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  #1  
Old 03-10-2018, 03:47 AM
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astronobob (Bob)
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Focus Capers - What's the go with these Star shapes ?

Can't win a trick these days ?

Apon having a recent 'rant' I now experience further unexplained phenomenon , Focusing

1st up, the gear used: 10" SW F4.7 Newt on Neq6p;; Zwo Asi 290 Mono, no filter wheel, just straight on as to take the filters out of the equation..
Note: Also does/looks similar using the QHY5L II,, ,

Pic attached shows the story of Inside focus -> through Focus -> and outside focus

NOTE B: 1st pic, Frame 5 & 6 are both at same focus, difference being that 6 is off a smaller less bright star, which shows it is not focused but resolves into 4 points of light ....

2nd pic: Showing best focus on M2 Glob Cluster

These star 'profiles' are screenshot grabs off Fire-Capture window @ %200

In conclusion with my Thunking abilities, I question whether I have just simply found the resolution ability of this scope considering both these cameras have tiny pixels and am presenting the Star at %200, or is there more to tell - primarily, mirror/s quality, or maybe simply 'collimation' surely it isnt collimation, it looks pretty darn good in an EP Airy disc and with the laser collimator ?

Insights, etc much appreciated
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  #2  
Old 03-10-2018, 04:42 AM
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troypiggo (Troy)
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What exposure times are these images? I have same scope and for focusing I use 2s exposure. Your shots look like seeing is affecting them with wobble.

And are you using a coma corrector like MPCC or Parracor?
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  #3  
Old 03-10-2018, 06:18 AM
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multiweb (Marc)
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Seems there are a few things going on in there. First I think you're slightly out of focus in your cluster shot. That would make coma and other aberrations worse in your field. I think you have coma and also astigmatism. The latter could be confirmed by comparing in and out of focus shots. Maybe pinched optics as well.

Here's what I'd do to eliminate possible causes.

1_ check the primary and secondary glass are not stressed (primary retaining clips and secondary bonding)

2_ check your on axis stars in and out of focus. If you have astigmatism rotate your primary until "it's better".

3_ once your on axis stars are sorted check your secondary position/tilt and collimation without corrector.

4_ tweak your coma corrector spacing to get the flatest field you can.

See how that goes.
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  #4  
Old 03-10-2018, 08:00 AM
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troypiggo (Troy)
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I was going to get to those next

If the weird star shapes were astig/coma, wouldn't they radiate out the shapes from the centre? His cluster shot they all seem to be same direction/orientation. That's why I was trying to rule out seeing if he had really short exposures and it was wobble of the stars due to seeing.
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  #5  
Old 03-10-2018, 09:35 AM
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billdan (Bill)
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With my 14 inch I get weird star shapes when I first start using the scope, but 1.5 hours later after the mirror has cooled down everything is OK.

So I was wondering did you allow enough time for the primary mirror to stabilise?
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  #6  
Old 03-10-2018, 12:56 PM
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Quote:
Originally Posted by troypiggo View Post
What exposure times are these images? I have same scope and for focusing I use 2s exposure. Your shots look like seeing is affecting them with wobble.

And are you using a coma corrector like MPCC or Parracor?
Cheers & Thanx Troy, yes these are 2sec's. No MPCC, cam connected straight on to rule out all other possible interferences, eg, MPCC & Filters.
Re: seeing and wobble,, agree there is some wobble from seeing, but on looking closely there are two consistent bright spots in the rings which I am suspect about, I also took about 5 shots at each focus position and the 'seeing wobble' was evident with the ring, but the bright spots were consistent in location on/in the ring and as two points at closer to focus.
What mainly gets me is the four points at focus, also 5 x 2sec shots showed these 4 points - therefore most likely ruling out seeing, not 100% sure though ?
Thanx again
'
'
'----------------------------------------------

Quote:
Originally Posted by multiweb View Post
Seems there are a few things going on in there. First I think you're slightly out of focus in your cluster shot. That would make coma and other aberrations worse in your field. I think you have coma and also astigmatism. The latter could be confirmed by comparing in and out of focus shots. Maybe pinched optics as well.

