Help identifying the cause of internal reflection in my Newtonian.

[MichaelBibby]

I am using a Sky-Watcher 6" F5 Newtonian (not the PDS version) with a Uranus-C (IMX 585) and need some advice isolating the cause of an internal reflection in my optics train.

Here is my first image of the Horsehead nebula, with Alnitak just out of the frame, and you can clearly see an internal reflection underneath the diffraction spike (ignore the tilt in my optics train, I'm working on it...).




After this image was taken I flocked the inside of the tube opposite the focuser, the inside of my dew shield, the inside of the aluminum focus tube, and took a black sharpie to the side of the secondary mirror which was facing the focuser (using a flashlight I determined that these were all possible sources of internal reflection). I then re-imaged the Horsehead with Anlitak clearly in frame and got this image (obviously nothing I did helped because you can still see the internal reflection).



The same night I caught that image I did a meridian flip and took this image immediately after (notice the absence of the internal reflection).




I'm not sure what else to try. I think I know what most people will say (and its something I'd rather avoid!): completely flock the inside of the OTA, but the inside of the OTA has already been blackened by the manufacturer (not very well, but better than many other newts), and even the aluminum draw tube was also blackened before I flocked it. Why is the internal reflection not apparent after the meridian flip? Any help would be greatly appreciated.

Edited December 29, 2022 by MichaelBibby

[MichaelBibby]

I think I already know: what else could it be but the inside of the OTA? I have a theory as to why it wasn't there after the meridian flip: because the position of Alnitak flipped around, and the small amount of flocking I already did on the OTA, opposite the focuser, probably blocked the reflection from that angel. Thoughts?

[Ratlet]

Are there any other components in the optical train?  Filters, etc?

After the meridian flip the reflection might be gone because it isn't on the sensor anymore.  If you imagine the reflection being on the opposite side of the star and the same distance it'll be out of the FOV?

[MichaelBibby]

5 minutes ago, Ratlet said:

Are there any other components in the optical train?  Filters, etc?

After the meridian flip the reflection might be gone because it isn't on the sensor anymore.  If you imagine the reflection being on the opposite side of the star and the same distance it'll be out of the FOV?

No filter wheel, just a uv/ir cut filter threaded onto the front of the camera. No OAG. Just a 2" to 1.25" adapter. Yes, your right, it might still be there but just out of view.

I guess what I really want to know, from people with more experience than me, is is this evidently and obviously an internal reflection from the inside of the OTA? I will eventually flock the entire OTA, but I'm not in any hurry (unless I know that it will resolve this issue).

[Ratlet]

It could be a reflection from the filter rather than the tube.

[MichaelBibby]

1 minute ago, Ratlet said:

It could be a reflection from the filter rather than the tube.

I don't see how: its just a 1.25" filter threaded onto the end of the barrel of the camera. I suppose I could test it with and without...

[MichaelBibby]

I should also mention that my focus tube is completely extended for critical focus so that there really isn't any part of it protruding into the light path, which would be another possible source of reflection.

The back and sides of the secondary mirror are naked (besides the side of the secondary facing the focuser which I blackened with a sharpie), as is the side of the primary (I have completely blacked out the back end of the OTA to prevent any stray light getting in from outside).

[Clarkey]

I would add that you are trying to image a very bright object with other bright stars just out of sight. Yes you could flock the whole tube and it might solve the problem, but in reality there are very few objects in the sky you are likely to get this issue with. I never had problems with reflections with my image train except when I imaged Alnitak. Personally I wouldn't worry too much unless you get the same problem with 'normal' areas of sky.

[david_taurus83]

I have to say, even with the minor reflection, they are very nice images of the Horse and Flame.😀

[barbulo]

If not reflections of a very bright star, cables/ropes in the line of sight?

[MichaelBibby]

40 minutes ago, barbulo said:

If not reflections of a very bright star, cables/ropes in the line of sight?

Interesting. I can't exactly remember, but I think that I waited for the target to clear some powerlines before I began imaging. I suppose one argument against that possibility is that the 'reflection' is in the exact same orientation as the spider-veins/diffraction spike in both images where it appears. I have seen the effect of powerlines in my images before, in the form of an extra diffraction spike (at least I am pretty sure that it was caused by the powerlines).

