RGB alignment

[Ken82]

24 minutes ago, fireballxl5 said:

potential issues with the optics aside, here's a 100% crop of the LRGB combined image using MaxImDL v6.09 for comparison.

HTH, Andy

 

Yea it’s nice Andy, very similar to mine yet I’ve changed histogram curves ever so slightly do warm the galaxy. Which I’m turn brings out the purple even more. 

[Ken82]

1 hour ago, ollypenrice said:

Here's a corner crop in luminance from my elderly FSQ106N on a 45mm diagonal chip resized up by the same amount as I did yours. (Atik 11000 camera.) It isn't perfect but I think it's significantly better. I never have any channel alignment issues arising in RGB.

Olly

Much better ! 
 

Ian king did mention about possible collimation of the focuser.  I wasn’t particularly happy to try this myself though so I left it. 
 

It just seems to me for the price you pay for these scopes you shouldn’t really need to adjust the focuser tension or collimation. Indeed there is a good reason the focuser tension screws have locktite on them ! Which is why I had initially sent it back for the focuser to be adjusted. 
 

The astigmatism does change Depending on where it’s pointing. So sounds to me like the focuser is maybe sagging or something. 
 

Ian king has said he is sending it to takahashi Europe (France) for evaluation so let’s see when it returns. It was sent today back to FLO who I’m assuming will send it to France. 
 

[Ken82]

4 hours ago, RayD said:

Nice!

I'll have a look at your images with APP later to see if that does anything different.  PI is generally pretty good though, so I'll run it the way I do mine in that.  

Do you do a manual process or do you run the batch pre-process script, and are you using default settings?

sorry I missed this question Ray -

I do a full manual process in pixinsight following the light vortex tutorial. So default settings work fine. Only change I’ve tried (which I’ve never had to before) is enable distortion corrections in star alignment. This hasn’t changed anything unfortunately. 
 

[ollypenrice]

 

8 hours ago, Ken82 said:

Much better ! 
 

Ian king did mention about possible collimation of the focuser.  I wasn’t particularly happy to try this myself though so I left it. 
 

It just seems to me for the price you pay for these scopes you shouldn’t really need to adjust the focuser tension or collimation. Indeed there is a good reason the focuser tension screws have locktite on them ! Which is why I had initially sent it back for the focuser to be adjusted. 
 

The astigmatism does change Depending on where it’s pointing. So sounds to me like the focuser is maybe sagging or something. 
 

Ian king has said he is sending it to takahashi Europe (France) for evaluation so let’s see when it returns. It was sent today back to FLO who I’m assuming will send it to France. 
 

One thing to check: does a single sub show the same distortion? It's just possible that the stack might be accumulating field rotation from polar misalignment if the guide star was closely on-axis.

Unfortunately there have been plenty of problem FSQs reported on the forums in the last few years and I've seen three here. 

Olly

[RayD]

14 minutes ago, ollypenrice said:

Unfortunately there have been plenty of problem FSQs reported on the forums in the last few years and I've seen three here. 

Olly

There certainly has.  I went over to Steve Collingwood's house/workshop with mine and saw how bad it was first hand on his equipment, and it was bad.  The issues were very noticeable in the images, similar to Ken's, and I even changed my focuser to a Feathertouch thinking it was most likely to be that (turns out that was an excellent choice anyway).  In the end Ian King sent mine off to Tak Europe and the guys there did a cracking job, with it coming back producing absolutely perfect images across the whole FOV.  

They definitely know what they are doing, so it should come back spot on.

Edited May 29, 2020 by RayD

[ollypenrice]

8 minutes ago, RayD said:

There certainly has.  I went over to Steve Collingwood's house/workshop with mine and saw how bad it was first hand on his equipment, and it was bad.  The issues were very noticeable in the images, similar to Ken's, and I even changed my focuser to a Feathertouch thinking it was most likely to be that (turns out that was an excellent choice anyway).  In the end Ian King sent mine off to Tak Europe and the guys there did a cracking job, with it coming back producing absolutely perfect images across the whole FOV.  

They definitely know what they are doing, so it should come back spot on.

Good to know. Maybe they should check all scopes before sending them on, though, since the problem is widespread. Taks are extremely expensive, after all.

Olly

[RayD]

1 hour ago, ollypenrice said:

Good to know. Maybe they should check all scopes before sending them on, though, since the problem is widespread. Taks are extremely expensive, after all.