Here's what I'd do to eliminate possible causes.

1_ check the primary and secondary glass are not stressed (primary retaining clips and secondary bonding)

2_ check your on axis stars in and out of focus. If you have astigmatism rotate your primary until "it's better".

3_ once your on axis stars are sorted check your secondary position/tilt and collimation without corrector.

4_ tweak your coma corrector spacing to get the flatest field you can.

See how that goes.
Appreciated this 'Check list' Marc

1_ I did do a main mirror retaining clip adjustment many months ago but the problem then was sorting the MPCC spacings & collimation - But will check them again ,, - Havent considered the 2ndary bonding ?

2_ Considering the chips on both cameras are so small I'd suspect all stars in the field would be well in the on axis zone, anyway, all stars doing the same thing across the field.
Astigmatism, havent notice any with my Ronchi testing ep fitting & have a mark on mirror so goes back in same position since new, but again, I may experiment with this ?

3_ Secondary I have considered, so I will be inspecting & adjusting this next chance.

4_ No Coma corrector in place

Thanx Marc
,
,
,
----------------------------------------------

Quote:
Originally Posted by troypiggo View Post
I was going to get to those next

If the weird star shapes were astig/coma, wouldn't they radiate out the shapes from the centre? His cluster shot they all seem to be same direction/orientation. That's why I was trying to rule out seeing if he had really short exposures and it was wobble of the stars due to seeing.
Cheers Troy, Astigmatism should present itself across the field, Coma affects the outfield more than the center, ,
The slight diagonal direction/orientation effect is most likely drift over that 5-10sec capture, wasnt guiding ?
,
,
,
----------------------------------------------
Quote:
Originally Posted by billdan View Post
With my 14 inch I get weird star shapes when I first start using the scope, but 1.5 hours later after the mirror has cooled down everything is OK.

So I was wondering did you allow enough time for the primary mirror to stabilise?
Cheers Bill, This was done around middnight, was cloud about and after sunset, cleared somewhat around 10pm, from then on I do usual routine - two hrs of struggling, bumbling, pulling hair out Then give up
Hence my other RANT Thread
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  #7  
Old 03-10-2018, 06:41 PM
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Ok, , Have finished 3 hrs (in between chores) on the 2ndary alignments :-

1) Centering in tube, the spider hub is out by 3-4mm, compensating for the 2ndary mirror not being mounted center in the 45deg mounting face on the 2ndary mount tube,, the hub is smaller than 2ndary mirror, no obstruction. The 2ndary mirror is now central to tube.

2) The distance the 2ndary in tube was about 3-4 mm too close to main mirror, (Using a cardboard sheet on inside optical-tube) this is now = inline with the Focus draw tube within 1mm,, I say within on average because the 2ndry mirror's glass ground outside edge has not been cut/done squarely and is producing some unwanted obstruction, ,, no worries, atleast the important reflective surface is now as truly as accurately possible centered

3) The rotation of 2ndry is now reflection back squarely parallel down the tube and an almost perfect reflecting circle matching the main mirror has been achieved.

4) Placed the pin-hole cheshire alignment tool in focuser and followed the main mirror's central dot by moving my eye around the edge of the focus tube but back/away enough to line things up - this produced a line-of-sight from the focus tube end closest to eye circled edge, to the 2ndary's outer edge, and to the main mirror's central spot, , completely all the way around...
This took some time as well, , tweaking tweaking...


