Strange light at all OIII antlia 3nm filter - Forums

Topic starter

Apr 23, 2023

Hi everybody,

Id like to ask here about strange behavior of my Pelican Nebula imaging.
Last night Ive get on all of my OIII images this strange scattered (?) light.
Could anyone tell me, what it can be?

thank you and CS!

Themis Karteris

Apr 23, 2023

Type of filter? unmounted?

andrea tasselli

Frequent contributor

Apr 23, 2023

Scattered light from a bright star off-field, most likely.

Taras_M

Topic starter

Apr 23, 2023

Themis Karteris:
Type of filter? unmounted?

1.25 Antlia 3nm V-Pro mounted

Bruce Donzanti

Apr 23, 2023 · Edited: Apr 23, 2023

I agree with Andrea. I get these now and then on both my refractor and SCT. It is usually reflective light from a very large, close by star. Sometimes, it can be from a local light source. And I have the Antlia 3nm V-Pro mounted filters but it also happens with my Chroma filters.

Taras_M

Topic starter

Apr 23, 2023

It’s very strange to me, because the same region with Ha filter (also antlia 3 nm) shows no reflections at all…

Thomas W

Apr 23, 2023 · Edited: Apr 23, 2023

It’s very strange to me, because the same region with Ha filter (also antlia 3 nm) shows no reflections at all…

That stray light points almost perfectly in the direction of... Deneb!

At magnitude 1 spectral type A2, it's both bright and blue. Flux from Deneb and camera QE are both greater at OIII compared to Ha, hence the difference between the frames. Small changes in pointing or a meridian flip could also result in the differences. What could be happening is light from Deneb is landing on some surface and reflected onto the sensor - it's a common issue most people will run into at some point. Sometimes this can be fixed by shining a bright light down the OTA then using baffling / masks / flocking material, though the success will be dependant on the exact opto-mechanics of your system.

Well Written Helpful Insightful Concise

Christian Koll

Apr 24, 2023

Taras,

as others have pointed out, this is stray light from your optical train and has nothing to do with the filters.

To check for glossy surfaces in your optical train, just point the telescope at a bright surface (white wall), remove the camera and peek through the draw tube.
Any reflecting surfaces will be visible.

Often, those black anodized aluminium spacer rings (to adjust backfocus with) are the culprit - these have a reflecting surface (even if threaded inside).
Having had the same problem, I removed the anodized finish and coated mine with Noctutec matte black lacquer. This solved the issue.
Noctutec is a German brand, I'm sure there is similar stuff available from astronomy stores from where you live.

These straylight reflections mainly occur in the green filter, a little in the blue and almost not in the red.
This depends on the properties of the reflecting surface within your optical train.

Again, this has nothing to do with a particular filter brand or filters at all.

CS
Chris

Well Written Helpful Insightful Engaging Supportive

Taras_M

Topic starter

Apr 25, 2023

Christian Koll:
Taras,

as others have pointed out, this is stray light from your optical train and has nothing to do with the filters.

To check for glossy surfaces in your optical train, just point the telescope at a bright surface (white wall), remove the camera and peek through the draw tube.
Any reflecting surfaces will be visible.

Often, those black anodized aluminium spacer rings (to adjust backfocus with) are the culprit - these have a reflecting surface (even if threaded inside).
Having had the same problem, I removed the anodized finish and coated mine with Noctutec matte black lacquer. This solved the issue.
Noctutec is a German brand, I'm sure there is similar stuff available from astronomy stores from where you live.

These straylight reflections mainly occur in the green filter, a little in the blue and almost not in the red.
This depends on the properties of the reflecting surface within your optical train.

Again, this has nothing to do with a particular filter brand or filters at all.

CS
Chris

Thanks to all who replied me. Strange to me is still that even if Im pixel exact at the target, Ill get this only through my OIII filter, but nothing to see through my Ha filter.

CS!

Michael Hornfeck

Apr 25, 2023

Thanks to all who replied me. Strange to me is still that even if Im pixel exact at the target, Ill get this only through my OIII filter, but nothing to see through my Ha filter.

CS!

A quick google search of Deneb's spectrum shows that it outputs very little light at the Hα wavelength, and is almost entirely centered on OIII. It's a blue-white supergiant that's no longer fusing hydrogen so I imagine that's why there isn't any visible light in hydrogen alpha.

Well Written Insightful Concise