Half donut on out-of-focus star test. Generic equipment discussions · Craig Dixon · ... · 11 · 1028 · 6

craigdixon1986 3.01
...
· 
·  Share link
I recently collimated my Mak-Newt and star tested it afterwards. It all seemed fine but the data from my last imaging session wasn’t great so I placed a home-made artificial star at the back of my garden (cloudy all week) and this is what I get.

The half donut rotates if I rotate the camera and if I put an eyepiece in, I see the half donut and it stays where it is regardless of me rotating the eyepiece.

This is in the middle of the frame but doesn’t change if I move it to the edge. I can’t see any obstructions in the imaging train either.

Really stumped by this one.IMG_5332.png
Like
andreatax 9.89
...
· 
·  Share link
Something is deeply wrong. Check the everything in the MN is where it should be. Incidentally, the distance  should be at least 20 times the focal length of the telescope. Is that far?
Like
wimvb 3.11
...
· 
·  1 like
·  Share link
Half a donut means that half of the light is obstructed.  Since it doesn't move when you move the field of view, try rotating the scope, and rotate the artificial star. Does the half donut rotate then?
How did you make the artificial star? Is the light source directly behind the pinhole, or do you use the reflection of a light source in a small sphere?
Edited ...
Like
craigdixon1986 3.01
Topic starter
...
· 
·  Share link
Thanks for taking the time to help!

This is a really weird one, I just can't work it out. Everything in the scope seems to be as it should be. I collimated it last week using an Ocal Pro collimation camera and this was the view down the focuser tube:
Screenshot_2023-10-14_at_13.09.45.png
 The red thing in the centre is the local with it's sensor on the crosshairs.

I also did a very quick star test using Deneb in poor seeing just after and got this:

IMG_5157.jpg

When I was trying to find my artificial start last night, the image of my back fence, shrubs, trees, etc looked normal and there was nothing being blocked out.

The "star" I was using was a quick and simple foil with a pin prick in it over the end of an LED torch DIY job. I was hoping to use an actual star but it was 100% cloudy. The star was placed about 30m away. the focal length is 1000mm so I'm over the 20m minimum. Or do you mean the focal length as in the aperture x focal length (f5.3 x 1000mm)? In that case, the star would need to be 106m away.

I've tried moving the torch and the scope in all directions and get exactly the same half donut.


Incidentally, here's a single auto-stretched sub from the other night (after collimation), which doesn't look as bad as my half-donut start test suggests:

Screenshot 2023-10-26 at 12.39.10.png
Like
andreatax 9.89
...
· 
·  1 like
·  Share link
I would suspend judgment (and action) until you get even a short glimpse of a star to confirm everything is OK,
Like
wimvb 3.11
...
· 
·  Share link
Craig Dixon:
The "star" I was using was a quick and simple foil with a pin prick in it over the end of an LED torch DIY job. I was hoping to use an actual star but it was 100% cloudy. The star was placed about 30m away. the focal length is 1000mm so I'm over the 20m minimum. Or do you mean the focal length as in the aperture x focal length (f5.3 x 1000mm)? In that case, the star would need to be 106m away.


Suyter, who literally wrote the book on star testing, recommends to put the star at 2-3 times the limit that @andrea tasselli mentioned earlier, ie 40-60 m from the scope. My earlier remarks were based on a hunch that the star itself could be the problem. Your test on a real star looks ok. The shape you get with the artificial star looks as if something was blocking part of the light. A star that is near the edge of the field of view can look like this, because the scope's aperture (or edge of a mirror) blocks part of the light.
Like
ScottBadger 7.63
...
· 
·  1 like
·  Share link
Wim van Berlo:
The shape you get with the artificial star looks as if something was blocking part of the light. A star that is near the edge of the field of view can look like this, because the scope's aperture (or edge of a mirror) blocks part of the light.

The one time I've seen a defocused star clipped cleanly like this, though not nearly as much as half, it was from a less than round dew shield. Craig, f you're doing the test with a dew shield on, but using an artificial star with the ota close to horizontal, maybe it's 'drooping'?.... It would be pretty coincidental, though, that it would clip the star exactly in half.

Cheers,
Scott
Like
andreatax 9.89
...
· 
·  1 like
·  Share link
Wim van Berlo:
Suyter, who literally wrote the book on star testing, recommends to put the star at 2-3 times the limit that @andrea tasselli mentioned earlier, ie 40-60 m from the scope. My earlier remarks were based on a hunch that the star itself could be the problem. Your test on a real star looks ok. The shape you get with the artificial star looks as if something was blocking part of the light. A star that is near the edge of the field of view can look like this, because the scope's aperture (or edge of a mirror) blocks part of the light.


The value recommended by Suiter is for when you want to evaluate the quality of the optics but in this case there is no need for that, just small enough to appear stellar (albeit slightly aberrated by 3rd order SA). The cut-off is very sharp and that usually means the "thing that is cutting the light beam" is close or at the pupil entrance which in our case should be at the corrector. Or half the mirror has dropped off...
Like
craigdixon1986 3.01
Topic starter
...
· 
·  5 likes
·  Share link
Just to follow up on this. The cause of the half donut was indeed an obstruction. It was the roof of my observatory. When the “star” was in focus, the roof was well out of the frame but it must have been in the way when de-focused. I’ve spend two days trying to figure this one out so feel like a complete idiot now. All’s well and ends well. here’s the whole donut.IMG_5342.jpeg
Like
wimvb 3.11
...
· 
·  1 like
·  Share link
Experience gained. Nice to have this sorted out.

cs,

Wim
Like
markjohn 0.00
...
· 
·  Share link
Craig Dixon:
I recently collimated my Mak-Newt and star tested it afterwards. It all seemed fine but the data from my last imaging session wasn’t great so I placed a home-made artificial star at the back of my garden (cloudy all week) and this is what I get. rice purity test

The half donut rotates if I rotate the camera and if I put an eyepiece in, I see the half donut and it stays where it is regardless of me rotating the eyepiece.

This is in the middle of the frame but doesn’t change if I move it to the edge. I can’t see any obstructions in the imaging train either.

Really stumped by this one.IMG_5332.png

It sounds like you're encountering an interesting optical issue! The half donut pattern you're seeing could be related to several factors, such as collimation misalignment or even the quality of the optics in your imaging train. Since it rotates with the camera but remains stationary with the eyepiece, it suggests that the problem might be related to the camera sensor or its alignment.
Have you checked for any potential dust or debris on the sensor or lenses? Additionally, ensuring that your collimation is spot on and that there are no misalignments in the optical path could help resolve this. It might also be worth experimenting with different cameras or eyepieces to see if the issue persists. Keep us updated on your findings!
Like
craigdixon1986 3.01
Topic starter
...
· 
·  Share link
I discovered shortly after experiencing this problem that there was a tiny obstruction between the scope and the artificial star, which is what caused the half donut. As the FOV is so small, it just didn't look like there was anything in the way but it was a left if I remember correctly.
Like
 
Register or login to create to post a reply.