Using UHC with Barlow

[Bugdozer]

I was testing out my new UHC filter last night by observing M27 and M57. It definitely provided some clarity and I was pleased with the results. Because M57 is small, I decided to add in my 2x Barlow to make it a little bigger. However, when I did that I found that the view became weirdly "obstructed", as if there were dark fringes that moved depending on the position of my eye. I have never experienced this before and wondered if it was because my filter was between the eyepiece and the Barlow. Is it better to put the filter directly in front of the Barlow? If not, can anyone explain what these strange dark areas were caused by? 

[Ricochet]

On 07/10/2023 at 09:54, Bugdozer said:

Is it better to put the filter directly in front of the Barlow?

As a general rule, it is always better to place the filter that reflects the most light as far forward in the optical chain as possible. Any light reflected of a component (in this case the UHC filter) could then be reflected back off an element in front of it (e.g. the lenses in the barlow) causing unwanted reflections. Some cheaper UHC filters also have issues caused by non-uniformity of the coatings. However, if you have any optical issue, and that issue moves in line with any component, in this case your eye, then it is usually the case that the issue lies within the component that you have moved.

From a quick look at your posts I see references to an F10 SCT and a 25mm eyepiece. Assuming that you were using these two items then you will have had an exit pupil of 2.5mm, which is about the smallest size that you want to use with a UHC filter, because the amount of light blocked by the UHC filter has to be counteracted by increasing the exit pupil to achieve a bright image. If you then add in a 2X barlow you will be decreasing the exit pupil to 1.25mm, which usually makes extended objects too dim even without a filter. With the additional light loss from the filter, the entire view will be so dim that my suspicion is that your brain doesn't have enough information to properly filter out the artefacts that it automatically removes under brighter conditions. Specifically, I would suggest that you may be seeing the blood vessels in the retina, which are in front of the photo-receptive cells, but are absent from the central region.

[Bugdozer]

OK, I will try putting the filter in front of the Barlow then. 

It's not the blood vessels in my retina I was seeing, it was definitely some kind of light distortion. 

[Bugdozer]

On 08/10/2023 at 11:29, Ricochet said:

As a general rule, it is always better to place the filter that reflects the most light as far forward in the optical chain as possible. Any light reflected of a component (in this case the UHC filter) could then be reflected back off an element in front of it (e.g. the lenses in the barlow) causing unwanted reflections. Some cheaper UHC filters also have issues caused by non-uniformity of the coatings. However, if you have any optical issue, and that issue moves in line with any component, in this case your eye, then it is usually the case that the issue lies within the component that you have moved.

From a quick look at your posts I see references to an F10 SCT and a 25mm eyepiece. Assuming that you were using these two items then you will have had an exit pupil of 2.5mm, which is about the smallest size that you want to use with a UHC filter, because the amount of light blocked by the UHC filter has to be counteracted by increasing the exit pupil to achieve a bright image. If you then add in a 2X barlow you will be decreasing the exit pupil to 1.25mm, which usually makes extended objects too dim even without a filter. With the additional light loss from the filter, the entire view will be so dim that my suspicion is that your brain doesn't have enough information to properly filter out the artefacts that it automatically removes under brighter conditions. Specifically, I would suggest that you may be seeing the blood vessels in the retina, which are in front of the photo-receptive cells, but are absent from the central region.

Further to this: I realised my diagonal has a fliter thread on it. So would that be the optimal place to put it? Certainly handy for changing eyepieces!

[John]

35 minutes ago, Bugdozer said:

Further to this: I realised my diagonal has a fliter thread on it. So would that be the optimal place to put it? Certainly handy for changing eyepieces!

Yes, the end of the diagonal barrel is a good place to put the filter. You just need to remember it is there so you take it off when you move to a target that the filter does not help with. I've done that quite a few times 🙄

Edited October 12, 2023 by John

[bosun21]

44 minutes ago, John said:

You just need to remember it is there so you take it off when you move to a target that the filter does not help with. I've done that quite a few times 🙄

You’re not alone John. I’ve done this multiple times and I now place them on my eyepiece due to this.

[RT65CB-SWL]

1 hour ago, Bugdozer said:

Further to this: I realised my diagonal has a fliter thread on it. So would that be the optimal place to put it? Certainly handy for changing eyepieces!

42 minutes ago, John said:

Yes, the end of the diagonal barrel is a good place to put the filter. You just need to remember it is there so you take it off when you move to a target that the filter does not help with. I've done that quite a few times 🙄

Agreed! 

Also, if you have a variable polarising filter, screw one half into the diagonal and the other half into the eyepiece.