I will have to move the secondary mirror down by 5.5 + 9.4 mm = 15 mm
towards the primary mirror.
The spider was installed at the wrong rotation.
Mike Sidonio's Newt. has the spider offset by 45 degrees to the focuser: https://pbase.com/strongmanmike2002/...th_the_catseye
It's obvious now that it's a better spot for the spider as it's not
placing direct strain on the focuser which could cause tilt.
Why didn't TS in Germany think of that too?
Gee wizz - moving the focuser tube up 15 mm could be a problem.
To move the primary mirror down 15 mm would mean having to make new
extension spacers for the mirror cell.
I have only just added anti-lateral movement springs which work
very well to stabilise the mirror cell.
The gap left over would be wide enough for those wooden blocks
as they could be placed partially over the metal ring.
This is starting to get very complicated.
see pictures.
Hi Alpal, I may be wrong but I think you may be overthinking the situation. In a newt if the spider is properly centred and you use a sight tube to centre your secondary mirror in the focuser and adjust your tilt to be able to just see all your primary mirror clips your offset should be automatically set. If you cant see all the primary it may be too close to the secondary and need to be moved down the tube.
I guess I may be over simplifying it but I like the KISS method. I may or may not have been any help here.
Cheers and regards, Richard
Hi Alpal, I may be wrong but I think you may be overthinking the situation. In a newt if the spider is properly centred and you use a sight tube to centre your secondary mirror in the focuser and adjust your tilt to be able to just see all your primary mirror clips your offset should be automatically set. If you cant see all the primary it may be too close to the secondary and need to be moved down the tube.
I guess I may be over simplifying it but I like the KISS method. I may or may not have been any help here.
Cheers and regards, Richard
click on "diagonal"
then click on "Diagonal Offset study".
A Newtonian telescope's diagonal must be offset in order to center the cone of illumination on the field of view. Because of geometry, there it is not possible to center the cone of illumination on the center of the field of view while also satisfying the desire for even illumination at the edge of the field. The diagonal is offset in relation to the focuser, so either the diagonal can be moved or the focuser can be moved. I assume a fixed focuser. Let's see why an offset is desired. etc
No worries alpal. I understand there needs to be offset. Im just saying if you use a sight tube and collimate as normal procedure this will happen as a matter of course. If you look at astrobaby's newtonion collimation page at the bottom this is demonstrated in picture and explainer of offset you will see in fast newt. I see this in my f5 newt and my star tests good. When I look through sight tube at secondary I see all mirror clips so must be getting whole light cone. Hope I make sense and if I'm wrong I apologize.
Cheers Richard
No worries alpal. I understand there needs to be offset. Im just saying if you use a sight tube and collimate as normal procedure this will happen as a matter of course. If you look at astrobaby's newtonion collimation page at the bottom this is demonstrated in picture and explainer of offset you will see in fast newt. I see this in my f5 newt and my star tests good. When I look through sight tube at secondary I see all mirror clips so must be getting whole light cone. Hope I make sense and if I'm wrong I apologize.
Cheers Richard
Rather than moving the primary or focuser to fix the offset, can't you just install a spacer of some sort between the spider and the secondary holder to move the secondary closer to the primary?
Hi Richard. Yeah I guess it depends what sort of secondary holder you have. In my Parks 8 inch it's on a threaded rod so I have a fair bit of adjustment. My 5 inch its on a stalk, so
no adjustment. Happily it's in the right place
Cheers mate
Last edited by croweater; 03-03-2023 at 07:30 PM.
Reason: spelling
At the moment I use my laser for collimation.
I may have to buy a Cheshire or some other device to get it perfect.
cheers
Allan.
I have the orion cheshire/sight tube (from Bintel) and find it easy to use and gets me pretty close. If it needs to be any better than that I can star test it but I'm visual and not that bothered usually. I've got a laser but haven't used it for years.
Cheers to you mate, Richard
Rather than moving the primary or focuser to fix the offset, can't you just install a spacer of some sort between the spider and the secondary holder to move the secondary closer to the primary?
Hi Richard,
Apart from introducing cantilever changes in angle causing instability -
moving the secondary mirror down towards the primary will
change the point at which focus is found.
That point is now setup so that focus in my camera
is achieved just as the coma corrector enters the inside of the tube.
I am experimenting with different designs here: https://stellafane.org/tm/newt-web/newt-web.html
Apart from introducing cantilever changes in angle causing instability -
moving the secondary mirror down towards the primary will
change the point at which focus is found.
That point is now setup so that focus in my camera
is achieved just as the coma corrector enters the inside of the tube.
