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Orion Optics UK VX8 Secondary Mirror Positioning and Collimation


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I had recently purchased a VX8 Newtonian that was shipped to me in the USA. I was not able to get much support from OOUK on this so I am posting here

with some questions.

1. Most collimating procedures I have read first want you to position the secondary to be centered in the focuser tube. When it arrived

from the factory the VX8 secondary was definitely not - it was at least 10mm further out from the primary than it would have been if centered. In fact to center

it in the focuser I had to put in longer adjustment screws. Is this normal? Am I missing something?

2. With the modification above, I have it centered the secondary and collimated using a Cheshire. The view through the Cheshire looks good, the crosshairs

are centered on the donut mark and all three clips that hold the primary are visible. No issues with visual observation. Yet when I use a camera

(ASI 294MC Pro), I see that the vignetting is quite asymmetric - i.e. much more vignetting on one side and almost none on the other. So the

"sweet spot" of the FOV is not centered on the camera sensor. Any ideas of what could be causing this? The camera is mounted just

as an eyepiece would be on the 2" focuser.

 

Thanks in advance for any help/pointers.

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The weight of the camera may be causing the focuser to sag under gravity, or even the OTA tube to flex.
 

OOUKs thin aluminium tubes are not known for their rigidity unfortunately.

Edit: you could check if this is the case by putting the scope flat on a table with the focuser pointing straight up and then repeating the Cheshire/camera test. If the issue goes away then you know it’s likely flex somewhere.

Edited by CraigT82
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48 minutes ago, rpnconsult said:

I had recently purchased a VX8 Newtonian that was shipped to me in the USA. I was not able to get much support from OOUK on this so I am posting here

with some questions.

1. Most collimating procedures I have read first want you to position the secondary to be centered in the focuser tube. When it arrived

from the factory the VX8 secondary was definitely not - it was at least 10mm further out from the primary than it would have been if centered. In fact to center

it in the focuser I had to put in longer adjustment screws. Is this normal? Am I missing something?

2. With the modification above, I have it centered the secondary and collimated using a Cheshire. The view through the Cheshire looks good, the crosshairs

are centered on the donut mark and all three clips that hold the primary are visible. No issues with visual observation. Yet when I use a camera

(ASI 294MC Pro), I see that the vignetting is quite asymmetric - i.e. much more vignetting on one side and almost none on the other. So the

"sweet spot" of the FOV is not centered on the camera sensor. Any ideas of what could be causing this? The camera is mounted just

as an eyepiece would be on the 2" focuser.

 

Thanks in advance for any help/pointers.

Sadly, this sounds familiar as my VX8 came with secondary collimation screws that made it physically impossible to collimate the scope properly and i too had to install properly fitting ones.

As for the vignetting part, there are many possible problems and solutions, some free some not. My focuser had a bit of side to side movement as the focuser was racked in and out, so in order to collimate properly you must collimate with the focuser in the same position as the camera will be. Preferably done with a laser as it is easier to place on the focal plane than a long cheshire that probably has your eye be far from it. And on the topic of the focuser, its not very stable, at least mine was not. You will find that the focuser itself sags under the weight of a camera and you will have trouble keeping collimation because of it. Another major source of sag is the thin aluminum tube itself, which can bend and take you out of collimation, and can also bend around the points where the secondary spider is connected to the tube (causing the secondary to wander at different tube orientations.

The focuser issue is solvable completely by changing the focuser, but that costs some money of course. A temporary fix would be to try and adjust the tension screw so that the focuser just barely is able to move in and out and the drawtube is under tension. Care must be taken to collimate under the same tension as the camera will be in. This reduces the focuser sagging effect somewhat, but for imaging purposes you will find that you get annoying mismatches between your flats and lights.

