6 replies to this topic

[Pistachio_Enjoyer]

Hello Everyone,
I recently finished acquiring subframes for IC 63, using a GSO RC 10 truss-tube reflector, Touptek IMX 571 camera and ZWO 36mm RGB/7nm Ha filters. After stacking my subframes however, I noticed a strong internal reflection in my subframes. The source of the internal reflection is the bright star Gamma Cassiopeiae. I have had similar internal reflections from capturing the Horsehead Nebula earlier this year from Alnitak. It was more pronounced in the green and blue filters compared to the red filter, so I am wondering if I have an issue with my filters or is it an issue with my telescope optics. I am planning on purchasing Antlia filters at the end of the year, but I'm not sure if it will fix the issue. Attached below are the stacked subframes, please let me know what I can do about this issue.

Red

 

Green

 

Blue

Edited by Pistachio_Enjoyer, 02 November 2024 - 03:14 PM.

[quilty]

Most likely culprits are always the baffles inside as well as diagonal and focusser strut internals

[luxo II]

Well, Aside from the scattered light from the bright star just above the field, there's an issue most visible in the blue frame - a very out-of-focus donut that resembles the off-axis aperture of the scope.

 

I'd guess this is an internal reflection of the same star from within each filter, noting the direction of its off-axis skew matches the direction of the bright star from the centre of the frame.

 

One way to solve it is to reposition the shot, the reflection artefacts will shift significantly and possibly out of the frame. 

Or new filters with better coatings (quite a challenge IMHO).

 

Another thought is to shuffle the spacers in your train to change the  spacing of the filter from the camera - either put the filter as close to the camera as possible, or as far away as possible. 

Attached Thumbnails

• 

Edited by luxo II, 03 November 2024 - 06:28 AM.

[Pistachio_Enjoyer]

Well, Aside from the scattered light from the bright star just above the field, there's an issue most visible in the blue frame - a very out-of-focus donut that resembles the off-axis aperture of the scope.

 

I'd guess this is an internal reflection of the same star from within each filter, noting the direction of its off-axis skew matches the direction of the bright star from the centre of the frame.

 

One way to solve it is to reposition the shot, the reflection artefacts will shift significantly and possibly out of the frame. 

Or new filters with better coatings (quite a challenge IMHO).

 

Another thought is to shuffle the spacers in your train to change the  spacing of the filter from the camera - either put the filter as close to the camera as possible, or as far away as possible. 

The reason I have the bright-star off-axis is because my telescope is zoomed in too far and thus I would not be able to image the object. I can't change the spacers in my image train as I need to actually reduce the backfocal distance. I am planning on replacing my filters with higher quality Antlia ones. Do you believe this would resolve my issue?

[luxo II]

Re Antlia filters - no idea, sorry. My cameras is OSC - ASI 2600MC DUO, with an Optolong L-Pro or L-Extreme. The only Antlia filter I tried was a quad-band one supposedly OK with an OSC, but the results were unusable - huge flares around every star, some extending across half the frame.

 

I have an observing buddy using a mono camera with LRGB filters on a 10" RC like yours, and he also has a lot of grief with both filters and guiding - to the extent he stopped using that scope and has reverted to using a 127mm APO.

 

He has seen how the 2600MC DUO works for me and is considering switching to one just to get a useable result, then adding subs from Ha or OIII filters to add color eg for galaxies.

Edited by luxo II, 03 November 2024 - 07:28 PM.

[Pistachio_Enjoyer]

Re Antlia filters - no idea, sorry. My cameras is OSC - ASI 2600MC DUO, with an Optolong L-Pro or L-Extreme. The only Antlia filter I tried was a quad-band one supposedly OK with an OSC, but the results were unusable - huge flares around every star, some extending across half the frame.

 

I have an observing buddy using a mono camera with LRGB filters on a 10" RC like yours, and he also has a lot of grief with both filters and guiding - to the extent he stopped using that scope and has reverted to using a 127mm APO.

 

He has seen how the 2600MC DUO works for me and is considering switching to one just to get a useable result, then adding subs from Ha or OIII filters to add color eg for galaxies.

I see. Everyone's mileage varies and I can't say I have guiding issues with my setup, just these internal reflections. Worst case scenario, if swapping out the filters has no effect, I will avoid imaging targets near bright stars with the RC and use a different scope for the job. Thank you for your input.

[luxo II]

I would suggest sit back in the daylight one day and take a critical look at your scope to identify all the things you could relocate or change that could influence the result. Looking for anything that has a reflective surface close to right-angles to the light path, and have a darn good look at the anti-reflective coating.

 

These include the cover glass of the camera, as well as filters, reducers and corrector plates (if any).

 

NB one aspect of the design of modern multi-element camera lenses is that they use multi-layer anti-reflective coatings, and if you look into them, each surface exhibits a reflection in a particular (and different) color. This is deliberate - to reduce the intensity of ghost images at the focal plane from double-reflections.

 

If you are using reflective (interference) filters, they are also highly reflective, so anything reflected to them will be reflected back. Hence the ghosts in your images may be due to a bounce from the camera cover glass to the backside of the filter, and then back to the camera. Putting the filter closer or further from the camera will confirm this by altering the size of the ghost. It's out-of-focus by virtue of the additional path length involved. It also suggests multilayer dielectric reflective filters should have the most reflective side towards the camera sensor.

 

Apart from anti-reflective coatings, other ways to eliminate ghosts include tilting flat surfaces (i.e. filters) so reflections are deflected well off to one side, or make them curved (not flat), so that a double-reflection is diffused very widely rather than causing a ghost. Unfortunately no-one makes filters with spherical surfaces, and unfortunately I dont think it has occurred to anyone yet to make filter trays with a deliberate tilt angle.

Edited by luxo II, 03 November 2024 - 10:30 PM.