IDAS D2 halos

[Ken82]

Following my previous post about some halos i did a more thorough test. Ignoring the exceptionally bad light pollution the difference between having no filter and connecting the Idas D2 into the optical train is very clear. This is 1min unguided exposure at iso 800. The D1 filter has the same result as having no filter. Should i try to position the filter in a different part of the optical train ? FLO tell me they are about to release a filter holder  that fits between the flattner  and focuser. If the halo is caused by reflections onto the back of the flattner this would solve the problem.

In terms of light pollution supression the D2 has been a better filter to the D1 although the halos are an obvious issue.

[michaelmorris]

I'm a little confused (not difficult).  Where in the optical train is your filter presently positioned?

1 - between your camera and your flattener, or

2 - between your flattener and your focuser?

[Ken82]

1 minute ago, michaelmorris said:

I'm a little confused (not difficult).  Where in the optical train is your filter presently positioned?

1 - between your camera and your flattener, or

2 - between your flattener and your focuser?

Between flattner and camera

[sharkmelley]

It's difficult to be sure of what is causing that halo.  If you take a number of exposures, with the star in a different position of the image each time, is the halo just above the star each time or does it move relative to the star?  It's odd to see a halo that is slightly off-centre from the star.  Maybe choose a target with more bright stars (e.g. the Pleiades).

What's the diameter of the halo (in pixels) and what is the focal length and F-ratio of the scope being used?  Does the flattener also have a reducing effect?  If so by how much.  The answer to these questions will help understand the imaging geometry.

Mark

[Ken82]

I may just have found my halo problem. FLO were excellent and sent me a new unrealeased filter holder cell for the esprit which arrived this morning. I wanted to test the filter before the flattener and this adapter allows me to do that.

This afternoon I took the filter off to give it a quick clean before placing the new adapter back on. I found there is a small circular imperfection in the glass (bottom of my picture). I’m not saying 100% this will solve it but what do others think ? Is this imperfection likely to cause my halos ? 

Thanks ken 

 

[sharkmelley]

I'm not sure I'm seeing the circular imperfection.    But if there is an area of glass where the AR coatings are failing then it could certainly cause the problem.

Mark

[Ken82]

I can definitely feel a textured edge when i run my finger over the imperfection. I hope this has been causing the issue as i do feel it has been an improvement over the D1 filter which doesnt cause halos in my imaging setup. 

[sharkmelley]

That is such a tiny imperfection.  I seriously doubt it will be causing your halo.

Mark

[Ken82]

That’s a shame I was getting hopeful. I still don’t understand why this particular filter gives me halos when my others don’t. 

It’s got to be worth a try as I’ve never seen this sort of imperfection on other filters. 

[Ken82]

I’ve tested putting the filter before and after the flattener but this doesn’t change anything. So it’s not reflection off the flattener. Anything else I can do ?

picture is a comparison D2 and D1 

[Ken82]

On 19/01/2019 at 13:40, sharkmelley said:

It's difficult to be sure of what is causing that halo.  If you take a number of exposures, with the star in a different position of the image each time, is the halo just above the star each time or does it move relative to the star?  It's odd to see a halo that is slightly off-centre from the star.  Maybe choose a target with more bright stars (e.g. the Pleiades).

What's the diameter of the halo (in pixels) and what is the focal length and F-ratio of the scope being used?  Does the flattener also have a reducing effect?  If so by how much.  The answer to these questions will help understand the imaging geometry.

Mark

Mark I can’t be sure of the diameter of the halo as I guess it changes due to the star size?? Focal length is 840mm at f7. The flattener isn’t a reducer and I’ve tried putting the filter in front of the flattener and behind without success. 

Since it’s not the flattener reflecting the light it could be one of the lenses on the scope ? Although I don’t get any halos with the IDas D1 filter or others. 

[sharkmelley]

You've ruled out everything I can think of.  You've changed the position of the filter and you still get the problem so it's not reflections between the filter and the reducer.  The other obvious cause of halos is internal reflections within the filter glass itself.  But that causes halos of the same size whereas you say yours have different  diameters.  It would be informative to see an example of the different diameter halos because that might provide further clues.

