Filters at separate positions in image train

[Juicy6]

Hey

What do experts think about my setup? I always have an IR/UV cut filter mounted first in image train, just after focusing draw tube and before flattener. Then I have a filter drawer near camera where I can switch between, clear focusing, CLS and Optolong L-enhance Duo filters.  Two questions:

Am I asking for trouble with internal reflection?

Is it good practise to always have IR/UV cut filter mounted? I only use scope for imaging.

This is my setup:

 

 

Christer, Sweden

[vlaiv]

1 hour ago, Juicy6 said:

Am I asking for trouble with internal reflection?

Using any sort of interference filters? You certainly are asking for reflection trouble . Good/bad thing about it (depends on how you look at it) is that you don't really have much control over it, or rather you have no idea for the most part of how your actions will affect end result. One configuration might lead to very bad reflections, then change something by very small amount and reflections are gone. This is because light interaction with itself is complex thing and depends on very short distances - order of wavelength of light in question (it is due to interference of light with itself).

Could be that you will have reflections in certain combination, but probably best attitude to have towards that fact is: "Cross that bridge when we come to it ....".

1 hour ago, Juicy6 said:

Is it good practise to always have IR/UV cut filter mounted? I only use scope for imaging.

In general no. Sometimes you need to have your IR/UV cut filter "permanently" mounted, but most of the times having double stacked filters hurts your efforts unless you have very specific reasons to stack filters.

In your above case - it would probably hurt more then help. If you look at transmission curves of filters you are using together, you will see that they are redundant. In fact, here is good example for and against having stacked UV/IR cut filter:

This is comparison between CLS-CCD and CLS (plain or visual) transmission curves. CCD version of CLS filter does not pass any light below 400nm and no light above 700nm (same as UV/IR cut filter would do) - so in case you are using CLS-CCD filter - UV/IR cut filter is not needed. In case of plain CLS filter used mainly for visual, things are different - that one does not filter out light above 700nm. This is IR part of spectrum and human eye can't see it, but sensor can detect it and refracting telescopes are not well corrected in that part of the spectrum. In this case you need UV/IR cut filter.

I've shown you example where you need to have UV/IR cut filter combined (other cases include some RGB filters and in general any filters that have "leaks" in UV or IR part of the spectrum and you are using refractor - then you need stacked UV/IR cut filter), and example where you don't need one - but does it hurt to have one?

Well it does. A bit - and again that will depend on filters. First thing - more possibility of reflections. In your case this is minimized by large spacing between filters. Second thing - you can see from the graph above that filters don't have 100% transmission and cause some light loss. If you don't need filters stacked - why block light more than you need to? 90% * 90% = 81%, so you can loose as much as 10% of light when you stack filters. Third thing is that filters are not ideal in optical performance - they distort light, and although that distortion is low and filters are usually 1/10 wavelength in wavefront aberrations - again such aberrations compound together like light loss - so why distort wavefront more than you need.

I want to address one more thing in the end - distance of filters from the sensor. That is sort of battle of two things - you want your filter close enough to sensor as not to introduce vignetting (which depends on sensor size, filter size and speed of telescope light cone) but you also want your filters far away enough to reduce impact of reflections.

Reflections are always there - it is just about amount of light that gets reflected and how concentrated that light is on chip. By having filter (or other source of reflected light) further away from sensor - reflected light reaching the sensor will be more out of focus and thus spread over larger surface - which means each pixel will receive less photons, and if level of photons from reflection is below noise floor - you will not see it in the image.

Since you are using 2" filters - you can move your filter drawer away from camera without much fear of introducing vignetting because of that. This means that you have some "room for maneuvering" if you get reflections from your filter in the drawer - you can always swap filter drawer and extension tube positions in your diagram above - that moves filter further away from sensor and yet keeps total distance between FF/FR and sensor the same.

[Juicy6]

2 hours ago, vlaiv said:

You certainly are asking for reflection trouble. Good/bad thing about it (depends on how you look at it) is that you don't really have much control over it, or rather you have no idea for the most part of how your actions will affect end result ... Could be that you will have reflections in certain combination, but probably best attitude to have towards that fact is: "Cross that bridge when we come to it ....".

Since you are using 2" filters - you can move your filter drawer away from camera without much fear of introducing vignetting because of that. This means that you have some "room for maneuvering" if you get reflections from your filter in the drawer - you can always swap filter drawer and extension tube positions in your diagram above - that moves filter further away from sensor and yet keeps total distance between FF/FR and sensor the same.

Thank you for many wise words. The reason for my questions was that my UV/IR cut filter was too thick for the filter drawer. As I had the filter cell adapter in my junk box, I came up with this idea. I will instead try to find a thinner UV/IR filter, which will fit in the drawer.

Christer, Sweden

[andrew s]

Some filters have quite high light leakage out of band either in the IR or UV so stacking in that case may not be such a bad idea.  If stacking I would go from narrowest to widest as the light goes target to camera to minimise reflections. 

Regards Andrew