Collimation problem on SCT

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Piet Le Roux South Africa
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Collimation problem on SCT

#1

Post by Piet Le Roux »


I wonder if anybody has had the same problem with their SCT : I have a 8” Meade LX90ACF of which the collimation differs from one side of focus to the other. I found a reference to this in “Reflecting telescope optics II” by R.N. Wilson on page 135 and he has this to say:

“The appearance of "geometrical adjustment error" means the following. It may happen that the secondary is not well centered in the telescope tube. If the primary is slightly inclined, the coma may well be excellently compensated giving a coma-free Schiefspiegler, as discussed above. If a beam corresponding to the field center is investigated, the pro­jection of the secondary may not be at the center of the entrance pupil. For a pupil plate on one side only of the focus, this effect cannot be distinguished from coma. However, it inverts on the other side of the focus whereas coma remains unchanged.”

I have tried to centre my secondary by measuring the distances from the secondary to the edge of the OTA, with a calliper, while the outside allen screws holding the gasket and 3 allen screws holding the secondary, have been loosed and then tightening the allen screws when its in the centre, while this improved it, it’s still not perfect.
Main Equipment : Tele Vue 27mm Panoptic, 7&13mm Nagler, Big Barlow : 8" Meade LX90ACF with Meade 2.0" Enhanced Diagonal : Camera Fuji XT100
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Re: Collimation problem on SCT

#2

Post by Greenman »


So the Airey disk looks centred, but even so the collimation is out at the edge on one side. It sounds as if you are taking the right steps. Hopefully someone who has more practical experience will be along. I wonder if the primary is off-centre?
Cheers,

Tony.

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Re: Collimation problem on SCT

#3

Post by yobbo89 »


yeah i have same dam problem with my scope. i wish i purchased a rc scope where you can collimate the primary .. to me it sounds like tilt error where it is so much that even a tilt ring can't correct it. may be some miss alignment from the secondary in the corrector plate and the tilt on the primary on the focuser plate/tube .... rotateing the secondary on the corrector plate might help a bit ....

you might be better off collimating closer to focuse then having a full defocused star ... to even out between inner and outer focuse tilt ..


i feel they just slap these mass produced scopes together even though they come with a qa data sheet and just don't really test them ....
scopes :gso/bintel f4 12"truss tube, bresser messier ar127s /skywatcher 10'' dob,meade 12'' f10 lx200 sct
cameras : asi 1600mm-c/asi1600mm-c,asi120mc,prostar lp guidecam, nikkon d60, sony a7,asi 290 mm
mounts : eq6 pro/eq8/mesu 200 v2
filters : 2'' astronomik lp/badder lrgb h-a,sII,oIII,h-b,Baader Solar Continuum, chroma 3nm ha,sii,oiii,nii,rgb,lowglow,uv/ir,Thousand Oaks Solar Filter,1.25'' #47 violet,pro planet 742 ir,pro planet 807 ir,pro planet 642 bp ir.
extras : skywatcher f4 aplanatic cc, Baader MPCC MKIII Coma Corrector,Orion Field Flattener,zwo 1.25''adc.starlight maxi 2" 9x filter wheel,tele vue 2x barlow .

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Re: Collimation problem on SCT

#4

Post by John Baars »


Like Yobbo says, check your collimation very near focus, and as a final step, in focus. At high magnification. The airy disc should be centered in the first diffraction ring. It is the only way to establish eventual other problems.
Refractors in frequency of use : *SW Evostar 120ED F/7.5 (all round ), * Vixen 102ED F/9 (vintage), both on Vixen GPDX.
GrabnGo on Alt/AZ : *SW Startravel 102 F/5 refractor( widefield, Sun, push-to), *OMC140 Maksutov F/14.3 ( planets).
Most used Eyepieces: *Panoptic 24, *Morpheus 14, *Leica ASPH zoom, *Zeiss barlow, *Pentax XO5.
Commonly used bino's : *Jena 10X50 , * Canon 10X30 IS, *Swarovski Habicht 7X42, * Celestron 15X70, *Kasai 2.3X40
Rijswijk Public Observatory: * Astro-Physics Starfire 130 f/8, * 6 inch Newton, * C9.25, * Meade 14 inch LX600 ACF, *Lunt.
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Re: Collimation problem on SCT

#5

Post by Piet Le Roux »


The way I am interpreting the reference in " Reflecting telescope optics II" is that if you have a SCT that is just out of collimation, resulting in coma, the unsymmetrical pattern of rings will look the same on both sides of focus, this can be rectified by just adjusting the secondary mirror angle, but if the pattern reverses or appear differently on different sides of focus this is not the same as just coma and it is known as "geometrical adjustment error" and indicates that the secondary mirror could be off centre, or the primary mirror is not perpendicular to the focus tube or even the focus tube is not perfectly centre. Mine clearly indicates this and yes I have been trying to do its collimation as close to focus as possible and then the result looks good but does differ between different eye pieces. I did see a post a while back on how to diagnose this problem, this involved making a cross hairs with two pieces of string across the face of the OTA and then focusing up and down to see if the primary mirror is at a angle and adjusting the primary mirror angle if necessary . Maybe someone has knowledge of this? At this stage I am not sure if it is worth the effort so I am trying find-out as much as I can about the procedure.
Main Equipment : Tele Vue 27mm Panoptic, 7&13mm Nagler, Big Barlow : 8" Meade LX90ACF with Meade 2.0" Enhanced Diagonal : Camera Fuji XT100
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Re: Collimation problem on SCT

