Deconstructing, Fixing and Upgrading a used OO VX8

[Captain Scarlet]

Some months ago I bought, from a member here, an Orion Optics VX8 1/10-wave-upgraded scope, originally made in June 2017. I am about to transport it from the UK to Ireland, where a new Helmerichs carbon tube awaits. Over the past couple of days I’ve dismantled it to fit into a suitcase for air-transport. The mirror-cell itself proved useful as a way of holding the primary mirror, with the front-clips handily providing clearance above the mirror surface. The two end-caps for the main tube also combined to make a convenient capsule to keep everything protected against any airline-manhandling.

Deconstructed, ready for "suitcasing up", surrounded by shock-absorbing clothes. Spot the clever way I've protected the secondary

Carbon tube awaits:

However, in the process of dismantling, I came across a fair number of things worthy of note. Things which perhaps explain why the seller sold such a nominally lovely scope, and of which the original manufacturer ought to be ashamed.

The first thing to prepare it for transportation was to remove the primary mirror and cell from the OTA. I’ve always rather liked the look of the smaller Orion cells. They comprise a 9-point whiffle-tree design with a well-engineered look about them. I discovered that, although the cell is indeed nice, the way it had been installed into this telescope was anything but.

-    The primary mirror was silicone-glued to the three “floating” triangles, such that many of the nylon pads on which the mirror was supposed to be supported weren’t even touching the mirror.

-    There was more silicone on the sides gluing the mirror to each of the three edge-supports, and in all cases it had by no means been sparingly applied.

-    There was tape wrapped all around the edge of the mirror OVER the side-supports.

-    On the main support base of the cell, there are three lugs with threaded holes to receive big bolts through the side of the tube, providing the means of securing the whole cell to the OTA. These main bolts were finger-tight only, and the smaller bolts securing the lugs to the cell-base were rattling loose.

-    In other words, within the cell as a whole, the things supposed to be loose and free-floating were glued to death, and the things supposed to be tight were loose!

-    The scope is sold as a 200mm f/4.5 scope with a nominal focal length of 900mm. The sticker on the side of the mirror states FL 870.9mm. The difference is enough to seriously alter the position of any holes you might want to drill in a virgin tube. (But actually this suits me, as this will serve part of its duty as a wide-ish field scope). I’m going to have to measure the focal length properly myself before making holes, as I’m not trusting the sticker. It does explain why I had to remove a spacer to achieve focus though.

-    Perhaps most egregiously, if that were possible, the centre “doughnut” on the main mirror I measured to be 3mm away from the true centre of the mirror. Even by eye I could tell it wasn’t quite in the centre. This makes the scope, to an unsuspecting owner, guaranteed to be miscollimated by no small degree! Using normal collimating techniques, a 3mm error at the mirror translates into a 6mm error back at the eyepiece, which means that the eyepiece centre-field will be looking at a spot fully 6mm away from the coma-free part of the image. This explained why I was seeing coma even after several attempts at collimation.

There are other problems with design rather than construction with this scope.

-    Obviously, the supplied tube being thin aluminium, it’s quite flexible, so a long or heavy eyepiece-stack causes droop. This was why I ordered the carbon tube in the first place.

-    Every single one of the fittings is posi-drive. I would have thought Hex or Torx would be more appropriate for something as modern as 2017? Perhaps I am wrong here.

-    The end-rings for the main tube, which help keep it round, wrap inwards over the tube-end causing the clear aperture at the front end to be exactly 200mm. In other words, 800mm or so in front of the mirror, the aperture is still the same as the mirror itself, clipping all off-axis rays and reducing illumination for anything other than the very centre-point of the image.

I feel so sorry for whoever has owned this scope before me, especially the person who bought it new. I can imagine the anticipation and excitement buying a scope with such a lovely mirror and cell, followed by terribly disappointing performance: a scope requiring remedial attention well beyond what a normal buyer of even a premium scope would expect to do; and providing horrid views of the sky. So far I’ve had to completely dismantle the cell to its constituent parts to remove all the silicone. I’ll have to soak off the centre-spot and place one actually in the centre: not too difficult to do but certainly not expected.

Pictures below with some annotation. And I’ll add more to the thread to document its journey into the carbon tube and subsequently to comment on its hopefully stellar performance.