At this stage I am assuming its reflection from the inside of the OTA, but as Clarkey pointed out, not really a problem with any other targets.

 

[AstroMuni]

22 hours ago, MichaelBibby said:

I should also mention that my focus tube is completely extended for critical focus so that there really isn't any part of it protruding into the light path, which would be another possible source of reflection.

The back and sides of the secondary mirror are naked (besides the side of the secondary facing the focuser which I blackened with a sharpie), as is the side of the primary (I have completely blacked out the back end of the OTA to prevent any stray light getting in from outside).

Any possibility that its reflection from the spider itself? One of your spider vanes is not exactly perpendicular to the light path causing the flare at the end of the spike pointing downwards. I have same problem with my spider but havent seen a reflection like yours (yet).

Edited December 30, 2022 by AstroMuni

[MichaelBibby]

18 minutes ago, AstroMuni said:

Any possibility that its reflection from the spider itself? One of your spider vanes is not exactly perpendicular to the light path causing the flare at the end of the spike pointing downwards. I have same problem with my spider but havent seen a reflection like yours (yet).

I'm pretty sure that the reason the diffraction spikes on Alnitak are splitting on the vertical axis (but not on the horizontal one) is because of a slight tilt in my optics train. Last time I checked, the spider veins were nice and straight.

 

Edited December 30, 2022 by MichaelBibby

[AstroMuni]

4 minutes ago, MichaelBibby said:

I'm pretty sure that the reason my diffraction spikes are splitting on the vertical axis (but not on the horizontal one) is because of a slight tilt in my optics train.

I dont believe tilt will cause this effect. It could certainly cause some stars to be slightly out of focus in corners where the tilt is highest. But I am no expert here, and I came to know cause of my flare just yesterday. See my post on diffraction spikes in this channel.

Edited December 30, 2022 by AstroMuni

[MichaelBibby]

3 minutes ago, AstroMuni said:

I dont believe tilt will cause this effect. It could certainly cause some stars to be slightly out of focus in corners where the tilt is highest. But I am no expert here, and I came to know cause of my flare just yesterday. See my post on diffraction spikes in this channel.

Ah, I see. I'm not sure how I got it in my head that it was the effect of camera tilt (a trouble I have been having). I'll look into it.

 

[skybadger]

Tilt will cause the vanes to go out of focus and thus broaden but not to split. 

I think that would take a significant tilt of several degrees though. 

[MichaelBibby]

I took a couple of photos of my spider vein, making sure that the camera sensor was the same height as the centre of the spider, and tried to line up, first, the secondary mirror so that it was concentric with the primary, and then, second, the hub of the spider with the primary. I'll take a closer look later when I get some time, but are any problems evident to others? A twist in one or more of the veins? Are the angle's between the veins perfect right angles? It looks like the vein's perpendicular to the axis of the focuser are not quite straight but pulled towards the focuser, so I'll correct that and do some testing.

 

 

 

So maybe this is the situation I am facing (thanks to @inFINNity Deck for the graphic). Though I'd doubt it would explain my reflection problem, but its good to at least sort this out. Thanks @AstroMuni.



 

Edited December 31, 2022 by MichaelBibby

[inFINNity Deck]

Hi Michael,

if I draw straight lines in both images along the vanes of the spider, it indeed seems that the hub is drawn towards the focuser:

 

It appears to me that the upper-right vane is more off than the lower-left one as if they are not in line with each other on the hub. The other two vanes look fine to me.

Nicolàs

 

[david_taurus83]

I would flock the whole tube. Look how bright it is compared to the flocked bit behind the secondary. Also look how the primary has reflected the flash onto those screws. Obviously the flash has magnified the effect but still, a bright star could still provide some rogue photons to bounce around in there.

[MichaelBibby]

Just to confirm that adjusting the screws on the spider veins did indeed fix the problem with my diffraction spikes, thanks guys! (I do love pristine spikes...)