5) Place in the laser collimator and it reflected back onto the collimators target 1/2 way off center - whoo,
Then tweaked the 2ndary to middle of main mirror's center dot, as normal, & was initially pointed to just touching the outside edge of main mirror's dot, , tweak to center, very touchy allowing for flex from the spider fins. Got that, and now sitting 1/3rd off center of collimators target, Whoo, - that's the "Trippple score Ring on a Dart Board
Might need take a compact scope to the Pub, haha !
Then fine tuned main mirror to centered laser in laser target

I have never spent so much time on collimating, not to this extent, must of spent 15-20mins on all those seperate areas of adjustment,,, !!!
Hopefully: This should eliminate the collimation factor of those star shapes, atleast ?
See what a star image look like directly, After dinner, Ill go out to find an overcast sky, , , , , , Just checked, looks clear apart from a hazy horizon with thin high cloud in west,

L8r . . . . .. .

Last edited by astronobob; 03-10-2018 at 06:48 PM. Reason: Add txt on note 4
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  #8  
Old 03-10-2018, 08:35 PM
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Hmmm, non focused star, in ep showing 2ndry not center, , Have been changing squareness of main mirror, chasing star but not changing the out-of-center alignment,,,

Now made/designed a double ring cardboard mask for inside of ota and will place it over the inside opening of focus tube , hopefully showing where reflective light is bouncing back off 2ndry to focus tube, maybe need rotation of 2ndary, looks good 90deg to main parallel path ?
Hopefully find some horizontal discrepancies ?

I am still with Stupid ha ho
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  #9  
Old 03-10-2018, 11:02 PM
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astronobob (Bob)
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Drat, Drat, , Double Dratt

ZZzzZZzz
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  #10  
Old 04-10-2018, 02:29 PM
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Going to change scope's - think this 10" F/4.7 mirror needs resting & dust off the 8" F/4 ---- Grrrr, back to the 'drawing board' ! !
What a fantastic hobbie - Can only go up from here
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  #11  
Old 04-10-2018, 02:41 PM
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multiweb (Marc)
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Quote:
Originally Posted by astronobob View Post
1) Centering in tube, the spider hub is out by 3-4mm, compensating for the 2ndary mirror not being mounted center in the 45deg mounting face on the 2ndary mount tube,, the hub is smaller than 2ndary mirror, no obstruction. The 2ndary mirror is now central to tube.
Do you mean your spider vanes are not square?

Do you have a catseye collimating kit or do you know someone who can lend you one?

What do your on axis stars look like now?
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  #12  
Old 04-10-2018, 07:38 PM
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Do you mean your spider vanes are not square?

Do you have a catseye collimating kit or do you know someone who can lend you one?

What do your on axis stars look like now?
Marc, If I center the Spider hub by using calipers from the edge, then the 2ndry mirror is not in center as it is not mounted centered on the 2ndry mount, which is connected to the spider hub. Soo, I needed to 'off-set' the spider's hub to center the 2ndry, is this a 'no-no' ?
I ask, because I have heard of some 'Off-Setting' which needed addressing rgrds the 2ndary, not sure if that is to do with the actual 2ndry mirror or the spider hub ?

Stars Look the Same this 10" is now 'off' the mount, resting, I may make a ronchi tester, knife edge and light test for a better squizz at this mirror's shape ? ? or, I might ask here if any mirror-makers close by ?

Cheers mate
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  #13  
Old 07-10-2018, 09:33 PM
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Ok, have just been reminded about '2ndary mirror off-set' for fast newts - re researching ....
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  #14  
Old 08-10-2018, 06:43 AM
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Quote:
Originally Posted by astronobob View Post
Marc, If I center the Spider hub by using calipers from the edge, then the 2ndry mirror is not in center as it is not mounted centered on the 2ndry mount, which is connected to the spider hub. Soo, I needed to 'off-set' the spider's hub to center the 2ndry, is this a 'no-no' ?
I ask, because I have heard of some 'Off-Setting' which needed addressing rgrds the 2ndary, not sure if that is to do with the actual 2ndry mirror or the spider hub ?

Stars Look the Same this 10" is now 'off' the mount, resting, I may make a ronchi tester, knife edge and light test for a better squizz at this mirror's shape ? ? or, I might ask here if any mirror-makers close by ?

