Collimation

[Jazzo111]

Hey friends! I was just hoping for some advise my gf recently joined the community and got herself a vt8 classic dobsonian telescope however i cant collimate the secondary mirror no matter how hard i try it seems like the secondary mirror is to far off. I tried tightening and loosening the middle nut instead and the mirror spins in a compete circle, is this normal? Would a local teleacope shop be able to fix or look at this maybe?

collimating my teleacope celesteon omni xlt 150 is a breeze so im thinking its a issue with the mechanics of the secondary mirror hopefully not considering the price payed for it..

 

any help will be appreciated

[RT65CB-SWL]

Hi @Jazzo111.

When you say: "...a local telescope shop..." - do they specialise in astronomy equipment only, then maybe worth it. If they specialise in other stuff, forget about them. Sorry to be so blunt with an answer. My best advice would be take it along to a local astro-society/club. I am sure if you ask, someone will be able to assist/advise.

[Helen]

Hi and welcome to SGL 🙂 

If you want to find a local society just plug your postcode into this page 😉 

https://www.firstlightoptics.com/locator.html

Hope you get it sorted - an 8inch dob will be fun to use!

Helen

[Jazzo111]

Aww cool theres one 25 miles from me thats good to know.

I may phone the shop and ask on the phone they specialise im cameras teleacopes and other such things.

[Peter Drew]

Knowing roughly where you are might help, there could be an experienced SGL member nearby who could help you.   😀

[Jazzo111]

South wales neath 😂 best country for stargazing brecon beacons not far from me is the top 5 places in the world for stargazing surprisingly pitch black no light pollution what so ever.

[Jazzo111]

Update.. tried a teleacope shop no joy and done my own research and found out secondary mirrors on fast telescopes are purposely off centre which makes things even more difficult

I did try a local astronomy centre phoned  no answer and only have meetings on certain days next one being late june.. any advise on how to focus a secondery mirror back in the exact spot? Putting in under the focuser is one thing but how do i work out the offset? 

Does a vt8 telescope have a slightly offset secondary mirror? I tried to email the manufacturer and didnt get a email back

[fiestazetecmk2]

Sounds a bit odd to me.never had a problem with my 200p with collumation or setting it.

[Jazzo111]

15 minutes ago, fiestazetecmk2 said:

Sounds a bit odd to me.never had a problem with my 200p with collumation or setting it.

The misses loosened the middle nut with the other nuts and spun the secondery 360 degrees it almost fell off it was her first attmept of collimation so not her fault, been trying to tix it since.

[mikeyj1]

Hi there,

firstly will it focus? Were you 'checking' collimation or is/was there a problem? Second, what are you using to collimate; Laser or Cheshire?  Laser wont do the secondary, especially if its a mile out. 

Use a cheshire and get everything concentric, important is that the mirror reflection is round, move the focuser out until you can see the sides to compare to the cheshire tube. 

Yes if you loosen the central screw the mirror can rotate, but you also need to do this to get the secondary mirror positioned under the focuser correctly.  (There is a possibility the focus tube is out of alignment, but probably don't need to go that far to start with).  Have you got links to astronomy shed's YT video and astro baby's collimation guide? they are great for explanation.  (I actually did wind the screws all the way in one time to make sure the were all at the same position, but remember the three screws act against the central screw, so be careful not to overtighten)

Once the secondary is set up, move on to the primary.

Sorry if you already know all that, just thought i had better put it all in there

Good luck, 

 

[Ricochet]

55 minutes ago, Jazzo111 said:

Does a vt8 telescope have a slightly offset secondary mirror?

It will have the offset away from the focuser built into the secondary stalk. So long as the secondary looks centred when you look through a cheshire/sight tube it will be fine. 

To get your mirror back in the right place put one hand into the telescope and take hold of the secondary mirror stalk (not the mirror) and turn it so that it faces the focuser. While still holding the stalk, tighten up the centre screw. 

[fiestazetecmk2]

If your unsure look up collumation on you tube .there are some easy to follow set ups to watch.

[Jazzo111]

I think im just paranoid i can collimate it, it seems. I use a laser and chershire but just worried after it spun so much that the secondary not in the optimum position with a offset tilt.. i seen YouTube videos explaining how a offset secondary can be still collimated with a laser making the situation worse.

Il look more into it but its information overload for me atm being new to this its so much to take in

[John]

Are we talking about the Orion Optics VX8 here ?

Is it the F/4.5 or F/6 version ?