Mark

[Ken82]

11 hours ago, sharkmelley said:

You've ruled out everything I can think of.  You've changed the position of the filter and you still get the problem so it's not reflections between the filter and the reducer.  The other obvious cause of halos is internal reflections within the filter glass itself.  But that causes halos of the same size whereas you say yours have different  diameters.  It would be informative to see an example of the different diameter halos because that might provide further clues.

Mark

Thanks Mark, the first picture I posted shows the halo towards the top of the star in my first post. In my more recent post (D1 and D2 filter comparison) the halo is central to the star. I’m not sure if this tells us anything ? The halos do look the same size to be fair. What would it mean if the halos were the same size ? 

[sharkmelley]

10 hours ago, Ken82 said:

Thanks Mark, the first picture I posted shows the halo towards the top of the star in my first post. In my more recent post (D1 and D2 filter comparison) the halo is central to the star. I’m not sure if this tells us anything ? The halos do look the same size to be fair. What would it mean if the halos were the same size ? 

It would almost certainly mean that the halo is caused by a double reflection within the filter glass causing those rays to travel extra distance to the sensor and arrive out of focus - the halo being the out of focus version of the star.  These double reflections are usually prevented by the anti-reflective coatings.  It might be worth contacting the supplier/manufacturer to see if it is normal for this filter.  The fact that you see the halo with the D2 filter but not the D1 could be an indication that something is not quite right.

Mark

[Ken82]

I did some further testing with a new D2 filter last night . As it turns out changing the filter doesn’t solve the issue . So I tried imaging through a different scope and the halo went away . 

Conclusion- the scope is causing the halos with the d2 filter .

why ?

[sharkmelley]

On ‎26‎/‎01‎/‎2019 at 10:01, Ken82 said:

I did some further testing with a new D2 filter last night . As it turns out changing the filter doesn’t solve the issue . So I tried imaging through a different scope and the halo went away . 

Conclusion- the scope is causing the halos with the d2 filter .

why ?

You have a new D2 filter? What is the second scope?  Where have you mounted the D2 filter on the second scope?  By the way, what camera are you using (so we can work out the pixel pitch).

Mark

[Ken82]

Thanks mark ! 

Yes the new filter gives the same halo. The second scope without the halo is a takahashi Fsq 85 at f5.3 with the filter screwed inside the t-adapter like I use on the esprit.

Its a modified canon 6d. I’ve also tried an unmoddified canon 1300d with the same results. 

Thanks ken 

[sharkmelley]

Do you have an estimate for the halo diameter (in pixels) on the Canon 6D?  Preferably a halo near the image centre.

Mark

[Ken82]

39 minutes ago, sharkmelley said:

Do you have an estimate for the halo diameter (in pixels) on the Canon 6D?  Preferably a halo near the image centre.

Mark

I think it’s about 67 pixels without the flattener and 80 pixels with. 

[sharkmelley]

Let's say 70 pixels.  The pixel pitch of the Canon 6D is 6.5microns.  You said your scope is f/7.  Then given the refractive index of glass as 1.5 and assuming the cause of the halo is a double reflection within the filter glass then that would give a filter thickness of just over 2mm.  2mm thickness sounds about right for a filter.

So your halo size is consistent with an internal reflection within the filter.  But I can't explain why you don't see a halo with your other scope.

Mark

[Ken82]

Just taken some images of bright stars without any filters. What do you think, is this normal for the Esprit range?

https://drive.google.com/file/d/1kZKkayjfDSaPiKQA_K-8dd6akRCymVW6/view?usp=sharing

 

[sharkmelley]

3 hours ago, Ken82 said:

Just taken some images of bright stars without any filters. What do you think, is this normal for the Esprit range?

https://drive.google.com/file/d/1kZKkayjfDSaPiKQA_K-8dd6akRCymVW6/view?usp=sharing

 

I have no experience with the Esprit range, so I can't tell if that is normal or not.

Mark