Collimating an Orion Optics VX12 F4

[Trentend]

Having a go collimating my recently purchased/used scope using the very thorough instructions on the astrobaby site. View through a collimating cap reveals a pretty circular view of the secondary mirror but this doesn’t appear to be centred and needs to move “down” (see below, first picture with focuser fully extended and second retracted). I’ve measured the length of the spider vains and a couple of these are out by around 5mm. Is this something I should adjust or am I ready to move on to the next phase aligning the secondary to primary mirror?

 

[johninderby]

It’s called secondary offset.

Plenty of info about it such as here.

https://www.lcas-astronomy.org/articles/display.php?filename=newtonian_secondary_offset&category=telescopes

[Grimbles]

Hi

Your images definitely show a few issues.   On an F4 scope collimation is pretty critical especially if you are going to be imaging, I would follow Astro Baby' s guide (which is excellent) closely, for example  if your spider vanes are not equal lengths to start with then your secondary will not be centred. 

Secondary offset is something that is usually set at manufacture in how the secondary mirror is mounted on the holder.  Once collimated the offset will be visible in the final views of the collimation process again Astro Baby explains this very well.

Just follow the guide and you will be fine, but you may need to have a couple of attempts, again with an F4 scope for imaging try to get all the adjustments as close as possible.  Secondary adjustment is definitely the trickiest part but take things slowly and once you have done it a few times it will become easier, and when correct shouldn't need touching again.

Good Luck

 

Martin

[Trentend]

Thanks John, must admit I’ve read this a few times and still unclear in my mind if I try and adjust or just move on.

[Trentend]

Thanks Martin, posted my reply to John just as your on advice came through. I’ll be doing visual only but looks like I need to adjust this. 

A related question, how tight should I tighten the screws on the secondary once I get back to that? Had the scope on it’s side applying quite a bit of pressure to get some movement.

[Grimbles]

7 hours ago, Trentend said:

Thanks Martin, posted my reply to John just as your on advice came through. I’ll be doing visual only but looks like I need to adjust this. 

A related question, how tight should I tighten the screws on the secondary once I get back to that? Had the scope on it’s side applying quite a bit of pressure to get some movement.

I assume you mean the secondary spider vanes, which shouldn't need much force at all, slacken one side then tighten the opposite side.  if you apply too much force you will start to distort the tube which will cause other problems.  Lightly finger tight is all I do to mine.

If you mean secondary adjustment screws, then again just nipped up is enough, you may have to loosen one to adjust another, but don't try to physically force anything you will risk stripping a thread or worse.

It is not nearly a bad or difficult as it may seem at first just go slow and do not be too heavy handed.

Martin

[AKB]

10 hours ago, Grimbles said:

Secondary adjustment is definitely the trickiest part

This is certainly the case.  However, you may want to try the rather simple and effective “milk bottle top” modification, which helps enormously...

 

 

[Trentend]

It was the secondary adjustment screws. I’d been applying a lot of force. Good to know I should stop and clearly something else misaligned. Any recommended links for adjusting the spider vanes?

[Moonshane]

It looks like you need to adjust the central screw to take the secondary towards the primary a but and also adjust the tilt of the secondary screws to move the mirror 'down' in the view shown. Re the vanes just asses which way it needs to go and loosen the opposite external nut and then tighten the one in the direction of movement. Don't overtighten though as the OOUK threaded bars are quite delicate. Don't sweat it though as their tubes are not round. Rely more on what you see not what you measure.

[Grimbles]

1 hour ago, Trentend said:

Any recommended links for adjusting the spider vanes?

Astro Babys guide starts with spider vane adjustment, I think you have already been there but here's the link any way.

http://www.astro-baby.com/astrobaby/help/collimation-guide-newtonian-reflector/

If you follow the whole collimation guide from start to finish you will not go far wrong, many people use and recommend this guide.

[NGC 1502]

 

I wouldn’t worry over much about the length of the spider vanes, the important bit is the position of the secondary itself in line with the focuser.  OO secondaries are normaly siliconed to the holder with the necessary offset away from rhe focuser. If you get the secondary central “under” the focuser you have automatically achieved the offset towards the primary.