I am experimenting with different designs here: https://stellafane.org/tm/newt-web/newt-web.html
cheers
Allan
Thanks for putting that link up Allan. Haven't seen that site before.Good stuff on there.
Cheers Richard (Croweater)
Apart from introducing cantilever changes in angle causing instability -
moving the secondary mirror down towards the primary will
change the point at which focus is found.
That point is now setup so that focus in my camera
is achieved just as the coma corrector enters the inside of the tube.
I am experimenting with different designs here: https://stellafane.org/tm/newt-web/newt-web.html
cheers
Allan
well moving the primary would also change the focus point. If the offset is wrong and you want to fix it then you'll have to change something. if you don't want to cut new holes in your tube for a new focuser position, and you don't want to change the position of the primary or extend the secondary down because that will alter the focal plane, then what is left...? You could get a new tube I guess, or the nuclear option of a whole new scope...
well moving the primary would also change the focus point. If the offset is wrong and you want to fix it then you'll have to change something. if you don't want to cut new holes in your tube for a new focuser position, and you don't want to change the position of the primary or extend the secondary down because that will alter the focal plane, then what is left...? You could get a new tube I guess, or the nuclear option of a whole new scope...
Hi Richard,
Moving the primary down 15mm and also the secondary down 15mm will
keep the focal point the same -
it will only fix the incorrect offset -
which is what I want.
Note: this telescope was taking pretty good pictures
even though it was set up incorrectly.
see here: https://www.flickr.com/photos/247194...in/photostream
A guy called Mark helped me out with a measuring method on another forum.
How to measure the offset away from the focuser?
I am glad I didn't disassemble my Newt before measuring it.
I used 2 rolled up pieces paper with masking tape around them and kept trimming them until
they would sit by themselves between the secondary and the other side of the tube.
I was expecting to find that the offset was away from the focuser by 5.5 mm
but instead the focuser side was shorter by 1.3 mm.
The offset is wrong in value and it's wrong in direction as set up by TS in Germany.
Now I'm going to have to work out how I will do that offset myself when the spider is reinstalled.
see pic.
Allan if you use method C as described in the "sky and telescope" article, using a sight tube (which is what I've been pushing I guess) you dont have to worry. Just centre your spider and secondary mirror, use a sight tube to set up your secondary(then the cheshire to do primary) and you are done. I get good collimation with that method. The amount of tilt introduced is negligable and undetectable as far as I know. Though if you want to go for method B thats fine too. Let us know how you get on.
Cheers mate, Richard
Allan if you use method C as described in the "sky and telescope" article, using a sight tube (which is what I've been pushing I guess) you dont have to worry. Just centre your spider and secondary mirror, use a sight tube to set up your secondary(then the cheshire to do primary) and you are done. I get good collimation with that method. The amount of tilt introduced is negligable and undetectable as far as I know. Though if you want to go for method B thats fine too. Let us know how you get on.
Cheers mate, Richard
Thanks Richard,
method C does seem to take care of the offset away from the focuser.
I don't have a sight tube or a Cheshire.
I think I need to get those devices.
It seems that the offset away from the focuser -
is built into the design of the spider hub on some telescopes?
It seems that most spider hubs don't have that - why?
This is becoming a huge mystery.
Thanks Richard,
method C does seem to take care of the offset away from the focuser.
I don't have a sight tube or a Cheshire.
I think I need to get those devices.
It seems that the offset away from the focuser -
is built into the design of the spider hub on some telescopes?
It seems that most spider hubs don't have that - why?
This is becoming a huge mystery.
cheers
Allan
I don't know why either Allan. I certainly don't claim to be an expert. I'm just doing what I've always done and the stars test good so that's good enough to me. I wonder if its imaging scopes or high end maybe that do it that way so there is no tilt in the system that would show up under very close scrutiny. Hopefully someone better qualified than me will chime in.
Cheers to you, Richard
I don't know why either Allan. I certainly don't claim to be an expert. I'm just doing what I've always done and the stars test good so that's good enough to me. I wonder if its imaging scopes or high end maybe that do it that way so there is no tilt in the system that would show up under very close scrutiny. Hopefully someone better qualified than me will chime in.
