Check your Newt. offset.

[alpal]

Embarrassing problem as I should have noticed this before.

I am putting my 10" f4 Newt back into service after 6 years and this is where I'm up to:

I hadn't cleaned the secondary mirror yet because I wanted to check it for offset -
just in case it might have a small error in it.
When I used my digital vernier – after about an hour of making difficult measurements -
I came up with a result that I thought must be wrong –
It tells me that the secondary mirror is offset in the wrong direction by 9.4 mm.
An online calculator says the offset should be 5.5 mm towards the primary mirror –

offset is approximated by: M/(4*F) where M is the minor axis of the secondary mirror and F is the F ratio.
So for my 10 inch f/4 with secondary minor axis diameter of 88mm, the offset will be 88/(4*4) = 5.5mm.

but it’s offset the other way by 9.4 mm towards the front opening of the telescope!
Then I looked in the focuser and sure enough I can see that it’s wrong
without even measuring it. See pic attached.
( I placed a red shirt on the opposite side of the tube to make a photo more obvious
and I focused on the edge of the secondary mirror using my DSLR on a tripod. )

I don’t know how this could have happened.
It was bought from TS in Germany in 2015 and they set up the position of the secondary mirror by
installing the spider and focuser.
How could professionals who charge a lot of money and make 100s of telescopes make such an error?
How did my telescope even work at all like that?

There is also the question of the other offset which would
be away from the focuser by the same amount 5.5mm.
The center of the spider is centralised so it may not have any offset
away from the focuser.
I'm not sure how to measure that.

Any comments?

cheers
Allan

[Carbon Brush]

Yes this big fast scope definitely calls for an offset.

Now there is the correct way (measuring everything) way to do this, which may mean drilling new holes in the tub for the spider.
The alternative is to move (more drilling) the focusser towards the open end of the tube.
Which, or even both, is called for dpends on the measurements.

There is a simpler way that may be good enough.

You could fit longer screws on the secondary allowing it move toward the primary mirror so placed under the focusser.

Intrigued....Looking forward to the next instalment.

David

[alpal]

7 minutes ago, Carbon Brush said:

Yes this big fast scope definitely calls for an offset.

Now there is the correct way (measuring everything) way to do this, which may mean drilling new holes in the tub for the spider.
The alternative is to move (more drilling) the focusser towards the open end of the tube.
Which, or even both, is called for dpends on the measurements.

There is a simpler way that may be good enough.

You could fit longer screws on the secondary allowing it move toward the primary mirror so placed under the focusser.

Intrigued....Looking forward to the next instalment.

David

Thanks David,

Yes - I don't see any other way out of this but drilling 4 new holes and

re-installing the spider further down the tube  5.5 + 9.4 = 14.9 mm.

I could fill the old holes with Araldite and black dye to cover them.

14.9 mm is too far for spacers or longer bolts to work.

Once I get the secondary out I'll have a closer look.

 

cheers

Allan

 

[markse68]

4 hours ago, alpal said:

The center of the spider is centralised so it may not have any offset
away from the focuser.
I'm not sure how to measure that.

The secondary is probably bonded to the hub with offset. If you get a sheet of paper and fold it so it fits between the wall of the tube and just touches the side of the secondary on the rear edge then move it to the focuser side, you should be able to judge by how much it is offset

Mark

[alpal]

3 hours ago, markse68 said:

The secondary is probably bonded to the hub with offset. If you get a sheet of paper and fold it so it fits between the wall of the tube and just touches the side of the secondary on the rear edge then move it to the focuser side, you should be able to judge by how much it is offset

Mark

That's a good idea - thanks.

[david_taurus83]

Sorry if I'm misinterpreting the problem but why does the secondary need to be offset and not directly beneath the focuser?

[alpal]

4 minutes ago, david_taurus83 said:

Sorry if I'm misinterpreting the problem but why does the secondary need to be offset and not directly beneath the focuser?

 

see here:

https://www.bbastrodesigns.com/NewtDesigner.html

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

[alpal]

The spider needs to be rotated.

Mike Sidonio's Newt. has the spider offset by 45 degrees to the focuser:

https://pbase.com/strongmanmike2002/collimating_with_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?

