Pronounced secondary mirror reflections in Skywatcher 200PDS after improvements

[Mark - Coventry]

I have been improving my Skywatcher 200 PDS in an attempt to obtain better the star shapes, generally minimise unwanted reflections and increase contrast. What I have done is:

• flock the inside of the tube
• flock the edge of the secondary mirror which is visible from the direction of the eyepiece/camera
• install a primary mirror baffle
• blacken all silvered surfaces (screw heads etc) with a Sharpie
• cut down the focusing tube by 15mm to prevent it protruding into the light path and causing odd-shaped stars

The star shapes are dramatically better than before thanks to the baffle and the cut-down focus tube. However I am getting really weird reflections which I'm struggling to track down.  I can see that the vanes look to be slightly twisted hence the split diffraction spikes and there is a very pronounced halo around the bright star Alnitak with what looks like a kaleidoscope-shaped pattern in the immediate area around the star. The halo is almost invisible in H-Alpha but really pronounced in R, G and B, especially G where you can even see the outline/shadow of the secondary mirror and spider. The kaleidoscope pattern is most visible in H-Alpha.

This is something I have not seen before making the so-called improvements! I used a ZWO 1.25" 7nm HA filter and the ZWO RGB filters optimized for ASI1600. I have read that ZWO filters are prone to halos and wondered if anyone else had experienced the same issues or, indeed, found a solution to such problems.

As an afterthought, could the odd reflections be caused by the spider not being 100% straight? it seems unlikely but I'm open to ideas.

Mark

[Tomatobro]

taken a while back with a 200mm SW.

[AbsolutelyN]

I'm curious as to the cause too. Possibly spider not centred and square? This particular star will likely always cause problems though, all the other stars look great. I recently imaged this area and simply framed it so that Alnitak was simply out of frame so it could not cause problems. 

Edited December 1, 2021 by AbsolutelyN

[Mark - Coventry]

Thanks for your input. I have seen a recommendation not to image bright stars when this problem was encountered. Not really the solution which I was hoping for! It's interesting to see you have the same issue. I will keep searching and in particular look at the straightness or otherwise of the spider. I'm tempted to invest in better filters when funds allow but I prefer to solve problems without throwing money at them if at all possible...

[ONIKKINEN]

I don't know about the halo, but i would suspect reflections somewhere between any of the lenses in your comacorrector(?), filter, sensor window and the sensor surface itself. The brighter the object the more likely these should be and many pictures of this area have such halos.

The split diffraction spikes are caused by a twisted/unsymmetrical spider and can be difficult to fix. Straightening out the spider veins is a huge pain and its not that easy to see visually which way you need to bend and what when you're fixing it. I had this issue with my VX8 and fixed this by just replacing the very thin but also weak spider with a sturdier one. The new sturdy spider doesn't buckle under tension to any direction and certainly doesn't deform easily. I still found it difficult to do the final adjustments by eye, so i took some pictures with my phone and overlaid some lines drawn in MS paint to make it more visually clear what is bent and to what direction.

Much easier to see the direction of the bend at a glance like this!

Do note that centering the spider symmetrically can take it out from under your focuser if the borehole has been drilled incorrectly. I fixed this by shimming the focuser (tilting it basically) so that it faces the newly positioned spider head on.

[AbsolutelyN]

36 minutes ago, Mark - Coventry said:

Thanks for your input. I have seen a recommendation not to image bright stars when this problem was encountered. Not really the solution which I was hoping for! It's interesting to see you have the same issue. I will keep searching and in particular look at the straightness or otherwise of the spider. I'm tempted to invest in better filters when funds allow but I prefer to solve problems without throwing money at them if at all possible...

I didn't take an image with Alnitak in frame to see how it handled the flare directly but did notice the HA image was completely free of issues but in RGB you could see artefacts from such a bright star at the edge of the frame despite being out of frame.  The cause of split diffraction spike is very nicely illustrated in previous post. Not ideal but I always try to avoid very bright stars in frame, saves a lot of headaches. 

[Clarkey]

On 01/12/2021 at 17:05, Mark - Coventry said:

However I am getting really weird reflections which I'm struggling to track down. 

I have started to get exactly the same problem with my TS Photon and have a thread in the imaging section on the same subject. Suggestion seems to be internal reflections between lenses or filters. I need to look at it when the next clear night happens.

 

[SpookyKatt]

I think the kaleidoscope-shaped pattern you are seeing is the 1600 microlensing issue.

Kathleen

[Mark - Coventry]

18 hours ago, SpookyKatt said:

I think the kaleidoscope-shaped pattern you are seeing is the 1600 microlensing issue.

Kathleen

Thanks Kathleen. That is the first I have heard of 1600 microlensing. I will do some research and report back in case it helps others.

[Clarkey]

13 minutes ago, Mark - Coventry said:

1600 microlensing

The 1600 does micro lens at times but this is a smaller localised effect. The large halo around the bright star is caused by internal reflections.

There is a link here with an example of microlensing.

https://www.cloudynights.com/topic/646966-zwo-asi1600mm-pro-bright-star-issue/

[Mark - Coventry]

3 minutes ago, Clarkey said:

The 1600 does micro lens at times but this is a smaller localised effect. The large halo around the bright star is caused by internal reflections.

There is a link here with an example of microlensing.

https://www.cloudynights.com/topic/646966-zwo-asi1600mm-pro-bright-star-issue/

Looks like two problems rather than one from the Cloudy Nights discussion. I will probably stay away from bright stars until I can afford to fix one or both of these issues! The following night I got some time on M1 with no such artefacts visible. The internal reflections have me a little stumped in fact as I have flocked all possible surfaces. I love my newtonian but maybe a refractor might prove easier to work with on bright objects.

[Clarkey]

14 minutes ago, Mark - Coventry said:

The internal reflections have me a little stumped in fact as I have flocked all possible surfaces

The internal reflections are between the sensor and filters or CC, so flocking does not help. It is also possible that you might get similar with a refractor. The other link to my SGL post has a bit more detail. Not easy to fix though.