#6

Post by yobbo89 »


I tried to pull my sct appart, and for what ever reason i can't get the tube out of the rear cell end.. it seems that meade that for some reason particularly on my scope might of glued it on which they are not ment to do, there is yellow seeping stuff around the edges ...

i can never have a inside and outiside focus symmetry on my sct .it's just so poorly put together ..

i've seen a video of someone running a camera in the focuser and checking the alighnment of the secondary with a crosshair on a computer. and they started sanding the inside of corrector plate and packing one side with cork.. i woudn't go as far as that .. but maybe machine up a slightly smaller secondary holder that i can shift around in the corrector plate hole ..

there is bound to be a few that has this type of problem but no real experts on the methode for corection, it seems people mostly op for the slightly more expensive sct scopes, " celestrone'' or other premiume brands ..

i've seen some mods go as far as putting 3 holes in the back of the scope with bolts and fixing the primary and collimating it keeping it perm locked down , treating the collimation like an rc scope .
scopes :gso/bintel f4 12"truss tube, bresser messier ar127s /skywatcher 10'' dob,meade 12'' f10 lx200 sct
cameras : asi 1600mm-c/asi1600mm-c,asi120mc,prostar lp guidecam, nikkon d60, sony a7,asi 290 mm
mounts : eq6 pro/eq8/mesu 200 v2
filters : 2'' astronomik lp/badder lrgb h-a,sII,oIII,h-b,Baader Solar Continuum, chroma 3nm ha,sii,oiii,nii,rgb,lowglow,uv/ir,Thousand Oaks Solar Filter,1.25'' #47 violet,pro planet 742 ir,pro planet 807 ir,pro planet 642 bp ir.
extras : skywatcher f4 aplanatic cc, Baader MPCC MKIII Coma Corrector,Orion Field Flattener,zwo 1.25''adc.starlight maxi 2" 9x filter wheel,tele vue 2x barlow .

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Re: Collimation problem on SCT

#7

Post by John Baars »


Same problem. Other cause.
I wonder if anybody has had the same problem with their SCT : I have a 8” Meade LX90ACF of which the collimation differs from one side of focus to the other.
yobbo89 wrote: Tue Oct 06, 2020 6:02 pm (...)

i've seen some mods go as far as putting 3 holes in the back of the scope with bolts and fixing the primary and collimating it keeping it perm locked down , treating the collimation like an rc scope .
Not exactly what we did at our public observatory with our 14 inch Meade SCT. But we used the same principle: locking the main mirror.

Focussing is done with an external focusser. We found out that as a result of generous dimensions of the original focussing system, the mirrorposition on the west position of the telescope differs from the mirrorposition when the telescope is in his east position. "The mirror-flip", we'd call it. After finding the best position of the mirror along its focusing axis (taking into account the external focuser and the various eyepieces) we locked the mirror with the mirror lock ( standard for transport occasions). Completed by collimating with the secondary. The telescope has a permanent position and mount under a dome.
Refractors in frequency of use : *SW Evostar 120ED F/7.5 (all round ), * Vixen 102ED F/9 (vintage), both on Vixen GPDX.
GrabnGo on Alt/AZ : *SW Startravel 102 F/5 refractor( widefield, Sun, push-to), *OMC140 Maksutov F/14.3 ( planets).
Most used Eyepieces: *Panoptic 24, *Morpheus 14, *Leica ASPH zoom, *Zeiss barlow, *Pentax XO5.
Commonly used bino's : *Jena 10X50 , * Canon 10X30 IS, *Swarovski Habicht 7X42, * Celestron 15X70, *Kasai 2.3X40
Rijswijk Public Observatory: * Astro-Physics Starfire 130 f/8, * 6 inch Newton, * C9.25, * Meade 14 inch LX600 ACF, *Lunt.
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Re: Collimation problem on SCT

#8

Post by yobbo89 »


I guess that's the way with these type of scopes, I'm going to probably try a simular approach as you and maybe rotate the secondary holder until I find something at it's best..
scopes :gso/bintel f4 12"truss tube, bresser messier ar127s /skywatcher 10'' dob,meade 12'' f10 lx200 sct
cameras : asi 1600mm-c/asi1600mm-c,asi120mc,prostar lp guidecam, nikkon d60, sony a7,asi 290 mm
mounts : eq6 pro/eq8/mesu 200 v2
filters : 2'' astronomik lp/badder lrgb h-a,sII,oIII,h-b,Baader Solar Continuum, chroma 3nm ha,sii,oiii,nii,rgb,lowglow,uv/ir,Thousand Oaks Solar Filter,1.25'' #47 violet,pro planet 742 ir,pro planet 807 ir,pro planet 642 bp ir.
extras : skywatcher f4 aplanatic cc, Baader MPCC MKIII Coma Corrector,Orion Field Flattener,zwo 1.25''adc.starlight maxi 2" 9x filter wheel,tele vue 2x barlow .