Cheers mate
The secondary offset should be built in and the spider vanes should be kept square and centered as much as possible to avoid double diffraction spikes. Maybe the secondary has been moved and glued in a different position. The glass might also be stressed and why you're getting funny star shapes. Is it siliconed on the mounting? That's where I'd start looking.
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Old 08-10-2018, 12:52 PM
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Quote:
Originally Posted by multiweb View Post
The secondary offset should be built in and the spider vanes should be kept square and centered as much as possible to avoid double diffraction spikes. Maybe the secondary has been moved and glued in a different position. The glass might also be stressed and why you're getting funny star shapes. Is it siliconed on the mounting? That's where I'd start looking.
Yes, thnx Marc, your right, the 2ndary has been originally mounted with the required 'offset' so, need to re center the spiders hub to center of tube, then adjust forward/back from main mirror to place squarely in the focus tube line-of-sight..
Cheers for your support
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Old 08-10-2018, 01:01 PM
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Quote:
Originally Posted by astronobob View Post
Yes, thnx Marc, your right, the 2ndary has been originally mounted with the required 'offset' so, need to re center the spiders hub to center of tube, then adjust forward/back from main mirror to place squarely in the focus tube line-of-sight..
Cheers for your support
Cool - so that means you need to center your spider vanes, then put an A4 piece of paper inside the tube and move the secondary up and down the tube and orient it until it looks like a circle centered in the focuser draw tube. Try to ignore all the reflections from the primary.
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Old 09-10-2018, 10:31 PM
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Quote:
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Cool - so that means you need to center your spider vanes, then put an A4 piece of paper inside the tube and move the secondary up and down the tube and orient it until it looks like a circle centered in the focuser draw tube. Try to ignore all the reflections from the primary.
That's Correct.
Did this with my 8" F4 last night. Then an 'out-of-focus star test' showed 2ndry obstruction in center (One Step Forward) but is somehow producing a double shadow from one of the spider veins, no biggy & hopefully twiddle it right next time or see how it corresponds with a focused exposure 1st ..
Mmmm, the plot thickens !

I am also changing cameras, the zwo 290 chip is way to small
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  #18  
Old 18-10-2018, 02:38 PM
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Hey Bob

Astigmatism is clearly there when you look at the difference between 1 & 7 and between 2 & 8 out of focus images. There is the 90 degree difference in the light pattern.

The star field shows a uniform star shape distortion. If it aint tracking error then maybe some tilt in the focusser / imaging train.
I have seen this star shape when using the old Meade DSI and I attributed it to oblong pixels! ZWO say their pixels are 2.9 x 2.9micron so that shouldnt be the cause - anyone else seeing this with the same camera?

miscollimation could cause that flared focussed star image. Final collimation visually at high mags should help there

No sign of coma there IMO.

Cheers
GlennB
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Old 20-10-2018, 11:15 PM
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Hey Bob

Astigmatism is clearly there when you look at the difference between 1 & 7 and between 2 & 8 out of focus images. There is the 90 degree difference in the light pattern.

The star field shows a uniform star shape distortion. If it aint tracking error then maybe some tilt in the focusser / imaging train.
I have seen this star shape when using the old Meade DSI and I attributed it to oblong pixels! ZWO say their pixels are 2.9 x 2.9micron so that shouldnt be the cause - anyone else seeing this with the same camera?

miscollimation could cause that flared focussed star image. Final collimation visually at high mags should help there

No sign of coma there IMO.

Cheers
GlennB
Interesting, thanx Glenn,, a combination of capers going on here nbo me thinks,, bit of everything

This scope is now resting with the Primary having no mounting clip pressure holding the mirror, could be a fact of possibly being to much torque on these causing astigmatism ?

I will also check the focus tube mounting alignment for squareness in relation to the tube, this & other collimating issues may/most likely have been attributing to these poor star shapes inside & outside of focus...

Cheers
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