Unmounted Filters - Which side should face the scope??

[teoria_del_big_bang]

I have just bought a set of Baader 36 mm unmounted filters (LRGBC & NB). I have also found the following information on the Baader website.

Always put the more reflective side towards the telescope side. On these filters were the position matters. This arrow indicates which face of the filter should be directed towards the sky (telescope-sided). All cell-mounted filters are already oriented in a way that the most appropriate filter face is facing the sky when the filter would be mounted directly on the front end of the nosepiece of a camera. 
If you mount your filter the other way, any reflected light would have a short to the camera sensor, resulting in a higher risk of getting some kind of back-reflections inside the camera field. Many sensors have highly reflective areas near the light sensitive area.

But:  this is true only for instruments without optical elements near the focal plane. If you have a coma corrector, field flattener, focal reducer, focal extender (to a lower degree due to concave surface), or in extreme cases a whole lens group for more complex field corrections a few centimeters in front of the filter it could be useful to flip the filter against the rule from above (thus having the arrow pointing away from the telescope). Cause in such cases the likelihood of reflections from the sensor could be better then fort and reflections from such glass-surfaces. If in doubt, it helps to make some test images from a star field with bright stars, using the filter in both ways for comparison.

I do have a flattener but just thought I would check whether others have found the above to be true or not.

Also I bought the full set of LRGBC filters my reasoning being that as I have a light pollution filter in the flattener which is applied which ever filter is selected I thought for Luminance I would be better using the clear filter. I can only fit 7 filters so need to either fit the clear or L filter, so am I right to use the clear one, or does it not really ,matter?

Steve

 

Edited August 13, 2019 by teoria_del_big_bang

[david_taurus83]

Theres a little arrow penciled onto the edge of the filters showing which way to point them. I have the same set by the way. Very close to parfocal. Only issue I've found is the halos on bright stars with the OIII. Regarding the L or C, I believe the L has UV and IR cut where the C is just that, a Clear filter that passes all wavelengths. Personally I shoot with the L and my LPS is permanently mounted in front of the flattener. Even for narrowband. To remove it means taking the image train apart. So i just shoot longer subs lol

[teoria_del_big_bang]

Thanks for reply @david_taurus83 and sorry for not asking earlier but I did not get a notice you had posted this. 

I was aware of the arrow and have already fitted then with the arrow pointing towards the scope (not the sensor) as I always thought should be the case.  The only thing that made me wonder was when I looked on the Baader site and saw the following:

If you have a coma corrector, field flattener, focal reducer, focal extender (to a lower degree due to concave surface), or in extreme cases a whole lens group for more complex field corrections a few centimeters in front of the filter it could be useful to flip the filter against the rule from above (thus having the arrow pointing away from the telescope). Cause in such cases the likelihood of reflections from the sensor could be better then fort and reflections from such glass-surfaces.

And would have just turned them around but thought I would see if others had followed this instruction or not. I have not had a real chance to try them as yet due to cloud cover. Are your arrows pointing towards the scope ?

Also was warned about the O111 halos prior to buying so went for the ultra NB Baader OIII to see if this was better.

Seems like my setup is very much identical to your own with the light pollution filter permanently fitted hence why I wondered if the L filter was really required or in fact was okay to use but made no real difference due to the Light Pollution filter.

Steve

[david_taurus83]

I have sent my OIII back to Baader for testing as none of the other filters give me halos. They are advertised as halo free after all. I'm a bit sceptical tbh as it's a known issue. Would be very interested to see how you get on with the 4.5nm version. Only one review I can find and that's on another thread here!

[teoria_del_big_bang]

On 18/08/2019 at 16:31, david_taurus83 said:

I have sent my OIII back to Baader for testing as none of the other filters give me halos. They are advertised as halo free after all. I'm a bit sceptical tbh as it's a known issue. Would be very interested to see how you get on with the 4.5nm version. Only one review I can find and that's on another thread here!

@david_taurus83  I just remembered this post of yours about how I got on with the ultra NB OIII. With one thing and another I have not managed too much imaging since buying them but the few I have done I do not seem to see much in the way of halos.

Here's a stack of 8 400S frames of OIII

And a close up of RH top corner

Steve

Edited October 22, 2019 by teoria_del_big_bang

[Physopto]

Last I read, Astronomik coat both sides of their filters so there is no preferred direction. I had a set of theirs a few years ago, and some of them did produce halos on Alnitak. They were returned under their guarantee and a new set procured. They were extremely good to deal with and pretty fast at replacing the filters. Great service. Shortly after I made enquiries about the actual refractive indexes on their glass substrate and exact (within experimental error) thickness  of the filters. If memory serves me the thickness is actually approximately 1.1mm.

Later on using Astrodon filters (3mm thick) and one  Astronomik caused too much trouble refocusing all the time as the backfocus between field flattener  and CCD sensor changed due to the refractive index differences. 

Astronomik state that for a small extra payment they will match the optical thicknesses of a set of filters for you. What that means in real terms is anybodies guess, as there will always be an error of some sort.

I have never used Baader as their 1.25mm filters at that time had  a very thick metal frame, much too thick for my QSI cameras causing too much vignetting. I see they now can supply unmounted filters.

Derek

Edited October 25, 2019 by Physopto