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Re: Collimation problem on SCT

#9

Post by Piet Le Roux »


I think one should start by making sure your secondary mirror is perfectly centre,I remember somebody calculated that if it were just 1.6 mm out you would have severe collimation problems. I did this with a caliper measuring the distance from the OTA lip to the secondary mirror starting at a point then measuring the side opposite and the the two sides 90 degrees on. The correcter plate is spaced on cork spacers and because your scope is horizontal the most of the time the corrector will compress the bottom spacer over time. I now store my scope facing up or down on a 50% basis. As I said this improved mine but did not totally solve the problem. I did it again and followed that with a star collimation and it is close enough now and I am not going to do anything more drastic.
Main Equipment : Tele Vue 27mm Panoptic, 7&13mm Nagler, Big Barlow : 8" Meade LX90ACF with Meade 2.0" Enhanced Diagonal : Camera Fuji XT100
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Re: Collimation problem on SCT

#10

Post by yobbo89 »


http://www.bbayle.com/page_modif_c8/ i' don't think i'll do this to my $5-6 k scope lol. .. interesting enough to have a look, and seems others have simular issues ..

good idea on the park positions for the cork compression .

oh didles. i just want a scope i can fully collimate ...
scopes :gso/bintel f4 12"truss tube, bresser messier ar127s /skywatcher 10'' dob,meade 12'' f10 lx200 sct
cameras : asi 1600mm-c/asi1600mm-c,asi120mc,prostar lp guidecam, nikkon d60, sony a7,asi 290 mm
mounts : eq6 pro/eq8/mesu 200 v2
filters : 2'' astronomik lp/badder lrgb h-a,sII,oIII,h-b,Baader Solar Continuum, chroma 3nm ha,sii,oiii,nii,rgb,lowglow,uv/ir,Thousand Oaks Solar Filter,1.25'' #47 violet,pro planet 742 ir,pro planet 807 ir,pro planet 642 bp ir.
extras : skywatcher f4 aplanatic cc, Baader MPCC MKIII Coma Corrector,Orion Field Flattener,zwo 1.25''adc.starlight maxi 2" 9x filter wheel,tele vue 2x barlow .

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Re: Collimation problem on SCT

#11

Post by yobbo89 »


you'll have to translate to english if it dosn't automaticly do it . he runs a laser collimater up in the baffle from the rear end to check its alighnment
http://www.astrosurf.com/jousset/page_t ... t_lame.htm
scopes :gso/bintel f4 12"truss tube, bresser messier ar127s /skywatcher 10'' dob,meade 12'' f10 lx200 sct
cameras : asi 1600mm-c/asi1600mm-c,asi120mc,prostar lp guidecam, nikkon d60, sony a7,asi 290 mm
mounts : eq6 pro/eq8/mesu 200 v2
filters : 2'' astronomik lp/badder lrgb h-a,sII,oIII,h-b,Baader Solar Continuum, chroma 3nm ha,sii,oiii,nii,rgb,lowglow,uv/ir,Thousand Oaks Solar Filter,1.25'' #47 violet,pro planet 742 ir,pro planet 807 ir,pro planet 642 bp ir.
extras : skywatcher f4 aplanatic cc, Baader MPCC MKIII Coma Corrector,Orion Field Flattener,zwo 1.25''adc.starlight maxi 2" 9x filter wheel,tele vue 2x barlow .

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Re: Collimation problem on SCT

#12

Post by andy brown »


Piet Le Roux wrote: Mon Sep 21, 2020 9:45 am I wonder if anybody has had the same problem with their SCT : I have a 8” Meade LX90ACF of which the collimation differs from one side of focus to the other. I found a reference to this in “Reflecting telescope optics II” by R.N. Wilson on page 135 and he has this to say:

“The appearance of "geometrical adjustment error" means the following. It may happen that the secondary is not well centered in the telescope tube. If the primary is slightly inclined, the coma may well be excellently compensated giving a coma-free Schiefspiegler, as discussed above. If a beam corresponding to the field center is investigated, the pro­jection of the secondary may not be at the center of the entrance pupil. For a pupil plate on one side only of the focus, this effect cannot be distinguished from coma. However, it inverts on the other side of the focus whereas coma remains unchanged.”

I have tried to centre my secondary by measuring the distances from the secondary to the edge of the OTA, with a calliper, while the outside allen screws holding the gasket and 3 allen screws holding the secondary, have been loosed and then tightening the allen screws when its in the centre, while this improved it, it’s still not perfect.
It is extremely difficult to keep the secondary holder centered whilst screwing down the holder halves onto the corrector!!

Have a look/read of these. They are from Roberts yellow book.
For those who don't know, Robert was once called Mr SCT by none other than Rod Molise!
All his books are available in E book format from himself.

I have his contact details but I will have to contact a mod or someone "In Charge" to ask for permission to post them.
Please note, I get nothing from Robert for highlighting his books OK.

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