Artifical star images, am I using it right, too low power, too close to scope?

[pipnina]

I have my artificial star torch (5 holes, 50 micron, up to 250 micron) and I'm looking at the smallest one in my 6mm eyepiece at a distance of about 13 meters (800mm focal length, 130mm triplet of unknown make and design).

As I adjust my focus from intra to extra, I notice that when it's focused it actually looks quite good, intra focal it looks like two strong rings concentric with the white dot in the middle... but the middle isn't well defined. it looks strong, but it also almost looks like three dots split up. Not sure if that's pinching or if it's miscolimation or something else! When I'm extra-focal, I see rings moving into one corner of the outermost ring, which seems like coma (cell misalignment) to me.

Should I be basing my adjustments of the tilt of the main cell on the extra focal value? Thanks

Here's the best pic I could get down the eyepiece extra-focal, sadly intra focal wasn't possible as getting the phone into position was very hard indeed and the stars were a white-out!

And this is what I'm seeing in images (albeit with corrector, but it shows that star shapes are not uniform across the field)

I have performed the paper-with-crosshair check over the front of the scope to align the focuser with a laser. And because I have tightened the grub screws holding the focuser that is now rock solid (can't use the rotation feature though now)

Should I adjust my cell with this artificial star setup, do I need more magnification, am I reading things right (that extra focal view is showing me the coma error?)

 

Many thanks!

[pipnina]

I've done some fiddling with it in daylight and I think I've solved the coma issue in the center. But now that that's solved I think I've introduced astigmatism... or worse, it's there when the cell is properly tilt collimated, meaning the problem lies within the arrangement of the triplet of lenses within... Far beyond my ability.

Hopefully this makes it better when imaging however, will report back if it is or isn't 😬

[Adam J]

20 hours ago, pipnina said:

I have my artificial star torch (5 holes, 50 micron, up to 250 micron) and I'm looking at the smallest one in my 6mm eyepiece at a distance of about 13 meters (800mm focal length, 130mm triplet of unknown make and design).

As I adjust my focus from intra to extra, I notice that when it's focused it actually looks quite good, intra focal it looks like two strong rings concentric with the white dot in the middle... but the middle isn't well defined. it looks strong, but it also almost looks like three dots split up. Not sure if that's pinching or if it's miscolimation or something else! When I'm extra-focal, I see rings moving into one corner of the outermost ring, which seems like coma (cell misalignment) to me.

Should I be basing my adjustments of the tilt of the main cell on the extra focal value? Thanks

Here's the best pic I could get down the eyepiece extra-focal, sadly intra focal wasn't possible as getting the phone into position was very hard indeed and the stars were a white-out!

 

And this is what I'm seeing in images (albeit with corrector, but it shows that star shapes are not uniform across the field)

 

I have performed the paper-with-crosshair check over the front of the scope to align the focuser with a laser. And because I have tightened the grub screws holding the focuser that is now rock solid (can't use the rotation feature though now)

Should I adjust my cell with this artificial star setup, do I need more magnification, am I reading things right (that extra focal view is showing me the coma error?)

 

Many thanks!

13m is not sufficient for a 130mm triplet assuming its about F7 then about 25m is the minimum.

Also you want to be viewing the star out of focus so you can see about 3-5 diffration rings to perform a proper star test.

Adam

Edited January 22, 2023 by Adam J

[Highburymark]

The image of the night sky you posted shows signs of astigmatism, but it’s difficult to diagnose the problem without more images - as Adam says you need more diffraction rings on both sides of focus, and the artificial star placed further away. You might need to use a more powerful eyepiece with such a large scope too. 

[newbie alert]

Are you sure you have your spacing correct, stars pointing to the center indicate that you need more

[Louis D]

On 22/01/2023 at 10:00, pipnina said:

I've done some fiddling with it in daylight and I think I've solved the coma issue in the center. But now that that's solved I think I've introduced astigmatism... or worse, it's there when the cell is properly tilt collimated, meaning the problem lies within the arrangement of the triplet of lenses within... Far beyond my ability.

Make sure the triplet is fully acclimated.  My 90mm triplet takes over 30 minutes to acclimate just 11 degrees C.  Until then, I see all sorts of star spikes.

[pipnina]

36 minutes ago, newbie alert said:

Are you sure you have your spacing correct, stars pointing to the center indicate that you need more

I have been tuning the spacing but since the star shapes are different in each corner it's been hard to get it exact.

 

I will have the chance to test it tonight so hopefully I will know if the tilt is now correct and will be able to dial the spacing in a bit better.

 

29 minutes ago, Louis D said:

Make sure the triplet is fully acclimated.  My 90mm triplet takes over 30 minutes to acclimate just 11 degrees C.  Until then, I see all sorts of star spikes.

A good shout. I have been keeping it outside for a while now as the weather is ok and it is protected from prying eyes. So it should remain decently acclimated to the temperature