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Secondary offset on a "fast" newtonian - how much?

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#1 Rudi Bjoern

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Posted 24 October 2019 - 12:14 PM

I have a Sky-Watcher f/3.9 8" Quattro that I am very happy with. I can collimate well, but still, I have an issue with the projected holographic image of the primary's centre ring on my Tublug using a Howie-Glatter collimator. If I center the projected ring and then rotate the collimator in the focuser, then the projected ring is not centered any more, but a bit (2-3 mm) offset. Can this be caused by secondary not "offset" away from the focuser proberly?

I have centered the secondary, bot by using the Howie-Glatter with the projected concentric rings on the primary, a "Ring Concenter Collimator", a hole in the telescope cap centre and by simply measuring distances from tube inner edge to secondary holder along the spiders - all methods confirms, that the secondary is perfectly centered.

 

But then I remembered hearing about "fast" newtonians, that their secondary's shuoldn't be centered, but slightly offset.

This is also mentioned here:

https://www.collimator.com/collimation

 

In Sky-Watcher's manual for my telescope, here: https://inter-static...01440182865.pdf

The same is also mentioned on p. 8:

QuattroSecondary.png

 

Unfortunately, Sky-Watcher didn't mention how many mm the secondary should be offset!!!

 

Can anyone help me with this - or perhaps an owner of the same telescope, measure the distances from tube inner side to secondary holder centre and the opposite distance?

 

QuattroSecondary2.png


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#2 Garyth64

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Posted 24 October 2019 - 01:25 PM

Just google, "secondary offset", there's lots of information out there.

 

Here's one from S&T:

 

https://www.skyandte...condary-mirror/

 

The formula to find out how much offset you scope should have is in the article.

 

If the offset is preset from the factory,  then I don't think you should use a centering collimator.

 

How do the stars look in the fov?


Edited by Garyth64, 24 October 2019 - 01:25 PM.

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#3 Rudi Bjoern

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Posted 24 October 2019 - 01:36 PM

Thanks for the help.
Using the formula at the site you linked to, the offset should be:

Offset = (secondary size)/(4*focal ratio)=200/4/3.9=12.8mm.
I try that.

My most recent image, shot through a hole in the clouds, is here:

https://www.dropbox....filter.fit?dl=0

#4 MitchAlsup

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Posted 24 October 2019 - 02:00 PM

Read this page and the next page: https: //www.telescope-optics.net/newtonian_collimation.htm


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#5 StrStrck

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Posted 24 October 2019 - 02:17 PM

Hey Rudi. Good to see you herecool.gif

In a fast Newt the secondary should already shifted off from the center of it’s holder, you only need to move the secondary closer or further away from the primary, in the tubes longitudinal axis. So the center of the secondary holder should be about dead centered. Look inside and see if the 2ndary is shifted on it’s holder already.


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#6 Garyth64

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Posted 24 October 2019 - 02:24 PM

Thanks for the help.
Using the formula at the site you linked to, the offset should be:

Offset = (secondary size)/(4*focal ratio)=200/4/3.9=12.8mm.
I try that.

My most recent image, shot through a hole in the clouds, is here:

https://www.dropbox....filter.fit?dl=0

12.8mm?, somethings wrong, it shouldn't be that much.

 

Recheck the math, and I will do it too.

 

Edit:  you used the primary size, use the secondary size.  If your secondary is 70mm then your offset should be more like 4.4mm


Edited by Garyth64, 24 October 2019 - 02:27 PM.

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#7 Rudi Bjoern

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Posted 24 October 2019 - 02:44 PM

Yes - you're right. The secondary is 70 mm, so it is:

Offset=(secondary size)/(4*focal ratio)=70/4/3.9=4.5mm

 

And thanks for the PM about the same Richard!


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#8 Rudi Bjoern

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Posted 24 October 2019 - 02:47 PM

Hey Rudi. Good to see you herecool.gif

In a fast Newt the secondary should already shifted off from the center of it’s holder, you only need to move the secondary closer or further away from the primary, in the tubes longitudinal axis. So the center of the secondary holder should be about dead centered. Look inside and see if the 2ndary is shifted on it’s holder already.

Hi Bjørn,

 

Actually, I was wondering about just that. Because if I use the spider vanes to shift the secondary, I will get double spikes from the two other spider vanes, that will no longer be aligned on the same line nor parallel.



