Baader OIII filters - A Cautionary Tale

[Melitastro]

Back in 2016 I bought the Baader LRGBC + Narrowband 1.25" filter set. For various reasons I only used the LRGB and didn't use the narrow band filters until a few weeks ago.  Those of you who've bought these filters will know they advertise 'NO halos, NO reflections, NO ghosting' on the filter case, so I was really surprised when the images through the OIII filter had a really well defined halo.

I looked on Baader's website and contact with them is by using a web form so I posted a description of my filters, a copy of my NB images (Ha, SII and OIII) and a list of the equipment in my imaging train, and asked for their feedback on the OIII image.  However, their reply was a copy and paste - or "Because we have such inquiries regulary, we wrote a complete, detailed explanation about such effects" - which more or less said that the problem was most likely due to other optics in the imaging chain (there were none, only their filters) or the window in front of the camera or spacing of the optical elements (none again).  The problem is everything but their filter.  It even suggested reversing the filter, something I've never seen suggested before for a mounted filter.  Are you supposed to dismantle the FW so you can insert it from the back of the carousel? They even had the cheek to try and sell me on the aesthetic value of having halos because they even occur around bright stars when imaging nebulae (ok, we've all seen Alnitak in the Horeshead nebula photos!).  They concluded with "Most likely you cannot get rid of the Halo - unless using extensive image processing such as described by our customer Andreas Bringmann in the attached pdf".

Having been in charge of management systems before I retired, my approach to problem solving is a bit different to theirs, and with so few variables to consider the use of a different OIII filter seemed obvious.  Although I can't really afford it, I opted for the Astrodon 5nm as it is supposed to be the standard by which others are judged. The filter arrived yesterday and these are last night's results comparing the Baader and Astrodon - identical 100s exposures for each, the only difference being the refocus between images.

I think the images speak for themselves, so when customer service tell you:

 

what they actually mean is "Most likely you cannot get rid of the Halo - unless..." you stop using our filters!!

PS: I've got a Baader OIII filter going cheap - I'm saving for some more Astrodon NB filters 🙂

Tony

Edited September 10, 2019 by Grant
Screenshot modified to remove Baader employee name / URL at the request of Baader.

[Xplode]

Looking at the small halo it's easy to tell it's a reflection from within a filter.
Did you try to reverse the filter?
It can for certain help because you will switch side of the reflective side of the filter.

[Melitastro]

5 minutes ago, Xplode said:

Looking at the small halo it's easy to tell it's a reflection from within a filter.
Did you try to reverse the filter?
It can for certain help because you will switch side of the reflective side of the filter.

No I didn't, as I say in my post I didn't try to screw it in from the other side. Is that really a solution that's used with mounted filters, I've only seen it discussed with unmounted ones?

[Xplode]

It's very easy to flip the glass in a mounted filter, just cut a plastic sheet to fit the locking ring to unscrew it and flip it

Edited August 24, 2019 by Xplode

[Hallingskies]

My experience with the Baader Olll was exactly the same as yours.  Reversing the filter made no difference.

[continuum]

I also have the same experience and have been pondering on other makes of OIII filters. Because Astrodon prices are stratospheric I have been considering Astronomik or Optolong.  Does anyone have experience of these brands?

Ron

[continuum]

I forgot ZWO also.

Ron

[Melitastro]

14 minutes ago, continuum said:

I also have the same experience and have been pondering on other makes of OIII filters. Because Astrodon prices are stratospheric I have been considering Astronomik or Optolong.  Does anyone have experience of these brands?

Ron

I'm starting to realise how widespread this issue is. Has anyone had good experience with this filter?

[Geoff Barnes]

22 minutes ago, continuum said:

I have been considering Astronomik or Optolong.  Does anyone have experience of these brands?

I think the general consensus on here would be that Astronomik are among the very best and, as I have an Optolong HBeta filter I would say steer clear.