Olly

Totally agree, Olly.  I bought mine with the view that it would be prefect, it's an expensive Takahashi after all!   You certainly get a sinking feeling when you realise it is actually an issue with the optics.  However, the 2 guys at Tak Europe really are excellent and can definitely rectify any issues.

Whether things get knocked about in transit I'm not sure but it is certainly possible, although they ar eextremely well packaged.  I would hope (expect) that every one is bench/quality tested before leaving the factory but, even if this is the case, boy can you see it when it isn't right!

I think one thing @Ken82 needs to be very mindful of when it comes back is the sensitity of some of them to temperature change.  I used to re-focus mine every 1 degree of change, but had to change that ealry on to every 0.5 degrees as, with the APS-H sensor, it became very noticeably out of focus at 1 degree. 

[Ken82]

This is from pixinsight developers, so it’s not alignment causing the colour issue. Although I do agree all frames have a problem with sagittal astigmatism. 
 

I think this is alignment issue.

I disagree. There are no alignment issues with these images. To demonstrate this statement, here are PSF measurements performed at the four corners of the green and registered blue channel images with your M106 data (click the links to open full-size screenshots):

Top-right corner
Bottom-right corner
Bottom-left corner
Top-left corner

Image registration has been applied with distortion corrections enabled and a thin-plate based registration model; see a screenshot of the tool with relevant parameters attached to this post. As you can see, registration is accurate up to about 0.05 px for the best sampled stars, and about 0.1 px for the dimmest ones. These are good results, considering the scale of the image and the deformed star profiles.

 

[Ken82]

Since the pixinsight developers said its definitely not alignment (previous post) i thought i'd connect up my FSQ 85 to see if i could replicate the problem and perhaps eliminate something.

Unfortunately the FSQ 85 could only be connected by a 2" nose piece and i haven't got the additional flattener so star shapes in the corners were very bad. But even with the alignment of the distorted stars in the FSQ 85 the star colours were much better and uniform all round. In the screen shot ive taken this is after an alignment and combination of the RGB data and a simple auto stretch in pixinsight. Note at this point no further post processing or saturation has been carried out but the difference is clear. 

This eliminates the problem being filter or camera related. I would also say with evidence from the pixinsight developers and the fact that i was able to align the data easily from the distorted stars in the FSQ 85 that alignment of the data is not the problem. Although i do agree there is a slight issue with sagittal astigmatism. 

Left hand side of screen shot shows auto stretched image from FSQ 85 with a nice uniform colour to bright stars regardless of distorted stars in the corners. Image on the right hand shows an unevenly coloured star from FSQ 106 with purple edge to top and right hand side. 

At this time I think its likely something has been misaligned (possibly during transportation) but RayD and ollypenrice have given me confidence the Takahashi engineers in France will be able to resolve this. 

Looking forward to getting it back

 

[Stub Mandrel]

Good luck Ken, hope it comes back cured from its trip to Lourdes...

[Ken82]

Thanks for your advice today Neil (We did socially distance, promise everyone!) so i had a look and indeed there is a rotation of the purple fringe which means its chromatic aberration.  

All images below are a simple RGB alignment with an accuracy of 0.05 pixel and an auto stretch in pixinsight. No saturation or further processing has been done at this stage. 

This is backing up the opinion of most people ive spoken to.

At this time im shocked to say the least but lets see what happens in France.  vlaiv has mentioned in a message it could simply be a tilt in one of the lenses which could be easily resolved like collimation. 

Fingers crossed 

Top right with the purple pointing inwards

Bottom right with the purple pointing inwards

Bottom left purple pointing inwards

Centre of image 

[Whistlin Bob]

I'm a little horrified reading this. I think we all probably dream of having one of these beautiful scopes and being freed of the flaws that go with lesser equipment (like mine!!!) Takahashi make a big deal about the excellence of their products and some very strong claims about its capability. I really hope that given the cost of these and the efforts you've gone to to diagnose the problem, they do the business and return the scope working as promised. 

[Skipper Billy]

That's shocking!!!

How old is the scope - have you spoken to the retailer??

[Ken82]

2 hours ago, Skipper Billy said:

That's shocking!!!

How old is the scope - have you spoken to the retailer??

Couldnt agree more David!