#9 StrStrck

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Posted 24 October 2019 - 03:16 PM

https://www.skywatch...tonian-8-205-mm

 

From the picture looking almost straight down the tube, it seems the 2ndary is shifted on it’s holder (as well as rotated away from a meaningful position). What’s advertized isn’t always what you get, I know. But I’m curious to know how your 2ndary sits. 


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#10 Vic Menard

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Posted 24 October 2019 - 03:17 PM

I have a Sky-Watcher f/3.9 8" Quattro that I am very happy with. I can collimate well, but still, I have an issue with the projected holographic image of the primary's centre ring on my Tublug using a Howie-Glatter collimator. If I center the projected ring and then rotate the collimator in the focuser, then the projected ring is not centered any more, but a bit (2-3 mm) offset. Can this be caused by secondary not "offset" away from the focuser properly?

No. To align the "projected ring" in the Tublug, you tilt the primary mirror. This alignment centers the primary mirror axis in the focuser. Of course, when you tilt, or rotate, the secondary mirror, you will most definitely change the primary mirror axis alignment. This is why primary mirror tilt alignment is always the last alignment.

 

I have centered the secondary, bot by using the Howie-Glatter with the projected concentric rings on the primary, a "Ring Concenter Collimator", a hole in the telescope cap centre and by simply measuring distances from tube inner edge to secondary holder along the spiders - all methods confirms, that the secondary is perfectly centered.

 

In Sky-Watcher's manual for my telescope, here: https://inter-static...01440182865.pdf
The same is also mentioned on p. 8:

"...all methods confirms, that the secondary is perfectly centered."

 

This doesn't make sense. Although the fully offset secondary mirror should appear "centered" under the focuser, it should appear offset away from the focuser when viewed from the front aperture (as illustrated in the manual).

 

...Unfortunately, Sky-Watcher didn't mention how many mm the secondary should be offset!!!

 

Can anyone help me with this - or perhaps an owner of the same telescope, measure the distances from tube inner side to secondary holder centre and the opposite distance?

 

The thing is, you don't need to know the offset amount if you've chosen to offset your secondary mirror--and you do want to offset your secondary mirror. (Incomprehensible as it may seem, you do need to know the offset amount if you choose a "centered" secondary mirror placement!)

 

To achieve a correctly offset secondary mirror placement, you need to make three circles concentric (as viewed from a centered pupil in the focuser):

The bottom edge of the focuser drawtube (or sight tube),

The actual edge of the secondary mirror, and

The reflected edge of the primary mirror.

 

When you have all three circles concentric, the secondary mirror will be offset, and the outgoing beam from a simple thin beam laser should already be very close to the primary mirror center marker. You should be able to fine adjust the secondary mirror tilt to bring the laser into alignment with the primary mirror center marker. This aligns the focuser axis, which makes the focal plane perpendicular to the primary mirror axis.

 

When you've got the three circles concentric and the outgoing laser beam aligned, the last alignment is the primary mirror tilt alignment. Use the Tublug, and get the projected silhouette of the primary mirror center marker centered relative to the aperture in the Tublug target. This alignment centers the coma "free" field diameter in the center of the focal plane.

 

And that's pretty much Newtonian collimation in a nutshell.


Edited by Vic Menard, 24 October 2019 - 03:28 PM.

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#11 StrStrck

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Posted 24 October 2019 - 03:34 PM

Just read the manual’s statement on the “spider vane offset”, that could apply to a centered 2ndary scenario, as mentioned by Vic. I do however come across some odd translations from China now and then, and they could also be saying “don’t worry if the spider vanes are not exactly dead center in your view while collimating”, that be if your 2ndary is already in an excentric position.



#12 Vic Menard

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Posted 24 October 2019 - 03:35 PM

Yes - you're right. The secondary is 70 mm, so it is:

Offset=(secondary size)/(4*focal ratio)=70/4/3.9=4.5mm

 

And thanks for the PM about the same Richard!

Actually, it depends on how far the focal plane is from the secondary mirror. The simple formula works well for visual use Newtonians, but imaging Newtonians have larger secondary mirrors and longer focal plane to secondary mirror distances to accommodate cameras and other imaging accessories. Still, the offset won't be much more than the simple formula--somewhere around 5.5mm...