For the secondary, make sure that the screws holding the spiders to the tube are tight, but here comes another problem. The tube itself dents very easily and the spider is lose again, so there may not be a tension that is just enough to keep things stable but also keep the tube round. I fixed this by fitting a heavier and sturdier spider that has an inner diameter the same as the tube itself, so the tube rests on the spider and no further denting is possible. I installed this one: https://www.teleskop-express.de/shop/product_info.php/info/p9317_TS-Optics-massive-Metal-Spider-for-8--Newtonian-Telescopes-D-223mm.html

For the flimsy tube you can get a couple of pipe clamps meant for AC tubing (like below) and so create tension on the tube that will help with the sagging. One between the focuser and finder shoe will strengthen the tube noticeably and may help with some collimation related issues. You might also want to put one more half way between the primary mirror screws and the rear tube ring.

33-212_xl_1.jpg

I upgraded to a Baader diamond steeltrack focuser and did the above modifications and found that random collimation issues (unrelated to transporting the scope) reduced to almost 0. The BDS focuser is half of the cost of the scope so its a bit of a stretch to ask, but it is a very good focuser and makes life much simpler. The spider costs a lot of money for what it is and im sure some local machine shop would fashion one out of scrap metal for a 5th of the price, but the one i linked works for the purpose very well.

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Was the spider fitted when it arrived or was it packed separately for you to assemble?  I ask because there's a right and wrong orientation for these spiders and if fitted 180 degrees out the secondary offset will be a mile out and that shows as difficulty collimating and uneven illumination.

Someone will no doubt jump in now with a link and say that offset is a myth and isn't needed to correctly collimate a scope.  Whist this is true from a technical standpoint (the scope can be collimated), if you want even illumination then you must have the correct offset, or close, otherwise you have to tilt the primary and the top of the tube cuts off part of the light beam = uneven illumination.

This isn't a big issue in scopes designed with a large enough tube diameter.  I've owned two VX10s and most recently a CT10, the VX series use very narrow diameter tubes, and without the correct secondary offset your attempts to collimate it lead the top of the tube cutting off part of the light beam.   If I remember rightly; not enough offset and the tube edge by the focuser will be visible, too much and tube edge opposite the focuser will be visible - this is the uneven illumination you're seeing.  

I mention this as I've not seen a secondary 'glued' in the wrong offset by OO, I have seen the spider flipped 180 degrees though.  Basically I'm not saying that the offset is wrong, it may be, but much more likely is that the spider isn't fitted correctly.

Can you post a pic or two showing the top of the tube with the spider and the position the secondary has been glued in?

 

Ian

 

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Posted (edited)

Ian - the VX8 came fully assembled. I've attached a couple of pictures. Does that help? The three secondary screws you see are the ones I had to replace

since the original ones weren't long enough. When I look at how the secondary is glued to the holder, it does not look well centered as the picture shows.

Regarding the tube itself being too narrow, how do I get the correct offset to get even illumination?

Also, could you explain "Glued in the wrong offset"? I have understood offset to be along the tube axis, correct? Is there

also offset along the perpendicular axis (i.e. the focuser axis) or should the spider just center the secondary wrt the tube?

thumbnail (1).jpg

thumbnail.jpg

Edited by rpnconsult
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I too have had to modify my VX8 quite seriously to get it working properly. The tube is, as mentioned, narrow and flexible. The entrance aperture created by the top-ring is exactly 200mm, meaning all off-axis viewing will be vignetted, the more so the more off-axis. I binned my tube and got a Helmerichs carbon one. But the most serious problem I had was the primary mirror centre-spot. It was 3mm away from the centre of the mirror. That guaranteed collimation impossibility. I strongly suspect that was the reason the person I bought it from sold it: a 1/10 OO mirror that produced appalling views. I re-spotted it and all was good. I would check yours if I were you (in fact I would urge all newt owners to check). My OO 300mm also had an off-centre spot, though by not quite as much.

