Bad Image from Newtonian

[Ramius (Bill)]

Hi,
This image is from my 8" Skywatcher Quattro Newtonian (just a few frames to illustrate the problem). The newt is relatively new to me but I have had a couple of previous images from it without issue. I just put the newt back on after using my refractor for a while and got this mess.
The stars - particularly bright ones - show trails that i initially thought was tracking but PHD2 says tracking is around 0.7" and the trails do not appear uniform across the image. Have since checked tilt and collimation but both appear good. Notably the aberration diminishes and then disappears as the scope tilts to the zenith.
I don't even know whether this is a mount or optical issue - hopefully somebody can help me out.

[bojan]

It could be deformed or "pinched" mirror...

How thick is it? How it is mounted/supported?

[Ramius (Bill)]

How thick? - really not sure. I am thinking around 25mm as a guess. The mirror is retained by clips. I used spacers to ensure they were not over tightened. Also I have not touched the clips since the last session when all was good.
The only thing that has obviously changed since the last session is the temperature - much colder now (like -5 here at the moment).

[bojan]

If the mirror didn't have enough time to cool down (assuming it was kept in warmer room prior to imaging), that could explain the issue..

[Ramius (Bill)]

Yes i had thought of that but the scope is stored outside and the aberration reduces through the night as it moves toward the zenith but then gets worse again as it moves away from the zenith.
The more i think about it the more it seems like a mount issue - except for the good PHD2 readings.

[Ramius (Bill)]

This crop from a single frame shows it a little better i think. The small stars appear fine while the larger brighter stars show a strange double image or reflection ??

[Startrek (Martin)]

I’ve only used 6”, 8”, 10” and 12” Newts
Looks like a combo of either pinched optics and bad tracking ( polar alignment) or mirror flop. A flare on one side of the Star is typical of pinched optics.

Maybe try a couple of quick tests during a clear stable night

Unguided Star test
Don’t rely on your guiding alone , take a 30 sec to 50 sec unguided exposure on a mag 2 star at 40 deg Alt and then at 70 deg Alt. Zoom in on the exposure and compare Star shape and diffraction spikes. Stars should be round if mount is tracking correctly and PA is below an arc min. Obviously the star shape at 70 degrees will be sharper than at 40 degrees.

Pinched Optics ( Defocus Star test )
Slew to a mag 2 Star and defocus the Star into a large donut. The Star or large donut should be circular and uniform and also be able to see the spider vanes and the 3 mirror clips. If not , check primary mirror for pinching and flop. Use cork strips to eliminate flop and ensure rubber mirror clamps are only just touching mirror.

Check Guiding
If using PHD2 for guiding run the guiding assistant to give you a sanity check on your optics and mount performance.Mount could have severe Dec backlash. Worm gear could be the reason why the optical problem changes in different parts of the sky. Only a hunch ??

Re check Collimation using both Cheshire and Laser as well

Finally there’s a load of photo examples of optical aberrations with Newtonians online , Stargazers Lounge , Cloudy nights and even on some old IIS posts , they may match something similar to yours

Good luck
Martin

[Ramius (Bill)]

Martin
Thanks for that.


Unguided Star test
Unguided images are good at 60s.

Pinched Optics ( Defocus Star test )
Slew to a mag 2 Star and defocus the Star into a large donut. The Star or large donut should be circular and uniform and also be able to see the spider vanes and the 3 mirror clips.
Per image the collimation is OK but not perfectly centred particularly in the edges of the image. There are no clips visible as I am using a mask. The donuts on those bright stars (no idea what magnitude - need to do some more homework) look good but the scope is near the zenith now and the problem is not there.




Check Guiding
Dec backlash is causing me a bit of grief with the bigger newt but hasn't effected images to date.

Re check Collimation using both Cheshire and Laser as well
I am using OCAL. I will re-check collimation but not sure this is the problem.

Finally there’s a load of photo examples of optical aberrations with Newtonians online , Stargazers Lounge , Cloudy nights and even on some old IIS posts , they may match something similar to yours
I had started looking but had not yet found anything that helped. Will continue focusing on recommendations above.


I have just captured another image with the scope defocused and it shows a definite trail to one side. Not clear to me whether its optics or tracking. However I am done for tonight but will try and post tomorrow.

[bojan]

Collimation is not good.. something is moving there.
You mentioned earlier the issue is no more when scope is pointing up ?

[Ramius (Bill)]

Yes the really obvious aberration disappears near the zenith. That last photo was taken near the zenith and the focused image taken at the same time did not show the bad aberration that appears in the first photo. Therefore it would seem that the degree of poor collimation indicated here is not responsible for the aberation in the first image.


That said the collimation is clearly off. Also I think I probably over tightened the screws on the primary mirror. I will have a go at fixing both and take another set of test images hopfully tomorrow.

[SB (Chris)]

Originally Posted by Ramius

Yes the really obvious aberration disappears near the zenith. That last photo was taken near the zenith and the focused image taken at the same time did not show the bad aberration that appears in the first photo. Therefore it would seem that the degree of poor collimation indicated here is not responsible for the aberation in the first image.


That said the collimation is clearly off. Also I think I probably over tightened the screws on the primary mirror. I will have a go at fixing both and take another set of test images hopfully tomorrow.

