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GSO 10" Collimation woes

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#1 weathermon

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Posted 13 April 2021 - 04:27 AM

Hi all,

 

I own a 10" F5 GSO reflector (GS-830) and over the years have done upgrades to it but have never really been happy with the stars, they have always been footballs on one side.  This is the list of mods I have done to it:

 

* Moonlite 2" Crayford focuser (2" travel)

* TS Optics carbon double spider

* aluminium inner brace to stiffen the inside of the tube around the focuser

* Black 3.0 paint to make it less reflective on the inside of the tube

 

I have a Howie Glatter with the Tublog for collimating the scope and I collimate as follows:

 

1) Put Howie Glatter collimator in the focuser with 1mm barlow'ed attachment, adjust secondary thumb screws to bring red laser dot onto center dot of primary mirror. 

2) Remove collimator and 1mm barlow and place Howie Glatter into the Tublog and adjust the primary mirror knobs (release white locking knobs, adjust black knobs to bring black circle around the hole of the tublog.

 

I put the cheshire collimator in and give it a visual.. I can see all 3 primary mirror clips and the secondary is offset nicely on the stalk.. should all be good right?

 

I use a MPCC MKIII coma corrector and it had horrible stars so I purchased a Skywatcher 0.9x F5 corrector in the hope to fix it which is hasn't.. I've completely started my collimation from scratch and when I defocus my image, one side of the frame always has the donuts cut off frown.gif My camera is an ASI 1600MM Pro.. the issue is only going to get worse when my ASI2600MM Pro eventually ships.. here's a preview of my donuts on the defocused image.. the Howie Glatter showed perfect collimation and I've even confirmed the laser is in collimation by putting it into a jig and spinning it, the red dot remained in the middle.

 

I thought I know enough about collimation but this is really doing my head in not being able to sort it out and one side always having horrible stars.. Is my secondary too far up/down the stalk? 

 

Cheers, Mike

Attached Thumbnails

  • defocused-130421.jpg

Edited by weathermon, 13 April 2021 - 04:29 AM.


#2 wrvond

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Posted 13 April 2021 - 06:22 AM

If it was mine, I'd be suspecting the primary at this point.

You might want to index the primary (mark the side at the 12 o'clock position) and then rotate it in the cell. Does the problem move around as the mirror is turned?


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#3 KBHornblower

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Posted 13 April 2021 - 01:01 PM

Hi all,

 

I own a 10" F5 GSO reflector (GS-830) and over the years have done upgrades to it but have never really been happy with the stars, they have always been footballs on one side.  This is the list of mods I have done to it:

 

* Moonlite 2" Crayford focuser (2" travel)

* TS Optics carbon double spider

* aluminium inner brace to stiffen the inside of the tube around the focuser

* Black 3.0 paint to make it less reflective on the inside of the tube

 

I have a Howie Glatter with the Tublog for collimating the scope and I collimate as follows:

 

1) Put Howie Glatter collimator in the focuser with 1mm barlow'ed attachment, adjust secondary thumb screws to bring red laser dot onto center dot of primary mirror. 

2) Remove collimator and 1mm barlow and place Howie Glatter into the Tublog and adjust the primary mirror knobs (release white locking knobs, adjust black knobs to bring black circle around the hole of the tublog.

 

I put the cheshire collimator in and give it a visual.. I can see all 3 primary mirror clips and the secondary is offset nicely on the stalk.. should all be good right?

 

I use a MPCC MKIII coma corrector and it had horrible stars so I purchased a Skywatcher 0.9x F5 corrector in the hope to fix it which is hasn't.. I've completely started my collimation from scratch and when I defocus my image, one side of the frame always has the donuts cut off frown.gif My camera is an ASI 1600MM Pro.. the issue is only going to get worse when my ASI2600MM Pro eventually ships.. here's a preview of my donuts on the defocused image.. the Howie Glatter showed perfect collimation and I've even confirmed the laser is in collimation by putting it into a jig and spinning it, the red dot remained in the middle.

 

I thought I know enough about collimation but this is really doing my head in not being able to sort it out and one side always having horrible stars.. Is my secondary too far up/down the stalk? 

 

Cheers, Mike

The out-of-focus disks which are cut off in the left side of the field suggest that the secondary is not positioned properly.  Your description "nicely offset" has me scratching my head.  The view through a sight tube peephole should make it appear concentric with the focuser.  However, this fault by itself should not degrade the focused stars.  Your primary tilt adjustment may be off despite your efforts so far.  I would need to see the focused stars before commenting any further.