Cheers, Magnus

***

Silicone everywhere. Whoever did this looks as though they were in a hurry:

 

Just in case the silicone isn't enough to hold the mirror rock solid:

 

some nylon-tip supports not even touching the mirror:

 

OTA attachment lug rattling loose:

 

Silicone and tape as much removed as possible:

 

Pic showing actual centre of the mirror (the little cross), vs the doughnut:

 

Edited March 31, 2022 by Captain Scarlet
more pics

[iantaylor2uk]

I have an Orion Optics CT-8, which is also advertised as a 900 mm focal length (f4.5) but my ASI Air Pro told me the actual focal length was also about 870 mm. 

However, if you use a coma corrector you will likely see a focal length above 900 mm (my Explore Scientific HR coma corrector will multiply the focal length by 1.06 when used at the correct spacing - so it should come up to about 922 mm). 

[Dave scutt]

The carbon tube looks so nice can't wait to see pictures of it finished 

[Captain Scarlet]

55 minutes ago, Dave scutt said:

The carbon tube looks so nice can't wait to see pictures of it finished 

Ironically length for length the carbon tube is heavier than the alu one: it’s lined with 6mm of hardfoam on the inside to make it extremely stiff. Not such a long drive for you to come and see it in person?

M

[Dave scutt]

1 hour ago, Captain Scarlet said:

Ironically length for length the carbon tube is heavier than the alu one: it’s lined with 6mm of hardfoam on the inside to make it extremely stiff. Not such a long drive for you to come and see it in person?

M

Thanks for the offer If I'm ever down that way I'll let you know or you can always pop up my way to the dark site 

[davidc135]

Captain Scarlet.

Do you think that the mirror deserves its 1/10 wave rating?  David

[Captain Scarlet]

11 minutes ago, davidc135 said:

Captain Scarlet.

Do you think that the mirror deserves its 1/10 wave rating?  David

That I couldn’t say, but I have no reason to think otherwise. The zygo report for it certainly supports the 1/10-ness. I also have an OO 300mm 1/10 mirror which I bought new a couple of years ago and it star-tested beautifully one night of good seeing and stable temperature, and gave a brief view of Mars of amazing clarity. It is after all “what they do” so I’d be reasonably confident the mirror is as claimed.

However the proof of the pudding is in the, er, observing, so once I’ve got it properly installed again I’ll report back.

M

[markse68]

I wonder if a previous owner might have applied the silicone and tape in an attempt to lock the collimation for AP Magnus? I know that with my AP scope which has the same mirror and cell, even tiny rotation or lateral motion of the mirror throws the collimation slightly. My mirror reads ~880mm in Siril’s photometric calibration (which uses platesolving) so it sounds like it’s common.

Hope you love your new scope- it’ll certainly look stunning

Mark

[jetstream]

7 hours ago, Captain Scarlet said:

    The primary mirror was silicone-glued to the three “floating” triangles, such that many of the nylon pads on which the mirror was supposed to be supported weren’t even touching the mirror.

-    There was more silicone on the sides gluing the mirror to each of the three edge-supports, and in all cases it had by no means been sparingly applied.

-    There was tape wrapped all around the edge of the mirror OVER the side-supports.

Magnus, years ago I purchased an 8" f3.8 OOUK and it arrived in the same way with respect to the mirror cell and had massive astigmatism. It gets better- after removing the silicone the mirror was too thick to be properly supported with the nylon tips... if supported properly the mirror was pushed up into the "L" clips that retain the mirror. This also added big restraint on the glass.

Make sure you have enough room to support the mirror properly while maintaining a bit of room to the clips.

I posted this years ago with pictures.

[Stu]

9 hours ago, markse68 said:

I wonder if a previous owner might have applied the silicone and tape in an attempt to lock the collimation for AP Magnus?

I believe this is fairly standard practise from OO, I took a mirror out of a 12” scope a while back and found the same tape, actually there was much more than in Magnus’ picture, but no silicone. It was a pain to remove, I don’t understand why it is needed if the cell is doing it’s job properly (which it looks to be capable of)

[markse68]

24 minutes ago, Stu said:

I believe this is fairly standard practise from OO, I took a mirror out of a 12” scope a while back and found the same tape, actually there was much more than in Magnus’ picture, but no silicone. It was a pain to remove, I don’t understand why it is needed if the cell is doing it’s job properly (which it looks to be capable of)

Interesting Stu- mine has no signs of tape or silicone but they may have been well cleaned off by po. I have found that i need to set a reasonable gap between the side mirror supports and mirror as the aluminium cell contracts in the cold and pinches the mirror otherwise. This does leave the mirror loose and prone to rotation which throws the collimation. I wonder if the taping would have any detrimental effect other than looking like a bit of a bodge? I saw they siliconed the nylon grub screws on Magnus’ cell- what i did was remove all the screws and squash the threads in the middle a bit with some needle-nose pliers so when reinserted they are a tight fit and stay where they’re set

Mark

[Captain Scarlet]

11 hours ago, jetstream said:

Magnus, years ago I purchased an 8" f3.8 OOUK and it arrived in the same way with respect to the mirror cell and had massive astigmatism. It gets better- after removing the silicone the mirror was too thick to be properly supported with the nylon tips... if supported properly the mirror was pushed up into the "L" clips that retain the mirror. This also added big restraint on the glass.