 

Also -  that if I move the secondary down 15mm I’ll need

to move primary further down  15mm too.

It’s not so easy:

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.

 

cheers

Allan

[Carbon Brush]

This is reminiscent of my very first reflector from Orion Optics UK.
I took it to a club for help with collimation.
3 of their experienced members turned screws and scratched their heads for an hour before reaching a conclusion. Send it back!
Fortunately I had only just received it. So it went back to OO and was returned to me with a new tube.
Being new to the game I did not understand everything about the lack of alignment and it was a long time ago.
The replacement tube gave very good results and I enjoyed using the scope.

Might it be worth you looking for another 10" tube with grotty mirrors, on sale cheap?
Either experiment with making lots of new holes, or if the focus and spider holes are near enough right, saw a bit of the back end?

In this case, if the primary to secondary distance is not quite right and there is a bit of light spill, does it matter?
I have not done the sums but if you have in effect 240mm of the 254mm main mirror hitting the secondary, will you notice?

But, do you know the mirror is F4 and not F3.9 or F4.1.
I remember seeing a big (24") newt at a club. To help with alignment and collimation.
The clever folks at the observatory made a device with motor drive, turning an LED on an arm to help establish mirror position.
I'm wondering if something similar might help. A light source (laser pen?) near the tube edge shines down the tube to hit the main mirror near the edge.
Then look where it hits (or misses) the secondary mirror. 

Sorry if this doesn't make a lot of sense. It is a very annoying problem.

 

[Peter Drew]

Ideally, a secondary should be pear shaped rather than elliptical s the cone radius below the centre line is wider as it approaches the lower portion of the secondary below its centre line.  A quick sketch soon shows this and vindicates the offset of an elliptical secondary in a "Fast" Newtonian.    🙂 

[alpal]

3 hours ago, Carbon Brush said:

This is reminiscent of my very first reflector from Orion Optics UK.
I took it to a club for help with collimation.
3 of their experienced members turned screws and scratched their heads for an hour before reaching a conclusion. Send it back!
Fortunately I had only just received it. So it went back to OO and was returned to me with a new tube.
Being new to the game I did not understand everything about the lack of alignment and it was a long time ago.
The replacement tube gave very good results and I enjoyed using the scope.

Might it be worth you looking for another 10" tube with grotty mirrors, on sale cheap?
Either experiment with making lots of new holes, or if the focus and spider holes are near enough right, saw a bit of the back end?

In this case, if the primary to secondary distance is not quite right and there is a bit of light spill, does it matter?
I have not done the sums but if you have in effect 240mm of the 254mm main mirror hitting the secondary, will you notice?

But, do you know the mirror is F4 and not F3.9 or F4.1.
I remember seeing a big (24") newt at a club. To help with alignment and collimation.
The clever folks at the observatory made a device with motor drive, turning an LED on an arm to help establish mirror position.
I'm wondering if something similar might help. A light source (laser pen?) near the tube edge shines down the tube to hit the main mirror near the edge.
Then look where it hits (or misses) the secondary mirror. 

Sorry if this doesn't make a lot of sense. It is a very annoying problem.

 

thanks,

hopefully this is the last iteration I'll need to go through and the Newt. will be right.

I want every penny's worth of the light entering my telescope tube so it will be working

the best that I can make it.

I have been checking the deign here:

https://www.bbastrodesigns.com/NewtDesigner.html

and here:

https://stellafane.org/tm/newt-web/newt-web.html

cheers

Allan

 

[alpal]

2 hours ago, Peter Drew said:

Ideally, a secondary should be pear shaped rather than elliptical s the cone radius below the centre line is wider as it approaches the lower portion of the secondary below its centre line.  A quick sketch soon shows this and vindicates the offset of an elliptical secondary in a "Fast" Newtonian.    🙂 

Thanks - I'm sure you're right.

[alpal]

Here is a good explanation of the offset and a picture too.
It's from here:

https://skyandtelescope.org/astronomy-equipment/offsetting-your-secondary-mirror/

 

[alpal]

On 03/03/2023 at 01:24, markse68 said:

The secondary is probably bonded to the hub with offset. If you get a sheet of paper and fold it so it fits between the wall of the tube and just touches the side of the secondary on the rear edge then move it to the focuser side, you should be able to judge by how much it is offset

Mark

Thanks Mark,

You finally gave me the best idea I could find 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.

