Narrowband filters - what way round?

[Hallingskies]

Should I have the knurled edge of narrowband filters (the bit you hold to screw it into the filter wheel) facing the camera or the scope?  Does it matter?  I’ve never thought about it before until my new EFW2 turned up and I realised I could mount the wheel either way round.

[Adreneline]

My Atik428 has a female thread immediately in front of the sensor window so I assumed filters went with their male thread toward to camera, so knurled edge away from the camera.

I'll be interested to see what others say.

Good question!

[Adam J]

 

51 minutes ago, Hallingskies said:

Should I have the knurled edge of narrowband filters (the bit you hold to screw it into the filter wheel) facing the camera or the scope?  Does it matter?  I’ve never thought about it before until my new EFW2 turned up and I realised I could mount the wheel either way round.

the thread faces towards the camera. 

[daz]

My AtikOne has the threaded part of filter facing towards the sensor, as this is how I access the wheel. If I were to thread the other way, I would need to disassemble the wheel completely...

Ultimately it doesn't matter, but it would depend on what clearance you have on the wheel, I guess.

[MarsG76]

I don't think it matters....

 

[simmo39]

Also read on other sites that it does not matter, mine face the sensor as it brings it closer to it.

[Stub Mandrel]

I can only fit 2" filters to my CC thread towards camera.

[Hallingskies]

Thanks to everyone who replied.  “Thread towards camera” seems to be the consensus, which certainly seems to be the way the ATIK EFW2 is built to be used.  Probably a no-brainer, really.

But in the short time since I joined this forum, the biggest thing I have learned is that (despite my best efforts) there are no stupid questions...?

[Adam J]

5 hours ago, simmo39 said:

Also read on other sites that it does not matter, mine face the sensor as it brings it closer to it.

6 hours ago, daz said:

My AtikOne has the threaded part of filter facing towards the sensor, as this is how I access the wheel. If I were to thread the other way, I would need to disassemble the wheel completely...

Ultimately it doesn't matter, but it would depend on what clearance you have on the wheel, I guess.

4 hours ago, Hallingskies said:

“Thread towards camera” seems to be the consensus.

 

In most cases it certainly does matter. But it depends on the type of filter. The filter will work in either direction dependent on type...for example, interference, absorption, dichromic. However, I grantee you that the anti reflection coating is optomised to work in one direction only with some cheaper filters only being coated on one surface. So the difference will only show in bright stars where you will get halos if its the wrong way around. Other types of filter such as an astronomic cls would not matter as they are double coated. It will also make a difference at very fast f-ratios <F3 where you will get more frequency shift (with loss of peak transmission) if you have it the wrong way around.

Still dont believe me??? Take a look here:

https://www.cloudynights.com/topic/609908-zwo-filter-orientation/

Adam J

[simmo39]

8 hours ago, Adam J said:

 

In most cases it certainly does matter. But it depends on the type of filter. The filter will work in either direction dependent on type...for example, interference, absorption, dichromic. However, I grantee you that the anti reflection coating is optomised to work in one direction only with some cheaper filters only being coated on one surface. So the difference will only show in bright stars where you will get halos if its the wrong way around. Other types of filter such as an astronomic cls would not matter as they are double coated. It will also make a difference at very fast f-ratios <F3 where you will get more frequency shift (with loss of peak transmission) if you have it the wrong way around.

Still dont believe me??? Take a look here:

https://www.cloudynights.com/topic/609908-zwo-filter-orientation/

Adam J

I use Baader filters and on there site it says it dosent matter. I have had no problems yet, not saying that may not change.

[Hallingskies]

52 minutes ago, simmo39 said:

I use Baader filters and on there site it says it dosent matter. I have had no problems yet, not saying that may not change.

 

[Hallingskies]

I did look on the Baader site before I posed the question.  Obviously not hard enough.

For unmounted filters, “which way round” could be a bit more difficult I suppose. The Cloudy Nights thread on this topic looked a bit equivocal to me...

[simmo39]

This is the bit from the Baader sight. First bit says to put the shiney side out but goes on to say:

But: this is true only for instruments without optical elements near to the focal plane. If you have f.e. 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 lesser then fort- and back- 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 have a CC fitted and went the other way to get the filter closer to the sensor.

[Adam J]

24 minutes ago, simmo39 said:

This is the bit from the Baader sight. First bit says to put the shiney side out but goes on to say:

But: this is true only for instruments without optical elements near to the focal plane. If you have f.e. 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 lesser then fort- and back- 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 have a CC fitted and went the other way to get the filter closer to the sensor.

Yes you are essentially only getting an anti-reflection coating in one direction, the filter works in both directions, but the anti reflection coating in only one direction. From above there are situations when it may be better the other way around. However, the end point is that it does make a difference. 

[simmo39]

Yep, don't disagree. But at the end of the day it depends on your set up. In time I may find that I may have to change mine but at the moment it seems to work for me.