#13 Vic Menard

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Posted 24 October 2019 - 05:39 PM

Just read the manual’s statement on the “spider vane offset”, that could apply to a centered 2ndary scenario, as mentioned by Vic. I do however come across some odd translations from China now and then, and they could also be saying “don’t worry if the spider vanes are not exactly dead center in your view while collimating”, that be if your 2ndary is already in an excentric position.

Actually, it's good advice to avoid using the reflection of the spider vanes as an alignment reference whether the secondary mirror is centered or offset. The spider centering is mechanical (as is the focuser "squaring"), and it's common for these mechanicals to be slightly misaligned relative to the optical "pointing" axis. I would stick with the basics--three circles and two axial alignments--unless you have a really good reason to complicate the collimation considerations.


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#14 MeridianStarGazer

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Posted 24 October 2019 - 10:44 PM

I once calculated that for 10" f5, the secondary should be 1/8" further from the focuser than center axis, if you want to get the most out of it.

#15 Rudi Bjoern

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Posted 24 October 2019 - 11:22 PM

Thanks for the input, advice and links. I will loosen the secondary holder screw and start alignment/positioning of the secondary all over again.

 

I took this picture last night (danish time), where I tried to place the reflection of iPhone camera aligned with secondary holder centre (eye-balled), from that, it looks like the spiders are parallel and the secondary looks like it's maybe pointing a bit downwards, 3:30 o-clock?

 

But as mentioned above, I will start all over with the co-center collimator to re-check that the secondary looks perfectly round and centered. Only then, I will start the Howie-Glatter collimation procedure.

 

(Full resolution: https://www.dropbox...._2843.jpg?raw=1)

IMG_2843_s.jpg?raw=1


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#16 StrStrck

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Posted 25 October 2019 - 12:04 AM

Remember to put a sheet of (coloured) paper in the tube to act as background for 2ndary. 

Good luck! 



#17 Asbytec

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Posted 25 October 2019 - 12:43 AM

"If I center the projected ring and then rotate the collimator in the focuser, then the projected ring is not centered any more, but a bit (2-3 mm) offset."

How well does your Glatter register (fit) in the focuser?

#18 Rudi Bjoern

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Posted 25 October 2019 - 01:42 AM

Remember to put a sheet of (coloured) paper in the tube to act as background for 2ndary. 

Good luck! 

Yes - will do. I have a piece of red cardboard cut for this purpose laying in my "collimation kit box".

This is what is looked like, a while back:

4.jpg

 

2.jpg

 

"If I center the projected ring and then rotate the collimator in the focuser, then the projected ring is not centered any more, but a bit (2-3 mm) offset."

How well does your Glatter register (fit) in the focuser?

Well, it fits as well as one can expect in Sky-Wacthers 2" crayford with two lock screws. But I never lock the Tublug. In stead, I hold the flanges on the Tublug and focuser tight together, then I imagine that they are as parallel as possible. There's no slack perpendicular to the focuser axis, it fits very snugly.



#19 airbleeder

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Posted 25 October 2019 - 08:34 AM

   I secure all my collimation tools the same as I secure my eyepieces. I remedied inconsistent registration with a Parallizer which I leave in the focuser with all my 1.25" eyepieces.


Edited by airbleeder, 25 October 2019 - 02:19 PM.

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#20 Vic Menard

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Posted 25 October 2019 - 09:49 AM

StrStrck, on 25 Oct 2019 - 01:04 AM, said:
Remember to put a sheet of (coloured) paper in the tube to act as background for 2ndary.

 

Yes - will do. I have a piece of red cardboard cut for this purpose laying in my "collimation kit box".

This is what is looked like, a while back:

4.jpg

 

I don't recommend placing a card between the secondary mirror and the primary mirror as this blocks the primary mirror reflection, which is one of the three circles you'll need to make concentric. I do recommend placing a piece of white paper behind the secondary mirror (opposite the focuser) against the inside wall of the telescope tube to improve the visibility of the actual edge of the secondary mirror. I believe this is what StrStrck was suggesting.