 

 

 

 

 

[markse68]

That’s not great Alpal! You’ll probably have a bit of adjustment on the spider vanes but with the height discrepancy as well i wonder if they fitted the wrong part!

Mark

[alpal]

2 minutes ago, markse68 said:

That’s not great Alpal! You’ll probably have a bit of adjustment on the spider vanes but with the height discrepancy as well i wonder if they fitted the wrong part!

Mark

Thanks Mark again.

I don't know.

So TS in Germany got both offsets in the wrong direction and both of them wrong in value.

Can you believe that? – they are master opticians  !  LOL

Allan

[alpal]

I think I have some good answers here:

https://catseyecolli...m/mccluney.html

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 where there is no offset outwards from the focuser so it's done via tilt:

https://skyandtelesc...condary-mirror/

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.

[alpal]

I made my own Cheshire eyepiece by drilling a 1.5 mm hole in the center of a 1.25" focuser end plug.
I post the picture of what I found.
I can see the 3 mirror holding clips so you'd think it would be OK as is?
How important is offset and collimation?


cheers
Allan

[Jason D]

Just follow the proper collimation steps and the "away-from-focuser" offset will be taken care of automatically. In other words, don't do anything special for the "away-from-focuser" offset.

Adjusting the spider vanes to move the secondary mirror away from the focuser might exacerbate diffraction spikes since you will end up with opposite spider vanes that are not inline/parallel with respect to each other.

The only valid reasons why you want to re-install the secondary mirror with the proper "away from focuser" offset are:

1- Your scope has a corrective lens at the OTA front opening

2- Improves setting circles accuracy but the improvement is minimal

3- Avoid front-end vignetting which happens when the OTA opening is almost as wide as the primary mirror diameter.

If the secondary mirror is centered in its stalk then following proper collimation steps will end up tilting the primary mirror towards the focuser as shown in the right diagram of the attachment.

Jason 

[Spile]

This is my interpretation of your image that shows the secondary offset error.

 

 

Edited March 6 by Spile

[alpal]

32 minutes ago, Spile said:

This is my interpretation of your image that shows the secondary offset error.

 

Thanks Spile,

I re-collimated the scope and -

 took a clearer image using my mobile phone through a Cheshire cap.

and a person on another forum made this image and agrees that there is an offset problem.

he says -

The current secondary mirror (dashed green circle) needs to be moved to the solid green circle (about 1/6 of the secondary mirror diameter, so ~15mm).

I am about to pull the secondary out for a closer look.

cheers

Allan

 

 

[alpal]

2 hours ago, Jason D said:

Just follow the proper collimation steps and the "away-from-focuser" offset will be taken care of automatically. In other words, don't do anything special for the "away-from-focuser" offset.

Adjusting the spider vanes to move the secondary mirror away from the focuser might exacerbate diffraction spikes since you will end up with opposite spider vanes that are not inline/parallel with respect to each other.

The only valid reasons why you want to re-install the secondary mirror with the proper "away from focuser" offset are:

1- Your scope has a corrective lens at the OTA front opening

2- Improves setting circles accuracy but the improvement is minimal

3- Avoid front-end vignetting which happens when the OTA opening is almost as wide as the primary mirror diameter.

If the secondary mirror is centered in its stalk then following proper collimation steps will end up tilting the primary mirror towards the focuser as shown in the right diagram of the attachment.

Jason 

Thanks Jason,

Yes I may have to go for - partially offset collimation.

as per the post further back.

cheers

Allan

[alpal]

I have some pics after I pulled the hub and spider out.

I think I have enough to work it out now.

 

 

[alpal]

What should I do about the dimple plate for the 3 adjustment screws?

So - the dimples didn't line up so they just reversed the plate -

or is that the normal setup?

 

Edited March 6 by alpal

[alpal]

Latest news,
I fitted a 15 mm spacer and used a longer central socket head bolt -
I also rounded the 3 screw ends a little bit.
After a lot of collimation help from another forum I achieved a better result.


cheers
Allan