#4 SteveG

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Posted 13 April 2021 - 01:16 PM

The 1 mm Glatter attachment is an aperture stop, to keep the beam tight and round.

 

Please post pictures taken through your “Cheshire collimator”. These out of focus images tell us nothing.



#5 weathermon

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Posted 13 April 2021 - 04:09 PM

Hi all,

 

Thanks for the suggestions - here's some more pics of the setup and also a few pics taken with my phone on 5x zoom looking down the cheshire pinhole. I'll get an image of the frame in focus as well tonight

 

Cheers, Mike

Attached Thumbnails

  • 172003678_288123396113114_309540798932011767_n.jpg
  • 171060521_470623667711374_6568966863909741944_n.jpg
  • 171243787_2232636103537777_1514917708882330797_n.jpg
  • 171313914_731481047523532_1302698399667832781_n.jpg

Edited by weathermon, 13 April 2021 - 04:17 PM.


#6 BradFran

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Posted 13 April 2021 - 04:30 PM

Did you check the center spot on your primary mirror? It could be off. If you need to replace it, I like these from CatsEye.


Edited by BradFran, 13 April 2021 - 04:32 PM.


#7 KBHornblower

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Posted 13 April 2021 - 09:44 PM

Your first photo is cropped too tight.  I cannot see whether or not the reflection of the primary is centered in the focuser drawtube.  


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#8 weathermon

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Posted 14 April 2021 - 12:13 AM

Did you check the center spot on your primary mirror? It could be off. If you need to replace it, I like these from CatsEye.

Yeah I checked the center spot a few months ago and it was all good, I thought it might have been the issue back then

 

Cheers, Mike


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#9 weathermon

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Posted 14 April 2021 - 12:31 AM

Your first photo is cropped too tight.  I cannot see whether or not the reflection of the primary is centered in the focuser drawtube.  

Hows this one? This was another that I took with the phone over the cheshire pinhole with no zoom

 

Cheers, Mike

Attached Thumbnails

  • 172649659_296394525536967_2229176663371801158_n.jpg


#10 weathermon

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Posted 14 April 2021 - 01:00 AM

Could I have a Type 2 error? I didn't even realise this was a thing.. when I installed the new double spider, I positioned the secondary smack bang in the middle of the tube - I didn't realise I needed to offset it away from the focuser or would the GSO secondary holder already have this accounted for? 

offset=minor axis/(4*focal ratio)
 

So for mine it would be:

 

offset=70/(4*5) = 3.5mm.  Could this be causing the issues? 

 

Cheers, Mike


Edited by weathermon, 14 April 2021 - 01:12 AM.


#11 SteveG

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Posted 14 April 2021 - 01:24 AM

Could I have a Type 2 error? I didn't even realise this was a thing.. when I installed the new double spider, I positioned the secondary smack bang in the middle of the tube - I didn't realise I needed to offset it away from the focuser or would the GSO secondary holder already have this accounted for? 

offset=minor axis/(4*focal ratio)
 

So for mine it would be:

 

offset=70/(4*5) = 3.5mm.  Could this be causing the issues? 

 

Cheers, Mike

Absolutely not with the spider. Yes, the secondary mirror might be glued on away from the focuser, but either way your offset will happen when you get the secondary centered under the focuser, and properly tilted. In your pictures, we can’t see the edge of the secondary.

 

Ill let the experts chime in. Hopefully Vic will give some advice.


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#12 BradFran

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Posted 14 April 2021 - 02:22 AM

Do the footballs change their orientation inside versus outside of focus?



#13 weathermon

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Posted 14 April 2021 - 03:29 AM

I'm about to get some test shots now as it's almost dark enough but here's another pic just looking directly down the focuser

 

Cheers, Mike

Attached Thumbnails

  • 172584127_6030390390365038_6521593317536225345_n.jpg


#14 weathermon

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Posted 14 April 2021 - 04:06 AM

Here is the in focus test sub

focus.jpg

 

Cheers, Mike



#15 weathermon

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Posted 14 April 2021 - 04:07 AM

Here is the inside focus test sub

inside.jpg

 

Cheers, Mike



#16 weathermon

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Posted 14 April 2021 - 04:08 AM

Here is the outside focus test sub

 

outside.jpg

 

Cheers, Mike



#17 BradFran

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Posted 14 April 2021 - 07:10 AM

Inside and outside focus have similar orientation and it is pronounced on one side of the field. Hmrph. It doesn't look like a collimation problem to me. I would try rotating things to eliminate the cause. First the primary mirror, as was suggested. If there is no change, flip the secondary mirror 180 degrees and see if the footballs move to the other side of the field. I'm not an imager and use the star test, so perhaps I'm missing something.

