Strange Reflections - any ideas?

[Clarkey]

Hi,

I am after some guidance from the collective mind of SGL.

Last night I was imaging in Ha and OIII using the Photon F4 newtonian, aplanatic coma corrector, zwo EFW and filters with a 1600mm pro. I have imaged with this same set up previously without issue, but this time there was a bright star in the field of view. Although I used to the usual problem of halo's round stars with the OIII, this time I had really bad reflections with both the Ha and OIII. In fact the Ha was much worse. For info there was no moon and there are no lights overlooking the scope. Additionally it can be seen on all the subs - not just some. Does anyone have any idea of what might have caused this reflection? Below is the stacked Ha channel.

 

[Budding Star Gazer]

5 hours ago, Clarkey said:

Hi,

I am after some guidance from the collective mind of SGL.

Last night I was imaging in Ha and OIII using the Photon F4 newtonian, aplanatic coma corrector, zwo EFW and filters with a 1600mm pro. I have imaged with this same set up previously without issue, but this time there was a bright star in the field of view. Although I used to the usual problem of halo's round stars with the OIII, this time I had really bad reflections with both the Ha and OIII. In fact the Ha was much worse. For info there was no moon and there are no lights overlooking the scope. Additionally it can be seen on all the subs - not just some. Does anyone have any idea of what might have caused this reflection? Below is the stacked Ha channel.

 

That looks like light reverberation from the inside of your OTA, id say check the reflection before & after adding dark material to the inside of the OTA. 

[Clarkey]

8 hours ago, Budding Star Gazer said:

That looks like light reverberation from the inside of your OTA, id say check the reflection before & after adding dark material

Should have added - the OTA is fully flocked already.

[Budding Star Gazer]

2 hours ago, Clarkey said:

Should have added - the OTA is fully flocked already.

I believe if the primary mirror is out of alignment it can have this effect as the light souce doesn't all train to the secondary mirror & travels straight back up the OTA to the point of entry.

Someone with way more exerience than me could put your problem right. 

[sharkmelley]

It is likely to be a reflection between the camera (sensor or front window) and filter.  If so, the distance between the 2 glass surfaces causing the problem can be calculated by halving the distance calculated by the dust donut calculator: Dust Donut Calculator

Mark

Edited December 3, 2021 by sharkmelley

[Clarkey]

3 hours ago, sharkmelley said:

It is likely to be a reflection between the camera (sensor or front window) and filter. 

Once I am back home I'll have a pixel count. The overall reflection just looks too big for a reflection of this type? It's nearly 1/3 of the frame.

[inFINNity Deck]

I agree with Mark that it most likely is reflection between the camera and filter. Increasing the distance between them will make the halos larger and weaker, so perhaps that is a solution. Also the ZWO filters have a front and rear side, as indicated by ZWO, and it is useful to test which side works best (I found it quite difficult to test it otherwise). I have done those test with the old style (pre June 2018) filters and turning them around made a difference (for the test I simply turned around the whole filter-wheel).

The old style filters also showed multiple concentric halos in H-alpha and S-ii, which are absent in the new style ones.

Details of my test are on this Dutch forum but translate quite well when opened in Chrome: https://www.starry-night.nl/forums/topic/halos-en-zwo-filters/

Nicolàs

[Clarkey]

12 minutes ago, inFINNity Deck said:

reflection between the camera and filter.

I will have to do some more investigating. It is a viable possibility - I just need to prove it.

It is a bit odd using similar set up (filters and camera) with a number of different scopes, flatteners and reducers and this is the first time I have seen this. I even imaged the western veil using the same scope and imaging train without issue. I am confident that the filters are the right way round. I was very careful when they were installed.

[inFINNity Deck]

The best test for proving it is camera-filter reflection is changing the distance between them and see if they grow or shrink accordingly. I had a 10mm spacer between the flattener and the filter-wheel, which I could move to sit between the filter-wheel and the camera. I did it that way as not to mess up the proper camera-flattener distance, but for a test that is not of great importance, adding a spacer will affect star quality, but immediately show (or not) the effect on the halos.

Good luck and please let us know what you find!

Nicolàs

[Clarkey]

I have two spacers before the filter wheel so I'll put one before and one after to test. If this is the problem it may not be an easy fix on the faster scope. New filters and wheel is not going to get passed the boss!

Just need a clear night to test!

[sharkmelley]

10 hours ago, Clarkey said:

The overall reflection just looks too big for a reflection of this type? It's nearly 1/3 of the frame.

That gives us something to work with. 1/3 of the 1600mm sensor is approx 6mm.  Your scope is f/4.  The total extra distance travelled by the reflected rays is therefore 6x4=24mm.  The distance between the two surfaces causing it would be half this i.e. 12mm

[Clarkey]

2 hours ago, sharkmelley said:

12mm

Hmmmm. That is probably about right for the sensor to filter distance. If this is the cause I'm not sure how easy it will be to fix. With the f4 scope I already get some vignetting with the 31mm filters and moving it further away will undoubtedly make it worse. I can also try turning the filters round.

Thanks for the help everyone. At least I have something go work with.