar. Ele ainda está na parte verde do espectro, mas tem uma faixa de passagem muito mais estreita em comparação com os filtros acima.

I received a double stack just two weeks ago, so I did not have time to test it calmly, but I realized that in the achromatic refrator AR 152mm it was the filter that gave me the best result. But as they are only 150mm nominal aperture this was already expected. I want to test now with the great opening of the C14!

[astroavani]

8 hours ago, vlaiv said:

Tudo gira em torno da refração da luz, afinal, é por isso que há afinal de contas.

Esta imagem explica isso quase completamente:

O ângulo de refração depende do comprimento de onda - o vermelho fica menos inclinado, enquanto a parte azul se curva mais. Ele mostra outra coisa importante - se você tiver um único comprimento de onda de luz, ele será dobrado no mesmo grau, mas se o filtro passar por uma banda larga com diferentes comprimentos de onda - cada um será dobrado por uma quantidade diferente, criando assim borrão - é por isso que os filtros de banda estreita devem funcionar melhor que a banda larga.

Há algumas coisas para equilibrar para melhor desempenho ao usar o filtro de banda estreita:

1. centro de comprimento de onda - como vimos, a atmosfera tem a maior influência em azul e a menor em vermelho e parte IV do espectro

2. Tamanho do disco arejado - mais uma vez depende do comprimento de onda, mas de maneira inversa em comparação com o desfoque atmosférico - o azul tem vantagem sobre a parte vermelha do espectro

3. QE do sensor também depende do comprimento de onda - aqui, a parte verde do espectro tem a ponta, pois a maioria dos sensores tem pico QE nesta parte e desce à medida que você se move na direção do azul e do vermelho. Um QE mais alto permite exposições mais curtas, o que é importante para obter tempo de coerência para congelar a visão.

4. Largura de faixa de passagem do filtro - mais larga a passagem da banda do filtro, comprimentos de onda mais diferentes e cada um deles terá diferentes ângulos de refração na atmosfera, mas também mais sinal será obtido permitindo melhor SNR e menor exposição

Eu não tenho idéia de quanto cada um deles é importante em comparação com os outros. O filtro contínuo da Baader Solar está praticamente no meio para a maioria das coisas - então eu acho que deve funcionar bem para este propósito. O CWL é de 540 nm, portanto, um bom equilíbrio entre o tamanho do disco e a influência atmosférica e na região onde a maioria dos sensores atinge o pico em seu QE. Tem FWHM de cerca de 10-12nm, portanto, nem muito estreito (como filtros de banda estreita de nebulosas de 3 ou 7nm em OIII ou Ha), mas ainda estreito o suficiente para ser considerado filtro de banda estreita.

Se alguém não tiver Solar Continuum, mas tiver filtros comuns de NB, como OIII e Ha, tenho certeza de que estes também seriam muito úteis para a captura da Lua. Na verdade, acho que seria interessante ver uma comparação entre esses dois, semelhante a um - certamente seria indicativo do que é mais importante - tamanho do disco ou ângulo de refração, já que o borrão devido à variedade de comprimentos de onda seria quase eliminado. de equação.

Excellent explanation friend Vlaiv!
In fact I'm looking forward to testing the Solar Continuum, the problem was that I bought it a little while ago and I have not yet had the opportunity to try it out as I would have liked to have it in the dark. But today I am preparing a session that should shed some light on this subject, I hope to post it briefly the evidence of our thinking.

[vlaiv]

8 hours ago, astroavani said:

I received a double stack just two weeks ago, so I did not have time to test it calmly, but I realized that in the achromatic refrator AR 152mm it was the filter that gave me the best result. But as they are only 150mm nominal aperture this was already expected. I want to test now with the great opening of the C14!

With refractors, especially achromatic refractors, this filter has additional advantage - it eliminates chromatic aberration as any narrow band filter does, but it also isolates wavelengths where most achromatic refractors have highest strehl ratio. Usually achromats have the least spherical aberration in green around 500-550nm.

I was able to get incredibly sharp high power lunar view from my short focal length achromat - skywatcher ST102 F/5 when using this filter.

[bottletopburly]

Ir pass 685 looks sharper to my eyes, there doesn’t look that much between them at all to me .