Are filters reversible or mono directional?

[Adam J]

Exactly what I just said really. Can you reverse a filter and get the same results. This has come about as I have a Astronomic CLS clip filter, the tape on the front (that retains the filter within its mounting and has the writing on it) started to peal away, so I removed the filter from the clip mount and remounted it holding it in place using a small amount of silicon. However, afterwards I was not entirely sure if I had mounted it the right way around. This is not an issue as the filter can be placed into the camera in either direction, however I just want to know if it actually matters?

 

 

 

[vlaiv]

I don't think it actually matters, but I might be wrong.

For absorption filters it does not matter for sure - it is the glass that absorbs the light and it should be homogeneous so same absorption from each side as it happens "inside".

For interference filters I'm not so sure. Here rejection happens not in glass itself but rather in coating layers - maybe ordering of layers has something to do with rejection - since it is interference based - one layer reflects light back and changes phase other just reflects back - no phase change and phases cancel out, on the other hand maybe single layer cancels certain wavelength - reflection from rear surface of layer interferes with incoming wave front, or it might be the actual boundary of two layers that does the trick. So it might be that the order of layers is important - if you flip the filter you flip also the order of all layers - back to front.

Found this on google:

" As a general rule, the highly reflective side of the filter should always face the source of radiation. This minimizes the thermal load on the absorbing glass blocking components and epoxies, thereby extending the lifetime of the filter. Apart from reduction of thermal effects, filter orientation is without influence on the spectral characteristics. "

So it might not be important

[ChrisLX200]

Interference filters need to point the right way, they reflect the rejected light back out the front (which in itself can be a problem with some - like the IDAS series - which can result in unwanted reflections coming back at you too from rear optical elements.

ChrisH

[Adam J]

19 minutes ago, ChrisLX200 said:

Interference filters need to point the right way, they reflect the rejected light back out the front (which in itself can be a problem with some - like the IDAS series - which can result in unwanted reflections coming back at you too from rear optical elements.

ChrisH

So is a CLS Clip filter likely to be an absorptive or interference type filter?

[ChrisLX200]

Just now, Adam J said:

So is a CLS Clip filter likely to be an absorptive or interference type filter?

interference I would think.

ChrisH

[Adam J]

Just now, ChrisLX200 said:

interference I would think.

ChrisH

Figures, it looks the same on both surfaces and when you hold it up to a white light source I cant see any difference either. Guess Ill just have to try it on a bright star and if I get reflections flip it over. 

[Adam J]

Annoyingly it seems that the clip is actually just a 1.25 inch filter in a different mounting....so why does it cost so much more???? 59 pounds Vs 106 pounds on modern astronomy.....feel slightly violated. 

[ronin]

If it is interferance then it will probably be directional for the best results and I would ecpect the CLS to be an interferance filter, as it passes 460 to 520 then blocks then passes 640 to 680. Absorbtion filters usually block/oass at a singel bit of the spectrum. As in Block IR then pass everything else of shorter wavelength.

You may get very little difference but I would expect a difference just the question is the degree of it.

[Adam J]

53 minutes ago, ronin said:

If it is interferance then it will probably be directional for the best results and I would ecpect the CLS to be an interferance filter, as it passes 460 to 520 then blocks then passes 640 to 680. Absorbtion filters usually block/oass at a singel bit of the spectrum. As in Block IR then pass everything else of shorter wavelength.

You may get very little difference but I would expect a difference just the question is the degree of it.

So one good question is how do people who use unmounted filters know which way around they are supposed to be placed into the filter wheel? If they are indeed directional....

[ChrisLX200]

2 hours ago, Adam J said:

So one good question is how do people who use unmounted filters know which way around they are supposed to be placed into the filter wheel? If they are indeed directional....

If they are unmounted they will be marked on the edge with a chevron or arrow which points towards the incoming light, a mounted (screw-fit) filter only fits one way.

ChrisH

[Adam J]

Sent an e-mail to astronomic. The CLS is the same in either direction.

[Gina]

This is interesting as two filter wheels I have are opposite ways round.  With the ZWO EFW the filters screw in from the camera side whereas with the Atik they screw in from the other side.  I have found that both NB and LRGB filters appear to work well in both filter wheels.  I also have some unmounted 36mm NB filters that have no indication as to which way round they go and I was unable to find any information either supplied with the filters or online that mentioned it.

[LukeSkywatcher]

I cant speak of photographic filters but i do know that most (if not all) of the filters i have for my scopes are threaded on both ends so can be used either way round and/or stacked together. Even my ND3.0 solar filter for my Hershel Wedge is threaded at both ends to allow it to be screwed into the wedge and then for other filters (polarising or continuum filter) to be stacked. 

Not much help, but my point is that i assume any filter for any purpose is reversible and yield the same results. Best to always check with the retailer or company who make them......as i see you already have done.

[Adam J]

On 16/11/2016 at 08:27, Gina said:

This is interesting as two filter wheels I have are opposite ways round.  With the ZWO EFW the filters screw in from the camera side whereas with the Atik they screw in from the other side.  I have found that both NB and LRGB filters appear to work well in both filter wheels.  I also have some unmounted 36mm NB filters that have no indication as to which way round they go and I was unable to find any information either supplied with the filters or online that mentioned it.

That is interesting as my Baader 2" 7nm H-A is definitely directional. The outside coating (facing the scope) is clearly a different to the inside coating. The man from Astronomik told me that the CLS is a reflection filter that works exactly the same in both directions. However he also indicated that this was not the case with all astronomik filters. He is also sending me a replacement sticker so I can properly fix the filter.