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#21 Rudi Bjoern

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Posted 25 October 2019 - 09:52 AM

I have just completed a total collimation, the three secondary screws removed all together, centering secondary, counting revolutions on three secondary screws, to make sure, that thei approximately were screwed the same distance in. Fineadjusting secondary "lontitudinal" position and rotation with concenter collimator. Then iterating Howie-Glatter point to center secondary and Howie-Glatter with Tublug to angle primary. Finally, it all seemed to come perfectly together and I got the projected primary center mark well centered in the Tublug, regardless og how the Tublug was rotated (could it be, that the focuser is crooked?)

 

Now the final test remains, an image of real stars wink.gif

 

Front and stern views of the Tublog projection:

 

HowieGlatterFront.jpg?raw=1

 

HowieGlatterBack.jpg?raw=1



#22 Vic Menard

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Posted 25 October 2019 - 10:17 AM

...I got the projected primary center mark well centered in the Tublug, regardless of how the Tublug was rotated (could it be, that the focuser is crooked?)

If the unBarlowed laser beam is accurately aligned to the primary mirror center marker, the focuser axis is correct (whether the focuser is perpendicular to the tube or not, or for that matter, if the focuser axis is perpendicular to the primary mirror axis or not). And assuming the focusing motion is linear on that same axis, the axial alignments will remain static.

 

On the other hand, if there's a load sufficient to cause significant gravitational flexure/torque in the focuser, drawtube, or the tube assembly itself--that would certainly impact both axial alignments. 



#23 Rudi Bjoern

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Posted 25 October 2019 - 10:26 AM

Vic, remember that You can't collimate a newton, exclusively using a "laser dot" collimator.

For example, assume that the laser dot is in the dead center of the primary, and You then rotate the secondary, say 20°, the laserdot will move away from the center, and may be brought back to the center again, just using the three secondary adjustment screws.

So it is extremely important, to have the secondary centered and rotated perfectly, before proceding using e.g. a Howie-Glatter.


Edited by Rudi Bjoern, 25 October 2019 - 10:27 AM.


#24 Rudi Bjoern

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Posted 25 October 2019 - 10:41 AM

I don't recommend placing a card between the secondary mirror and the primary mirror as this blocks the primary mirror reflection, which is one of the three circles you'll need to make concentric. I do recommend placing a piece of white paper behind the secondary mirror (opposite the focuser) against the inside wall of the telescope tube to improve the visibility of the actual edge of the secondary mirror. I believe this is what StrStrck was suggesting.

Vic, it's this device I use:

https://www.teleskop...Telescopes.html

concenter-2z-kollimation.jpg

 

TS writes:

Collimation of the secondary mirror to the focuser for optimum illumination
This step is made with covered primary mirror. Pay attention to a good illuimination when using the concenter eyepiece. Now the secondary mirror, a rectangular mounting of the focuser supposed, is adjusted so that it appears between the circles of the collimation cicles and without offset. With the focus control of the focuser you can bring the outer edge of the secondary mirror and the matching circle to cover. Even small deviations from the ideal position are recognized immediately. By doing this, you place the secondary mirror in the optimal position to the focuser to millimetre accuracy. This step is necessary only once, you can remove the cover of the primary mirror afterwards.


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#25 Vic Menard

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Posted 25 October 2019 - 12:34 PM

Vic, remember that You can't collimate a newton, exclusively using a "laser dot" collimator.

For example, assume that the laser dot is in the dead center of the primary, and You then rotate the secondary, say 20°, the laserdot will move away from the center, and may be brought back to the center again, just using the three secondary adjustment screws.

So it is extremely important, to have the secondary centered and rotated perfectly, before proceding using e.g. a Howie-Glatter.

As I noted above (post #10), there are three alignments normally associated with Newtonian collimation--secondary mirror placement, focuser axial alignment, and primary mirror axial alignment.

 

You are correct that there are an infinite number of fully corrected axial alignments for the infinite number of possible secondary mirror placements. But given the mechanical "centerings" of the primary and secondary mirrors and the "squaring" of the focuser (I put quotations around "centering" and "squaring" because it's more likely that this condition will not be true to any real precision), there is only one optimal secondary mirror placement--and that's defined by the three circles.

 

But it's not extremely important to have the secondary mirror centered or even rotated to any high precision, and certainly not to perfection. A less than optimal secondary mirror placement will have minimal impact on the balanced illumination at the focal plane--while the axial alignments deliver the real image performance.




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