Magnus

Edited by Captain Scarlet
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It's interesting to note that Mewlons have a rolled inwards lip at the top entrance of the tube which defines the aperture of the telescope.  I don't know, but this detail might be intended to reduce off axis light to enhance the on axis contrast.  Mewlons are legendary for their contrast.    🙂

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7 hours ago, Captain Scarlet said:

I too have had to modify my VX8 quite seriously to get it working properly. The tube is, as mentioned, narrow and flexible. The entrance aperture created by the top-ring is exactly 200mm, meaning all off-axis viewing will be vignetted, the more so the more off-axis. I binned my tube and got a Helmerichs carbon one. But the most serious problem I had was the primary mirror centre-spot. It was 3mm away from the centre of the mirror. That guaranteed collimation impossibility. I strongly suspect that was the reason the person I bought it from sold it: a 1/10 OO mirror that produced appalling views. I re-spotted it and all was good. I would check yours if I were you (in fact I would urge all newt owners to check). My OO 300mm also had an off-centre spot, though by not quite as much.

Magnus




Great post👍      The factory fitted end trim can be modified if you’re ok with DIY.  Remove it and cut the lip so it becomes a band that can be refitted to make a neat job.  This means the entrance aperture of the tube is now larger than the primary mirror.  That’s how EVERY other Newtonian manufacturer make theirs!

A jig saw (sabre saw) works well for that, finish of with files and paint.

 

 

Edited by NGC 1502
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22 hours ago, rpnconsult said:

thumbnail.jpg

Hard to say anything about how the mirror is glued from the image. Maybe looks like its a bit off, but you should be able to adjust the position with the collimation screws and the secondary spider screws.

But i see the same issues i had with mine with your spider. The left and right vanes are bent and the bottom vane is twisted so that it presents a larger surface to incoming light. The left and right vanes will produce double diffraction spikes at slightly lower angles and the bottom one will produce a thicker diffraction spike that also appears on the other side.

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On 25/06/2022 at 19:32, rpnconsult said:

ideas of what could be causing this

Hi

None of the low end rolled steel tubes we've witnessed hold the mirror cell and focuser fixed relative to each other sufficiently well so as  to hold collimation as the tube changes angle, especially with a camera attached.

There are some simple modifications which are needed on such telescopes to make them into reliable astrographs, These include fitting  stronger primary mirror springs, sealing the mirror to its cell, and replacing the secondary support with a light plastic version. However, before replacing the focuser [1], I believe in the case of the vx, the most urgent change you'll need is to spread the tube rings at least 50cm apart on a Losmandy dovetail plate. Then tie the top of the same using rigid aluminium profile. We use 2mm wall 2 x 6 cm aluminium box section for this. This should make the tube rigid enough to obviate the need for the CF upgrade and so eliminate tube flexure from your debugging.

Install the other modifications until the collimation holds at all angles.

Visualisation here.

Cheers and HTH

[1] IIRC the focuser on the vx example we used was fine after adjustment of the tension, but note we only tried it with a dslr. If you're monochrome, you're probably going to have to reinforce the tube around the focuser circumference as well as the other modifications we outlined.

Edited by alacant
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On 26/06/2022 at 13:36, rpnconsult said:

Ian - the VX8 came fully assembled. I've attached a couple of pictures. Does that help? The three secondary screws you see are the ones I had to replace

since the original ones weren't long enough. When I look at how the secondary is glued to the holder, it does not look well centered as the picture shows.

Regarding the tube itself being too narrow, how do I get the correct offset to get even illumination?

Also, could you explain "Glued in the wrong offset"? I have understood offset to be along the tube axis, correct? Is there

also offset along the perpendicular axis (i.e. the focuser axis) or should the spider just center the secondary wrt the tube?

thumbnail (1).jpg

thumbnail.jpg

Sorry for the slow response, I've been away for a couple of days.

It's hard to tell from the photos but the centre bolt of the secondary holder looks closer to the focuser than the tube wall opposite it.  The four brass threaded bolts show a lot of variation on the amount of thread showing, these should be much more even.

