SHO Filters and Star Halation

Glenn Mitchell

I sold my Baader 6.7nm 2" SHO filters. While they advertise no halos, that was not my experience.

I receive Astronomik 6nm 2" SHO filters today. Astronomik advertises no reflections. Does anyone have experience with halos from their 6nm filters?

Gerhardt at Astronomik tells me no halation is impossible as a matter of physics. He also remarked that he makes filters for use, not for testing (whatever that means, since I'm buying them to use.)

My mono camera is a ZWO ASI2600MM. My EFW is made by ZWO. 7 x 2". The inside of the entire optical train to the sensor is anodized matte black. The protective window for the sensor is anti-reflective coated glass. 

I could be wrong. It seems to me that halos are likely to come primarily from reflection between the sensor and the back of the filter. I suspect the light is mostly orthogonal to the filter so that any reflection would not stray.

Another possible source is light reflecting from the front of the filter and back up the OTA to the rear lens element. I use a pair of ZWO APO refactors. 65mm and 107mm. Sometimes I use field reducers. The rear element could be part of a quintuplet or quadruplet or FR. That light would then have to reflect back down the optical train to and through the filter and on to the sensor. That seems like a less likely source. Seems that way to me, if for no other reason than parsimony. More needs to converge just right for halos to result from light bouncing around in front of the filter.

My whole optical train between scope and camera is M54. The 2" filters are the only time the optical train diameter drops to 2"".

Sorry about all the detail. The problem with halos, it seems to me, is the problem is over-determined. It's likely not just a single, simple explanation. 

Is it a fool's errand to avoid halos around high magnitude stars with NB filters?

First clear night, I 'll grab Alnitak and a bright star without ionized gas like Deneb.

Cheers all!

Mitch

BlockHead

How light behaves with different filters in the presence of other optical surfaces is complex. For example you note that one (of many) paths light can take is to come through the filter, be reflected up to the backside of the filter from the sensor window and back down to the sensor. In addition, some fraction the light can also enter the filter... reflect off of  the backside of he first surface and then back through the sensor window to the sensor. Depending on the optical thickness of the filter in wavelengths of light- this can cause an interference (ugly!) pattern. So just specifying a particular filter or single optics doesn't help. It really is the complete set of optical components and surfaces that matters.

Focal correctors and reducers almost always in my experience cause issues. You need to weigh their benefits with the downsides of reflecting and scattering light which will show up typically with bright stars. What you say is a less likely source is often a very big source. It depends on a number of factors- but the optical coatings (their type, thickness, quality...etc) play a big role. 

I prefer not to use and glass elements like correctors and reducers if possible.

The Takahashi Epsilon 180 system I use does have a corrector... oh well.

-the Blockhead

Glenn Mitchell

Thanks, Adam, for clarifying that the reflection of light between the filter and the rear scope element can also be significant.

The reason I thought reflections between filter and sensor cover might be more significant is the angle of reflection to keep the light scatter in tight proximity to the star.

My scopes are both Petzval designs. Spherical correction is built in. However, the FR screws in. I am queasy every time I reach for it. For the reason you mention. Anytime I add glass to the optical train, I'm uneasy.

Both sides of the filter appear highly reflective. I would hope that Astronomik uses some sort of AR coating. I don't know if or the nature. I know my camera does but the type and its effectiveness, no clue. Likewise, I don't know the substrate, the details of its polishing.

It sounds like the match between any filters (SHO filters in particular, perhaps) and the optical train, and the sensor window is this: you spend your money, you take your chances owing to all the variables. You have to rely on the precision of the vendor and factors about our optical train for which we have little control.

Glenn Mitchell

Upon reflection (pardon the pun), I believe I understand why you mentioned the reflection from the front of the filter back toward the scope.

The dichroic nature of the filters and the 6nm bandpass means nearly all of the light is reflected back.

My Astronomik filters are fully reversible. Same dichroic coatings on both sides of the substrate.

My EFW is screwed to my camera. There's not nearly as much black anodizing inside the camera housing to absorb reflections like there is between filter and scope.

My camera has approx. 91% QE. Where do the uncaptured photons end up? Bouncing around randomly (contributing to noise?) until they end up in a charge well or absorbed by the surrounding black anodized walls?