Hi Bill
I had the exact same problem as you with the Quattro 800 spending ages thinking it was the mirrors. After much head banging I bought a Backyard Universe Secondary Mirror Spider for Sky-Watcher 200mm from First Light Optics in the UK.

Replacing the flimsy stock spider just revolutionised the ease of use of the 800 making it a reliable bit of kit.

Hope that helps.

Chris

[Nikolas (Nik)]

I think the secondary mirror spider is the culprit, tried loosening tightening the screws on the spider veins? You may have to recollimate. Better to get a solid spider assembly instead of the rubbish skywatcher uses.

[Pierre_C]

Originally Posted by bojan

...something is moving there.

Originally Posted by Ramius

Yes the really obvious aberration disappears near the zenith.

Random thoughts:

• Gravity dependent movement of the primary mirror or primary mirror housing - could there be lateral movement of the mirror, apart from vertical movement from loose clips?
• Gravity dependent movement of the secondary mirror? Check tension of secondary mirror and spider vane system?
• Flexure in the optical tube affecting the relationship betweeen the primary mirror and the secondary mirror?
• Gravity dependent movement in the focusser or imaging train?

Check collimation at the zenith and then in each cardinal direction after adjusting each component to see if there is any difference?

[Ramius (Bill)]

Thanks for the responses.
The scope has a CYCK spider so I do not think that is the problem. For the record it also has a carbon fibre tube with CYCK focuser and a Sharpstar F4 coma corrector.

I re-collimated yesterday (with OCal). I am very inexperienced but my impression is that it works well with the least reliable aspect being the final primary mirror adjustment.

I then spent a lot of time last night faffing with the scope. Long story short I now believe the issue was primarily tilt combined with some poor collimation. I managed to dial these out and got to a nice clean image with very minimal tilt detected by ASIStudio. Pretty happy with that at about 2am. I then thought i was good to capture some data. I slewed to the target and rotated about 1.5 degrees - and image is crap again.

Frustrating but progress made. I will now concentrate on identifying movement in the image train.

[bojan]

This problem should be easily visible using just an eyepiece instead of camera..

Maybe just visual monitoring of optical adjustments results could be more practical ?

[Ramius (Bill)]

Eyepiece? I think that might introduce new variables if the problem is associated with image train and weight. More to the point I am exclusively set up for imaging - not sure i even have an eyepiece!

[Startrek (Martin)]

Bill,
I just thought of something which hasn’t been raised as yet
Make sure your focuser tube is not obstructing the light path of your optical light cone. This can cause flares on one side of your stars !!
A coma corrector with long barrel can cause the same issue.
Also from memory SW Newts have focuser tubes painted Matt silver ( got me buggered why they’re not Matt black to minimise reflection etc… is yours silver ?
I feel your pain , there have been so many folk on this forum over the years starting off imaging with entry to mid level f4 Newts ( Skywatcher , GSO and similar ) and running into problem after problem.
The higher end f4 to f3.8 Astrographs never seem to have too many issues as they are a precision quality engineered scope and nil internal reflection.
When I started imaging back in 2017 , a bloke at Bintel recommended I go f4 as it provides more light gathering power and a wider field of view. After research on many forums I decided to stick with f5 for all my Newts as the products they were offering me were entry level budget type f4 Newts plus I didn’t want to spend big bucks on a high end premium f4. The f5 is so much more forgiving than the f4 and that was the determining factor as a newbie at the time.
I’ve never had an issue with any of my f5 Newts ( 6” , 8” , 10” and 12” ) and any minor issues have always been cockpit user errors.
I check collimation every few months , they just hold collimation these f5’s irrespective of temperature ( Ive use carbon tubes in the last 3 years and built all my Newts from GSO parts )
I still reckon your issue is a combo of many factors that have been mentioned so far. Another point is that when your up at the zenith your punching through less atmosphere so any optical aberration whether reflective or mechanical will be marginally less in effect.

Anyway have a look at your focuser tube to see it protrudes into your optical path.

I do hope you resolve the issue sooner than later but it’s going to take time and plenty of clear nights

Good luck
Martin

[Ramius (Bill)]

Martin
Thanks for the input. I have overcome the worst of the issue but the image is still not quite where i want it (and I am not seeking perfection). I will definately check the focuser tube (location and colour) as I am seeing some asymmetry that doesn't seeem to be due to collimation.
Part of my problem is tilt - which means every time i swap the camera between rigs i will need to adjust the tilt. To be honest that sounds painful - i will need to see if i can dial it out at the focuser.

I went back and forth on the F4 v F5 thing before electing for the F4 - partly due to pragmatic size and weight issues. I was a little surprised how smoothly things went at first but this current business has me regretting not going with the F5 - and investigating a 5" refractor if I am honest.

I think i have turned the corner though so hopefully it will be back to capturing some images - the last several days have been a bit tiring.
Cheers
Bill

[Nikolas (Nik)]

Maybe time to fork out for a better focuser?

[Ramius (Bill)]

Crossed my mind but what would do the job? I don't know that the Skywatcher / GSO crayfords would necessarily be an upgrade and Feather Touch isn't happening.

Irrespective need to run a full nights imaging before deciding what's next - and weather looks bad for a week.