Cheers to you, Richard
Offset Away from the Focuser -
Unless the spider assembly or secondary mirror holder is specifically designed to include it, offsetting in this direction can be difficult. For a typical 4-vane spider the mounting holes may be drilled slightly shifted in the tube wall in order to accommodate the offset. It may also be possible to offset the spider using the spider leg mounting hardware by loosening the leg(s) nearest the focuser and tightening the farthest leg(s). This solution is less desirable as it may tend to increase the width of diffraction spikes seen around bright objects, or even make each existing spike 'branch' into two spikes. The primary reason for offsetting the diagonal away from the focuser is to keep the optical centerline and the telescope tube centerline coincident and prevent vignetting at the front entrance of the telescope. If this could be a problem, or if the most perfectly possible aligned system is desired, then include this offset dimension. When offset in this direction is not included, the optical centerline will be reflected by the secondary mirror by slightly more than 90 degrees. This will be compensated for by primary and secondary mirror tilt with no detriment to the telescope's performance.
And here as Richard pointed out:
And here where there is no offset outwards from the focuser so it's done via tilt:
But what if you want the fully illuminated field centered in the eyepiece,
but must leave the secondary mirror centered in the telescope tube?
It can be done, as shown in diagram C, by slightly adjusting the tilt of both mirrors. Now the optical axis is slightly tilted within the telescope tube. In practice, this is not a problem because the tilt is never more than a small fraction of a degree. Since the secondary is offset down the tube, this is known as partially offset collimation. It is no doubt the most common situation, even among telescope owners who may not even realize that their scope's secondary is offset at all.
Offset Away from the Focuser -
Unless the spider assembly or secondary mirror holder is specifically designed to include it, offsetting in this direction can be difficult. For a typical 4-vane spider the mounting holes may be drilled slightly shifted in the tube wall in order to accommodate the offset. It may also be possible to offset the spider using the spider leg mounting hardware by loosening the leg(s) nearest the focuser and tightening the farthest leg(s). This solution is less desirable as it may tend to increase the width of diffraction spikes seen around bright objects, or even make each existing spike 'branch' into two spikes. The primary reason for offsetting the diagonal away from the focuser is to keep the optical centerline and the telescope tube centerline coincident and prevent vignetting at the front entrance of the telescope. If this could be a problem, or if the most perfectly possible aligned system is desired, then include this offset dimension. When offset in this direction is not included, the optical centerline will be reflected by the secondary mirror by slightly more than 90 degrees. This will be compensated for by primary and secondary mirror tilt with no detriment to the telescope's performance.
And here as Richard pointed out:
And here where there is no offset outwards from the focuser so it's done via tilt:
But what if you want the fully illuminated field centered in the eyepiece,
but must leave the secondary mirror centered in the telescope tube?
It can be done, as shown in diagram C, by slightly adjusting the tilt of both mirrors. Now the optical axis is slightly tilted within the telescope tube. In practice, this is not a problem because the tilt is never more than a small fraction of a degree. Since the secondary is offset down the tube, this is known as partially offset collimation. It is no doubt the most common situation, even among telescope owners who may not even realize that their scope's secondary is offset at all.
Thanks Allan. It seems to me its almost an overkill for perfectionists but thats ok. This hobby does tend to harbour many people of that ilk. Look how we pay exorbitant amounts for a few degrees extra fov or round stars right at the edge of the field. I used to chase that too once. When showing my wife some globulars with a Nagler t6 I once had she said" so you payed hundreds extra to see a bit more black around it" I laughed a bit sheepishly I now have more modest eyepieces and I'm quite happy with 60 degree fields.
Anyway I find the Orion combination cheshire/sight tube very good and I think the Celestron one the same but a bit cheaper. My laser makes a very good cat toy and he goes beserk running around after the dot.
Cheers, Richard
Thanks Allan. It seems to me its almost an overkill for perfectionists but thats ok. This hobby does tend to harbour many people of that ilk. Look how we pay exorbitant amounts for a few degrees extra fov or round stars right at the edge of the field. I used to chase that too once. When showing my wife some globulars with a Nagler t6 I once had she said" so you payed hundreds extra to see a bit more black around it" I laughed a bit sheepishly I now have more modest eyepieces and I'm quite happy with 60 degree fields.
Anyway I find the Orion combination cheshire/sight tube very good and I think the Celestron one the same but a bit cheaper. My laser makes a very good cat toy and he goes beserk running around after the dot.
Cheers, Richard
I'm like that.
if someone posts a picture I go straight to the 4 corners and start pixel peeping.
Can I find any out of round stars or stars with comet tails on them?
I've looked through Carl Zeiss eyepieces and there was a large difference
compared to ordinary eyepieces especially when looking at planets.
Even when looking at Orion - you could see more nebulosity.
You could get a new tube I guess, or the nuclear option of a whole new scope...
I now have more modest eyepieces and I'm quite happy with 60 degree fields. 
