Filter distance to sensor. Does it really matter?

[Rocket Stars]

Hi! 

Ive read that some of the bloating/overexposed stars. Can be traced to a problem with light reflecting between filter and the sensor. 

If so, is it better to have the filters close to the sensor? (usually the normal solution)  Or does it really matter? 

 

[vlaiv]

Bloating of stars can be related to filter - but can happen only if:

- improper filter is used for a given telescope, so bloating is feature of the telescope not corrected by filter - this happens when for example when telescope is not well corrected for chromatic aberration and wide luminance filter is used in place where narrower luminance would block light towards UV and IR part of the spectrum most likely to be out of focus.

- there is optical distortion due to filter. This can happen if filter surface is not flat and parallel (front and back of filter to each other) - this is equal to having optically poor telescope - stars are distorted and bloated because of poor optics.

Over exposed stars can only happen if you over expose them - no feature of filter or telescope can make more light appear then there is reaching the telescope.

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Halo around bright stars can be associated with filters or other optical elements. Whenever you have something in optical train in front of the sensor that has reflective surfaces (and poor anti reflexive coating) - this can create halos. If filters are causing this - moving them closer to sensor will make things worse.

Example of star halo due to reflection:

Here is another example - filter is closer and halos are smaller but more intense:

Here is diagram of what is (usually) happening in these cases:

Light beam is coming down and is focused on sensor (comes to single point - star). There is some surface in front of sensor, or even sometimes sensor itself (micro lens or other feature of sensor) that reflects part of the light back. Even with very good anti reflective coatings some percent of the light is reflected back (less than 1% in case of good coatings). This light goes back - but hits filter from back side (or other optical element like field flattener / coma corrector / reducer - whatever) and there it also some of the light bounce back (again depends on coatings on that particular element - interference filters are the worst because they are made to reject some frequencies by reflecting them) and continues back towards the sensor.

When it reaches sensor again - it is very much out of focus and presents itself like large defocused star around original focused star. Sometimes it can be a bit to the side - if incoming light ray was at an angle - star was away from optical axis, and very rarely it can even be on opposite side of sensor - this happens if reflecting element is far enough and only on the brightest stars - but in this case, offending element is usually not filter.

On right side of above diagram - same thing, except this time filter was placed further away. Only difference between two cases is level of defocus that halo will have. More it is defocused - it is larger and that means it is spread over more pixels. This means that each individual pixel will receive less light (again - we only have so much light reaching us from particular star - it is fixed per time interval) and halo will be fainter. If it is faint enough - at the level of background noise, it will not be seen.

To reiterate - better optical coatings means less percentage of light is reflected (less intensity of effect), and further away filter is - light is spread over larger surface - again less intensity per pixel.

We can argue that this sort of thing always happens, and that it's just matter of how much light there is in a star halo that makes difference if it will be seen or not (this explains why it is seen only on bright stars in the image and not all).

Btw, there is reason why you might want to place filters closer to sensor and that is related to vignetting, so it is balance - place it close enough to avoid strong vignetting, but not too close to avoid strong reflections.

 

[Rocket Stars]

Wow vlaiv!! 

You are big, big book of knowledge!  

I do really appriciate your very, very good explaination! 

Halos, was the first thing on my mind. Are still setting up a hyperstar with filter holder. And I read a pdf on Baader´s sit about halo effects from filters. 

That made me wonder if I should set the filterholer as close as possible to the sensor. Or as far as possible. Now, there is less than 20mm difference of the options. 

Thank you so much for your time and effort, with diagrams and all!