[Hallingskies]

32 minutes ago, continuum said:

I also have the same experience and have been pondering on other makes of OIII filters. Because Astrodon prices are stratospheric I have been considering Astronomik or Optolong.  Does anyone have experience of these brands?

Ron

I am using the 6nm Astronomik OIll.  There is some halation around bright stars but it is bearable and definitely superior to the Baader in my opinion, which put doughnuts around everything.  Astronomik do not boast that there are “no haloes” with their filter and I think you get what you pay for.

[PaulR1]

after seeing this baader are now off my shopping list .

I am looking at getting filters in the near future complete set LRGB , Ha SII and OIII.

So thanks for the heads up on this manufacturer and I'm sorry for your hassles

[dave_galera]

So reading this thread got me worried as I also have a Baader OIII filter. Just checked 5 different sets of OIII subs with at least one very bright star in the frame, and .......... I haven't got any halos around any bright star, slight halation yes but not definite halos, but mine are the 2" version not the 1.25", so maybe this makes a difference.

Edited August 24, 2019 by dave_galera

[continuum]

Based on replies so far, it looks like it has to be Astrodon as even Astronomik suffer with haloes even if smaller than Baader. 

Ron

[MarsG76]

How unfortunate.. I always saw Baader ad premium filters... personally I haven't experienced this with my Baaders... but I don't use a camera chilled to well below freezing.

 

[dave_galera]

5 minutes ago, continuum said:

Based on replies so far, it looks like it has to be Astrodon as even Astronomik suffer with haloes even if smaller than Baader.

There are quite a few reports that imagers getting halos with Astronomik filters seem to be camera related, and there are a lot of imagers who don't get any halos with these filters, and like I commented above I don't get halos with 2" Baader filter/Atik camera combination

[Whirlwind]

19 minutes ago, dave_galera said:

There are quite a few reports that imagers getting halos with Astronomik filters seem to be camera related, and there are a lot of imagers who don't get any halos with these filters, and like I commented above I don't get halos with 2" Baader filter/Atik camera combination

It comes down to what halos are and what causes them.  Baader are correct in this statement to some extent, although the commentary on professional telescopes also having halos isn't really relevant (professional telescope filters are set for maximising throughput not pictures).  

Halos are in effect defocussed images of the star.   To get a halo off a filter you need it to reflect light from a source.  This could be, for example light reflecting off the cover slip back on to the filter that then gets re-reflected back towards the CCD.  The light path for this is much longer than the focal length and hence by the time the light gets to the CCD it is way out of focus (hence the halo).  Alternatively the light may reflect off the filter onto a reducer/flattener glass and then be reflected back towards the CCD.  The same effect is seen.  A cover slip that transmits almost 100% of the light and has a very effective anti reflective coating hence shouldn't have halo issues with *any* filter as nothing is being reflected back on to the filter.  On the other hand a filter with extremely effective anti-reflective coating would not reflect light as well and prevent the halos (e.g. astrodon).  If both have anti reflection issues then you will get halos.  

As such it is not impossible that the some combinations work and some don't with any filter.  I'm not sure of the set up here but it is some form of reflector so if there is a flattener/reducer/coma corrector in the way you could try removing this and seeing if you still have the halos.  If they are still there then the chances the reflection is between the CCD cover slip and the filter.  If it goes away then you may find that a flattener/reducer/coma corrector with excellent AR properties (or possibly longer back focus to spread the light so far that it is not visible) may solve the problem.  

If it is between the cover slip and the filter then you probably will need new filters or a camera with better anti-reflective characteristics (which might be cheaper given the price of astrodons!)

[Melitastro]

8 minutes ago, Whirlwind said:

It comes down to what halos are and what causes them.  Baader are correct in this statement to some extent, although the commentary on professional telescopes also having halos isn't really relevant (professional telescope filters are set for maximising throughput not pictures).  