The Takahashi FSQ was bought last summer with a view to imaging with the new full frame ZWO when released. I got the camera early this year and it was then i started to see problems. So scope is brand new although it was send back initially as the focuser was slipping (see previous video in thread)

Ive had a few other minor issues with this setup unrelated to scope that have been resolved with help from vlaiv (I should write up my findings as it may help others considering this camera and scope) positive i may add

Then turning my attention to fixing the purple colour problem Ive had from the beginning hasn't been easy. At first i thought it was something i was doing in the observatory or alignment but vlaiv and others helped me eliminate things one by one. 

It wasnt until Stub Mandrel told me the other day that if its chromatic aberration then the colour will rotate. Makes sense really but its not something i had initially looked at as like most people you expect these scopes to be free of these kinds of problems. 

But a quick search for chromatic aberration astronomy indeed gives the following which is exactly what i have- "In astrophotography, chromatic aberration often shows up as a coloured edge on one side of a star".  I guess in some ways i've been spoilt as my other refractors don't have this issue (FSQ 85 + 72ed)

You can clearly see this in the centre of my image bearing in mind this is just an auto stretch and no saturation has been altered. Indeed once you start to change saturation then horrible two tone purple stars appear. 

To answer your question, the scope was bought from FLO / Ian King so im sure if they cant resolve it in France they will replace the scope. 

Its not what i expected with a potential lifetime investment in a Takahashi FSQ 106 but fingers crossed im back imaging soon

I dont think ive slept a wink for months 😬 im sure buying a tak must be bad for your health. 

 

[Skipper Billy]

On 29/05/2020 at 09:45, RayD said:

I used to re-focus mine every 1 degree of change, but had to change that ealry on to every 0.5 degrees as, with the APS-H sensor, it became very noticeably out of focus at 1 degree. 

I too noticed this - refocus every 0.5 degree temp change is essential.

[Ken82]

4 minutes ago, Skipper Billy said:

I too noticed this - refocus every 0.5 degree temp change is essential.

Yea agreed I’ve been keeping a tight lock on the focus with the sesto senso. 👍

[Stub Mandrel]

On chromatic abberration this is what I figured...

If you use an RGB sensor and have CA some colours are out of focus, so you get halos.

If you use RGB filters with a  mono camera all your subs will be sharp, so no haloes.

BUT the focal length for each colour will be slightly different so the image scale for each colour will be different.

Say the blue focuses further away from the lens, then the blue image will be smaller.

This means that in the composite images there will be blue fringes on the inside edge of the stars in the corners, and the outside edge will look yellow.

I'd expect bad alignment to cause a consistent shift in the same direction across the image.

 

That said, having seen the odd star shapes Ken's getting, I think it's one or more lens elements out of alignment, so hopefully fixable on an optical bench.

 

[Stub Mandrel]

I found this.. the little blur diagram looks a bit like the distorted stars..

http://50.87.30.54/goldfocus/collimation_refractor.php

[Ken82]

16 hours ago, Stub Mandrel said:

On chromatic abberration this is what I figured...

If you use an RGB sensor and have CA some colours are out of focus, so you get halos.

If you use RGB filters with a  mono camera all your subs will be sharp, so no haloes.

BUT the focal length for each colour will be slightly different so the image scale for each colour will be different.

Say the blue focuses further away from the lens, then the blue image will be smaller.

This means that in the composite images there will be blue fringes on the inside edge of the stars in the corners, and the outside edge will look yellow.

I'd expect bad alignment to cause a consistent shift in the same direction across the image.

 

That said, having seen the odd star shapes Ken's getting, I think it's one or more lens elements out of alignment, so hopefully fixable on an optical bench.

 

I think your right stub-

I tested the focal length at the different RGB channels and the FSQ 106 is 0.51mm different in length. Whereas  the FSQ 85 was only 0.07mm different between the channels. 

I tested individual subs and they don’t differ from night to night so temperature and focus point don’t change the focal length. 
 

Takahashi FSQ 106 

Red focal length - 530.99mm

Blue focal length - 530.48mm 
 

Takahashi FSQ 85

Red focal length - 450.58mm

Blue focal length - 450.51mm

So at the moment the fsq 106 isn’t focusing RGB at the same focal length hence the purple stars. 
 

I also agree it’s likely an element out of alignment so should be repaired easily. Hope it’s not dust between the elements like your diagram 😩

Ken