Make sure you have enough room to support the mirror properly while maintaining a bit of room to the clips.

I posted this years ago with pictures.

I did put it the top half back together again before putting it into its travel-capsule, and it fits in fine with enough clearance to allow it to all work as it should. I'll demonstrate in a few days when I resume the build.

I do plan on a couple of modifications though. I'll grind off the "hook-over" bits of the restraining clips: they're far too big for their job and will introduce quite large needless diffraction artifacts. I'll replace them with a much thinner (from the point of view of the light-path) arrangement, perhaps such as Obsession suggest for their big dobs. And if I'm only ever going to use it on an alt-az mount, I might fashion a sling between a pair of those side-posts to spread the load more evenly. It won't make too much difference on a mirror as small as this but everything counts and it's all practise for my impending 20" dob build for which such an arrangement is truly necessary.

1 hour ago, markse68 said:

... I have found that i need to set a reasonable gap between the side mirror supports and mirror as the aluminium cell contracts in the cold and pinches the mirror otherwise. ...

My calculations might be wrong but on an initial lookup Aluminium's expansion coefficient (dL/L = 23e-6 per K) suggests that a, say, 160mm length will contract by 1/270th of a millimeter per degree change in temperature. Or for 10 degrees change, that's 1/27th of a mm. I can't believe that's enough to noticeably pinch from the standard suggested "paper-thin" gap unless the nylon side-grubs have already been tightened onto the mirror? (Which I used to do myself before someone asked me innocently why all my stars were triangular )

Cheers, Magnus

Edited April 1, 2022 by Captain Scarlet
typo

[ONIKKINEN]

2 hours ago, Stu said:

I believe this is fairly standard practise from OO, I took a mirror out of a 12” scope a while back and found the same tape, actually there was much more than in Magnus’ picture, but no silicone. It was a pain to remove, I don’t understand why it is needed if the cell is doing it’s job properly (which it looks to be capable of)

My 8'' dated 2020 has the tape and some blobs of silicone, but not nearly as much silicone as this one here, i think the mirror cell in question here might be a bit of a monday morning product?

The tape in mine seems a bit redundant, but at least it doesn't feel like its in the way. It doesn't really hold the mirror tightly, it just sort of loosely sits there around the mirror and the clip holders so for me i doubt it could pinch the mirror, or really hold it in place so its just useless. I can wiggle the mirror a bit on the cell if i try, but i do have to try so i guess my cell is working more or less as intended.

[Stu]

39 minutes ago, ONIKKINEN said:

My 8'' dated 2020 has the tape and some blobs of silicone, but not nearly as much silicone as this one here, i think the mirror cell in question here might be a bit of a monday morning product?

The tape in mine seems a bit redundant, but at least it doesn't feel like its in the way. It doesn't really hold the mirror tightly, it just sort of loosely sits there around the mirror and the clip holders so for me i doubt it could pinch the mirror, or really hold it in place so its just useless. I can wiggle the mirror a bit on the cell if i try, but i do have to try so i guess my cell is working more or less as intended.

The tape was not a problem at all, until I wanted to clean the mirror, then it was a tight pain, leaving stickiness all over the cell which I had to clean up.

[Captain Scarlet]

54 minutes ago, Stu said:

The tape was not a problem at all, until I wanted to clean the mirror, then it was a tight pain, leaving stickiness all over the cell which I had to clean up.

Yes this is a worry for me too. The residues of silicone especially on the sides add friction when dry and might reduce friction when wet,  making misjudging grasping it all the easier. Will have to be extremely careful.

[jetstream]

2 hours ago, Captain Scarlet said:

I'll grind off the "hook-over" bits of the restraining clips:

Thats where I had the issues, the hook overs

[Captain Scarlet]

OK I’m TRYING to get on with this, but I have some difficulties…

 

 

… HE thinks he’s helping

Edited April 14, 2022 by Captain Scarlet

[ukskies]

Don't saw the cat in half!😆