Of course at F4 collimation needs to be spot on for visual or imaging.   Although I own two OO UK Newtonians one feature of them I don’t like is the tube trim at the upper end of the tube. The circular aperture is only just a fraction larger than the primary mirror.  Almost all DIY Dob builders will have the internal diameter of the upper tube at least 10% larger than the primary.   Your scope can be accurately collimated and yet not be exactly in line with the tube. If that’s the case then having the opening at the upper end of the tube barely larger than the primary means that some light is not reaching the mirror.  How significant that is I’m not so sure, but there’s no real reason to not have a larger entrance at the top of the tube.

I can hear the sound of incoming missiles ? but I’m definitely not panning OO UK, I like the two I own.

Cheers, Ed.

 

[Scooot]

On my OO VX10 I seem to recall I tweaked the collimation screws on my focuser to centre the secondary under it. As long as the flat still appears round, & if not it can be rotated slightly, good collimation is achievable. I can still see all the mirror clips so don’t lose any light. Saved adjusting the spider vanes.   

[Trentend]

Ok, had a further fiddle this evening including loosening the central screw on the secondary and also making a minor adjustment to the vanes. Looks more centred this time around (first image below) compared to yesterday (bottom). Had a quick peak at the primary also. I can  see two clips, the third is visible but not in front of the mirror, more within the tube. Cheshire ordered today and the journey continues..

[cletrac1922]

Had  a guy recently who rocked up to club meet with a SW 8'' collapsible dob, had brought off E-Bay not long before, and said everything was blurry when looked through the eyepiece

I put in my laser collimator, and things would not line up

Grab a bit of paper, and measured from centre of secondary mounting screw, to outer edge of the vain

Found the secondary, was about 10cms to one side

How got like that no idea

Using my trusty bit of paper, marked off each vain, as loosening one side, tightening the other

After a few minutes secondary was dead centre all direction

Dropped the laser collimator back in, and a quick twig of the 3 outer screws of the secondary, and was centre of the primary

Never, Never, loosen the centre mounting screw of the secondary, as can cause the entire secondary unit to slew any direction, then you up the creek without a paddle

Fine tune of the primary, and aimed scope at Saturn, and rings crisp as crisp, 

Admit was in a fairly light free location, and clear crisp evening

Lot of people say use a Cheshire, others a laser, use plastic tube in eyepiece film canister 

My preference is laser collimator 

John

 

[Carbon Brush]

Is it possible that among the alignment issues, the focusser is not pointing to the right place?

I had an OO scope where the focusser pointed to a place other than the tube centre.
This was just one of a few issues on a new from the factory scope.

There are various ways to to check this.

A simple one being with the tube horizontal on a table.

Place a spirit level across the focusser eyepiece end and set level.
Drop a plumb line from the tube centre into the tube.
It should line up on the optical axis with the secondary mirror central screw
 

Hope this helps, David.

[NGC 1502]

3 hours ago, Carbon Brush said:

Is it possible that among the alignment issues, the focusser is not pointing to the right place?

I had an OO scope where the focusser pointed to a place other than the tube centre.
This was just one of a few issues on a new from the factory scope.

 

 

Good idea to check focuser alignment, although I’m thinking it has to be quite a bit off to really matter. Of course “quite a bit off” may mean different things to different people ?.   If it’s “quite a bit off” it can produce an issue during collimation when the out of focus star can look different inside of focus compared with outside of focus. That’s because the focuser and eyepiece are passing through focus obliquely rather than in an exact line towards the centre of the optical train.

Having said that, and if anyone feels put off from buying a Newtonian, please don’t take it that way. There’s no doubt in my mind that a good Newtonian is very hard to beat unless you’re spending an absolute fortune on a different optical design.

And I do agree that OO UK could do more to address some issues that need sorting by the purchaser, however I wouldn’t let that put me off buying from them again. But you could spend far less and get something very worthwhile..........

Ed.