 

edit: Vic's on the case! (see below)


Edited by BradFran, 14 April 2021 - 04:10 PM.

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#18 Vic Menard

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Posted 14 April 2021 - 07:11 AM

When I looked at your first image, I suspected you might have a "Type 2" error (FAE), but then I read, "I collimate as follows: 1) Put Howie Glatter collimator in the focuser with 1mm barlow'ed attachment, adjust secondary thumb screws to bring red laser dot onto center dot of primary mirror." and, "I've even confirmed the laser is in collimation by putting it into a jig and spinning it, the red dot remained in the middle." The allowable Type 2 error for a coma corrected Newtonian is about 0.005 times the primary mirror diameter, so about 0.05-inch (1.25mm). Given this additional information, I believe you're axial alignments are good.

 

As far as the clipped donuts in your far out of focus stars toward the edge of the field of view, you should expect to see this with a secondary mirror that doesn't deliver 100-percent illumination to the edge of the field of view. When focused, the star images should be good to the edge of the (coma corrected) field of view. There will be some light loss as you move away from the center of the field, but that's easily corrected once you understand the illumination profile. Your offset looks like it might be a bit more than 4mm in the Cheshire image you provided (compare the yellow circle to the violet circle), but this is also part of the illumination profile (it has little or no impact on your star image performance). The outermost light blue circle is puzzling, but more than likely it's a reflection/artifact associated with the Cheshire.

 

(Edit: After reassessing the light blue circle, I suspect it may be the actual edge of the secondary mirror. As I noted earlier, I suspect you have a bit too much offset, which would explain the offset light blue circle (which should be centered is it's the actual edge of the secondary mirror). But you should be able to see this in your Cheshire. Regardless, as I already noted, this offset error will have no real impact on your star/image performance.)

Attached Thumbnails

  • post-293246-0-97194500-1618348093_thumb.jpg

Edited by Vic Menard, 14 April 2021 - 08:28 AM.

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#19 weathermon

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Posted 14 April 2021 - 09:03 PM

When I looked at your first image, I suspected you might have a "Type 2" error (FAE), but then I read, "I collimate as follows: 1) Put Howie Glatter collimator in the focuser with 1mm barlow'ed attachment, adjust secondary thumb screws to bring red laser dot onto center dot of primary mirror." and, "I've even confirmed the laser is in collimation by putting it into a jig and spinning it, the red dot remained in the middle." The allowable Type 2 error for a coma corrected Newtonian is about 0.005 times the primary mirror diameter, so about 0.05-inch (1.25mm). Given this additional information, I believe you're axial alignments are good.

 

As far as the clipped donuts in your far out of focus stars toward the edge of the field of view, you should expect to see this with a secondary mirror that doesn't deliver 100-percent illumination to the edge of the field of view. When focused, the star images should be good to the edge of the (coma corrected) field of view. There will be some light loss as you move away from the center of the field, but that's easily corrected once you understand the illumination profile. Your offset looks like it might be a bit more than 4mm in the Cheshire image you provided (compare the yellow circle to the violet circle), but this is also part of the illumination profile (it has little or no impact on your star image performance). The outermost light blue circle is puzzling, but more than likely it's a reflection/artifact associated with the Cheshire.

 

(Edit: After reassessing the light blue circle, I suspect it may be the actual edge of the secondary mirror. As I noted earlier, I suspect you have a bit too much offset, which would explain the offset light blue circle (which should be centered is it's the actual edge of the secondary mirror). But you should be able to see this in your Cheshire. Regardless, as I already noted, this offset error will have no real impact on your star/image performance.)

Hi Vic,

 

Thanks so much for the information! I never realised that when I defocus the stars, that's when it'll have issues.  I've re-adjusted the secondary on the stalk now and will give it another go tonight.  I'll be upgrading to a Skywatcher 12" F4 soon anyway 

 

Cheers, Mike




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