With many newtonians, including the Orion Optics ones, the secondary offset is built into the way that the secondary mirror is fixed to it's mount.

I find a simple cheshite eyepiece works well with my Orion Optics 12 inch F/5.3 dobsonian.

 

[Jazzo111]

5 minutes ago, John said:

Are we talking about the Orion Optics VX8 here ?

Is it the F/4.5 or F/6 version ?

With many newtonians, including the Orion Optics ones, the secondary offset is built into the way that the secondary mirror is fixed to it's mount.

I find a simple cheshite eyepiece works well with my Orion Optics 12 inch F/5.3 dobsonian.

 

Apolgies its not my scope its a Orion skyquest xt8 classic dobsonian its a f5.9 i believe

Edited May 28, 2019 by Jazzo111

[John]

Ok - thats basically the same scope as the Skywatcher Skyliner 200P dobsonian. The secondary mirror offset is built in to the secondary mount design with those.

 

[Jazzo111]

6 minutes ago, John said:

Ok - thats basically the same scope as the Skywatcher Skyliner 200P dobsonian. The secondary mirror offset is built in to the secondary mount design with those.

 

Oh really so just turn it into the focuser and its set? What about the celestron omni xlt 150? Just for furure reference.. thats a lot of weight off my shoulders thank you lol 

[John]

The Celestron is the same scope as the Skywatcher Explorer 150P. The same applies to the secondary.

What collimation tools do you have ?

[Jazzo111]

[Jazzo111]

I managed to get the collimator more aligned it was way off i dont often use the laser 

[John]

If the laser is itself well collimated, it can be useful to get the secondary tilt adjusted so that the laser spot is in the "donut" ring on the primary. Then you go on to do the primary tilt with the cheshire collimator.

If you think it is OK try and star test on Polaris and see how it looks a bit inside and outside of focus. The dark shadow of the secondary mirror should be pretty much central both sides of focus.

 

[wookie1965]

Use the Cheshire unless you know the laser is dead on most cheap ones are not as John has said.

Position the secondary under the focuser so you can see a round of circle as you can and tighten up the centre screw making sure you have backed off the 3 collimation screws.

from there follow Astro Baby's and this site I used them when I did my 150p and 200p.

http://www.schlatter.org/Dad/Astronomy/collimate.htm

 

[bluesilver]

I have just been through this exact process with my Skywatcher Explorer 250P,  so exactly the same,  mine was a bit worse as i took the secondary completely out.

Have you got a empty film canister?  I found this made it easier for myself.

I used the Cheshire get the secondary square to the focuser first,  should end up looking like a nice round circle dead centre when looking through the Cheshire.

Then tighten up the centre screw, using the film canister,  drill a small hole in the cap and cut the bottom off,  then looking through the small hole in the canister cap,  adjust the 3 screws until you can see the primary mirror clips in the secondary mirror,  keep adjusting until all mirror clips are viewed nice and even.

Then put the cheesier back in and adjust the primary screws until the centre mark in the primary lines up with the centre of the Cheshire cross.

Should then spot on.  

At least this is what i did in the end and worked spot on for myself.

 

 

 

[wookie1965]

6 hours ago, bluesilver said:

I have just been through this exact process with my Skywatcher Explorer 250P,  so exactly the same,  mine was a bit worse as i took the secondary completely out.

Have you got a empty film canister?  I found this made it easier for myself.

I used the Cheshire get the secondary square to the focuser first,  should end up looking like a nice round circle dead centre when looking through the Cheshire.

Then tighten up the centre screw, using the film canister,  drill a small hole in the cap and cut the bottom off,  then looking through the small hole in the canister cap,  adjust the 3 screws until you can see the primary mirror clips in the secondary mirror,  keep adjusting until all mirror clips are viewed nice and even.

Then put the cheesier back in and adjust the primary screws until the centre mark in the primary lines up with the centre of the Cheshire cross.

Should then spot on.  

At least this is what i did in the end and worked spot on for myself.

 

 

 

Perfect method.

[JOC]

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

@Jazzo111 Click the link above and follow it to the letter - do absolutely every stage as written, even it seems daft to do so - use a basic Cheshire collimator - no fancy laser needed.  I was terrified when I did my first one, but this guide is faultless and the most recommended guide on SGL.  The only tip it doesn't give and what I found with my secondary is that as you tighten the screws on it gives one final movement and I just found that I needed to account for this when I positioned it.  You will note if you read the article to the end that some Newtonians are still perfectly collimated when the mirror reflections do seem very slightly off-centre.