The centre bolt on the spider should be exactly ( as close as you can get it) in the centre of the tube opening.  If you set the nuts on the four brass bolts to have an even amount of thread showing inside the tube then the centre bolt should be centred.  If you try this and it's not centred then the spider may be fitted wrongly and you can take it out (with the tube horizontal to avoid the risk of anything falling on the primary), spin it 180 degrees, refit the bolts and set it up again.

The secondary shouldn't be attached centrally on its holder, it should be around 4 or 5mm (rough guess) closer to the primary, this is the offset.  I'd expect the secondary to be glued correctly.

In summary, the spider should be dead central, the offset is taken care of by the secondary being glued off centre to the secondary holder.

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I got all of that except:

"The secondary shouldn't be attached centrally on its holder, it should be around 4 or 5mm (rough guess) closer to the primary, this is the offset. "

Not sure I understand that...shouldn't that distance be controlled by the position of the central bold (i.e. movement towards the primary?

Or do you mean that the secondary HOLDER is centered perfectly in the tube but the secondary mirror is glued with a small offset towards the focuser?

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8 hours ago, rpnconsult said:

the secondary HOLDER is centered perfectly

Exactly that.

On the vx -and most other modern Newtonians-  the secondary is fixed such that when the holder is centred and the telescope is collimated, the offset ensures as evenly illuminated field as possible.

Be sure to read Telia's collimation myths and Seronik's no nonsense guide, both of whom tell it just as it is; simple.

HTH

Edited by alacant
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13 hours ago, rpnconsult said:

 

I got all of that except:

"The secondary shouldn't be attached centrally on its holder, it should be around 4 or 5mm (rough guess) closer to the primary, this is the offset. "

Not sure I understand that...shouldn't that distance be controlled by the position of the central bold (i.e. movement towards the primary?

Or do you mean that the secondary HOLDER is centered perfectly in the tube but the secondary mirror is glued with a small offset towards the focuser?

This page deals with replacing a secondary mirror and has good information on offset.

Basically the secondary holder is centred in the tube and the way the secondary is attached, slightly away from the focuser and closer to the primary, gives the offset

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Very informative, thanks. From the picture I posted above, it looks like the secondary is offset towards the focuser rather than away from it? Is that an assembly error?

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I think this is a decent representation of offset. It's showing the same distance away from the focuser, and, towards the primary.

post-19446-14073932632217.jpg.a10c702699c907cbe476e138d8c722a6.jpg

I use a concentre collimation eyepiece which allows you to fine tune this offset.

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Traditional Newtonian offset is achieved by offsetting away from the focuser and nearer the primary by equal amounts. The excellent diagram in the previous post shows that. Offset can be achieved by fixing the secondary non symmetrically, or having unequal length spider vanes.

However there’s another school of thought that says correct collimation can be achieved with non-offset.  In that case it means that the optical train is correct but won’t be concentric within the tube.

Both methods work but with non-offset collimation it will cause a problem with go-to scopes. That’s because the optical train is not pointing in the exact same direction as the tube.  Another problem will arise with Orion Optics UK Newtonians. The front end trim opening is the same diameter as the primary mirror. Light will be cut off from reaching one edge of the primary mirror, unless the lip is removed as previously mentioned.

For me I prefer traditional offset, it’s not hard if you take your time.

I know all this can be a deterrent for some folk to choose a Newtonian. I think that’s a shame because the Newtonian is definitely the “best bang for the buck” when compared to the alternatives.  And once grasped collimation is not difficult but becomes routine.

 

 

Edited by NGC 1502
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  • 4 weeks later...

Hello all. Thank heavens for this thread ! I was just about to pull the trigger on one of these with 1/10th glass. To say I'm uneasy would be this years' understatement.

 

Think I'll just keep saving up 🙂

Thanks ALL

Collin

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