Halos are in effect defocussed images of the star.   To get a halo off a filter you need it to reflect light from a source.  This could be, for example light reflecting off the cover slip back on to the filter that then gets re-reflected back towards the CCD.  The light path for this is much longer than the focal length and hence by the time the light gets to the CCD it is way out of focus (hence the halo).  Alternatively the light may reflect off the filter onto a reducer/flattener glass and then be reflected back towards the CCD.  The same effect is seen.  A cover slip that transmits almost 100% of the light and has a very effective anti reflective coating hence shouldn't have halo issues with *any* filter as nothing is being reflected back on to the filter.  On the other hand a filter with extremely effective anti-reflective coating would not reflect light as well and prevent the halos (e.g. astrodon).  If both have anti reflection issues then you will get halos.  

As such it is not impossible that the some combinations work and some don't with any filter.  I'm not sure of the set up here but it is some form of reflector so if there is a flattener/reducer/coma corrector in the way you could try removing this and seeing if you still have the halos.  If they are still there then the chances the reflection is between the CCD cover slip and the filter.  If it goes away then you may find that a flattener/reducer/coma corrector with excellent AR properties (or possibly longer back focus to spread the light so far that it is not visible) may solve the problem.  

If it is between the cover slip and the filter then you probably will need new filters or a camera with better anti-reflective characteristics (which might be cheaper given the price of astrodons!)

As I mention, there are no other optics in the path, these are taken with just the filter and no reducers or flatteners etc. and the camera is ASI 1600MM-Pro at -10°C that has what ZWO refer to as the 'protective window' over the sensor

[dave_galera]

https://www.cloudynights.com/topic/634098-halos-around-stars-with-astronomik-oiii-6nm-filter/

Might be helpful as it does refer to the same camera

Edited August 24, 2019 by dave_galera

[Petergoodhew]

I had halos with Baader OIII filters too.  I then discovered that they had made a faulty batch of OIII filters and this was the reason for the halos.  Eventually Baader agreed to replace my filter free of charge on condition that I returned the faulty filter. My new Baader OIII filter has no halos.

[Melitastro]

53 minutes ago, Petergoodhew said:

I had halos with Baader OIII filters too.  I then discovered that they had made a faulty batch of OIII filters and this was the reason for the halos.  Eventually Baader agreed to replace my filter free of charge on condition that I returned the faulty filter. My new Baader OIII filter has no halos.

Mine will have come from stock held by Teleskop Service in 2015, I wonder if they are from the same batch?

[Petergoodhew]

11 minutes ago, Melitastro said:

Mine will have come from stock held by Teleskop Service in 2015, I wonder if they are from the same batch?

Quite possibly. Mine were bought then too.

[Hughsie]

Hi Tony,

I bought the same filter from FLO in the last month and used it last night on the Western Veil Nebula. I was processing the images this morning and whilst waiting for Pixinsight to do its thing I came across this thread. Suffice to say i did a quick stacki and other than the glow from 52 Cyg which is pretty much in your face, there were no obvious signs of halos around the other stars. Plenty of egg shaped ones but no halos

John

 

[ApophisAstros]

I suppose it wasn't anything to do with Focus ,  sorry to ask but how did you focus with the Baader as opposed to the Astrodon it was a different focus point ?

Stars are naff in the best NB images apparently anyway.

Regards

Roger

[Melitastro]

8 hours ago, apophisOAS said:

I suppose it wasn't anything to do with Focus ,  sorry to ask but how did you focus with the Baader as opposed to the Astrodon it was a different focus point ?

Stars are naff in the best NB images apparently anyway.

Regards

Roger

I can't remeber 100% which star, there are plenty available from my site. It would have been a 2nd or 3rd mag using APT with Bahtinov aid and mask. Recorded the focus point for each (I'm using a Pegasus focus cube) and then used the same difference between focus positions when imaging.

Tony

[G4EQZ]

On 24/08/2019 at 11:22, continuum said:

I forgot ZWO also.

Ron

I have ZWO 1,25 filters. Their Oiii filter is awful and it’s the newer variety so avoid!